Method and device for generating a focused strong-current charged-particle beam

The invention relates to a method for generating a focused charged-particle beam, comprising at least the steps of: a) generating a charged-particle beam (10); b) emitting a laser pulse (40); c) generating a focusing magnetic field structure in a target (50) by means of an interaction between the laser pulse and the target; and d) making the charged-particle beam penetrate the focusing magnetic field structure at least partially.

The present invention relates to methods for generating a focused beam of charged particles of high current and to devices for generating such beams.

More particularly, the invention pertains to a method for generating a focused pulsed beam of charged particles of high current, the beam of particles having for example a duration of the order of a picosecond, a current of the order of a kilo-ampere and being formed of particles having an energy of the order of a megaelectronvolt.

It is for example possible to generate such beams by means of an interaction between a laser of high power and a solid or gaseous target.

These beams are usually highly divergent and it is desirable to be able to focus them for applications such as for example the probing of physical phenomena, inertial fusion or the generating of intense radiations.

Unfortunately, the intensity of such beams renders them difficult to focus. Thus, the four-pole magnets commonly used to focus charged particle beams in particle accelerators are perturbed by the electromagnetic field of the intense beam and do not operate appropriately.

Chromatic focusing devices, for example that described in “Ultrafast laser-driven microlens to focus and energy-select mega-electron volt protons” by T. Toncian et al. (SCIENCE, vol. 312, 21 Apr. 2006) are known, however such a device selects an energy in the spectrum of the particle beam and a large part of the beam is therefore not focused.

There therefore exists a need for a generating device capable of generating a focused pulsed beam of charged particles of high current.

For this purpose, according to the invention, a method for generating a focused beam of charged particles comprises at least the steps of

a) generating a beam of charged particles;

b) emitting a laser pulse;

c) generating a focusing magnetic field structure in a target by means of an interaction of said laser pulse with said target; and

d) causing at least partial penetration of the beam of charged particles into said focusing magnetic field structure.

By virtue of these provisions, an intense and compact structure of magnetic fields may be generated in the target. The amplitude of these fields is sufficient to focus a pulsed beam of charged particles of high current without them being substantially perturbed by the field generated by said beam. The focusing may be stable for the whole of the duration of passage of the charged particle beam, for example several picoseconds, thereby allowing achromatic focusing of the pulsed beam of charged particles. The focusing intensity is adjustable as a function of the intensity of the laser pulse. The focusing of positively or negatively charged particles is possible simply by changing the direction of propagation of the laser pulse generating the magnetic field structure with respect to the direction of propagation of the pulsed beam of charged particles.

In preferred embodiments of the invention, it is optionally possible to have recourse furthermore to one and/or the other of the following provisions:the laser pulse possesses a power lying substantially between a terawatt and about a hundred terawatts;the laser pulse possesses a duration lying substantially between about ten femtoseconds and about ten picoseconds;in the course of step c), the laser pulse is focused on the target at the level of a focal spot and in the course of step d), the beam of charged particles passes at least partially through said focal spot;the target is made at least in part of a metal;the target is made at least in part of a metal chosen from a list comprising gold, copper and aluminum;the target extends substantially along a plane of extension between a front face and a rear face, said faces being opposite to one another in a thickness direction perpendicular to the plane of extension and separated by a thickness measured in said thickness direction, and in the course of step d), said beam passes through the target substantially in said thickness direction;the thickness of the target lies substantially between 500 nanometers and about a hundred micrometers;step a) of generating a particle beam comprises the emission of a generating laser pulse and the generation of a non-focused beam of particles by means of an interaction of said generating laser pulse with a generating target.

The subject of the invention is also a device for generating a focused beam of charged particles comprising

means for generating a beam of charged particles;

a laser source for emitting a laser pulse;

a target for generating a focusing magnetic field structure by means of an interaction of said laser pulse with said target, said beam of charged particles penetrating at least partially into said magnetic field structure.

In preferred embodiments of the invention, the means for generating a beam of charged particles may optionally comprise

a laser source for emitting a generating laser pulse; and

a generating target for generating a beam of charged particles upon an interaction of said generating laser pulse with said generating target.

In the various figures, the same references designate identical or similar elements.

The invention pertains to a method for generating a focused pulsed beam of charged particles of high current10.

Such a beam of particles10may have a duration of the order of a picosecond, for example between a few tens of femtoseconds and a few tens of picoseconds, for example three hundred femtoseconds.

Such a beam of particles10may have a current of the order of a kilo-ampere, for example of a few amperes to a few mega-amperes, and be formed of particles having energy of up to as much as a few tens of megaelectronvolts, for example up to sixty megaelectronvolts.

Advantageously the beam of particles10may comprise a significant fraction of particles with an energy greater than a megaelectronvolt, for example more than half the particles.

Such beams are for example used in applications such as the probing of physical phenomena, inertial fusion or the generating of intense radiations.

With reference toFIGS. 1 to 5, such a beam10may for example be generated by an interaction between a high power generating laser pulse20and a generating target30.

The generating laser pulse20may have a high power, for example about a hundred terawatts.

The laser beam may for example consist of a pulse having an energy of about thirty Joules and a duration of about three hundred femtoseconds. In other embodiments, the intensity of the first laser pulse may for example lie between a few Joules and a few kilojoules, and the duration of the laser pulse may lie between a few tens of femtoseconds and a few tens of picoseconds.

The generating laser pulse20may be generated1100by a first laser source21of high power and propagate in a direction of propagation XL1.

The generating target30may be a solid, liquid or gaseous target, for example an aluminum film 15 micrometers in thickness, as described in “Ultrafast laser-driven microlens to focus and energy-select mega-electron volt protons” by T. Toncian et al. (SCIENCE, vol. 312, 21 Apr. 2006) and the references cited in this article.

It may extend substantially along a plane of extension YT1ZT1.

An interaction1200between the generating pulse20and the generating target30may be obtained by at least partially focusing said pulse on said target.

Thus, the generating laser pulse20is focused, by means of optical focusing devices, on a front face31of the generating target30at the level of a focal spot32of restricted dimensions, for example around 6 micrometers in width at half the maximum intensity (“FWHM”).

This laser pulse20creates a plasma34at the level of the front face31of the generating target30by ionizing the atoms of the target30that are situated at the level of the focal spot32.

The laser pulse20heats the generating target30and communicates to the electrons of said target30a significant thermal energy which may lead a part35of said electrons to pass through the target so as to escape therefrom at the level of the rear face33, said rear face33being a face of the generating target30opposite with respect to the front face31in a thickness direction XT1of the first target, said thickness direction XT1being for example substantially perpendicular to the plane of extension of the first target TT1ZT1.

In one embodiment, the thickness direction XT1of the generating target30and the direction of propagation of the first laser pulse XL1may be substantially collinear.

As a variant, the direction of propagation XL1of the laser may be inclined with respect to said thickness direction of the first target XT1, for example by 45° or more. The first laser pulse20therefore generates a displacement of electrons35in the thickness of the generating target30which constitutes a beam of electrons35set into motion substantially in the thickness direction XT1of the generating target30.

By extending outside of the target at the level of the rear face, these electrons may produce significant electric fields36at the level of said rear face33(of the order of a tera-volt per meter).

These electric fields36may in particular be sufficiently intense to strip ions11from the rear face (for example impurities trapped on the opposite surface) and thus produce1200a beam10of charged particles11.

The energy of said charged particles11may for example reach as much as sixty or a hundred megaelectronvolts and the doses may for example be of the order of 10^11 to 10^13 particles per pulse.

A pulse of such a beam10may for example last less than a picosecond, that is to say substantially the duration of the first laser pulse and the current generated may thus be of the order of a few kilo-amperes to a few hundreds of kilo-amperes.

The beam of electrons35set into motion in the thickness of the generating target30by the first laser pulse20may be divergent. The beam of charged particles10created may thus likewise be divergent.

This makes it necessary to focus said beam of particles so as to be able to use it in several applications, including those mentioned hereinabove.

Thus, with reference toFIGS. 1 to 5, a method for generating a focused beam of charged particles of high current may comprise the following steps.

A step a) comprises the generation of a beam of particles10, for example by means of the operation described hereinabove.

A second step b)2100may comprise the emission of a second laser pulse40.

This second laser pulse40may have a power of a few terawatts, a few tens of terawatts or more.

This second laser pulse40may have a duration lying between about ten femtoseconds and a few tens of picoseconds.

The second laser pulse40may be emitted by a second laser source41, as illustrated inFIG. 1or, alternatively, it may be emitted by the first high power laser source21as illustrated inFIG. 3aand for example refocused by means of focusing devices42such as for example mirrors, circumventing the first target30.

The second step b)2100may also comprise the increasing of the laser contrast of said second laser pulse40such as will now be described in greater detail.

The second laser pulse40usually comprises pre-pulses of second laser pulse40propagating just before the main laser pulse of the second laser pulse40.

A device for increasing the laser contrast may in particular increase the laser contrast of the second laser pulse40.

In one embodiment of the invention, a device for increasing the laser contrast is a device able to significantly decrease the intensity of the pre-pulses of the second laser pulse40with respect to the main laser pulse of the second laser pulse40.

An incoming ratio is defined for example as being a ratio between the maximum intensity of the main laser pulse of the second laser pulse40and the maximum intensity of the pre-pulses of second laser pulse40, for a second laser pulse40propagating upstream of the device for increasing the laser contrast.

An outgoing ratio is defined for example furthermore as being a ratio between the maximum intensity of the main laser pulse of the second laser pulse40and the maximum intensity of the pre-pulses of second laser pulse40for a second laser pulse40propagating downstream of the device for increasing the laser contrast.

A device for increasing the laser contrast may for example be such that the outgoing ratio is about ten times greater than the incoming ratio.

In a variant, a device for increasing the laser contrast may for example be such that the outgoing ratio is about a hundred times greater than the incoming ratio.

The device for increasing the laser contrast may in particular be integrated into a focusing device42in the following manner.

The focusing device42may for example comprise a plate that is transparent for the wavelength of the laser, for example a transparent glass plate.

The second laser pulse40may strike said focusing device42with an angle of incidence tilted from the normal.

The second laser pulse40may furthermore have a fluence such that pre-pulses of the second laser pulse40are of sufficiently low intensity to pass through said focusing device42, or be reflected only by a few percent of intensity.

The intensity of the main laser pulse of the second laser pulse40being higher, the main laser pulse of the second laser pulse40, in particular a rising edge of said main laser pulse of the second laser pulse40, may trigger a plasma on a surface of the focusing device42.

Said plasma on the surface of the focusing device42may in particular be able to reflect, for example to reflect by fifty percent to eighty percent of intensity, the main laser pulse of the second laser pulse40as a second reflected laser pulse.

By “plasma on a surface of the focusing device” is thus meant a plasma mirror able to reflect at least a portion of the main laser pulse of the second laser pulse40.

Said second reflected laser pulse may then constitute the second laser pulse40refocused by means of focusing devices42for the remainder of the present description.

Such a device for increasing the laser contrast, comprising a transparent plate, may for example be such that the outgoing ratio is about ten times greater than the incoming ratio.

A device for increasing the laser contrast, comprising a transparent plate furnished with an antireflection treatment, may for example be such that the outgoing ratio is around a hundred times greater than the incoming ratio.

A third step c)2200may comprise the generation of a focusing magnetic field structure60in a second target50by means of an interaction of the second laser pulse40with said target50.

The second target50may for example be a solid target. It may be a metallic target.

The second target50may for example comprise a part made of gold, aluminum or copper.

The second target50may for example extend substantially along a plane of extension YT2ZT2, and comprise a front face51and a rear face53which are opposite with respect to one another in a thickness direction XT2perpendicular to said plane of extension YT2ZT2.

Said front face51and rear face53may be separated by a thickness measured in the thickness direction XT2and for example lying between 500 nanometers and about a hundred micrometers, for example about ten micrometers.

An interaction between the second pulse40and the second target50may be obtained by at least partially focusing said pulse on said target.

Thus, the second laser pulse40may be focused on the front face51of the second target at a focal spot52of restricted dimensions, for example around 6 micrometers in width at half the maximum intensity (“FWHM”).

In one embodiment, the second laser pulse40may propagate in a direction of propagation XL2, for example substantially collinear with the horizontal thickness direction XT2.

As a variant, the direction of propagation XL2of the laser may be inclined with respect to said thickness direction of the second target XT2.

With reference toFIG. 4, the interaction between the second laser pulse40and the second target50created a first displacement of electrons55according to a mechanism similar to the mechanism described hereinabove in relation to the interaction between the first laser pulse and the first target.

In one embodiment, the front face51of the second target50may be sculpted, for example by patterns in relief, so as to control said first displacement of electrons55.

This first displacement of electrons55may be directed from the front face51toward the rear face53of the second target50and may generate displacement currents in the second target50which are oriented substantially in the thickness direction XT2of the second target and are located in the prolongation of the focal spot52when following the thickness direction XT2of the second target50.

On account of said first displacement of electrons55, the electron density in a zone54of the second target50situated in proximity to the focal spot52on the front face51of the second target may be lowered.

This lowering of the electron density may produce a second displacement of electrons56, this time from the second target50as a whole toward said zone54of the second target situated in proximity to the focal spot, so as to re-establish electron neutrality in said zone54.

This second displacement of electrons56may generate return currents in the second target.

These return currents may be oriented differently from the displacement currents.

The displacement currents and the return currents may then produce magnetic fields60in the second target50.

These magnetic fields60may constitute a focusing magnetic field structure60which will now be described.

The displacement currents may be oriented in the thickness direction XT2of the second target50, the magnetic fields60may therefore be perpendicular to said thickness direction XT2of the second target50.

The return currents may be oriented at least in part in a direction radial to the thickness direction XT2of the second target (that is to say having at least one non-zero component in a direction radial to the thickness direction XT2) said magnetic fields60may thus comprise at least one non-zero component in a circumferential (or ortho-radial) direction, perpendicular to the thickness direction XT2of the second target50and to a direction radial to said thickness direction XT2.

The magnetic fields60situated on either side of an axial direction substantially collinear with the thickness direction XT2of the second target50may thus comprise components of opposite senses.

The focusing magnetic field structure60formed by said magnetic fields60may thus exhibit an axial symmetry with respect to an axis collinear with the thickness direction XT2of the second target50.

Thus, the focusing magnetic field structure60formed by the magnetic fields60may have a toroidal or solenoidal geometry about the thickness direction XT2of the second target50.

In the course of a fourth step d)2300, a beam of charged particles of high current10such as that described hereinabove may penetrate at least partially into said focusing magnetic field structure60.

The beam of particles10may for example propagate in a direction of propagation Xp, for example a direction of propagation substantially collinear with the thickness direction XT2of the second target50.

The direction of propagation of the beam of particles10may for example be understood to be the vector average of the directions of propagation of the particles11of which the beam is composed.

The beam of particles10may be placed so as to penetrate at least partially into the second target50, for example at the level of its front face51, for example at the level of the focal spot52situated on the front face51.

The particles11of which the beam10is composed being charged, they may be deviated by the focusing magnetic field structure60.

In particular, the focusing magnetic field structure60generated by the interaction between the second laser pulse40and the second target50may thus make it possible to focus said beam of charged particles10by deviating at least a significant fraction of the particles of the beam11.

Said particles11may be in particular deviated in the direction of the direction of propagation Xpof said beam10. That is to say the particles11may be deviated in a direction radial to the direction of propagation Xpof the beam.

Depending on the sign of the charge of each of the particles11of which the beam of particles10is composed, the focusing magnetic field structure60may deviate said particle11of the beam in the direction of the direction of propagation Xpof said beam or in the opposite direction, that is to say may focus or defocus said beam of particles.

In an alternative embodiment illustrated inFIG. 3b, the particle beam10may be placed so as to penetrate at least partially into the second target50at the level of its rear face53and propagate in the second target50in the direction of the front face51.

In this embodiment, the focusing magnetic field structure60is inverse to the structure60described in the embodiment ofFIGS. 1 and 3a, that is to say the directions of the magnetic fields60of the structure are opposite to the directions of the magnetic fields60of the structure of the previous embodiment. The deviation of each of the particles of the beam11is thus inverted with respect to the previous embodiment and the beam10will be defocused or focused according to the charge of the particles11of which it is composed in an inverse manner with respect to the embodiment ofFIGS. 1 and 3a.

The focusing distance of such a focusing device100or generating device200may be modulated.

Thus for example, by decreasing the intensity of the second laser40, the displacements of electrons55,56and therefore the currents generated in the second target50may be decreased. In this manner, the magnetic fields generated60may be decreased and the deviation of the particles11of the beam of particles10will be smaller.

The focusing carried out by the focusing device100or the generating device200may thus be less significant and the focal distance larger.

Conversely, by increasing for example the intensity of the second laser40, the focusing carried out by the focusing device100or the generating device200may be increased and the focal distance decreased.

The use of different materials for the second target50also makes it possible to influence the focusing carried out by the focusing device100or the generating device200.

The person skilled in the art will be able to choose various materials making it possible to vary the size of the magnetic field generated, in particular as a function of the resistivity of said material and of the dynamics of ionization and of heating of the material such as described for example in the article “Dynamic Control over Mega-Ampere Electron Currents in Metals Using Ionization-Driven Resistive Magnetic Field” of Y. Sentoku et al. (Physical Review Letters, vol. 107, 135005, 2011) and the references cited in this article.

A device for focusing a beam of charged particles of high intensity100or a device for generating a focused beam of charged particles of high intensity200according to an embodiment of the invention may furthermore comprise various extra modules.

Thus, a vacuum chamber70may accommodate said devices100,200and in particular at least a laser40and a target50.

The vacuum chamber70may be furnished with a window71allowing said beam of charged particles10to leave the vacuum chamber.

The vacuum chamber70may be furnished with a collimator80making it possible to stop peripheral radiations or particles at the exit of the device100,200.

The vacuum chamber70may be furnished with a module for stopping radiations, for example comprising a material with high atomic number such as iron, lead or uranium.

The vacuum chamber70may also be furnished with a beam deviation module making it possible to separate the charged particle beam and radiations having a similar direction of propagation, for example a deviation module based on magnetic fields.

The vacuum chamber70may be evacuated and kept under vacuum by means of one or more vacuum pumps72.