Laser chamber gas flow and acoustic control

Abstract

In an apparatus for generating laser radiation from discharges in a discharge chamber, with opposed surfaces of electrode supports defining at least a portion of a gas flow channel, in which at least one flow insert is positioned in the gas flow channel to regulate the flow of gas in the gas flow channel. The at least one flow insert may also be arranged to regulate acoustic phenomenon in the discharge chamber.

Description

LASER CHAMBER GAS FLOW AND ACOUSTIC CONTROLCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Application No. 63 / 611,442, filed December 18, 2023, titled LASER CHAMBER GAS FLOW AND ACOUSTIC CONTROL, which is incorporated herein in its entirety by reference.FIELD

[0002] The disclosed subject matter relates to systems for controlling the circulation of and acoustic waves in gas in a laser discharge chamber such as for a deep ultraviolet (DUV) radiation laser source.BACKGROUND

[0003] Photolithography is a process by which semiconductor circuitry is patterned on a substrate such as a silicon wafer. A photolithography radiation source provides the deep ultraviolet (DUV) radiation (wavelengths in a range of about 100 nanometers (nm) to about 400 nm) used to expose a photoresist on the wafer. Often, the radiation source is a laser source and the radiation is a pulsed laser beam. The radiation beam is passed through a beam delivery unit, then a reticle or a mask, and then projected onto a silicon wafer coated with photoresist. In this way, a chip design is patterned onto a photoresist that is then etched and cleaned.

[0004] In such sources the laser beam is produced by causing discharges in an interelectrode discharge region of one or more laser discharge chambers. During a discharge the gas in the discharge region between the electrodes becomes depleted of one or more components and must be replenished. To accomplish this a flow of gas must be established through the discharge region, with an inflow of fresh gas continuously displacing an outflow of spent gas. This spent gas is a complex mixture of different species including incompletely reassociated atomic fluorine and discharge products such as metal electrode debris in the form of ions, neutral atoms, or molecular clusters. An excessive amount of residual gas in the discharge region can interfere with the next discharge. The spent gas can also interfere with preionization of the fresh gas in the discharge region. The spent gas can also promote a phenomenon called downstream arcing in which undesirable discharges occur with improper timing and in a position downstream of the desired discharge position. Downstream arcing typically leads to energy dropouts, such as incidents where an electrical pulse applied to electrodes fails to produce an adequate laser light pulse. These problems become more acute as repetition rates increase and the time between discharges becomes shorter.

[0005] One technical challenge in the design and operation is thus carrying out gas replenishment sufficiently before the next discharge event. There is thus a need to regulate the flow of gas into, through, and out of the discharge region.

[0006] Another challenge in the design and use of these systems is that the discharge which produces the laser radiation also generates strong acoustic waves inside the discharge region. These acoustic waves create gas density modulations that propagate within the laser discharge chamber. Surfaces in the laser discharge chamber may reflect these acoustic waves back into the discharge region and adversely affect the laser’s performance. In particular these reflected waves may result in a round trip time-of-flight resonance depending on the inter-pulse delay or discharge repetition rate at which the laser system is operated.

[0007] It would be advantageous to improve the flow of gas on the discharge regions. It would also be advantageous to mitigate the adverse effects of acoustic disturbances. It is in this context that the need for the subject matter of the present disclosure arises.SUMMARY

[0008] The following presents a succinct summary of one or more embodiments in order to promote a basic understanding of the presently disclosed subject matter. This summary is not an extensive overview of all contemplated embodiments and is not intended to identify as key or critical any elements of any embodiments nor delineate the scope of any or all embodiments. Its sole purpose is to present some concepts relating to one or more embodiments in a streamlined form as a prelude to the more detailed description that is presented later.

[0009] According an aspect of an embodiment there may be disclosed a gas flow control system for a discharge chamber of a laser system, the gas flow control system comprising a first electrode support, a second electrode support arranged in an opposed relationship to the first electrode support to define a gas flow channel arranged to cause gas flow substantially in a gas flow direction, and a flow insert positioned in the gas flow channel between the first electrode support and the second electrode support.

[0010] The flow insert may comprise at least one guide vane arranged to extend codirectionally with the gas flow direction. The first electrode support may support a first electrode and the second electrode support may support a second electrode at a position opposed to the first electrode to define a discharge gap and the at least one guide vane may be arranged at a position in the gas flow channel downstream with respect to the gas flow direction from the discharge gap.

[0011] The first electrode support may support a first electrode and the second electrode support may support a second electrode at a position opposed to the first electrode to define a discharge gap and the at least one guide vane may be arranged at a position in the gas flow channel upstream with respect to the gas flow direction from the discharge gap. The at least one guide vane may have at least one curved surface arranged to extend codirectionally with the gas flow direction.

[0012] The flow insert may comprise a plurality of guide vanes arranged to extend codirectionally with the gas flow direction, the plurality of guide vanes being arranged such that the guide vanes are substantially collinear with one another. The flow insert may comprise a plurality of guide vanesarranged to extend codirectionally with the gas flow direction, the plurality of guide vanes being arranged such that the guide vanes are substantially parallel with one another.

[0013] The flow insert may comprise a first plurality of guide vanes arranged to extend codirectionally with the gas flow direction, the first plurality of guide vanes being arranged such that the guide vanes are substantially collinear with one another, and a second plurality of guide vanes arranged to extend codirectionally with the gas flow direction, the second plurality of guide vanes being arranged such that the guide vanes are substantially collinear with one another, the first plurality being displaced from the second plurality in a direction transverse to the gas flow direction.

[0014] The flow insert may comprise a first plurality of guide vanes arranged to extend codirectionally with the gas flow direction, the first plurality of guide vanes being arranged such that the guide vanes are substantially collinear with one another, and a second plurality of guide vanes arranged to extend codirectionally with the gas flow direction, the second plurality of guide vanes being arranged such that the guide vanes are substantially collinear with one another, the first plurality being displaced from the second plurality codirectionally with the gas flow direction. The flow insert may comprise at least one vortex generator on one of the first electrode support and the second electrode support and extending into the gas flow channel.

[0015] The first electrode support may support a first electrode and the second electrode support may support a second electrode at a position opposed to the first electrode to define a discharge gap and wherein the at least one vortex generator may be arranged on one of the first electrode support and the second electrode support at a position in the gas flow channel upstream with respect to the gas flow direction from the discharge gap. The first electrode support may support a first electrode and the second electrode support may support a second electrode at a position opposed to the first electrode to define a discharge gap and wherein the at least one vortex generator may be arranged on one of the first electrode support and the second electrode support at a position in the gas flow channel downstream with respect to the gas flow direction from the discharge gap.

[0016] The flow insert may comprise a plurality of vortex generators arranged to extend codirectionally with the gas flow direction, the plurality of vortex generators being arranged such that the respective vortex generators extend substantially collinearly with one another. The flow insert may comprise a plurality of vortex generators each arranged to extend codirectionally with the gas flow direction, the plurality of vortex generators being arranged such that the respective vortex generators extend substantially parallel with one another. The flow insert may comprise a first plurality of vortex generators each arranged to extend codirectionally with the gas flow direction, the first plurality of vortex generators being arranged to be substantially collinear with one another, and a second plurality of vortex generators arranged to extend codirectionally with the gas flow direction, the second plurality of vortex generators being arranged to be substantially collinear with one another, the first plurality being displaced from the second plurality in a direction transverse to the gas flow direction.

[0017] The flow insert may comprise a first plurality of vortex generators each arranged to extend codirectionally with the gas flow direction, the first plurality of vortex generators being arranged to be substantially collinear with one another, and a second plurality of vortex generators arranged to extend codirectionally with the gas flow direction, the second plurality of vortex generators being arranged to be substantially collinear with one another, the first plurality being displaced from the second plurality in a direction parallel to the gas flow direction.

[0018] According to another aspect of an embodiment there is disclosed a gas flow control system for a discharge chamber of a laser system, the discharge chamber including an anode and a cathode, the gas flow control system comprising a cathode support mechanically coupled to the cathode, an anode support mechanically coupled to the anode and arranged in an opposed relationship to the cathode support to define a gas flow channel arranged to cause gas flow substantially in a gas flow direction, and at least one guide vane positioned in the gas flow channel between the cathode support and the anode support and arranged to extend codirectionally with the gas flow direction.

[0019] The anode support may support the anode at a position opposed to the cathode to define a discharge gap and the at least one guide vane may be arranged at a position in the gas flow channel downstream with respect to the gas flow direction from the discharge gap. The anode support may support the anode at a position opposed to the cathode to define a discharge gap and wherein the at least one guide vane may be arranged at a position in the gas flow channel upstream with respect to the gas flow direction from the discharge gap. The at least one guide vane may have at least one curved surface arranged to extend codirectionally with the gas flow direction.

[0020] According to another aspect of an embodiment there is disclosed a gas flow control system for a discharge chamber of a laser system, the discharge chamber including an anode and a cathode, the gas flow control system comprising a cathode support mechanically coupled to the cathode, an anode support mechanically coupled to the anode and arranged in an opposed relationship to the cathode support and having a surface defining a gas flow channel arranged to cause gas flow substantially in a gas flow direction, and at least one vortex generator positioned on the surface and extending into the gas flow channel.

[0021] The gas flow control system may further comprise a plurality of vortex generators arranged to extend codirectionally with the gas flow direction, the plurality of vortex generators being arranged such that the respective vortex generators extend substantially collinearly with one another. The gas flow control system may further comprise a plurality of vortex generators each arranged to extend codirectionally with the gas flow direction, the plurality of vortex generators being arranged such that the respective vortex generators extend substantially parallel with one another.

[0022] Further embodiments, features, and advantages of the subject matter of the present disclosure, as well as the structure and operation of the various embodiments, are described in detail below with reference to the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the presently disclosed subject matter and, together with the description, further serve to explain the principles of the presently disclosed subject matter and to enable a person skilled in the relevant art(s) to make and use the presently disclosed subject matter. The drawings are not to scale unless otherwise indicated.

[0024] FIG. 1 is a schematic diagram of an overall broad conception of a photolithography system.

[0025] FIG. 2 is a schematic diagram of an overall broad conception of an illumination system such as might be used in the photolithography system of FIG. 1.

[0026] FIG. 3 is a cross section of a discharge chamber such as might be used in the illumination system of FIG. 2.

[0027] FIG. 4 is a cross section of a portion of the discharge chamber according to an aspect of an embodiment.

[0028] FIG. 5 is a cross section of a portion of the discharge chamber according to an aspect of an embodiment.

[0029] FIG. 6 is a cross section of a portion of the discharge chamber according to an aspect of an embodiment.

[0030] FIG. 7 is a cross section of a portion of the discharge chamber according to an aspect of an embodiment.

[0031] FIG. 8 is a cross section of a portion of the discharge chamber according to an aspect of an embodiment.

[0032] FIG. 9A is a cross section of a portion of the discharge chamber according to an aspect of an embodiment. FIG. 9B is a top view of a portion of the discharge chamber of FIG. 9A.

[0033] FIGS. 10A - 10E are top views of anodes and anode support surfaces with vortex generators according to an aspect of an embodiment

[0034] FIG. 11 is a cross section of a portion of the discharge chamber according to an aspect of an embodiment.

[0035] Further features and advantages of the presently disclosed subject matter, as well as the structure and operation of various embodiments of the presently disclosed subject matter, are described in detail below with reference to the accompanying drawings. It is noted that the scope of the presently disclosed subject matter is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art based on the teachings presented herein.DETAILED DESCRIPTION

[0036] Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, forpurposes of explanation, numerous specific details are set forth in order to promote a thorough understanding of one or more embodiments. It may be evident in some or all instances, however, that any embodiment described below can be practiced without adopting the specific design details associated with it below.

[0037] Systems such as those described herein may render benefits in a wide range of applications and implementations. For the sake of having a specific nonlimiting example to facilitate description, one such application is in semiconductor photolithography. FIG. 1 shows a photolithography system 100 that includes an illumination system 105. As described more fully below, the illumination system 105 includes a radiation source that produces a pulsed radiation beam 110 and directs it to a photolithography exposure apparatus 115 such as a scanner that patterns microelectronic features on a wafer 120. The wafer 120 is placed on a wafer table 125 constructed to hold wafer 120 and connected to a positioner 130 configured to accurately position the wafer 120 in accordance with certain parameters.

[0038] The pulsed radiation beam 110 may have a wavelength in the DUV range, for example, with a wavelength of 248 nm or 193 nm. The scanner 115 includes an optical arrangement 135 having, for example, one or more condenser lenses, a mask, and an objective arrangement. The mask is movable along one or more directions, such as along an optical axis of the pulsed radiation beam 110 or in a plane that is perpendicular to the optical axis. The objective arrangement includes a projection lens and enables an image transfer to occur from the mask to photoresist on the wafer 120. The illumination system 105 adjusts the range of angles for the pulsed radiation beam 110 impinging on the mask. The illumination system 105 also homogenizes (makes uniform) the intensity distribution of the pulsed radiation beam 110 across the mask.

[0039] The scanner 115 can include, among other features, a lithography controller 140 that controls how layers are printed on the wafer 120. The lithography controller 140 may include a memory that stores information such as process recipes that determine the parameters of the beam including a length of the exposure on the wafer 120 based on, for example, the mask used, as well as other factors that affect exposure. During lithography, a burst of pulses of the pulsed radiation beam 110 illuminates the same area of the wafer 120 to constitute an illumination dose.

[0040] The photolithography system 100 also preferably includes a control system 145. In general, the control system 145 includes one or more of digital electronic circuitry, computer hardware, firmware, and software. The control system 145 can be centralized or be partially or wholly distributed throughout the photolithography system 100.

[0041] FIG. 2 shows a pulsed laser source that produces a pulsed laser beam as the radiation beam 110 as an example of an illumination system 105. FIG. 2 shows a two-chamber laser system as a nonlimiting example but it will be understood that the principles explained herein are equally applicable to a single chamber laser system or a laser system having more than two chambers. The gas discharge laser system may include, e.g., a solid state or gas discharge master oscillator (“MO”) seedlaser system 200, an amplification stage, e.g., a power ring amplifier (“PRA”) stage 205, relay optics 210, and laser system output subsystem 215. The seed system 200 may include, e.g., an MO chamber 220 which includes a pair of electrodes 222 and 224.

[0042] The MO seed laser system 200 may also include a master oscillator output coupler (“MO OC”) 230, which may comprise a partially reflective mirror, forming an MO discharge chamber 220 with an oscillator cavity, defined in part by a reflective grating (not shown) in a line narrowing module (“LNM”) 235, that oscillates to form the seed laser output pulse. The MO seed laser system 200 may also include a line-center analysis module (“LAM”) 240. A MO wavefront engineering box (“WEB”) 245 may serve to redirect the output of the MO seed laser system 200 toward the amplification stage 205. The beam path through the LNM 235, the MO discharge chamber 220, the MO OC 230, and the LAM 240 defines an optical axis 237 for each of these components.

[0043] The amplification stage 205 may include, e.g., a PRA discharge chamber 250, which also may be an oscillator, e.g., formed by seed beam injection and output coupling optics (not shown) that may be incorporated into a PRA WEB 255. The beam may be redirected back through the gain medium in the PRA discharge chamber 250 by a beam reverser (“BR”) 260. The PRA WEB 255 may incorporate a partially reflective input / output coupler (not shown) and a maximally reflective mirror for the nominal operating wavelength (e.g., at around 193 nm for an ArF system) and one or more prisms. The PRA discharge chamber 250 may also include a pair of electrodes 252 and 254.

[0044] A bandwidth analysis module (“BAM”) 265 may receive the output laser radiation beam of pulses from PRA discharge chamber 250 and pick off a portion of the radiation beam for metrology purposes, e.g., to measure the output bandwidth and pulse energy. The laser output radiation beam of pulses then passes through the PRA WEB 255 to an optical pulse stretcher (“OPuS”) 270 and an autoshutter, in this case a combined autoshutter metrology module (“CASMM”) 275. One purpose of the OPuS 270 may be, e.g., to convert a single output laser pulse into a pulse train. The OPuS 270 may accordingly be arranged to receive the laser beam from the PRA WEB 255 and direct its output to the CASMM 275. The beam path through the BR 260, the PA discharge chamber 250, and the BAM 265 defines an optical axis 267 for each of these components.

[0045] The PRA discharge chamber 250 and the MO discharge chamber 220 are configured as chambers in which electrical discharges between the electrodes cause lasing gas discharges in a lasing gas to create an inverted population of high energy molecules or excimers, including, e.g., ArF, KrF, F2, XeF, and / or XeCl, to produce relatively broad band radiation that may be line narrowed to a relatively very narrow bandwidth and center wavelength selected in the LNM 235.

[0046] Turning now to FIG. 3, there is shown a laser discharge chamber 300 which may serve, for example, as the PRA discharge chamber 250 or the MO discharge chamber 220 described above. The chamber 300 has a chamber interior 315. Chamber interior 315 is filled with a chamber gas. In various implementations, the chamber gas is a gas mixture that is a suitable gain medium for an excimer laser.For example, the chamber gas may be a mixture of argon, fluorine, neon, and xenon. As another example, the chamber gas may be a mixture of krypton, fluorine, and neon.

[0047] In this specification including the claims the terms “up,” “down,” “upper,” “lower,” “top,” “bottom,” “vertical,” “horizontal,” and like terms are intended to connote relative orientation only and not any absolute orientation such as orientation with respect to gravity unless otherwise indicated or clear from context.

[0048] Contained within the chamber interior 315 is, e.g., a gas discharge system comprising two elongated (along the X axis in and out of the plane of the figure) opposing electrodes, in the example shown a cathode 310 and an anode 320, defining between them an elongated gas discharge gap or region 340, wherein, in response to a sufficient voltage being imposed across the cathode 310 and anode 320, a discharge occurs in the gas discharge region 340 resulting in the production of radiation at or near a characteristic center wavelength, that is optically directed along the optical axis of the output laser radiation pulse generally aligned to the longitudinal optical axis of the laser discharge chamber 300 in the direction of the positive X axis (into the plane of the figure) as indicated by the inset.

[0049] The chamber interior 315 may also contain, e.g., a cathode support 312, which may be an insulator, and an anode support 325. The cathode support 312 has a surface that faces the surfaces of the anode support 325 as shown in FIG. 4. These surfaces are generally configured to establish the characteristics of the flow of gas into and out of the discharge region 340 and are sometimes referred to as fairing surfaces.

[0050] Also within the chamber interior 315 may be, e.g., a preionizer 365, which may include, for example, a preionizer tube, adjacent to the cathode 310. The preionizer 365 may be configured as an elongate hollow tube made of dielectric material that is aligned parallel to the discharge electrodes 310, 320 and positioned near the discharge region 340. A conductive preionization electrode (typically made of copper or brass), is positioned in the bore of the tube and used to create a potential difference between the preionization electrode and one of the main discharge electrodes. This potential difference extends across the dielectric tube radially and results in a substantially uniform emission of photons that are emitted from the outer surface of the tube. More information about preionizers may be found in U.S. Patent No. 7,542,502, titled “Thermal-expansion Tolerant, Preionizer Electrode for a Gas Discharge Laser,” issued June 2, 2009.

[0051] All patent applications, patents, and printed publications cited herein are incorporated herein by reference in their entireties, except for any definitions, subject matter disclaimers, or disavowals, and except to the extent that the incorporated material is inconsistent with the express disclosure herein, in which case the language in this disclosure controls.

[0052] In some implementations the preionizer 365 may be placed downstream with respect to gas flow (indicated by arrow B in FIGS. 3 and 4) of the cathode support / insulator 312 (or, equivalently,downstream of the discharge region 340) as shown in FIGS. 3 and 4. In other implementations the preionizer 365 may be placed upstream of the cathode support / insulator 312.

[0053] Also within the chamber interior 315 may be a gas circulation system comprising a gas circulation fan 370, which may be, e.g., a generally cylindrical crossflow fan 370. The fan 370 moves gas within the chamber interior 315 by turning in a clockwise direction in the figure as indicated by the arrow A to drive gas through the discharge region 340 in order to expel gas that contains ionized particles and debris and is depleted of F2from the discharge region 340 between successive discharges, and thus to replenish the discharge region 340 with fresh gas before the next gas discharge.

[0054] FIG. 4 is a schematic close up view of the discharge gap 340 that further illustrates how the electrodes and adjacent components are situated in the gas discharge laser chamber. As shown in FIG. 4, anode 320 and cathode 310 are disposed facing one another with opposed surfaces extending in the X direction to define the discharge gap 340. The discharge region 340 typically has a height (direction of the Y axis) in a range of about .25 inches to about .75 inches, e.g., 0.5 inches. In one example, the length of the anode 320 and the cathode 310 may for example lie in a range of about 20 inches to about 30 inches. As noted above, the width of the discharge area and the length of the electrodes may be varied to suit the needs of particular applications. Also visible in FIG. 4 is preionizer 365. Also as shown, the cathode support 312 has fairing surfaces 314 and 316 and the anode support 325 has fairing surfaces 329 and 327. The fairing surfaces together define a gas flow channel that includes the discharge region 340. One of ordinary skill in the art will appreciate that the fairing surfaces may be provided on alternate or additional structures such as shoulders, insulators, and the like or even structures that are provided primarily for the purpose of shaping the flow of gas in the discharge region 340.

[0055] As mentioned, future needs for improvements in imaging and throughput of the scanners will drive demand for higher pulse repetition rates. Increasing pulse repetition rates will in turn entail overcoming several technical challenges, including controlling acoustic disturbances in the discharge chamber and undesirable arcing between the electrodes at positions downstream (with respect to gas flow) of the intended arcing position.

[0056] For example, one of the challenges encountered when the preionizer 365 is placed downstream of the cathode 310 is that the gas in the vicinity of the preionizer 365 has a large fraction of spent gas which has been carried away from the discharge region. When the laser fires a next pulse, spent gas is still present in the discharge region as indicated by filled shaded oval 410 in FIG. 4. The spent gas can interfere with preionization of the fresh gas in the discharge region 340. The spent gas can also promote downstream arcing, as indicated by the jagged line 420. As mentioned, this downstream arcing typically leads to energy dropouts and is one of the main factors limiting the ability to increase the repetition rate scaling in various configurations of excimer lasers. Finding a way to reduce and even to eliminate the downstream arcing issue will enable repetition rate scalingwithout increasing gas velocity (which requires increased blower motor power and hence drives energy consumption significantly higher).

[0057] According to an aspect of an embodiment downstream arcing is reduced and the acoustic resonance performance of the discharge chamber is improved through provision of a flow insert in the gas flow channel between the cathode and the anode. An example of a configuration using a downstream flow insert 430 is shown in FIG. 4. In the embodiment shown the downstream flow insert 430 is positioned in the downstream flow, extending in the direction of the flow through the area with spent gas 410.

[0058] According to another aspect of an embodiment, the downstream flow insert 430 is shaped in such a way that it imposes limited interference with upstream gas flow. For example, the downstream flow insert 430 can be a vane shaped as an airfoil similar to a wing of an airplane to reduce air drag forces. As another example, also as described more fully below, the downstream flow insert 430 can take the shape of an array, regular or irregular, of vanes spread vertically and / or horizontally in the plane of FIG. 4 or transverse to the plane of FIG. 4. In the embodiment of FIG. 4 the downstream flow insert 430 extends in and out of the plane of the figure in the X direction and may or may not be coextensive in that direction with the cathode 310 or the anode 320. The flow insert 430 may be made of or made from an insulating material to dismpt or prevent downstream arcing. For example, the flow insert may be formed of a ceramic material (or other material) that is mechanically, thermally, and chemically suitable for the gas and plasma environment in the vicinity of discharge region 340.

[0059] Various parameters relating to the flow inserts 430 can be varied including their number, size, shape, and distribution. For example, FIG. 5 shows an arrangement in which the flow inserts 432 are arranged collinearly in a line extending in the direction of gas flow as indicated by the arrow B. FIG. 6 shows an arrangement in which flow inserts 432 arranged side-by-side in an array extending transverse to the direction of gas flow.

[0060] FIG. 7 shows an arrangement in which flow inserts 432 are arranged parallel to one to one another with a smaller flow insert 434 between them. This is an example of an arrangement in which the flow inserts are not all of the same size. Also, in the arrangement of FIG. 7, there are two cambered airfoils 440 and 442 and two symmetrical airfoils 444. This is therefore an example of an arrangement in which the airfoils are of varying sizes and configurations. It is also possible to place an airfoil such as airfoil 432 upstream of the discharge region 340. This is also shown in FIG. 7.

[0061] Also, the discharge occurring in the discharge region 340 generates acoustic waves inside the discharge chamber 300. The acoustic waves generated by the discharge propagate outward from the discharge region 340, reflect off of internal surfaces of the discharge chamber 300, and then return to the discharge region 340 where they distort the production of subsequent laser pulses. Distributing the airfoils in horizontal and vertical planes can dephase acoustic wave reflections leading to improved acoustic performance of the chamber. Herein, to “dephase acoustic reflections” means to cause at least some of the acoustic reflections to be out of phase with one another.

[0062] According to another aspect of an embodiment, one or more flow inserts configured as one or more vortex generators may be provided to extend into the gas flow channel to control aspects of the gas flow such as flow separation from surfaces defining the gas flow channel.

[0063] Flow separation is a fluid dynamics phenomenon that occurs when a boundary layer detaches from a surface. A boundary layer exists whenever there is relative movement between a fluid and a solid surface with viscous forces present in the layer of fluid close to the surface. Vortex generators may be used to improve aerodynamic surface flows. They include protruding devices such as inclined elements, inclined blades, and grooves as well as Wheeler inclined vortex generators and the like. The protruding devices take energy from the higher energy upper layers of the thickened boundary layer or undisturbed flow at lower Reynolds numbers, but then pass upward over the thinner boundary layer at higher Reynolds numbers and cause high induced aerodynamic drag at this characteristic point. These devices are typically characterized as having heights that make up a significant part of the thickness of the boundary layer, for example, in the range of 35-100% or more of the maximum thickness of the boundary layer in a traditional vortex generator.

[0064] For example, U.S. Patent No. 2,800,291, issued July 23, 1957 and titled “Solid Boundary Surface for Contact with a Relatively Moving Fluid Medium” discloses the use of triangular ramptype vortex generators designed to delay or prevent flow separation by energizing the boundary layer through a pair of induced vortices. U.S. Patent No. 3,578,264 issued November 19, 1991, and U.S. Patent No. 3,741,285 issued June 26, 1973, both titled “Boundary Layer Control of Flow Separation and Heat Exchange,” show similar triangular ramp-type vortex generators. A similar “V” shaped vortex generator is disclosed in U.S. Patent No. 5,058,837, issued October 22, 1991 and titled “Low Drag Vortex Generators” but with the apex of the vortex generator oriented towards downstream. U.S. Patent No, No. 4,455,045 issued June 19, 1984 and titled “Means for Maintaining Attached Flow of a Flowing Medium” discloses a channel / groove type vortex generator.

[0065] Another class of vortex generators includes small-scale vortex generators (SSVG) also referred to as small-scale turbulence generators, small eddy generators, or generators of small-scale turbulence. See, for example, U.S. Patent No. 4,655,419, issued April 7, 1987, and titled “Vortex Generator,” U.S. Patent No. 5,058,837, issued October 22, 1991, and titled “Low Drag Vortex Generators,” and U.S. Patent No. 6,427,948, issued August 8, 2002, and titled “Controllable Vortex Generator.”

[0066] According to an aspect of an embodiment the vortex generators are selected and arranged to mitigate separation of the flow of gas from one or more surfaces over which the gas flows. According to one aspect of an embodiment, one or more vortex generators are installed on the downstream fairing surface of the anode support to mitigate the flow separation there. Such an arrangement is shown in FIG. 8 in which a vortex generator 500 is placed on the anode support surface 327.

[0067] Various parameters relating to the vortex generators 500 can be varied including their number, size, shape, and distribution. For example, FIGS. 9 A (side view) and 9B (top view) show anarrangement in which the vortex generators 500 are arranged in a linear array running parallel to the anode 320. FIGS. 10A through 10E, all side views, show other possible arrangements of vortex generators. FIG. 10A shows an array 520 of vortex generators 500 with the array 520 being a regular (periodic) array extending in both the X (parallel to the anode 320) and Z (transverse to the anode 320) directions. The vortex generator 500 may be made of or made from a material that is mechanically, thermally, and chemically suitable for the environment where the vortex generator is deployed. Vortex generator 500 may be configured with dimensions suitable to mitigate flow separation of the chamber gas along a surface such as upstream anode support surface 327. These dimensions may be selected based upon factors such as, for example, the gas properties and flow dynamics of the flow along the surface, such as the density, pressure, temperature, viscosity, or velocity profile of the gas and the geometry of the surface, or combinations thereof. In various implementations, vortex generator may have a length of approximately 2 mm, 3 mm, 5 mm, 7 mm, 10 mm, 12 mm, 15 mm, 20 mm, 25 mm, or 30 mm along a surface in a direction of chamber gas flow (e.g., the direction of arrow B in FIG. 8.). In various implementations, vortex generator may have a height of approximately 0.5 mm, 1 mm, 2 mm, 3 mm, 5 mm, 7 mm, 10 mm, 12 mm, 15 mm, 20 mm, 25 mm, or 30 mm normal to a surface and perpendicular to a chamber gas flow (e.g., the +Y direction in FIG. 8.).

[0068] FIG. 10B shows an array 530 of vortex generators 534, 536 which are angled with respect to the direction of gas flow. The vortex generator 534 is angled by an angle Oi with respect to the direction of gas flow and the vortex generator 536 is angled at an angle 02 with respect to gas flow. The angles 0i and 02 may be equal and in the range of 10 degrees to 60 degrees. In other examples, the angles 0i and 02 are not equal to each other but still in these ranges.

[0069] FIG. 10C shows an array 540 of triangular vortex generators 542 oriented to face upstream. FIG. 10D shows an array 550 of triangular vortex generators 554, 546 oriented to face downstream. The arrangement of FIG. 10D is also an example of the use of differently sized vortex generators, with the vortex generator 556 being a different size than the vortex generator 554. Differently shaped vortex generators may also be used together.

[0070] It is also possible to place the vortex generators on the upstream anode support surface 329. Such an arrangement is shown in FIG. 10E. In the arrangement shown in Figure 10E there is an array 560 of vortex generators 500 on the upstream anode support surface 327. There is also an array 510 of vortex generators 500 on the downstream anode support surface 327 but it will be understood that the upstream array 560 may be used with or without associated vortex generators on the downstream support surface 327.

[0071] Additional variations are possible. For example, it is possible to place the vortex generators on one or both of the cathode support surfaces. Such an arrangement is shown in FIG. 11 in which a vortex generator 500 is positioned on the downstream cathode support surface 316. FIG. 11 also shows vortex generators 500 being positioned on the downstream anode support surface 325, theupstream anode support surface 329, and the upstream cathode support surface 314. It will be appreciated that a given implementation could use any of these vortex generators so placed, singly or in combination. Also, it will be understood that any of the arrangements of vortex generator shown in FIGS. 9A, 9B, and 10A - 10E could also be used as well as other arrangements.

[0072] Also, the arrangement shown in FIG, 11 is an example of an arrangement using both an airfoil type flow insert 430 and flow insert in the form of vortex generators 500. Again, it will be understood that any of the arrangements of flow inserts shown in FIGS. 4, 5, 6, and 7 could also be used together with vortex generators as well as other arrangements.

[0073] Some of the above description is in terms of functional block diagrams with some functions allocated to some blocks and other functions allocated to other blocks. It will be understood that the division between blocks and the allocations are arbitrary and that different divisions and allocations are possible so long as the overall functions are carried out as described above.

[0074] The above description includes examples of multiple embodiments. It is, of course, not possible to describe every conceivable combination of components or methodologies for each of these embodiments, but one of ordinary skill in the art may recognize that many further combinations and permutations of elements of the various embodiments are possible based on the disclosure.Accordingly, the described embodiments are intended to be representative of and encompass all such alterations, modifications, and variations that fall within the spirit and scope of the appended claims.

[0075] Furthermore, to the extent that the term “includes” is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term “comprising” as “comprising” is constmed when employed as a transitional word in a claim. Also, although elements of the described aspects and / or embodiments may be described or claimed in the singular, the plural is contemplated unless limitation to the singular is explicitly stated. Additionally, all or a portion of any aspect and / or embodiment may be utilized with all or a portion of any other aspect and / or embodiment, unless stated otherwise.

[0076] Features, materials, characteristics, or groups described in conjunction with a particular aspect, embodiment, or example are to be understood to be applicable to any other aspect, embodiment or example described in this section or elsewhere in this specification unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract, and drawings may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. The protection is not restricted to the details of any foregoing embodiments. The protection extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0077] Furthermore, certain features that are described in this disclosure in the context of separate implementations can also be implemented in combination in a single implementation. Conversely,various features that are described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations, one or more features from a claimed combination can, in some cases, be excised from the combination, and the combination may be claimed as a subcombination or variation of a subcombination.

[0078] Moreover, while operations may be depicted in the drawings or described in the specification in a particular order, such operations need not be performed in the particular order shown or in sequential order, or that all operations be performed, to achieve desirable results. Other operations that are not depicted or described can be incorporated in the example methods and processes. For example, one or more additional operations can be performed before, after, simultaneously, or between any of the described operations. Further, the operations may be rearranged or reordered in other implementations. Those skilled in the art will appreciate that in some embodiments, the actual steps taken in the processes illustrated and / or disclosed may differ from those shown in the figures. Depending on the embodiment, certain of the steps described above may be removed, others may be added.

[0079] Furthermore, the features and attributes of the specific embodiments disclosed above may be combined in different ways to form additional embodiments, all of which fall within the scope of the present disclosure. Also, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described components and systems can generally be integrated together in a single product or packaged into multiple products.

[0080] For purposes of this disclosure, certain aspects, advantages, and novel features are described herein. Not necessarily all such advantages may be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the disclosure may be embodied or carried out in a manner that achieves one advantage or a group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein.

[0081] Conditional language, such as “can,” “could,” “might,” or “may,” unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements, and / or steps. Thus, such conditional language is not generally intended to imply that features, elements, and / or steps are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without user input or prompting, whether these features, elements, and / or steps are included or are to be performed in any particular embodiment.

[0082] Conjunctive language such as the phrase “at least one of X, Y, and Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to convey that an item, term, etc. may be either X, Y, or Z. Thus, such conjunctive language is not generally intended toimply that certain embodiments require the presence of at least one of X, at least one of Y, and at least one of Z.

[0083] Language of degree used herein, such as the terms “approximately,” “about,” “generally,” and “substantially” as used herein represent a value, amount, or characteristic close to the stated value, amount, or characteristic that still performs a desired function or achieves a desired result. For example, the terms “approximately”, “about”, “generally,” and “substantially” may refer to an amount that is within less than 10% of, within less than 5 % of, within less than 1% of, within less than 0.1 % of, and within less than 0.01 % of the stated amount. As another example, in certain embodiments, the terms “generally parallel” and “substantially parallel” refer to a value, amount, or characteristic that departs from exactly parallel by less than or equal to 15 degrees, 10 degrees, 5 degrees, 3 degrees, 1 degree, or 0.1 degree.

[0084] The scope of the present disclosure is not intended to be limited by the specific disclosures of preferred embodiments in this section or elsewhere in this specification, and may be defined by claims as presented in this section or elsewhere in this specification or as presented in the future. The language of the claims is to be interpreted broadly based on the language employed in the claims and not limited to the examples described in the present specification or during the prosecution of the application, which examples are to be construed as non-exclusive.

[0085] The implementations can be further described using the following clauses.1. A gas flow control system for a discharge chamber of a laser system, the gas flow control system comprising: a first electrode support; a second electrode support arranged in an opposed relationship to the first electrode support to define a gas flow channel arranged to cause gas flow substantially in a gas flow direction; and a flow insert positioned in the gas flow channel between the first electrode support and the second electrode support.2. The gas flow control system as in clause 1, wherein the flow insert comprises at least one guide vane arranged to extend codirectionally with the gas flow direction.3. The gas flow control system as in clause 2, wherein the first electrode support supports a first electrode and the second electrode support supports a second electrode at a position opposed to the first electrode to define a discharge gap and wherein the at least one guide vane is arranged at a position in the gas flow channel downstream with respect to the gas flow direction from the discharge gap.4. The gas flow control system as in clause 2, wherein the first electrode support supports a first electrode and the second electrode support supports a second electrode at a position opposed to the first electrode to define a discharge gap and wherein the at least one guide vane is arranged at a position in the gas flow channel upstream with respect to the gas flow direction from the discharge gap.5. The gas flow control system as in clause 2, wherein the at least one guide vane has at least one curved surface arranged to extend codirectionally with the gas flow direction.6. The gas flow control system as in clause 1, wherein the flow insert comprises a plurality of guide vanes arranged to extend codirectionally with the gas flow direction, the plurality of guide vanes being arranged such that the guide vanes are substantially collinear with one another.7. The gas flow control system as in clause 1, wherein the flow insert comprises a plurality of guide vanes arranged to extend codirectionally with the gas flow direction, the plurality of guide vanes being arranged such that the guide vanes are substantially parallel with one another.8. The gas flow control system as in clause 1, wherein the flow insert comprises a first plurality of guide vanes arranged to extend codirectionally with the gas flow direction, the first plurality of guide vanes being arranged such that the guide vanes are substantially collinear with one another, and a second plurality of guide vanes arranged to extend codirectionally with the gas flow direction, the second plurality of guide vanes being arranged such that the guide vanes are substantially collinear with one another, the first plurality being displaced from the second plurality in a direction transverse to the gas flow direction.9. The gas flow control system as in clause 1, wherein the flow insert comprises a first plurality of guide vanes arranged to extend codirectionally with the gas flow direction, the first plurality of guide vanes being arranged such that the guide vanes are substantially collinear with one another, and a second plurality of guide vanes arranged to extend codirectionally with the gas flow direction, the second plurality of guide vanes being arranged such that the guide vanes are substantially collinear with one another, the first plurality being displaced from the second plurality codirectionally with the gas flow direction.10. The gas flow control system as in clause 1, wherein the flow insert comprises at least one vortex generator on one of the first electrode support and the second electrode support and extending into the gas flow channel.11. The gas flow control system as in clause 10, wherein the first electrode support supports a first electrode and the second electrode support supports a second electrode at a position opposed to the first electrode to define a discharge gap and wherein the at least one vortex generator is arranged on one of the first electrode support and the second electrode support at a position in the gas flow channel upstream with respect to the gas flow direction from the discharge gap.12. The gas flow control system as in clause 10, wherein the first electrode support supports a first electrode and the second electrode support supports a second electrode at a position opposed to the first electrode to define a discharge gap and wherein the at least one vortex generator is arranged onone of the first electrode support and the second electrode support at a position in the gas flow channel downstream with respect to the gas flow direction from the discharge gap.13. The gas flow control system as in clause 10, wherein the flow insert comprises a plurality of vortex generators arranged to extend codirectionally with the gas flow direction, the plurality of vortex generators being arranged such that the respective vortex generators extend substantially collinearly with one another.14. The gas flow control system as in clause 10, wherein the flow insert comprises a plurality of vortex generators each arranged to extend codirectionally with the gas flow direction, the plurality of vortex generators being arranged such that the respective vortex generators extend substantially parallel with one another.15. The gas flow control system as in clause 10, wherein the flow insert comprises a first plurality of vortex generators each arranged to extend codirectionally with the gas flow direction, the first plurality of vortex generators being arranged to be substantially collinear with one another, and a second plurality of vortex generators arranged to extend codirectionally with the gas flow direction, the second plurality of vortex generators being arranged to be substantially collinear with one another, the first plurality being displaced from the second plurality in a direction transverse to the gas flow direction.16. The gas flow control system as in clause 10, wherein the flow insert comprises a first plurality of vortex generators each arranged to extend codirectionally with the gas flow direction, the first plurality of vortex generators being arranged to be substantially collinear with one another, and a second plurality of vortex generators arranged to extend codirectionally with the gas flow direction, the second plurality of vortex generators being arranged to be substantially collinear with one another, the first plurality being displaced from the second plurality in a direction parallel to the gas flow direction.17. A gas flow control system for a discharge chamber of a laser system, the discharge chamber including an anode and a cathode, the gas flow control system comprising: a cathode support mechanically coupled to the cathode; an anode support mechanically coupled to the anode and arranged in an opposed relationship to the cathode support to define a gas flow channel arranged to cause gas flow substantially in a gas flow direction; and at least one guide vane positioned in the gas flow channel between the cathode support and the anode support and arranged to extend codirectionally with the gas flow direction.18. The gas flow control system as in clause 17, wherein the anode support supports the anode at a position opposed to the cathode to define a discharge gap and wherein the at least one guide vane isarranged at a position in the gas flow channel downstream with respect to the gas flow direction from the discharge gap.19. The gas flow control system as in clause 17, wherein anode support supports the anode at a position opposed to the cathode to define a discharge gap and wherein the at least one guide vane is arranged at a position in the gas flow channel upstream with respect to the gas flow direction from the discharge gap.20. The gas flow control system as in clause 17, wherein the at least one guide vane has at least one curved surface arranged to extend codirectionally with the gas flow direction.21. A gas flow control system for a discharge chamber of a laser system, the discharge chamber including an anode and a cathode, the gas flow control system comprising: a cathode support mechanically coupled to the cathode; an anode support mechanically coupled to the anode and arranged in an opposed relationship to the cathode support and having a surface defining a gas flow channel arranged to cause gas flow substantially in a gas flow direction; and at least one vortex generator positioned on the surface and extending into the gas flow channel.22. The gas flow control system as in clause 21, comprising a plurality of vortex generators arranged to extend codirectionally with the gas flow direction, the plurality of vortex generators being arranged such that the respective vortex generators extend substantially collinearly with one another.23. The gas flow control system as in clause 21, comprising a plurality of vortex generators each arranged to extend codirectionally with the gas flow direction, the plurality of vortex generators being arranged such that the respective vortex generators extend substantially parallel with one another.

[0086] The above-described implementations and other implementations are within the scope of the following claims.

Claims

CLAIMS1. A gas flow control system for a discharge chamber of a laser system, the gas flow control system comprising: a first electrode support; a second electrode support arranged in an opposed relationship to the first electrode support to define a gas flow channel arranged to cause gas flow substantially in a gas flow direction; and a flow insert positioned in the gas flow channel between the first electrode support and the second electrode support.

2. The gas flow control system as in claim 1, wherein the flow insert comprises at least one guide vane arranged to extend codirectionally with the gas flow direction.

3. The gas flow control system as in claim 2, wherein the first electrode support supports a first electrode and the second electrode support supports a second electrode at a position opposed to the first electrode to define a discharge gap and wherein the at least one guide vane is arranged at a position in the gas flow channel downstream with respect to the gas flow direction from the discharge gap.

4. The gas flow control system as in claim 2, wherein the first electrode support supports a first electrode and the second electrode support supports a second electrode at a position opposed to the first electrode to define a discharge gap and wherein the at least one guide vane is arranged at a position in the gas flow channel upstream with respect to the gas flow direction from the discharge gap.

5. The gas flow control system as in claim 2, wherein the at least one guide vane has at least one curved surface arranged to extend codirectionally with the gas flow direction.

6. The gas flow control system as in claim 1, wherein the flow insert comprises a plurality of guide vanes arranged to extend codirectionally with the gas flow direction, the plurality of guide vanes being arranged such that the guide vanes are substantially collinear with one another.

7. The gas flow control system as in claim 1, wherein the flow insert comprises a plurality of guide vanes arranged to extend codirectionally with the gas flow direction, the plurality of guide vanes being arranged such that the guide vanes are substantially parallel with one another.

8. The gas flow control system as in claim 1, wherein the flow insert comprises a first plurality of guide vanes arranged to extend codirectionally with the gas flow direction, the first plurality of guide vanes being arranged such that the guide vanes are substantially collinear with one another, and a second plurality of guide vanes arranged to extend codirectionally with the gas flow direction, the second plurality of guide vanes being arranged such that the guide vanes are substantially collinear with one another, the first plurality being displaced from the second plurality in a direction transverse to the gas flow direction.

9. The gas flow control system as in claim 1, wherein the flow insert comprises a first plurality of guide vanes arranged to extend codirectionally with the gas flow direction, the first plurality of guide vanes being arranged such that the guide vanes are substantially collinear with one another, and a second plurality of guide vanes arranged to extend codirectionally with the gas flow direction, the second plurality of guide vanes being arranged such that the guide vanes are substantially collinear with one another, the first plurality being displaced from the second plurality codirectionally with the gas flow direction.

10. The gas flow control system as in claim 1, wherein the flow insert comprises at least one vortex generator on one of the first electrode support and the second electrode support and extending into the gas flow channel.

11. The gas flow control system as in claim 10, wherein the first electrode support supports a first electrode and the second electrode support supports a second electrode at a position opposed to the first electrode to define a discharge gap and wherein the at least one vortex generator is arranged on one of the first electrode support and the second electrode support at a position in the gas flow channel upstream with respect to the gas flow direction from the discharge gap.

12. The gas flow control system as in claim 10, wherein the first electrode support supports a first electrode and the second electrode support supports a second electrode at a position opposed to the first electrode to define a discharge gap and wherein the at least one vortex generator is arranged on one of the first electrode support and the second electrode support at a position in the gas flow channel downstream with respect to the gas flow direction from the discharge gap.

13. The gas flow control system as in claim 10, wherein the flow insert comprises a plurality of vortex generators arranged to extend codirectionally with the gas flow direction, the plurality of vortex generators being arranged such that the respective vortex generators extend substantially collinearly with one another.

14. The gas flow control system as in claim 10, wherein the flow insert comprises a plurality of vortex generators each arranged to extend codirectionally with the gas flow direction, the plurality of vortex generators being arranged such that the respective vortex generators extend substantially parallel with one another.

15. The gas flow control system as in claim 10, wherein the flow insert comprises a first plurality of vortex generators each arranged to extend codirectionally with the gas flow direction, the first plurality of vortex generators being arranged to be substantially collinear with one another, and a second plurality of vortex generators arranged to extend codirectionally with the gas flow direction, the second plurality of vortex generators being arranged to be substantially collinear with one another, the first plurality being displaced from the second plurality in a direction transverse to the gas flow direction.

16. The gas flow control system as in claim 10, wherein the flow insert comprises a first plurality of vortex generators each arranged to extend codirectionally with the gas flow direction, the first plurality of vortex generators being arranged to be substantially collinear with one another, and a second plurality of vortex generators arranged to extend codirectionally with the gas flow direction, the second plurality of vortex generators being arranged to be substantially collinear with one another, the first plurality being displaced from the second plurality in a direction parallel to the gas flow direction.

17. A gas flow control system for a discharge chamber of a laser system, the discharge chamber including an anode and a cathode, the gas flow control system comprising: a cathode support mechanically coupled to the cathode; an anode support mechanically coupled to the anode and arranged in an opposed relationship to the cathode support to define a gas flow channel arranged to cause gas flow substantially in a gas flow direction; andat least one guide vane positioned in the gas flow channel between the cathode support and the anode support and arranged to extend codirectionally with the gas flow direction.

18. The gas flow control system as in claim 17, wherein the anode support supports the anode at a position opposed to the cathode to define a discharge gap and wherein the at least one guide vane is arranged at a position in the gas flow channel downstream with respect to the gas flow direction from the discharge gap.

19. The gas flow control system as in claim 17, wherein anode support supports the anode at a position opposed to the cathode to define a discharge gap and wherein the at least one guide vane is arranged at a position in the gas flow channel upstream with respect to the gas flow direction from the discharge gap.

20. The gas flow control system as in claim 17, wherein the at least one guide vane has at least one curved surface arranged to extend codirectionally with the gas flow direction.

21. A gas flow control system for a discharge chamber of a laser system, the discharge chamber including an anode and a cathode, the gas flow control system comprising: a cathode support mechanically coupled to the cathode; an anode support mechanically coupled to the anode and arranged in an opposed relationship to the cathode support and having a surface defining a gas flow channel arranged to cause gas flow substantially in a gas flow direction; and at least one vortex generator positioned on the surface and extending into the gas flow channel.

22. The gas flow control system as in claim 21, comprising a plurality of vortex generators arranged to extend codirectionally with the gas flow direction, the plurality of vortex generators being arranged such that the respective vortex generators extend substantially collinearly with one another.

23. The gas flow control system as in claim 21 , comprising a plurality of vortex generators each arranged to extend codirectionally with the gas flow direction, the plurality of vortex generators being arranged such that the respective vortex generators extend substantially parallel with one another.

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