PULSE-TRAIN LASER-PLASMA ACCELERATOR

Abstract

A method for producing energetic electron beams using a laser-plasma accelerator including a laser and a device for producing a gas cloud in a vacuum chamber, the method including a step of generating a laser pulse which is focused into the gas cloud to create a plasma. The step of generating a laser pulse includes at least the generation of a laser pulse-train with a delay between two successive laser pulses of between three times and thirty times the plasma period TP, such that: TP=?p/c, ?p being the plasma wavelength defined by: ?p=(2?/C)*(n e2/(m ?0))??, where c is the speed of light, n is the electron density of the plasma in cm3, e=1.6e?19 C is the charge of an electron, m=9.1e?31 kg is the mass of an electron, and ?0=8.85?10?12 m?3 kg?1 s4 A2 is the permittivity of vacuum.

Claims

1. A method for producing energetic electron beams by means of a laser-plasma accelerator comprising a laser and a device for generating a gas cloud in a vacuum chamber, the method comprising: a step of generating at least one laser pulse that is focused into the gas cloud so as to create a plasma: generating at least one laser pulse including at least generating a laser pulse train with a delay between two successive laser pulses comprised between three times and thirty times the plasma period T.sub.p, such that:
T.sub.p=?.sub.p/c ?.sub.p being the plasma wavelength defined by: ?.sub.p=(2?/c)*(ne.sup.2/(m?.sub.0)).sup.?1/2, where c is the light celerity, n is the plasma electron density in cm.sup.?3, e=1.6 e?19 C is the electron charge, m=9.1 e?31 kg is the electron mass, and ?.sub.0=8.85?10.sup.?12 m.sup.?3 kg.sup.?1 s.sup.4 A.sup.2 is the vacuum permittivity.

2. The method according to claim 1, characterized in that the duration of each pulse is comprised between 5 femtoseconds and 100 femtoseconds.

3. The method according to claim 1, characterized in that the total number of pulses in the laser pulse train is comprised between 2 and 200.

4. The method according to claim 1, characterized in that the total laser energy is comprised between 100 mJ and 20 J.

5. The method according to claim 1, characterized in that the energy per laser pulse is comprised between 25 mJ and 2 J.

6. The method according to claim 1, characterized in that the laser emits a laser beam having a wavelength of 800 nm.

7. The method according to claim 1, characterized in that all the laser pulses have one and the same wavelength or different wavelengths comprising a wavelength and harmonics.

8. The method according to claim 1, characterized in that the laser beam is focused so that each pulse of the laser pulse train reaches an illumination greater than 10.sup.18 Wcm.sup.?2 in the gas cloud.

9. The method according to claim 1, characterized in that the gas comprises one or a mixture of the following gases: He, H2, Ar, N2.

10. The method according to claim 1, characterized in that the plasma electron density n is comprised between 10.sup.18 cm.sup.?3 and 10.sup.21 cm.sup.?3.

11. The method according to claim 1, characterized in that the gas cloud is produced either continuously or in pulsed fashion at the frequency of the laser pulses.

12. The method according to claim 11, characterized in that the gas cloud is emitted in pulsed fashion at the frequency of the laser pulses with an opening duration greater than 1 ms.

13. The method according to claim 1, characterized in that the plasma length is comprised between 0.02 mm and 100 mm.

14. A laser-plasma accelerator for producing energetic electron beams by implementing athe method according to claim 1; the laser-plasma accelerator comprising: a laser for emitting a laser beam; a laser compressor; a splitter of the laser beam into a pulse train; a device for producing a gas cloud in a vacuum chamber; and focusing optics.

15. The laser-plasma accelerator according to claim 14, characterized in that said laser is a laser incorporating the chirped pulse amplification technique (CPA).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Other advantages and features of the invention will become apparent on reading the detailed description of implementations and embodiments that are in no way limitative, and from the following attached drawings.

[0047] FIG. 1 is a diagrammatic general view of a laser-plasma accelerator according to the invention:

[0048] FIG. 2 is a diagrammatic view of a gas cloud passed through by a laser pulse train according to the invention:

[0049] FIG. 3 is a diagrammatic view of a plasma wave formed after a laser pulse according to the invention:

[0050] FIG. 4 is a graphical representation illustrating an ionic cavity produced following a laser pulse according to the invention.

DETAILED DESCRIPTION OF THE FIGURES

[0051] The embodiments that will be described hereinafter are in no way limitative: it is possible in particular to implement variants of the invention comprising only a selection of characteristics described hereinafter, in isolation from the other characteristics described, if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art. This selection comprises at least one, preferably functional, characteristic without structural details, or with only a part of the structural details if this part alone is sufficient to provide a technical advantage or to differentiate the invention with respect to the state of the prior art.

[0052] In particular, all the variants and all the embodiments described are intended to be combined together in all the combinations where there is no objection thereto from a technical point of view.

[0053] FIG. 1 shows a CPA laser 1 emitting at 800 nm with an energy of 1 J. This laser is provided to emit an initial laser pulse with a duration of 30 fs. A laser compressor 2 compresses this initial laser pulse before feeding a splitter 3 that generates a train of 8 pulses each having an energy of the order of 125 mJ. In other words, at each initial laser pulse, this pulse is divided into several pulses so as to constitute a pulse train according to the invention. The objective is to reduce the energy for each pulse of the pulse train and to multiply the number of laser pulses that accelerate electrons in their wake.

[0054] The splitter 3 can be placed downstream of the laser compressor 2 as shown in FIG. 1 or upstream as shown by the dashed-line frame 3 in FIG. 1. It is also possible to envisage the use of a laser capable of generating femtosecond pulses at desired energy levels. Such a laser can be considered as incorporating a pulse train splitter as shown by the dotted line frame 3.

[0055] FIG. 1 also shows a vacuum chamber 4 in which are arranged an optical focusing assembly 5 and a gas injector 6.

[0056] The gas injector 6 is capable of producing a gas cloud 8, such as helium, along for example a vertical axis inside the vacuum chamber.

[0057] The optical focusing assembly 5 comprises two mirrors the arrangement of which makes it possible to guide and focus the pulse train 7 originating from the laser-compressor-splitter assembly into the gas jet 8. Ideally, the pulse train 7 passes through the gas cloud 8 at a right angle but other arrangements making it possible to have different angles may be envisaged. In particular, the pulse train 7 can come into collision with the gas cloud 8 at an oblique angle so as for example to increase the distance travelled by the pulse train 7 in the gas cloud 8.

[0058] Furthermore, the cross section of the gas cloud 8 can have different shapes such as circular, rectangular, square, oval, elliptical, etc.

[0059] The laser-compressor-splitter assembly is configured so that the intensity of each pulse reaching the gas cloud 8 is equal to or greater than 10.sup.17 Wcm?2. For each pulse train, the gas is ionized by the rising edge of the first pulse of the train. Then, all the other laser pulses of the train directly see a plasma. Each pulse train encounters a new gas cloud, for example every s, 10 ms, or 100 ms, etc. according to the cadence.

[0060] On leaving the gas, the pulse train 7 as well as an electron beam 9 originating from the gas cloud, are encountered.

[0061] FIG. 2 shows the injector 6 diffusing a gas cloud having a circular cross section along a vertical axis. The pulse train 7 passes through the gas cloud 8, creating electron bunches that accompany the passage of the pulse train and are transformed into an electron beam 9 on leaving the gas cloud.

[0062] The plasma electron density can be calculated or estimated as a function of the gas used. In the case in point, in the example described using helium, the plasma electron density is of the order of 2 e19 cm.sup.?3. The plasma period TP can thus be calculated, such that:

T.sub.p=?.sub.p/c

[0063] ?.sub.p being the plasma wavelength defined by: ?.sub.p=(2?/c)*(n e.sup.2/(m ?.sub.0)).sup.?1/2, where c is the light celerity, n is the plasma electron density in cm.sup.?3, e=1.6 e?19 C is the electron charge, m=9.1 e?31 kg is the electron mass, and ?.sub.0=8.85?10.sup.?12 m.sup.?3 kg.sup.?1 s.sup.4 A.sup.2 is the vacuum permittivity.

[0064] The invention is noteworthy in particular in that the frequency of the pulses in the pulse train is comprised between three times and thirty times the plasma period. With a frequency defined within this interval, the successive pulses in the pulse train make it possible to produce energetic electron beams with a maximum of electrons.

[0065] With the system according to the invention, as a function of the application concerned, it is possible to define an optimum laser energy making it possible to produce electrons having characteristics necessary for the application concerned. This laser energy is optimum, since it makes it possible to produce these electrons in the most efficient manner per laser Joule. For example, it differs if the production of electrons at 5 MeV or at 100 MeV is concerned.

[0066] For example in industrial radiography, the accelerator according to the invention makes it possible to easily control the electron beam generated, while maintaining an average energy of approximately 4 MeV.

[0067] Unlike the prior art, rather than a single high-energy pulse, a pulse train is used, with a delay between two pulses of the order of approximately one hundred femtoseconds for example. Each pulse accelerates electrons in its wake so that a train of electron bunches is created.

[0068] Each laser pulse creates a plasma wave constituted by several ionic cavities.

[0069] FIG. 3 illustrates such a plasma wave in which the laser pulse 10, a first ionic cavity 11 and a second ionic cavity 12 can be seen in the wake of the laser pulse 10. The accelerated electrons are inside, close to the optical axis. Initially they are injected at the rear, then advance towards the front during acceleration. In some cases, electrons are injected continuously: they can then eventually occupy all of the optical axis inside the ionic cavity. For each ionic cavity, a ponderomotive force expels the electrons from the optical axis.

[0070] FIG. 4 shows the ionic cavity 11 formed in the wake of the pulse 10. This is for example an ellipsoid elongated in the direction of propagation. Part of the ionic cavity overlaps with a rear part of the pulse. In the example in FIG. 4, most of the ionic cavity extends outside the pulse. The backward or forward direction is defined as a function of the direction of propagation of the laser pulse.

[0071] This ionic cavity is the site of competition between two electric fields. A decelerating electric field is present at the front of the ionic cavity in the zone of overlap with a part of the pulse. An accelerating electric field is present at the rear within the cavity and accelerates the electrons.

[0072] The electrons that form the ionic cavity are not the same over time: at each instant new electrons form the ionic cavity. They do not follow the laser pulse.

[0073] In certain cases, electrons that form the cavity gain enough energy to be injected at the rear, as illustrated in FIG. 4. This is not always the case, in particular when Ar or N2 are used, or a mixture containing one of these gases. In this case, it is core electrons of these atoms that are extracted from the ions and injected directly into the ionic cavity.

[0074] Typically, the ionic cavity 11 in FIG. 4 has a diameter along the axis of propagation of approximately 10 ?m.

[0075] Thus in the present invention there is proposed a pulse train, with a delay of the order of approximately one hundred femtoseconds between two successive pulses. Each pulse accelerates electrons in its wake, so that a train of electron bunches is produced.

[0076] Of course, the invention is not limited to the examples that have just been described and numerous modifications may be made to these examples without departing from the scope of the invention.