Electrically Charged Pulse Generator and Method of Using an Electrically Charged Pulse Generator

Abstract

The invention relates to a pulsed generator of electrically charged particles comprising: a pulsed light source; a vacuum chamber (V); said generator being characterized in that: the vacuum chamber (V) is configured to maintain an internal operating pressure between 10⁻⁶ mbar and atmospheric pressure; the vacuum chamber (V) is configured to accommodate a photocathode (PH) and an anode (AN), the photocathode (PH) and the anode (AN) being separated by an adjustable distance (L) less than or equal to 30 mm; the vacuum chamber (V) has a window allowing the pulsed light to reach one face of the photocathode; the anode has an orifice adapted for the passage of electrically charged particles; the electrically charged particle generator includes means for applying an electrical voltage between the photocathode and the anode, said electrical voltage being configured to accelerate the charged particles.FIGURE 1.

Description

Title of the invention: PULSED GENERATOR OF ELECTRICALLY CHARGED PARTICLES AND METHOD FOR USING A PULSED GENERATOR OF ELECTRICALLY CHARGED PARTICLES TECHNICAL FIELD OF THE INVENTION

[0001] The invention lies in the field of generators of electrically charged particles for chemical, physical-chemical or biological analyses. An object of the invention is a pulsed generator of electrically charged particles capable of generating packets of particles having a time duration of less than or equal to 1 nanosecond or even less than or equal to 5 picoseconds according to one embodiment of the invention, or even less than or equal to 100 femtoseconds.

[0002] The generator according to the invention is capable of producing packets of electrons or ions. The applications of the device according to the invention concern all fields of electron-matter interaction: characterizations by diffraction (LEED, RHEED, gas phase, nano-objects, UED), surface physics studies, Auger spectroscopy, ionization experiments and sources (aerosols, mass spectrometry), curing of materials, irradiations (scintillator tests, simulation irradiations, microfabrication, biological materials), NDT, analysis of contaminants, microscopy, electron-induced desorption. Another subject of the invention is a method of using the charged particle generator according to the invention. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0003] The field of electron or charged particle generators, relatively recent, is segmented into several technological families which communicate little with each other and each have systematic working habits, hardly questioned, and which arise from the technological origins of each family of researchers. In each, decades of work and experiments have led to the favoring of technical choices representing good compromises, and as such never questioned. Thus each family produces an optimized type of source, but in relation to the said compromises or working habits. This leads to technological choices strongly orienting the characteristics of the invented source.

[0004] A first family is that of Accelerator or High Energy Physicists. It develops robust and very heavy equipment, whose dimensions are significant. It assumes very high vacuums (pressures lower than 107 mbar, and most often ultra-high vacuum with pressures lower than 109 mbar). distances are in decimeters or meters, or even much more, and the voltages applied to the various electrodes are in tens of kV, or even in MV or GV. The insulators are also massive and made of tested materials, not always easy to machine.

[0005] On a synchrotron, the particles obtained are very energetic, in a continuous train or in packets of many particles, at a high repetition rate (300 GHz). These particles are not used, because they do not leave the ring, but they are used to produce photons. The free electron laser (or XFEL according to the English translation X-ray Free Electron Laser) is a machine which tends to replace synchrotrons and which is also available to photon users.

[0006] On an accelerator, the repetition rates are 25 to 50 MHz due to the complexity of the events to be detected and the operation of the detectors.

[0007] The space charge is then high, and the Coulomb repulsions between the particles of the same packet require the use of bulky and powerful compression and focusing systems (such as electrostatic lenses or RF cavities), which greatly extend the time duration of the emitted particle packets. In addition, the equipment must cope with a generally harsh radiative environment. In this family, electron guns can be pulsed but never in a single-electron regime (i.e. a few electrons per packet) because high current intensities are required.

[0008] For example, the document "A pulsed electron gun for ultrafast electron diffraction at surfaces" by A. Janzen et al. published in "Review of Scientific Instruments" No. 78 in 2007 discloses an electron gun for the analysis of surfaces by electron beams. This device makes it possible to maintain the ultrahigh vacuum necessary for the intended application. In this gun, the cathode and the anode are as spaced as possible so as to minimize the risk of electrical breakdown and the focusing of the electron beam is carried out using electrostatic lenses.

[0009] The paper “Intrinsic Emittance Reduction of an Electron Beam from Metal Photocathodes” by CP Hauri et al. published in “Physical Review Letters” No. 104 in 2010 discloses the use of different metallic conversion targets such as Mo, Nb or Al usually used in high energy physics. However, this paper refers exclusively to large devices.

[0010] The second family is that of Optical Physicists. It of course uses a primary laser source and a conversion target, usually metallic or semiconducting, but also plasma. It is often pulsed, at a recurrence rate of 10 Hz to a few MHz, but always fixed. The achievements strive to obtain particle packets significantly smaller than for the first family, in order to reduce the space charge. For this family, the main objective displayed is the race to obtain the ultimate duration of the particle packet (of the hundred attoseconds to hundred femtoseconds), without considerations for the degree of vacuum, simplicity or comfort of use. The energies involved may be lower than those of the first family, but the beam energies targeted are generally from 30 keV to 200 MeV. In this family, electron guns can be pulsed in the single-electron regime but not necessarily (they can deliver packets of several tens of thousands of electrons, like the 1st family), the distances are in decimeters or meters and a high or ultrahigh vacuum is required.

[0011] For example, the paper "On the physics of ultrashort single-electron pulses for time-resolved microscopy and diffraction" by P. Baum published in "Chemical Physics" in 2013 discloses methods for reducing the duration of electron packets, in particular by reducing the number of particles composing each packet. However, these methods are applicable only to large devices and under strict operating conditions.

[0012] The document "Ultrashort pulse electron gun with a MHz repetition rate" by D. Wytrykus et al. published in Applied Physics B in 2009 describes an electron gun generating particle packets at a frequency of 2.7 MHz. These packets can contain a very small number of electrons. However, the energy dispersion of the generated electrons is large with an AE / E ratio of the order of 0.12 eV. Furthermore, the system requires a distance between the photocathode and the sample of the order of 300 mm, which greatly limits the compactness of the device. Furthermore, the time duration of the electron packets is not measured and only a theoretical value is given.

[0013] The third technological family is that of the Physical Chemists. It strives to produce particles energetic enough to ionize matter, but does not control the primary source. It aims to place the sample as close as possible to the production of the particles and does not necessarily seek high vacuum or ultra-high vacuum for its applications. The instruments thus developed have low temporal resolutions and poorly defined energies. When these two parameters become important for studies, physical chemists must resort to instruments from the first family.

[0014] For example, the paper "Femtosecond electron diffraction: 'making the molecular movie'" by JR Dwyer et al. published in Phylosophical Transactions of the Royal Society A in 2006 provides a review of the generation of femtosecond electron packets, directly related to femtosecond electron diffraction and ultrahigh vacuum applications. This paper further teaches that there is currently no detection system with sufficient resolution to monitor the new types of electron packets produced.

[0015] The paper "Medium Vacuum Electron Emitter as Soft Atmospheric Pressure Chemical Ionization Source for Organic Molecules" by S. Liedtke et al. describes an electron packet source with a high internal operating pressure. However, this device uses an electron source by heating a cathode or thermionic emission. This has poor time resolution and a high operating temperature, which is not compatible with fragile or liquid samples.

[0016] A fourth technological family is that of Electron Microscopy Physicists. This can be illustrated by document WO 2010 / 042629 A2 which aims to make a thermionic source and a photon source cooperate in an electron microscope to modulate the emission of the first source. Document DE 10245052 A1 concerns an electron microscope but whose electron source is produced according to the techniques of the second family with a train of electron packets chopped attosecondly by interaction with an ultra-fast laser. The whole is under ultra-high vacuum with electrostatic focusing lenses.

[0017] The applicant has also developed an electron gun capable of generating particle packets by photoelectric emission. This device, called LUBIOL, has a vacuum chamber with a very low operating pressure of the order of 107 mbar. The electron packets comprise a high number of particles of the order of 103. The kinetic energy of the electrons is of the order of keV with a repetition rate of the order of kHz. A system of electrostatic lenses is used to maintain these characteristics of the electron beam at the sample level. This system has a certain number of drawbacks, in particular the impossibility of avoiding high vacuum, i.e.to increase the operating pressure, the impossibility of extracting the beam in the presence of a gaseous atmosphere, the impossibility of working with fragile or liquid or desorbent samples, the impossibility of reducing the dimensions of the system and the impossibility of obtaining a robust and easy-to-use instrument.

[0018] More generally, state-of-the-art electrically charged particle generators have constraints that prevent their use for the analysis of certain samples. For example, devices that require a high vacuum do not allow the analysis of biological, liquid or desorbent samples. In addition, systems that allow a high operating pressure and are therefore compatible with fragile samples have low time and energy resolution.

[0019] Furthermore, most electrically charged particle generators known to those skilled in the art use very high electron acceleration voltages. This results in high energy consumption. In addition, the distance between cathode or The photocathode and anode must be increased considerably to reduce any risk of electrical breakdown. Therefore, there are no small-sized fast pulsed generators of electrically charged particles.

[0020] To at least partially resolve these technical problems, the invention aims to produce an ultrafast pulsed source of electrically charged particles which is portable, therefore having dimensions and energy consumption as small as possible, and which has a time resolution of the order of a picosecond, while destroying fragile samples, such as liquid or biological samples, as little as possible. Summary of the invention

[0021] To this end, the invention relates to a pulsed generator of electrically charged particles comprising: • a pulsed light source; • a vacuum chamber; said generator being characterized in that: • the vacuum enclosure is configured to maintain an internal operating pressure between 106 mbar and atmospheric pressure; • the vacuum enclosure is configured to accommodate a photocathode and an anode, the photocathode and the anode being separated by an adjustable distance less than or equal to 30 mm; • the vacuum chamber has an optical window allowing the pulsed light to reach one face of the photocathode; • the anode has an orifice suitable for the passage of electrically charged particles; • said pulsed generator of electrically charged particles comprises means for applying an electrical voltage between the photocathode and the anode, said electrical voltage being configured to accelerate the charged particles.

[0022] Electrically charged particles are understood to mean electrons or ions intended for the chemical, physicochemical or biological analysis of a sample to be characterized.

[0023] According to one embodiment, the gun initially produces electrons which are possibly converted into ions by interaction with an appropriate medium.

[0024] Advantageously, the generator according to the invention makes it possible to produce packets of charged particles having an ultra-short time duration, for example less than or equal to 5 picoseconds. According to one embodiment, the packets generated comprise a small number of charged particles, or even a single charged particle. More generally, the invention makes it possible to obtain packets of charged particles having a temporal duration less than or equal to 1 nanosecond or even less than or equal to 5 picoseconds according to an embodiment of the invention, or even less than or equal to 100 femtoseconds.

[0025] A pulsed light source is understood to mean a light source capable of providing pulses of light. According to one embodiment, the pulsed light source is a laser providing pulses of 120 fs duration with a repetition rate of 80 MHz. According to one embodiment, the pulses have an average wavelength centered in the ultraviolet, for example around 266 nm. According to one embodiment, the light used is incoherent light, such as a pulsed LED emitting pulses having a temporal duration less than or equal to 70 picoseconds.

[0026] A vacuum enclosure is understood to mean a device for maintaining an internal pressure less than or equal to atmospheric pressure. The vacuum enclosure is further configured to accommodate the elements necessary for producing the packets of charged particles. According to one embodiment, the vacuum enclosure is configured to also accommodate the sample to be characterized. According to another embodiment, the vacuum enclosure is provided with a window transparent to the charged particles and placed on the axis of their exit path from the enclosure, and the sample is placed on this path outside the enclosure, preferably as close as possible to it.

[0027] The vacuum chamber is configured to maintain an operating pressure of between 106 mbar and atmospheric pressure. In particular, the device according to the invention can be configured to work at an operating pressure of between 103 mbar and atmospheric pressure, which allows the analysis of fragile, liquid or biological samples.

[0028] A photocathode is an electrode intended to emit electrons by photoelectric effect or photoelectrons. The photoelectrons are obtained by sending pulsed light onto the photocathode. According to one embodiment, the photocathode is metallic.

[0029] The orifice present on the anode allows the passage of electrically charged particles accelerated by the acceleration voltage applied between the photocathode and the anode.

[0030] The charged particles leaving the anode can then be sent to a sample to be analyzed.

[0031] An optical window is understood to mean an optical element allowing the pulsed light to reach the photocathode. The optical window may for example comprise a porthole, an optical fiber passage, a lens or a combination of these elements. The optical window is sealed, ensuring that the working pressure is maintained inside the vacuum enclosure.

[0032] The distance between cathode and anode is adjustable, which allows the distance to be adapted to the acceleration voltage used. Advantageously, this makes it possible to obtain a very compact device depending on the voltage applied.

[0033] For example, the distance L between the photocathode and the anode has a value of 300 pm for an acceleration voltage of 5 kV. When this voltage is of the order of 10, 20 or 30 kV, this distance is increased by a proportional factor of 2, 4 or 6 to avoid electrical breakdown, but the size of the device remains of the order of magnitude of mm, which is much smaller than the prior art.

[0034] Advantageously, the reduction in the distance between the photocathode and the anode allows a higher operating pressure inside the vacuum chamber. The particle generator according to the invention operates with a higher working pressure than the particle generators of the state of the art. This makes it possible to analyze biological or liquid samples which are not compatible with vacuum or ultra-high vacuum.

[0035] Advantageously, thanks to a high operating pressure, the sample to be characterized can be placed inside the vacuum chamber and closer to the anode than in the prior art. This makes it possible to reduce the distance between the photocathode and the sample to be analyzed and therefore to reduce the effects of temporal dispersion of the packets of charged particles. In other words, the generator according to the invention makes it possible to obtain packets of charged particles having very short temporal durations of less than 5 picoseconds, or even less than one picosecond.

[0036] Advantageously, the charged particle generator according to the invention therefore makes it possible to analyze fragile samples such as liquid or biological samples with a very high temporal resolution, which is not possible with the devices known to those skilled in the art. In addition, the generator according to the invention has reduced dimensions and low energy consumption.

[0037] When the samples to be analyzed cannot withstand a pressure lower than atmospheric pressure, it is possible to use the variant of the invention comprising a window transparent to the charged particles and located in their path. The sample is then placed against this window while being outside the vacuum enclosure. If the sample is liquid, the generator according to the invention can be oriented so that the beam exits from the top, the particle exit window then being horizontal.

[0038] According to one embodiment, the photocathode comprises an Au film, which allows a much longer lifetime without maintenance in degraded vacuum conditions than the photocathodes used by those skilled in the art in semiconductor material. The invention can use this type of photocathode with Au film, abandoned by those skilled in the art, due to the deliberate choice of a low number of electrons per impulse, which is compensated by the fact that these impulses are renewed at high rate.

[0039] According to one embodiment, the elements present inside the vacuum enclosure are obtained by depositing thin or ultra-thin layers on blades having an optical quality polish. This makes it possible to apply electric fields having intensities greater than 30 MV / m while reducing the risk of electrical breakdown.

[0040] According to one embodiment, the charged particle generator according to the invention further comprises means for modifying the direction of the pulsed light beam, so as to modify the point and angle of attack of the photocathode. In other words, the generator according to the invention makes it possible to modify the focusing point of the light beam as well as its angle of arrival on the photocathode.

[0041] According to one embodiment, the generator according to the invention further comprises a breakdown detector. Advantageously, the breakdown detector makes it possible to determine, for a given acceleration voltage, the electrical breakdown pressure. It is therefore possible to choose a high working pressure while avoiding electrical breakdown.

[0042] It is important to note that, in the usual design of electron guns, the aim is to be outside the breakdown conditions with a safety margin, which is why the electrodes are very far apart and the guns have dimensions of a few tens of cm. Conversely, the device according to the invention avoids electrical breakdown not by a more advanced vacuum as in the prior art, but on the contrary by a slightly marked, or even almost non-existent, vacuum associated with a breakdown detector consisting of a spark gap set to a voltage slightly lower than the breakdown voltage of the device, and giving an alert signal making it possible to slightly distance the operating conditions (pressure of the vacuum enclosure or anode-cathode distance) from the breakdown conditions.

[0043] Advantageously, the device according to the invention makes it possible to generate packets of particles having a time duration less than or equal to 1 nanosecond or even less than or equal to 5 picoseconds according to one embodiment of the invention, or even less than or equal to 100 femtoseconds.

[0044] The device according to the invention may also comprise one or more of the following characteristics according to all technically possible combinations: • the distance between photocathode and anode is less than or equal to 2 mm and the acceleration electrical voltage is less than or equal to 30 kV so as to obtain packets of electrically charged particles, each packet having a time duration less than or equal to 5 ps; • the distance between photocathode and anode is less than or equal to 2 mm and the acceleration electrical voltage is less than or equal to 10 kV so to obtain packets of electrically charged particles, each packet having a time duration less than or equal to 5 ps; the acceleration voltage is less than or equal to 5 kV and the distance between photocathode and anode is less than or equal to 300 pm; the acceleration voltage is less than or equal to 10 kV and the distance between photocathode and anode is less than or equal to 600 pm; the acceleration voltage is less than or equal to 20 kV and the distance between photocathode and anode is less than or equal to 1.2 mm; the acceleration voltage is less than or equal to 30 kV and the distance between photocathode and anode is less than or equal to 1.8 mm; the pulsed light source produces light pulses having a temporal duration less than or equal to 5 ps; the generator further comprises a breakdown detector; The breakdown detector is placed near the anode and includes: • a substrate made of insulating material with a high dielectric constant; • a ring and a rod made of conductive material brought to the same potential as the anode; • a rod made of conductive material, said rod being movable and able to approach the conductive ring; the breakdown detector is configured to determine the electrical breakdown pressure under the acceleration voltage used; the pulsed generator of electrically charged particles is adapted to accommodate a sample to be analyzed, the distance between the photocathode and the sample to be analyzed being less than or equal to 60 mm and according to one embodiment less than or equal to 0.7 mm; the sample to be analyzed is placed inside or outside the vacuum chamber; the generator according to the invention has in the axis of the beam of charged particles a window transparent to said charged particles and making it possible to maintain in the enclosure a pressure lower than atmospheric pressure; the window transparent to charged particles is a membrane; the membrane is made of silicon nitride Si3N4 or graphene (C)n and has a thickness between a few nm and a few hundred nm, typically between 2 and 200 nm; the photocathode comprises a metallic nanoscale stack on a thin optical plate type substrate; • the anode orifice is anisotropic and has a first characteristic dimension and a second characteristic dimension, the second characteristic dimension being at least twice as large as the first characteristic dimension; • the two characteristic dimensions are orthogonal to each other and both orthogonal to the axis of the beam of charged particles; • the anisotropic orifice of the anode is slit-shaped; • the slit-shaped orifice has at least two parallel edges, the two parallel edges being at two different electrical potentials. • the generator further comprises a diagnostic module for determining the time duration of the packets of electrically charged particles; • the breakdown detector is configured to provide an alert signal and / or stop the particle generator if the operating pressure inside the vacuum enclosure is not low enough to eliminate the risk of electrical breakdown; • the charged particles are ions and the generator according to the invention further comprises the following elements: • a gas capable of emitting positive ions during electronic bombardment; • at least one anode for focusing the ion beam.

[0045] Another object of the invention is a method of using the pulsed generator of charged particles according to the invention.

[0046] The method according to the invention comprises a step of adjusting the internal operating pressure using the breakdown detector, the adjustment comprising the determination, for a given acceleration voltage, of the operating pressure at the limit of electrical breakdown.

[0047] In other words, the method according to the invention makes it possible, for a given acceleration voltage, to find the operating pressure at the limit of electrical breakdown. BRIEF DESCRIPTION OF THE FIGURES

[0048] Other characteristics and advantages of the invention will emerge clearly from the description given below, for information purposes only and in no way limiting, with reference to the appended figures.

[0049] [Fig. 1] illustrates an embodiment of the particle generator according to the invention.

[0050] [Fig.2] illustrates an embodiment of the breakdown detector according to the invention.

[0051] [Fig.3] illustrates an embodiment of the electrical circuit of the breakdown detector of the particle generator according to the invention.

[0052] Figures 4 and 5 illustrate two embodiments of the anode of the particle generator according to the invention.

[0053] [Fig.6] illustrates a sectional view of the diagnostic module, the section passing through the smallest diameter of the anisotropic anode.

[0054] [Fig.7] illustrates different photo trigger configurations of the charged particle generator according to the invention. DETAILED DESCRIPTION

[0055] [Fig. 1] [Fig. 1] illustrates an embodiment of the generator G of electrically charged particles according to the invention. In the remainder of this document, the generator G will be referred to interchangeably as a generator G of charged particles or Mini Gun. The generator G comprises a pulsed light source L emitting light pulses LP. The light pulses LP are sent to a tripler device T making it possible to obtain light pulses LP_UV having a shorter wavelength than the pulses LP. The beam LP_UV is then sent to a photocathode PH. According to the embodiment illustrated in [Fig.l], the beam LP_UV is focused using a lens LE so as to be concentrated on a precise point of the photocathode PH.

[0056] According to one embodiment, the source is a 1064 nm pulsed source and a frequency quadrupling system is used to obtain light pulses having higher energy.

[0057] According to one embodiment, the generator G further comprises means for modifying the point of arrival and the angle of attack of the LP_UV beam on the photocathode.

[0058] According to one embodiment, the pulsed light source LP emits pulses having a central wavelength in the infrared. For example, the pulses have a central wavelength of 800 nm with a repetition rate of 80 MHz. The time duration of the pulses is of the order of a few tens of fs. According to one embodiment, the light source L is a pulsed laser such as a femtosecond laser. Alternatively, the trigger laser is a UV, VIS or IR pulsed laser, or any other pulsed UV, VIS or IR light source.

[0059] In a manner known to those skilled in the art, the tripler T makes it possible to obtain LP_UV light pulses having a central wavelength approximately three times smaller than the wavelength of the pulses emitted by the source L. For example, the light pulses from the tripler T have a central wavelength of the order of 266 nm. Advantageously, the light pulses of the LP_UV beam are adapted to generate photoelectrons at the level of the PH photocathode.

[0060] The charged particle generator according to the invention further comprises a vacuum enclosure V configured to maintain within it a working pressure of between 103 mbar and atmospheric pressure. The vacuum enclosure V comprises an optical window F allowing the passage of the LP_UV pulses intended to be focused on the photocathode PH.

[0061] According to one embodiment, the PH photocathode consists of a metallic nanometric stack on a thin optical plate type substrate, the nature of which depends on the trigger wavelength and the desired vacuum resistance. The surface state of the constituent metal can be smooth or nanostructured. The substrate can be flat or shaped depending on the desired spatial shaping of the beam.

[0062] The generator G according to the invention comprises an anode A positioned at a distance L from the photocathode PH and means for applying an electrical voltage between the photocathode and the anode. This electrical voltage is also called electron acceleration voltage.

[0063] The distance L between photocathode PH and anode AN is adjustable and can be adapted to the chosen acceleration voltage. For example, the distance L between the photocathode and the anode has a value of 300 pm for an acceleration voltage of 5 kV. When this voltage is of the order of 10, 20 or 30 kV, this distance is increased by a proportional factor of 2, 4 or 6 to avoid electrical breakdown, but the size of the device remains of the order of magnitude of mm which is much smaller than the prior art.

[0064] According to one embodiment, the module comprising the photocathode PH and the anode AN is machined, the electrodes not being movable. The change in the distance L between the electrodes is therefore done by substituting the module comprising the electrodes, by choosing a module having a distance L adapted to the chosen acceleration voltage. According to another embodiment, the electrodes are movable and the distance L is adjusted by moving the two electrodes apart or closer together without changing the module.

[0065] According to one embodiment, the distance L between photocathode PH and anode AN is less than 30 mm.

[0066] According to one embodiment, the distance L between the photoemissive cathode and the accelerating anode has a numerical value less than LMax (mm) = 1 + V^vj where V is the acceleration voltage in kV, LMax being in millimeters.

[0067] Advantageously, the invention makes it possible to reduce the distance L between photocathode PH and anode AN while using high operating pressures. This makes it possible to reduce the temporal dispersion of the generated electron packets and to obtain electron packets with a small temporal duration, for example less than or equal to 5 ps.

[0068] The anode AN has an orifice or opening to allow the passage of charged particles accelerated by the acceleration voltage applied between the photocathode PH and the anode AN.

[0069] According to one embodiment, the anode AN has an electron passage orifice having an anisotropic section in the plane normal to the electron beam.

[0070] [Fig.4] As illustrated in [Fig.4] in the case of a circular anode, the section The anisotropic orifice has two characteristic dimensions: a smaller diameter PD and a larger diameter GD, perpendicular to each other.

[0071] Such an anode allows the use of the electron beam without going through a downstream focusing lens which would be difficult to implement.

[0072] The smallest diameter is substantially smaller than according to the embodiments of the prior art. The largest is greater than or equal to twice the smallest diameter.

[0073] According to one embodiment, the small diameter is between 100 μm and 2 mm and the large diameter is between 100 μm and 20 mm.

[0074] Advantageously, the anisotropic anode AN plays a role in filtering the electrons generated off-axis, so as to improve the emittance of the beam at the cost of the transmitted electron flux. Advantageously, this makes it possible to reduce the number of electrons and to limit the space charge effect which would reduce the temporal resolution of the Mini Gun.

[0075] According to a variant, the anode is not presented as a ring or a pierced plate but as a juxtaposition in the same plane of two flat plates facing each other by two parallel lips so as to obtain an orifice for the passage of charged particles in the form of a slot.

[0076] [Fig.5] This embodiment is illustrated in [Fig.5].

[0077] The generator G according to the invention further comprises a breakdown detector CL or spark gap.

[0078] The detection of a breakdown makes it possible to ensure that, despite a low vacuum inside the Mini Cannon, there will be no degradation of the components, in particular the cathode and the anode. There is no need for a real pressure measurement, but simply the assurance that at the working pressure chosen for the interior of the Mini Cannon, there is no risk of breakdown. The use of a breakdown detector is counter-intuitive for a person skilled in the art who would seek to use a means of measuring the pressure inside the vacuum enclosure, while adjusting the pressure and acceleration voltage to move as far away as possible from breakdown conditions.

[0079] To do this, a spark gap is placed in the enclosure of the Mini Canon, having a disruptive distance less than that between the anode and the cathode, so that a spark occurs on this spark gap. According to one embodiment, the CL breakdown detector or spark gap consists of a conductive ring placed opposite a tip, the assembly being inserted just after the anode according to the direction of propagation of the electrons. This spark gap is brought to the voltage at which the Mini Canon enclosure is to be operated, between 1 and 30 kV, preferably during an initial adjustment phase of a work campaign. The appearance of a breakdown on the spark gap protects the functional elements, namely cathode and anode, separated by a distance L slightly greater than the distance separating the elements of the spark gap. The CL breakdown detector makes it possible, for a chosen working voltage, to get very close to the breakdown without exposing the functional components in operating mode.

[0080] Advantageously, thanks to the breakdown detector or CL spark gap, the charged particle generator according to the invention provides packets of charged particles having a very short time duration, while working with a high pressure inside the vacuum enclosure. In other words, thanks to the breakdown detector it is possible to analyze fragile, liquid or biological samples, with a time resolution of the order of a picosecond.

[0081] The generator G according to the invention further comprises a diagnostic module D for determining the time duration of the electron or charged particle packets. According to one embodiment, the diagnostic module D is used with an anisotropic anode having a slot-shaped opening.

[0082] The sample to be analyzed E can be placed both inside or outside the vacuum enclosure V.

[0083] According to one embodiment, the distance d between the photoemissive cathode and the sample is between 30 mm for an acceleration voltage of less than 5 kV, and 60 mm for an acceleration voltage of the order of 30 kV.

[0084] Advantageously, a short distance between the photocathode PH and the sample to be analyzed E makes it possible to limit the temporal dispersion of the electron packets and to obtain a high temporal resolution.

[0085] According to one embodiment, the electrically charged particles are gas ions. In this case, the following elements are added downstream of the electron beam to obtain a gas ion generator: • a gas capable of emitting positive ions during electronic bombardment; • at least one anode for focusing the ion beam.

[0086] [Fig.2] [Fig.2] illustrates an embodiment of a breakdown detector or CL spark gap according to the invention.

[0087] In the usual design of electron guns, we seek to be outside the breakdown conditions with a safety margin, this is the reason why the guns have dimensions of a few tens of cm generally. A value of 10 kV mm1 corresponds to the vacuum breakdown threshold.

[0088] Conversely, the Mini Canon comes very close to the breakdown condition: the device according to the invention works at the limit of breakdown, on the one hand because it is necessary to bring high voltages to reduced dimensions, on the other hand because the insertion of a pressure gauge sufficiently miniaturized for the system is not possible.

[0089] Electrical breakdown inside the gun is avoided not by a higher vacuum as in the prior art, but on the contrary by a low, or even almost non-existent, vacuum associated with a breakdown detector CL comprising a spark gap set to a voltage slightly lower than the breakdown voltage of the gun, and giving an alert signal making it possible to slightly distance the operating conditions from the breakdown conditions.

[0090] The CL breakdown detector according to the invention comprises: • a substrate 1 made of insulating material with a high dielectric constant, for example Peek; • an electrically conductive ring 2 connected to an equally conductive rod 3, the two being electrically connected and brought to the potential of the anode; • a conductive rod 4 sliding in its bore, capable of moving radially, for example under the effect of a micrometric screw not shown.

[0091] The end of the rod 4, which can barely be seen protruding into the central bore, is placed at a distance slightly less than the distance of the shortest electrical path between anode AN and photocathode PH. As this distance between anode AN and photocathode PH varies according to the voltage at which the gun enclosure is chosen to operate, the distance between the end of the rod 4 and the ring at the anode potential must be adjusted accordingly, so as to have the same value or be very slightly below. According to a preferred variant, the rod 4 is integral with the cathode PH, so that when the distance between anode AN and photocathode PH is adjusted, the distance between the apex of the rod 4 and the ring 2 is modified at the same time and in the same proportions.

[0092] According to one embodiment, the anode AN and the photocathode PH are separated by a distance of 300 pm. The electrical resistance of air at atmospheric pressure is 36 kV cm1 which is equivalent to 300 pm for a potential difference of 1080 V. The device according to the invention can operate up to 10 kV, this is not sufficient to avoid breakdown, so creating a vacuum makes it possible to increase this voltage. maximum before creating an electric arc which could damage the system and create a risk for the user.

[0093] It is important to note that the vacuum is very low or non-existent depending on the needs of the sample to be analyzed. This is done by a pumping device, but which is used from time to time, for example before a measurement campaign.

[0094] Advantageously, the device according to the invention does not require any heavy and constantly active device such as devices working under a vacuum greater than 105 mbar, and often greater than 107 mbar.

[0095] Advantageously, thanks to the CL breakdown detector, exact pressure measurement is not necessary for the use of the device according to the invention.

[0096] Advantageously, the device according to the invention makes it possible to alert the user or to stop the system when the pressure is no longer low enough and this may represent a risk of breakdown.

[0097] In order to fulfill this function, the system includes a ground connected to an electronic card. This ground is placed so as to be closer to the anode than the cathode, thanks to a calibrated ring which serves as an insulating spacer. The breakdown between the anode and the ground will therefore occur earlier and it will thus be possible to process the information before the breakdown of the system.

[0098] [Fig.3] The breakdown detection circuit, illustrated in [Fig.3], receives in “Vin” the voltage of the photocathode PH, while the rest of the circuit and the anode AN are referenced to ground. With each spark, a pulse is generated on the output "Vout". The signal at the output "Vout" can therefore be used to alert the user to the risk of electrical breakdown.

[0099] Figures 4 and 5, discussed above, illustrate the two embodiments of the anode AN having an anisotropic opening.

[0100] [Fig.6] [Fig.6] illustrates a sectional view of the AN anode and the device of diagnosis D, the section passing through the smallest diameter of the anode AN. According to one embodiment, the anode is deposited on an insulating sapphire substrate 700 μm thick, orthogonal to the z axis corresponding to the propagation axis of the electron beam. According to one embodiment, the anode AN is a single electrode consisting for example of a conductive film affixed to a perforated substrate.

[0101] The opening for the passage of charged particles can be in the form of a slit, as illustrated in [Fig.5]. In this case, the large diameter GD extends to the ends of the anode so as to cut it into two separate elements mechanically linked together by an insulating element. The anode AN then comprises two parallel lips, one subjected to an acceleration voltage E - Av , and the other to E + Av.

[0102] According to one embodiment, it is also possible to envisage several parallel slots for several source points in parallel.

[0103] According to one embodiment, such a slot-shaped anode may consist of a conductor affixed to a substrate, the assembly having been cut by an elongated slot along y of very small dimensions along x, with parallel edges. According to this configuration, it is possible to obtain two separate electrodes brought to a potential of one or more kV relative to the emissive cathode, but electrically isolated from each other. They may thus have between them a small potential difference relative to the voltage of the cathode, but which may be a few tens or hundreds of volts, thus creating an electric field E between their lips. This electric field E may vary over time in synchronism with the arrival of the electron packets, so that the two ends of the electron packet do not see the same field E. Thus, in the diagram of [Fig.6], the first electron of each packet sees a strong field E which deflects it towards the electrode E2, and this field decreases for the following electrons until a zero field for the last one of the packet. This has the effect of rotating the axis of the electron packet, and therefore increasing its projection perpendicular to the propagation axis, and allowing a spatial measurement from which a temporal measurement will be deduced. Preferably, the electrodes E1 and E2 constitute a microstrip line "coplanar microstrip line" with an impedance of 50 Q which allows extremely fast rising or falling edges along these electrodes.

[0104] In other words, when the anode consists of two flat lips facing each other, these two lips spaced by a small and strictly constant distance are brought to different potentials. This is advantageous when the generator G is pulsed with short duration pulses ps or fs resulting in small packets of electrons. The anode then fulfills several functions simultaneously.

[0105] According to a variant, the constant potentials VE1 and VE2 applied to the electrodes E1 and E2 have a relatively small but time-varying difference AVE. The anode then has, in addition to its accelerating function, that of a deflector which allows a temporal diagnosis of the electron beam. This is the principle of this temporal diagnosis, called "streaking" or scanning, which can be summarized as follows: the electron packet is passed through a small domain with 2 elements at a different potential subjected to an electric field oscillating as a function of time, transverse to the direction of propagation of the electron packet. Thus the electron packet will be deflected as a function of time relative to the initial propagation axis z.The duration measurement is indirect, deduced from the spatial dimension of the electron packet projected onto a microchannel plate and phosphor screen type detector: the longer the packet is in time, the larger the image of the packet on the detector. Here we . passes the electron packet into a domain whose dimension is given by the thickness of the blades which constitute the anode.

[0106] Applying voltage to the two lips of the anode makes it possible to obtain, for example for a voltage of + / -20V, a beam displacement of 30 pm and 180 pm for + / -100V.

[0107] Advantageously, this embodiment allows the integration of a scanning system directly at the anode.

[0108] Example of an anode with integrated scanning system: the electrodes are designed to form a 50 Q line in "coplanar microstrip line". This configuration will allow testing the time-resolved scanning mode of this device. The anode plate is the most complex part of the device. It is a 700 pm sapphire substrate cut to allow the electrodes to exit and slit in the middle to allow the electrons to pass through, for example with a 100 pm slit. The rear face is possibly thinned in the middle to avoid any contact with the sample plate.

[0109] In static mode, without differential voltage on the anode electrodes, the beam maintains a reduced dimension of less than 20 pm. The dimension of the electron beam is greater than the dimension of the photon beam incident on the photocathode, but of the same order of magnitude. In scanning mode, with voltages on the anode electrodes, this size is maintained with a sensitivity of the order of 100 pm / 100 V, i.e. a simulated displacement of 200 pm at the sample level for voltages + / -100 V for the electrodes. The maximum deflection depends on the geometry of the slit and is of the order of 1 mm for voltages of + / -500 V. These values ​​are valid for a displacement perpendicular to the slit and measured at 700 pm from the anode, at the sample level.

[0110] The distance between the anode AN and the sample E to be analyzed is small and less than 60 mm. Consequently, it may be advantageous to place the sample E inside the enclosure of the charged particle generator G or outside.

[0111] Advantageously, it is possible to use the Mini Canon in a horizontal or vertical position on an optical bench or on any other assembly, given that the incidence of the triggering laser beam can be perfectly controlled. Liquid samples can then be analyzed with a beam in a vertical position.

[0112] Using the system in a horizontal position offers in particular the possibility of working with a liquid sample: a drop of the sample is positioned on an output interface adapted to the energy of the electrons, for example an ultra-thin membrane of Si3N4 or graphene having a thickness of between 2 nm and 200 nm.

[0113] In the prior art, the electron beam is directed onto a sample placed in a vacuum chamber. Advantageously, the device according to the invention offers the possibility for the sample to remain at atmospheric pressure, since the electron beam is forced out of the mini gun which constitutes a miniaturized vacuum chamber.

[0114] A variant for the study of samples supporting vacuum consists of placing the sample directly in the enclosure of the mini gun, as close as possible to the emission, the detection of the phenomenon to be studied, for example scintillation, being able to be carried out by a detector placed directly behind the sample.

[0115] The sample can be designed as a module of the mini gun, just like the photocathode, the anode or the detector.

[0116] [Fig.7] [Fig.7] illustrates different geometric configurations for the photo triggering of the device.

[0117] It is important to be able to isolate the laser beam from the electron beam, which is collinear with it. One way of doing this is to send the LP_UV laser at a different incidence than the normal incidence on the photocathode PH, so that the laser beam and the electron beam are not collinear on the target. In addition, the slit of the anode AN is sufficiently closed so that the LP_UV beam does not pass through the system. According to one embodiment, an angle of 30 to 45° can be used on this geometry.

[0118] According to one embodiment, the firing axis of the laser impacts the target at a point and at an adjustable angle. According to one embodiment, the impact angle is other than 90°, and preferably between 30 and 45°.

[0119] According to this variant of the Mini Canon, designed to operate in a modular manner and several modes of use are envisaged:

[0120] In particular, it is possible to choose to attack the entrance face of the Mini Canon with the LP_UV laser with an angle of incidence far from the normal, with an angle close to the normal or exactly on the normal, but with a latitude of displacement of the beam along the x axis due to the anode split along x with a relative displacement parallel to the slit. It is thus possible to work with an impact on different zones of the photocathode.

[0121] We can choose to work with backlighting of the sample, by attacking the output face of the Mini Canon.

[0122] Advantageously, these embodiments make it possible to increase the precision and reliability of the analyses thanks to better separation of the electron beam and the LP_UV beam at the level of the sample and the detection module used for the analysis.

[0123] According to a first variant, the Mini Canon is kept fixed and the axis of the incident laser beam on the entry or exit face of the Mini Canon is modified.

[0124] According to a second variant, the laser beam is kept fixed and the position of the Mini Cannon is modified relative to the axis of the laser beam.

[0125] According to a third variant, the point of impact on the photocathode is placed at an eccentric point, and the photocathode is rotated manually without disassembly (and without modifying the pressure in the enclosure) when the point of impact is to be modified.

[0126] In other words, the modification of the point of impact of the laser can be done either by moving the photocathode in its cradle or by modifying the direction of the laser on the slit. Advantageously, the modification of the point of impact of the laser on the photocathode is done without breaking the vacuum inside the enclosure V.

Claims

Claims

1. Pulsed generator of electrically charged particles comprising: - a pulsed light source; - a vacuum enclosure (V); said generator being characterized in that: - the vacuum enclosure (V) is configured to maintain an internal operating pressure of between 10 6 mbar and atmospheric pressure; - the vacuum enclosure (V) is configured to accommodate a photocathode (PH) and an anode (AN), the photocathode (PH) and the anode (AN) being separated by an adjustable distance (L) less than or equal to 2 mm; - the vacuum enclosure (V) comprises an optical window allowing the pulsed light to reach a face of the photocathode; - the anode has an orifice adapted for the passage of the electrically charged particles;- said pulsed generator of electrically charged particles comprises means for applying an electrical voltage between the photocathode (PH) and the anode (AN), said electrical voltage being configured to accelerate the charged particles.;

2. Pulsed generator of electrically charged particles according to the preceding claim, characterized in that the electrical acceleration voltage is less than or equal to 30 kV so as to obtain packets of electrically charged particles, each packet having a time duration less than or equal to 5 ps.

3. Pulsed generator of charged particles according to one of the preceding claims, characterized in that the pulsed light source produces light pulses having a time duration less than or equal to 5 ps.

4. Pulsed generator of electrically charged particles according to one of the preceding claims, characterized in that it further comprises a breakdown detector (CL).

5. Pulsed generator of electrically charged particles according to the preceding claim, characterized in that the breakdown detector (CL) is placed close to the anode and comprises: - a substrate (1) made of insulating material with a high dielectric constant; - a ring (2) and a rod (3) made of conductive material brought to the same potential as the anode; - a rod (4) made of conductive material, said rod being movable and able to approach the conductive ring.

6. Pulsed generator of electrically charged particles according to claim 4 or claim 5 characterized in that the breakdown detector is configured to determine the electrical breakdown pressure under the acceleration voltage used.

7. Pulsed generator of electrically charged particles according to one of the preceding claims, characterized in that said vacuum enclosure is configured to accommodate a sample (E) to be analyzed, the distance between the photocathode and the sample to be analyzed being less than or equal to 0.7 mm.

8. Pulsed generator of electrically charged particles according to one of claims 1 to 6, characterized in that said vacuum enclosure comprises a window transparent to the charged particles, the sample (E) to be analyzed being placed outside the vacuum enclosure.

9. Pulsed generator of electrically charged particles according to one of the preceding claims, characterized in that the photocathode (PH) comprises a metallic nanometric stack on a thin optical plate type substrate.

10. Pulsed generator of electrically charged particles according to one of the preceding claims, characterized in that the orifice of the anode (AN) is anisotropic and has a first characteristic dimension and a second characteristic dimension, the second characteristic dimension being at least twice as large as the first characteristic dimension.

11. Pulsed generator of electrically charged particles according to the preceding claim characterized in that the anisotropic orifice of the anode is in the form of a slot.

12. Pulsed generator of electrically charged particles according to the preceding claim, characterized in that the slot comprises at least two parallel edges, the two parallel edges being at two different electrical potentials.

13. Pulsed generator of electrically charged particles according to the preceding claim, characterized in that it comprises a diagnostic module (D) for determining the temporal duration of the packets of electrically charged particles.

14. Method of using the pulsed charged particle generator according to one of claims 4 to 6 and 7 to 13 in that they depend on one of claims 4 to 6, characterized in that the method comprises a step of adjusting the internal operating pressure using the breakdown detector, the adjustment step comprising the determination, for a given acceleration voltage, of the operating pressure at the limit of electrical breakdown.

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