Generation, Handling and Transport of Laser-Driven Heavy Ion Beams [thesis]

Johannes Ding
Demonstration der effizienten zeitlichen Komprimierung von laserbeschleunigten F 7+ Ionenstrahlen resultierte in (1, 3 ± 0, 1) ns (FWHM) kurzen Ionenpaketen. Die erfolgreiche effiziente Erzeugung, Handhabung und der Transport von lasergetriebenen Schwerionenstrahlen rückt eine lasergetriebene Ionenstrahlführung für beispielsweise die Untersuchung des Bremsvermögens von dichten Plasmen im Bezug auf Ionen in greifbare Nähe. Abstract The thesis at hand addresses the generation, handling and
more » ... rt of laser-driven heavy ion beams. The presented research has been conducted within the laser and plasma physics group at the institute of nuclear physics of the Technical University of Darmstadt. The experimental campaigns contributing to the results in this thesis have been carried out at the GSI Helmholtz Center for Heavy Ion Research and were supported by their plasma physics department. The laser-driven ion acceleration scheme target normal sheath acceleration (TNSA) enables the compact generation of intense ion bunches with kinetic energies in the range of 10s of MeV. The laser-accelerated ion beams have unique properties, such as extremely small transverse and longitudinal emittance, high numbers of ions per bunch, but also broad exponentially decaying energy spectra and high divergence. Due to the nature of the acceleration process the composition of ion species in the beam is defined by the atom population in the source area and is generally dominated by protons stemming from ever present hydro-carbon contaminations. Many anticipated applications of laser-driven ion beams rely on the efficient acceleration of heavy ions and on controlling the energy spread and the divergence of the initial TNSA beam. In this work the efficient acceleration of carbon and fluorine ion beams by means of TNSA was demonstrated. The hydrocarbon contaminations on the surface of the coated targets were removed by means of Joule heating. Utilizing this method and the 100 TW beamline of the PHELIX laser system at GSI, kinetic energies of fourfold positively charged carbon ions (C 4+ ) of up to 68.5 +4.8 −4.3 MeV and of sevenfold positively charged fluorine ions (F 7+ ) of up to 180 +32 −26 MeV were observed. With the help of a pulsed high-field solenoid magnet from Helmholtz Center Dresden-Rossendorf incorporated into the prototype beamline of the LIGHT (Laser Ion Generation, Handling and Transport) collaboration, the efficient collimation and transport as well as the energy selection by means of chromatic focussing of the laser-driven carbon and fluorine ion beams could be achieved. At an average energy of (14.9 ± 0.1) MeV a number of 9.9 +2.7 −3.1 · 10 8 C 4+ ions were detected at a distance from the ion source of more than 6.0 m by a combination of radiochromic films and Thomson parabolas. Challenging problems for the temporal bunch compression of laser-driven heavy ion beams with a three gap spiral resonator as well as viable solutions to these hurdles have been identified and already incorporated into the LIGHT beamline within the scope of this thesis. A first successful proof-of-principle of temporal bunch compression of laser-driven fluorine ion beams resulted in bunches as short as (1.3 ± 0.1) ns (FWHM). By successfully demonstrating the efficient generation, handling and transport of laser-driven heavy ion beams an anticipated laser-driven ion beamline for the investigation of for example the stopping power of ions in dense plasmas is within reach. Conclusion and outlook 105 List of abbreviations 109 References 111 Publications 125 Danksagung 127 Curriculum Vitae 129 6 The history of the laser, its rapid and ever continuing improvement, is a great success story. More than 50 years after the discovery of the first ruby laser by T. Maiman in 1960 [1] the 21 st century is generally being viewed at as the "century of the photon", because the laser is seen as a key technology to the majority of modern research and development. Shortly after the groundbreaking discovery by T. Maiman the techniques of Q-switching [2] and mode-locking [3] allowed for the generation of short laser pulses with pulse durations as low as femtoseconds. The chirped pulse amplification (CPA), developed by G. Mourou and D. Strickland in 1985 [4], enabled the amplification of these pulses up to several hundreds of Joules. Further development in numerous high power laser facilities around the world resulted so far in several petawatt class laser systems [5, 6, 7]. Focussing of the resulting high power laser pulses leads to intensities as high as 10 20 W cm −2 to 10 21 W cm −2 . With the imminent extreme light infrastructure (ELI) [8], laser facilities with pulse power exceeding 10 petawatt will be available. These very intense laser pulses of typically less than one picosecond duration have opened a completely new scientific field, the relativistic laser-plasma-interaction. The laser pulse interacts with an initially solid or gaseous medium, which gets ionized at least partially. Free electrons in the resulting plasma are accelerated to velocities close to the speed of light in the time span of a single laser cycle [9]. Among others, phenomena such as the generation of high harmonics of the laser pulse [10], x-rays caused by synchrotron radiation [11], electron acceleration [12, 13] and ion acceleration [14, 15, 16] all originate from relativistic laser-plasma-interactions. Almost two decades after the discovery of compact acceleration of ions by means of ultra-intense laser irradiation of solid targets [14, 17] , the mechanism is still subject to research. The new source for intense MeV-range ion bunches is considerted to be a driver for many applications. These range from: igniting inertial confinement fusion [18, 19] , driving isochoric heating to achieve warm dense matter [20, 21, 22] to the generation of isotopes with table-top devices [23], and particle therapy [24, 25] . Due to the high particle numbers and initially short pulse durations of the ion bunches the monitoring of transient phenomena is an interesting field of work [26] . Lately the generation of laser-driven neutron beams has been investigated by several groups [27, 28] Despite theoretical evidence in particle in cell (PIC) simulations for potentially game-changing new acceleration mechanisms such as the breakout-afterburner (BoA) [29, 30] , enabled by radiation induced transparency (RIT) [31, 32] or the radiation pressure acceleration (RPA) [33], up to now the only reliable mechanism is target normal sheath acceleration (TNSA) [14, 17, 15] . By irradiating micrometer thick targets with ultra-intense shortpulse lasers, which are focussed down to more than 10 18 W cm −2 , electrons from the front surface are pushed through the bulk material and form an electron sheath on the rear face of the target. The resulting electrical field, which is in the order of 10 12 V m −1 , ionizes the atoms on the rear surface and subsequently accelerates the ions up to several tens of MeV. Due to the initially cold ion temperature, the absence of collisions during the acceleration process and the process taking place on a picosecond time scale, the ion beams exhibit some exceptional characteristics. Total 7 particle numbers exceed 10 12 and stem from a sub-mm source area with transverse emittances as low as 0.01 mm mrad. While the energy spectrum of the ions has an exponentially decaying distribution, there is a very high degree of order in the longitudinal structure of the beam, manifesting itself in the longitudinal emittance being below 10 −4 eV s for specific energies [34] . Many of the above mentioned applications, such as isochoric heating, could be improved by separating the TNSA source spatially from the application, because highly relativistic electrons, x-rays, electro-magnetic pulses (EMP) and plasmas in general lead to strong interference for any measurement. Ion beam therapy imposes high demands on the maximum energy spread and brilliance of the applied ion beam. Thus, utilizing a TNSA ion beam for these kinds of applications, either the TNSA source has to be specifically tailored to allow for precise beam tuning, or the TNSA source has to be incorporated into a collimating and energy selective ion optical system. The former was attempted by tailoring the target, but this method could only influence the ion beam on a microscopic scale [20, 35] . The latter has shown promising results by utilizing conventional magnetic focussing devices like quadrupoles [36] or solenoids [37, 38, 39] . The combination of a laser-driven ion source with conventional accelerator technology seems to be the way to go and several projects are investigating in this field. Among others the ELI has dedicated a substantial work package, , to the transport and handling of laser-driven ion pulses with ion optical systems. Within the joint research project onCOOPtics 1 investigations are under way to take advantage of laser-driven ion acceleration in order to make ion beam therapy more compact and affordable [41] . One of the most successful collaborative efforts has been undertaken by German universities and Helmholtz centres to examine laser ion generation, handling and transport (LIGHT) [42] . This thesis is part of an ongoing investigation within the LIGHT project to selectively collimate a highly divergent laser-driven ion beam in such a way as to reduce the energy spread and compress the bunch temporally. In contrast to previous works by K. Harres [38, 43], T. Burris-Mog [37, 44], S. Busold [39, 42, 45, 46] and F. Kroll [47] and ongoing research [48], this thesis investigates laser-driven mid-Z ion beams instead of proton beams. 1.1 The LIGHT project The LIGHT collaborators from the Technical University of Darmstadt, GSI Helmholtz-center for heavy ion research (GSI), Helmholtz-center Dresden-Rossendorf (HZDR), Johann Wolfgang Goethe University Frankfurt, Helmholtz Institute Jena and Ludwig-Maximilians-University Munich have chosen the Z6 experimental area at GSI as a testbed for a laser-driven ion beamline. The choice was made based on the unique capabilities offered by the petawatt high-energy laser for heavy ion experiments (PHELIX) laser system [7] and the existing accelerator expertise and infrastructure. In particular the radiofrequency (rf) power supply, required for temporal bunch compression with resonators, could be provided by the universal linear accelerator (UNILAC), thus making the Z6 experimental area at GSI a unique experimental site. The high energy laser nhelix [49] enhances the future capabilities for applications of the LIGHT beamline. After demonstrating the effectiveness of pulsed solenoids for the collimation of laser-driven proton beams [37, 38] with experiments at the PHELIX Petawatt experimental area, further work was conducted at the Z6 experimental area. S. Busold et al. built and characterized 1
doi:10.15120/gsi-2019-00213 fatcat:sgl2qi6yljhl5h2bqbare65c7u