Device and method for producing ions using a plasma

Abstract

The present invention provides a simple and reliable way of producing ions. According to the invention, the ions produced can be mixed and / or neutralized simply and without a large number of additional parts and apparatuses. The device (10) required for this purpose is of very compact and cost-effective construction since only one excitation system (44) is required for two spatially separated and / or electrically isolated regions (24, 26) of a plasma vessel (20). As a result, it is possible to dispense with at least one complete excitation system, thereby gaining space and degrees of freedom (e.g. for the movement of the device itself). The device (10) according to the invention can be used both as an ion thruster and for material processing, that is to say can be used universally, with the kinetic energy being precisely adjustable by means of the grid system (28, 30) and the current density being precisely adjustable by means of the gas flow and the power of the plasma excitation.

Description

[0001] DEVICE AND METHOD FOR ION GENERATING BY PLASMA

[0002] The present invention relates to a device for ion generation by a plasma according to the preamble of claim 1 and a method for ion generation by a plasma according to the preamble of claim 9.

[0003] Produced ions can be used in a variety of applications.

[0004] For example, ions are used for material processing. After their generation, the ions are usually accelerated using a tailored electric field to acquire a specific kinetic energy. To prevent shielding caused by charge buildup, especially with non-conductive materials (e.g., insulating substrates or targets), it is desirable to neutralize the ions.

[0005] In addition, electric propulsion systems based on lattice ion engines have been used in spaceflight for many years, both for orbiting and attitude correction of telecommunications satellites, as well as for interplanetary and interstellar missions. Many future missions can also only be flown with electric propulsion due to the flight time, mission profile, and required fuel quantity.

[0006] What all electric engines have in common is that the thrust they generate is produced by accelerating ions into free space in an electric field. Due to the law of conservation of momentum, the missile (e.g., a satellite or similar) experiences a momentum of the same magnitude as the repelled ions, but in the opposite direction. To generate the highest possible thrust, the ions must have a large mass and be easily ionized. For this reason, xenon has been used as a fuel for electric propulsion for many years. However, due to the ever-increasing demand for xenon in other industries and the associated rising price, alternative fuels such as krypton, iodine, and heavy molecules are being discussed. However, these are generally more complex to handle and cannot be ionized as effectively as xenon. Low-pressure plasma discharges are used to ionize the propellant gas.The ions generated in this process always have a positive charge and are accelerated through a multi-aperture grid system. The grid system itself consists of a stack of electrically isolated grids that can be subjected to positive and negative voltages of up to several thousand V. In the simplest case, a grid system consists of only two grids. However, configurations with up to four or five grids are also possible.

[0007] Generating thrust by ejecting positively charged ions leads to an excess of negative charge carriers on the satellite, creating a negative potential field around the satellite. This prevents further ejection of positive ions and thus the generation of thrust. Therefore, every satellite equipped with an ion thruster has at least one neutralizer that generates the same amount of electrons as the ion thruster, thus removing the excess negative charge. The neutralizer is positioned so that the beam of electrons is injected obliquely into the ion beam.

[0008] In addition to the hollow cathodes commonly used in space travel, plasma bridge neutralizers (PBNs) have become established neutralizers. In these systems, a high-density plasma is generated in a small space, from which electrons are extracted, which are used, as described above, to neutralize the space charge or ions. The same excitation mechanisms are used for plasma generation in the neutralizer as for ion generation (hollow cathode, high-frequency, and ECR discharges). Simple thermionic cathodes are conceivable, but due to their short service life, they are not suitable, particularly for space applications. Operating the neutralizers requires a similar system of supply units as the ion source, but with different parameters such as gas flow, voltages, and power. Due to redundancy requirements, these systems are often installed in multiple locations.Another disadvantage of this design is that the generated electron beam is injected into the ion beam at an angle, resulting in an inhomogeneous or non-axisymmetric charge distribution.

[0009] Alternatively, beam switches are used, in which the voltages on the extraction grids are reversed in the kHz range so that ions and electrons are extracted alternately. On average, this reduces the effectively extracted proportion of ions, so that in the case of surface treatment, the process times must be increased accordingly to achieve the same result. Furthermore, the use of a beam switch represents another potential source of error with regard to highly stable process control.

[0010] The object of the present invention is therefore to provide a simple and safe method for ion generation. In particular, the ions should be neutralized easily and without the need for numerous additional parts and devices. Preferably, production should be possible using a cost-effective, compact ion beam source. Preferably, a highly stable process with precisely adjustable kinetic energies and current densities should be enabled.

[0011] This object is achieved with the device according to the invention according to claim 1 and the method according to the invention according to claim 9. Advantageous further developments are specified in the dependent claims and in the following description together with the figures.

[0012] The inventors recognized that this problem can be solved in a surprising and particularly simple way if the same means for plasma generation are used for both the generation of the ions and the generation of the other charged particles. In other words, the discharge chamber for generating the plasma is divided into at least two spatially and / or electrically separate areas. The device can then be kept very compact because only one means for plasma generation is used. Preferably, the discharge chamber for generating the plasma is manufactured in such a way that two electrically separate, in particular closely spaced, plasma spaces are formed, with one plasma feeding the ion extraction and the other plasma feeding the generation of the other charged particles.

[0013] The device according to the invention for ion generation by a plasma, wherein there is a first chamber which serves for ion extraction and which is surrounded by means for generating the plasma, is characterized in that there is a second chamber which serves for the extraction of further charged particles which were generated by a plasma, wherein the second chamber is also surrounded by the means for generating the plasma, so that the means for generating the plasma feed both the plasma in the first chamber and the plasma in the second chamber.

[0014] In an advantageous development, the means for generating the plasma comprise an induction coil, with the first chamber and the second chamber being arranged inside the induction coil. This makes the design particularly simple and requires little installation space.

[0015] In an advantageous development, the second chamber is arranged within the first chamber, with the first chamber preferably being arranged concentrically around the second chamber. This allows the charged particles from the second chamber to be radiated coaxially into the ions generated in the first chamber.

[0016] In an advantageous development, the first chamber is arranged so as to be electrically insulated from the second chamber, with the first chamber and / or the second chamber preferably comprising an electrically insulating wall material, the wall material being, in particular, a ceramic material. This allows the structure to be kept particularly simple and compact.

[0017] In an advantageous development, the first chamber and / or the second chamber have at least one extraction opening, wherein the extraction opening preferably has at least one aperture. This allows the ions or charged particles to be easily extracted from the device and also allows beam shaping.

[0018] In an advantageous development, at least one grid, preferably a grid system, is arranged in the extraction opening. This allows the parameters of the beam currents to be easily defined.

[0019] In an advantageous development, the grating system comprises a bias grating, preferably in the form of a metal foil, in particular a molybdenum foil, and an extraction grating, preferably with one or more additional gratings. This makes the grating system particularly simple in design. If more than two gratings are used, more defined beam shaping can be achieved. Furthermore, such additional gratings can also serve as erosion protection. Instead of a molybdenum foil, other high-melting metals and alloys such as titanium and tungsten, or even graphite, pyrographite, and CFC can be used.

[0020] In an advantageous further development, the second chamber serves for electron extraction and thus forms a neutralizer for the ions produced.

[0021] This means that the device can be used to process insulating materials or as an ion drive.

[0022] In an advantageous further development, the second chamber serves for ion extraction. This allows a second ion source to be provided for other ion species.

[0023] Independent protection is claimed for the method according to the invention for ion generation by a plasma, wherein there is a first chamber which serves for ion extraction and which is surrounded by means for generating the plasma, which is characterized in that further charged particles are generated and extracted in a second chamber, wherein the further charged particles are generated by a plasma in such a way that the means for generating the plasma feed both the plasma in the first chamber and the plasma in the second chamber.

[0024] In an advantageous further development, it is provided that the device according to the invention is used.

[0025] In an advantageous embodiment, the plasma is generated by applying a high frequency, preferably in the range of 0.9 to 100 MHz, to an induction coil. This allows the discharge to be achieved particularly efficiently.

[0026] In an advantageous development, electrons are generated in the second chamber and extracted from it in order to neutralize the ions generated in the first chamber. The neutralized ions are preferably used in an ion beam source for material processing or in an ion propulsion system. This makes both the material processing of insulating materials and the ion propulsion particularly efficient and process-stable. In an advantageous development, ions are generated in the second chamber and extracted from it in order to mix these ions with the ions generated in the first chamber. This makes it particularly easy to generate mixtures of different ion species in an ion source.

[0027] The features and further advantages of the present invention will become clear below from the description of a preferred embodiment in conjunction with the figures. These show, purely schematically:

[0028] Fig. 1 shows the device for ion generation according to the invention according to a preferred embodiment in a perspective view from the front,

[0029] Fig. 2 shows the device according to Fig. 1 in a perspective view from behind,

[0030] Fig. 3 the device according to Fig. 1 in plan view from the front and

[0031] Fig. 4 shows the device according to Fig. 1 in a sectional view.

[0032] In Figs. 1 to 4, the device 10 according to the invention for ion generation is shown in various views according to a preferred embodiment.

[0033] It can be seen that the device 10 has a housing 12 comprising a front 14, a rear wall 16, and struts 18 connecting them. A plasma vessel 20 is arranged between the front 14 and the rear wall 16. The front 14 forms the end of the plasma vessel 20 and centers and fixes it.

[0034] The plasma vessel 20 is integrally connected to a coaxial wall formation 22, which encloses a first discharge chamber 24 in a first annular section and a second discharge chamber 26 in a second cylindrical section. These discharge chambers 24, 26 are thus spatially separated from one another.

[0035] Preferably, the lengths of the discharge chambers 24, 26 are approximately 40 mm, the diameter of the first discharge chamber 24 is 40 mm, and the diameter of the second discharge chamber 26 is 14 mm. The first discharge chamber 24 has an extraction opening 27, which is defined by the wall 22 and the front 14 and is closed by a perforated foil 28 acting as a screen grid, which is preferably a molybdenum foil. This perforated foil 28 preferably has a thickness of 0.2 mm. This perforated foil 28 is held at a potential of 1.4 to 1.5 keV and thereby draws off a beam current of approximately 20 mA. This perforated foil 28 thus acts as a bias grid.

[0036] The wall formation 22 preferably consists of an insulator, in particular of a ceramic, so that the two discharge chambers 24, 26 are electrically insulated from one another.

[0037] An extraction grid 30, which has numerous apertures 32, is arranged in front of the perforated foil 28 at a short distance of, for example, 0.5 mm. This extraction grid 30 is preferably made of graphite and has a thickness of 1 mm. It is at a potential of approximately -400 eV, whereby the generated ions are extracted and accelerated through the apertures 32.

[0038] The terminals 28a, 30a are provided for applying the respective potential to the perforated foil 28 and the grid 30.

[0039] The second discharge chamber 26 is closed by the rear cover 34, which houses the central gas inlet 36 for the second discharge chamber 26 and the gas inlet 38 for the first discharge chamber 24. The gas inlet 36 for the second discharge chamber is electrically conductive, so that a potential can be applied to it. This allows the positive charges to be dissipated during the extraction of electrons. Furthermore, the electrons generated in the second discharge chamber 26 could be accelerated, if necessary.

[0040] At the front, there is a hole 40 with a diameter of 0.5 mm in the wall recess 22. The electrons generated in the second chamber 26 are extracted through this hole 40. No separate grid is required for this because the ions generated in the first chamber 24 build up a space charge zone which, via a plasma bridge, draws the electrons generated in the second chamber 26 out through the hole 40. Very high currents of approximately 20 mA flow. This corresponds to the beam current drawn via the foil 28, so that the total charge is balanced again. The design of the device 10 thus guarantees that the same amount of current of ions and electrons is generated. The electrons drawn from the hole 40 neutralize the ions extracted from the first chamber 24.

[0041] In principle, all typical working gases can be used for ion generation, i.e., all ionizable gaseous species, such as noble gases, oxygen, nitrogen, and reactive gases. Xenon and krypton are typically used in ion engines. Electron generation can be achieved using the same working gas or a different one—for example, a lighter one in space travel, because this gas does not need to be used for thrust, thus saving weight.

[0042] Instead of a single hole 40, multiple holes could be used. Alternatively or additionally, an accelerating grid for the electrons could be used.

[0043] Turns 44 of an induction coil 46 are placed in corresponding recesses 42 around the outer wall shape 22; these turn- ings serve to generate plasma in the two chambers 24, 26. Since both chambers 24, 26 are located within the induction coil 46, both discharges are fed by a single excitation, which operates independently of one another due to the electrical insulation. The induction coil 46 is supplied with a high frequency, preferably in the range 0.9 to 100 MHz, via the electrical connection 48a, 48b. Thus, due to the spatial arrangement of the two plasma spaces in the chambers 24, 26, only one induction coil 46 and one high frequency supply (not shown) are required to excite both discharges, which considerably reduces the complexity of the system. In addition to the common coil geometries such as cylindrical, conical and planar coils, other cross-sectional shapes such as rectangular, trapezoidal orAny round and n-sided shapes are possible, the geometric dimensions of which can also vary along the longitudinal axis L of the device 10.

[0044] The invention has been explained above using an example in which the second

[0045] Chamber 26 is arranged coaxially within the first chamber 24. The first chamber could also be arranged coaxially within the second chamber. Furthermore, any other arrangements and geometries of the two chambers could be used, including two or more first chambers and / or two or more second chambers. These first and second chambers could each generate different charged particles. It is essential that the first chamber and the second chamber have separate discharges, but are fed by the same excitation.

[0046] Furthermore, the invention has been described using an example in which the ions generated in the first chamber 24 are neutralized by electrons generated in the second chamber 26.

[0047] Alternatively, ions could also be generated in the second chamber, but they could be a different ion species.

[0048] In addition, there could be several different first chambers in which different ion species are generated and there could be one or more second chambers for generating electrons for their neutralization.

[0049] From the above description it has become clear that the present invention provides a simple and safe method for ion generation. The generated ions can be mixed and / or neutralized easily and without many additional parts and devices. The device required for this is very compact and cost-effective, since only one excitation system is required for two spatially and / or electrically separated regions 24, 26 of a plasma vessel 20. This eliminates the need for at least one complete excitation system, thereby gaining space and degrees of freedom (e.g., in the movement of the device itself). The device 10 according to the invention can be used both as an ion thruster and for material processing, and is therefore universally applicable, with the kinetic energy being precisely adjustable via the grid system 28, 30, and the current density being precisely adjustable via the gas flow and the power of the plasma excitation.

[0050] Used as an ion thruster, the device 10 simultaneously generates a beam of positive

[0051] Ions and the electrons neutralizing them, whose current strengths are identical in magnitude. Due to the law of conservation of momentum, the ion beam generates the necessary thrust for the missile, and the electrons carry off the excess negative charges created by the ion generation in the plasma, thus preventing the missile from becoming charged. Since the electron beam largely exits in the direction of the ion beam, good coupling between ions and electrons and continuous beam neutralization are achieved.

[0052] When used as an ion source in materials processing, device 10 also simultaneously generates a beam of positive and negative charge carriers. These can then be used on electrically insulating materials (substrates or targets). They impinge on the surface and lead to interaction processes, such as material removal, layer and surface modification, and, secondarily, layer growth. Because equal numbers of positive and negative charge carriers impinge on the material, charging of the material is avoided, which, with electrically insulating materials, would otherwise lead to ion repulsion and, consequently, unstable processing.

[0053] All features presented in the general description of the invention, the description of the exemplary embodiments, the following claims and the figures can be essential to the invention both individually and in any combination with one another. These features or combinations of features can each form the basis of an independent invention, the right to claim which is expressly reserved. Individual features from the description of an exemplary embodiment do not necessarily have to be combined with one or more or all of the other features stated in the description of this exemplary embodiment; in this regard, each sub-combination is expressly disclosed. Furthermore, physical features of a device can be reformulated to also be used as method features, and method features can be reformulated to be used as physical features of a device. Such a reformulation is therefore automatically disclosed.List of reference symbols.

[0054] 10 Device according to the invention for ion generation according to a preferred

[0055] Design

[0056] 12 housings

[0057] 14 Front

[0058] 16 Rear wall

[0059] 18 struts

[0060] 20 plasma vessel

[0061] 22 Wall shaping

[0062] 24 first discharge chamber, first chamber

[0063] 26 second discharge chamber, second chamber

[0064] 27 Opening in first discharge chamber 24, extraction opening

[0065] 28 Perforated foil, bias grid or screen grid

[0066] 28a Connection for perforated foil 28

[0067] 30 extraction grids

[0068] 30a Connection for extraction grid 30

[0069] 32 apertures

[0070] 34 lids

[0071] 36 central gas inlet for the second discharge chamber 26

[0072] 38 Gas inlet for the first discharge chamber 24

[0073] 40 Hole in second discharge chamber 26, extraction opening

[0074] 42 recesses in the outer wall shape 22

[0075] 44 turns

[0076] 46 Induction coil

[0077] 48a, 48b electrical connection of the induction coil 46 for high frequency

[0078] L Longitudinal axis of the device 10

Claims

Patent claims 1. Device (10) for ion generation by a plasma, comprising a first chamber (24) for ion extraction and surrounded by means (46) for generating the plasma, characterized in that a second chamber (26) is comprised for the extraction of further charged particles generated by a plasma, wherein the second chamber (26) is also surrounded by the means (46) for generating the plasma, such that the means (46) for generating the plasma feed both the plasma in the first chamber (24) and the plasma in the second chamber (26).

2. Device (10) according to claim 1, characterized in that the means (46) for generating the plasma comprise an induction coil (44, 46), wherein the first chamber (24) and the second chamber (26) are arranged inside the induction coil (44, 46).

3. Device (10) according to claim 1 or 2, characterized in that the second chamber (26) is arranged in the first chamber (24), wherein the first chamber (24) is preferably arranged concentrically around the second chamber (26).

4. Device (10) according to one of the preceding claims, characterized in that the first chamber (24) is arranged electrically isolated from the second chamber (26), wherein the first chamber (24) and / or the second chamber (26) preferably have an electrically insulating wall material (22), wherein the wall material (22) is in particular a ceramic material.

5. Device (10) according to one of the preceding claims, characterized in that the first chamber (24) and / or the second chamber (26) have at least one extraction opening (27, 40), wherein the extraction opening (27) preferably has at least one aperture (32).

6. Device (10) according to claim 5, characterized in that at least one grid (28, 30), preferably a grid system, is arranged in the extraction opening (27).

7. Device according to claim 6, characterized in that the grid system comprises a bias grid (28), preferably in the form of a metal foil, in particular a molybdenum foil comprising and an extraction grid (30), preferably comprising one or more further grids.

8. Device (10) according to claim 7, characterized in that the second chamber (26) serves for electron extraction and thereby forms a neutralizer for the generated ions or that the second chamber (26) serves for ion extraction.

9. A method for generating ions by a plasma, comprising a first chamber (24) for ion extraction and surrounded by means (46) for generating the plasma, characterized in that further charged particles are generated and extracted in a second chamber (26), wherein the further charged particles are generated by a plasma such that the means (46) for generating the plasma feed both the plasma in the first chamber (24) and the plasma (26) in the second chamber.

10. Method according to claim 9, characterized in that the device (10) is used according to one of claims 1 to 8 and / or that the plasma is generated by applying a high frequency, preferably in the range of 0.9 to 100 M Hz, to an induction coil (44, 46) and / or that electrons are generated and extracted from the second chamber (26) to neutralize the ions generated in the first chamber (24), wherein the neutralized ions are preferably used in an ion beam source for material processing or in an ion thruster and / or that ions are generated and extracted from the second chamber (26) to mix these ions with the ions generated in the first chamber (24).

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