Patent Publication Number: US-2022240364-A1

Title: Efficient production of high-energy ultrashort ion-, especially proton bunches

Description:
The present invention is directed to the acceleration of electrically charged particle bunches, that is, ion bunches, especially proton bunches. 
     Ion bunches (including proton bunches) with energies in the energy range of 10 keV to 20 MeV might be tools for important applications such as various material structure studies, nuclear transmutations, positron sources for PET examinations, or initiating nuclear fusion. In addition, said ion bunches can serve as sources of microwave accelerators. After post-acceleration, furthermore, proton therapy—as a medical application—can be an important application. 
     Directing laser beams of ultra-high intensity (10 15 -10 22  W/cm 2 ) to a solid or gas target is a common technique for accelerating electrically charged particles. Laser ion acceleration (proton acceleration) dates back to nearly two decades. A paper by B. Qiao et al. in “ Revisit on ion acceleration mechanisms in solid targets driven by intense laser pulses ” [Plasma Phys. Control. Fusion 61, 014039 (2019)] provides an up-to-date overview of the study of the mechanisms occurring simultaneously in acceleration processes with high-intensity laser fields for solid targets. The various effects considered include the so-called Coulomb explosion which is the ultrafast dispersion of a volume of material with excess electric charge created previously by a laser beam due to the Coulomb interaction, i.e. the acceleration of ions that make up said volume of material. The authors of this paper study the dynamics of the various mechanisms for varying target thicknesses. 
     One of the promising applications of ions (proton, deuterium, tritium, other ions) created by Coulomb explosion is to produce fast neutrons to initiate nuclear fusion. Patent documents KR101574203B1 and WO2014/019929A1 proposes respective solutions. It should be emphasized that both documents assume multi-cycle laser pulses for the acceleration, and the use of single- or few-cycle laser pulses is not mentioned or even hinted at. Moreover, patent document RU2534507C1 specifically teaches a conventional laser acceleration technique, however, the Coulomb explosion process is not mentioned at all. Here, and from now on, a laser pulse in which the number of optical cycles is at most five is referred to as a few-cycle pulse, while laser pulses comprising more than five optical cycles will be referred to as multi-cycle laser pulses. 
     T. Ditmire et al. report in “ High - energy ions produced in explosions of superheated atomic clusters ” [Nature 398, 489 (1999)] on the experimental study of Coulomb explosion in xenon clusters (atomic cluster) and the determination of the ion energy distribution. The kinetic energy of the ions also reached the level of 1 MeV. This is four orders of magnitude higher than the energy of Coulomb explosion experiments with molecules. Using molecular and atomic clusters, ion energy values typically below 100 keV are reported by I. Last et al. in “ Electron and nuclear dynamics of molecular clusters in ultraintense laser fields I. Extreme multielectron ionization ” [The Journal of Chemical Physics 120, 1336 (2004)]. Like the above-referred papers, this paper also assumes the application of a laser source which emits multi-cycle ionizing laser pulses. 
     The peculiarity of the above-referred solutions is the spherically symmetrical nature of the expansion, which can be a disadvantage for many applications, making said applications difficult. In many applications, a particle beam of low divergence is advantageous, such a beam can be achieved from starting with a bunch of charge with a geometry differing from that of a sphere. T. Morita studies disk-shaped initial charge bunches in his simulations in “ Laser ion acceleration by using the dynamic motion of a target ” [Phys. Plasmas 20, 093107 (2013)]. This choice provides a more favorable directional distribution for applications. The two-layer disk target structure shown is particularly effective for accelerating ions of low mass numbers (protons). Simulation studies assumed 20 J pulses of extremely high power (500 TW) lasers. It should be here emphasized that the accelerating process is dominated by radiation pressure, with the Coulomb explosion responsible for only a small fraction of the accelerating effect. The energy distribution characteristic of the spherically symmetric Coulomb explosion, according to which the particle number density is proportional to the square root of the energy, varies advantageously: the distribution function peaks at about the average value of 200 MeV, and the width of the peak is preferably small, approx. 2%. 
     The results of the analytical calculations and the numerical simulations performed for spherically symmetric ion clusters by M. Murakami et al. show that a favorable effect on the ion energy distribution can be obtained if one starts with a homogeneous mixture of ions derived from two various types of atoms with significantly different masses, instead of a single atom; see in “ Efficient generation of quasimonoenergetic ions by Coulomb explosions of optimized nanostructured clusters ” [Phys. Plasmas 16, 103108 (2009)]. The peak of the energy distribution function shifts in the direction of large energy values, while its width decreases. By tuning the charge/mass ratios defined for each individual component in case of the two components, as well as the ratio of the charge densities, high-energy, essentially monoenergetic ions (i.e. with an energy variance being practically less than 1%) can be obtained. 
     To create Coulomb explosion, it is a requisite to remove the electrons that get released during ionization. To this end, high-energy multi-cycle laser pulses have been used in previous experiments and simulation studies. This is, however, disadvantageous as the removal of electrons takes place in a disordered manner, and not along a distinguished direction. 
    
    
     It is an object of the present invention to remedy this problem, i.e. to increase the Coulomb explosion induced by removing the electrons released during ionization along a preferred direction. 
     In our studies, we have come to the surprising conclusion that this problem can be remedied by using ionizing laser pulses which consist of one or a few (at most five) optical cycles, in accordance with the method of claim  1  and the assembly of claim  10 . Further preferred variants of the process according to the invention are defined by claims  1  to  9 . Preferred exemplary embodiments of the assembly of the present invention are set forth in claims  11  to  15 . 
     Far infrared (also known as terahertz, from now on THz) pulse generation techniques have undergone enormous development in the last decade. Due to the development and improvement of the so-called tilted-pulse-front excitation technique (details of which can be found, e.g., in J. Hebling et al., “ Velocity matching by pulse front tilting for a large area THz - pulse generation ” [Opt. Expr. 21, 1161 (2002)]), it is now possible to generate single-cycle THz pulses with energy in the range of a few mJ and a peak electric field strength of the order of 10 MV/cm in lithium niobate crystals (the details can be found, e.g., in J. A. Fülöp et al., “ Efficient generation of THz pulses with  0.4  mJ energy ” [Opt. Expr. 22, 20155 (2014)]). A recently designed pulse source based on a stair-step echelon-faced lithium niobate crystal (see, e.g., Gy. Tóth et al., “ Numerical investigation of a scalable setup for efficient terahertz generation using segmented tilted - pulse - front excitation ” [Opt. Expr. 25, 29560 (2017)]) is capable of generating a THz beam consisting of high-energy, single-cycle terahertz pulses and having a perfect beam profile; the main feature of said THz beam is its good focusability. By means of focusing, a peak electric field strength of 10 to 100 MV/cm can be achieved without distorting said beam profile, which allows efficient particle acceleration. It should be here also noted that, e.g., U.S. Pat. No. 9,497,848 teaches, as a solution based on a different concept, a method and a device for accelerating and manipulating particle acceleration by means of the evanescent field of THz pulses. 
     Terahertz pulses generated in lithium niobate (or other material with good nonlinear optical properties) are suitable for accelerating or manipulating electrically charged particles, especially protons, as in their case
         the peak electric field strength is in the range of MV/cm or larger;   the characteristic wavelength exceeds 300 micrometers (i.e. the frequency is below 1 THz), which corresponds to the characteristic size of the particle bunches to be ionized, allows a sufficient amount of charge to be accelerated and ensures synchronizability between the particles and the ionizing pulse;   single- or near-single-cycle pulses can be generated.       

     In our studies, it has also been found advantageous to make use of two different ion types with various mass/charge ratios in the proposed solution. In our experience, in accordance with the findings of M. Murakami et al., in “ Efficient generation of quasimonoenergetic ions by Coulomb explosions of optimized nanostructured clusters ” [Phys. Plasmas 16, 103108 (2009)] cited above, this choice also advantageously increases the kinetic energy of the generated charged particles or reduces the variance in their energy. 
     In what follows, the invention will be discussed in more details with reference to  FIG. 1 . 
     The electrically charged particle bunches (from now on, ion bunches) are generated in the sealed inner volume of a vacuum chamber  2  of a setup  1  for efficient production of high-energy ultrashort ion, preferably proton bunches, into which a gas flow is introduced through a nozzle  3  from a suitable source of material (not shown). The gas is provided as atomic or molecular gas. To generate the ion bunches, preferably, hydrogen gas is used, of course gases of other material quality can equally be used. The incoming gas is ionized by an ionizing beam  4  obtained by focusing a laser pulse with a duration of less than 2.0 picoseconds and a wavelength of less than 2.5 micrometers, i.e. falling typically in the near infrared, visible or ultraviolet range; said ionizing beam  4  is introduced into the vacuum chamber  2  in a manner known by a person skilled in the art. Said laser pulse is generated by a suitable laser source. Focusing said laser pulse is performed by a suitable focusing device (not shown in the drawing) known to a person skilled in the art. The spatial distribution of the plasma  5  obtained by ionization in the vacuum chamber  2 , i.e. the geometry of the plasma volume, is set by properly choosing the duration of the ultrashort laser pulse generated by the laser source, by properly focusing said laser pulse, that is, by suitably choosing the beam neck and the Rayleigh length. An elongated plasma volume is created by focusing a longer (in duration) laser pulse, while a substantially spherically symmetrical plasma volume is created by focusing a shorter laser pulse. Given the material quality of the gas used and knowing the parameters of the laser source for generating said laser pulse, it is obvious to a person skilled in the art to choose and set the above-referred parameters of the required focusing. To focus laser pulses in practice, a great number of techniques is known in the art (see, e.g., the text book by T. Verdeyen,  Laser Electronics  [ISBN-13: 978 0137066667]). 
     Regarding the geometry of the plasma volume, the present invention encompasses cases wherein the plasma volume is nearly spherical or has a flat shape that can be approximated by a disk or ellipsoid—these geometries are of high relevance in terms of practical applications. After generating the plasma  5 , as the next step, the electrons are torn off, i.e. removed, from the plasma  5 . To this end, the electromagnetic fields of linearly polarized THz electromagnetic pulses  6   a ,  6   b  with identical beam properties propagating towards each other is used as being superimposed into a standing wave in such a way that, in said field, the electric field strengths of the two pulses add up and the magnetic field strengths of the two pulses cancel out each other (i.e. the resulting magnetic field strength is zero). The condition for operation is that the ratio of the central wavelength of the pulses  6   a ,  6   b  to the dimension of the plasma  5  along the propagation direction of the THz pulses  6   a ,  6   b  is at least two. As an example: for a plasma  5  with a spatial dimension of 250 μm, according to the condition, a wavelength of at least 500 μm is required. The electromagnetic pulses needed for generating the standing wave in question are produced by e.g. two suitable electromagnetic pulse generators synchronized with one other, or provided by e.g. a synchronized optical splitting of a single electromagnetic pulse that is produced by a suitable single electromagnetic pulse generator. Preferably, the electromagnetic pulse generator comprises a pulse source based on a stair-stepped echelon-faced lithium niobate crystal, and the generation of the THz pulses  6   a ,  6   b  takes place in the electromagnetic pulse generator by a tilted-pulse-front excitation technique known in the art. 
     The peculiarity of this technical solution is that the resulting electric field (E) of the THz pulses  6   a ,  6   b  has a direct role in said removal of the electrons, in contrast to the previously known solutions, wherein the ponderomotive force plays a decisive role in the removal. A feature and a great advantage of the invention is that the removal of electrons (contrary to the prior art solutions) is carried out not in a disordered manner, but in an ordered manner, along a straight line which is defined by a resulting electric field vector characterizing the electric field  7  of the electromagnetic pulses  6   a ,  6   b  (i.e. directive removal is performed), as is indicated in  FIG. 1  by the direction  8  of movement of the electrons. 
     In our studies, we have come to the conclusion that to effect the Coulomb explosion simultaneously with the directive removal of free electrons of the plasma  5  created, electromagnetic pulses  6   a ,  6   b  with the following properties are suitable:
         their peak electric field strength exceeds 1 MV/cm;   their frequency is in the THz range, i.e. between 0.1 and 10 THz, their wavelength is preferably at least ten times the wavelength of the laser pulses used for the ionization;   they consist of a single or a nearly-single, but at most five, optical cycles.       

     This is because efficient electron removal is possible with an electric field strength of at least in the order of MV/cm, which can be achieved in the frequency range of 0.1 to 10 THz by the methods discussed above. Furthermore, the range of cycle times belonging to this frequency range allows the removal of electrons during a single optical period. The advantage of applying single- or few-cycle pulses versus multi-cycle pulses is that the electrons can be removed from the plasma along a well-defined direction, which direction is much less disrupted by a change (alternation) taking place less frequently in the direction of the electric field strength of the single- or few-cycle pulses than in case of multi-cycle pulses. 
     Nowadays, THz electromagnetic pulses with the features allowing practicing of the invention can be generated in lithium niobate. Other materials with nonlinear optical properties, such as lithium tantalate or various semiconductors, are also suitable for this purpose. 
     After the directive removal of electrons from the plasma  5 , the remaining positively charged particle cloud spreads over due to the Coulomb interaction (i.e. Coulomb explosion takes place), thereby the kinetic energy of the particles increases. Due to the feature of the invention that the electrons are torn off in an ordered manner, an additional advantage is obtained: the electrostatic field of the electron bunch formed by the electrons torn off by ionization also contributes to the acceleration of the ions (protons). In our studies, it has been found that, as a result, the positively charged particles gain an additional energy increment in the order of 10 keV to a few times 10 MeV. 
     In our studies, it has also been found that the acceleration of the positively charged particles, especially protons, can be made more efficient if the plasma  5  contains other ions of higher mass number (e.g. carbon) in addition to the positively charged particles to be accelerated. Thus, the present invention is also advantageous in such cases wherein the plasma  5  contains more than one types of ion. Accordingly, in a possible preferred embodiment of the invention, two gases of different material quality are introduced into the vacuum chamber  2 , preferably through two nozzles  3  arranged in close proximity to one another, wherein each nozzle is connected with a source of gas of different material quality. 
     In our studies, it has also been found that the particle number distribution in terms of energy differs favorably from the square root distribution for the possible applications, and also has a smaller variance in energy. 
     An important further advantage of the solution according to the invention, e.g. over microwave accelerators, is that the setup  1  suitable/necessary for carrying out the process for accelerating charged particles is small and compact, and can be operated in a cost-effective manner in either continuous or batch mode.