Florian Kroll

Towards highest peak intensities for ultra-short MeV-range ion bunches

A laser-driven, multi-MeV-range ion beamline has been installed at the GSI Helmholtz center for heavy ion research. The high-power laser PHELIX drives the very short (picosecond) ion acceleration on μm scale, with energies ranging up to 28.4 MeV for protons in a continuous spectrum. The necessary beam shaping behind the source is accomplished by applying magnetic ion lenses like solenoids and quadrupoles and a radiofrequency cavity. Based on the unique beam properties from the laser-driven source, high-current single bunches could be produced and characterized in a recent experiment: At a central energy of 7.8 MeV, up to 5 × 108 protons could be re-focused in time to a FWHM bunch length of τ = (462 ± 40) ps via phase focusing. The bunches show a moderate energy spread between 10% and 15% (ΔE/E0 at FWHM) and are available at 6 m distance to the source und thus separated from the harsh laser-matter interaction environment. These successful experiments represent the basis for developing novel laser-driven ion beamlines and accessing highest peak intensities for ultra-short MeV-range ion bunches.

Preliminary investigations on the determination of three-dimensional dose distributions using scintillator blocks and optical tomography

PURPOSE Clinical QA in teletherapy as well as the characterization of experimental radiation sources for future medical applications requires effective methods for measuring three-dimensional (3D) dose distributions generated in a water-equivalent medium. Current dosimeters based on ionization chambers, diodes, thermoluminescence detectors, radiochromic films, or polymer gels exhibit various drawbacks: High quality 3D dose determination is either very sophisticated and expensive or requires high amounts of effort and time for the preparation or read out. New detectors based on scintillator blocks in combination with optical tomography are studied, since they have the potential to facilitate the desired cost-effective, transportable, and long-term stable dosimetry system that is able to determine 3D dose distributions with high spatial resolution in a short time. METHODS A portable detector prototype was set up based on a plastic scintillator block and four digital cameras. During irradiation the scintillator emits light, which is detected by the fixed cameras. The light distribution is then reconstructed by optical tomography, using maximum-likelihood expectation maximization. The result of the reconstruction approximates the 3D dose distribution. First performance tests of the prototype using laser light were carried out. Irradiation experiments were performed with ionizing radiation, i.e., bremsstrahlung (6 to 21 MV), electrons (6 to 21 MeV), and protons (68 MeV), provided by clinical and research accelerators. RESULTS Laser experiments show that the current imaging properties differ from the design specifications: The imaging scale of the optical systems is position dependent, ranging from 0.185 mm/pixel to 0.225 mm/pixel. Nevertheless, the developed dosimetry method is proven to be functional for electron and proton beams. Induced radiation doses of 50 mGy or more made 3D dose reconstructions possible. Taking the imaging properties into account, determined dose profiles are in agreement with reference measurements. An inherent drawback of the scintillator is the nonlinear light output for high stopping-power radiation due to the quenching effect. It impacts the depth dose curves measured with the dosimeter. For single Bragg peak distributions this leads to a peak to plateau ratio of 2.8 instead of 4.5 for the reference ionization chamber measurement. Furthermore, the transmission of the clinical bremsstrahlung beams through the scintillator leads to the saturation of one camera, making dose reconstructions in that case presently not feasible. CONCLUSIONS It is shown that distributions of scintillation light generated by proton or electron beams can be reconstructed by the dosimetry system within minutes. The quenching apparent for proton irradiation, and the yet not precisely determined position dependency of the imaging scale, require further investigation and corrections. Upgrading the prototype with larger or inorganic scintillators would increase the detectable proton and electron energy range. The presented results show that the determination of 3D dose distributions using scintillator blocks and optical tomography is a promising dosimetry method.

First application studies at the laser-driven LIGHT beamline: Improving proton beam homogeneity and imaging of a solid target

Abstract In the last two decades, the generation of intense ion beams based on laser-driven sources has become an extensively investigated field. The LIGHT collaboration combines a laser-driven intense ion source with conventional accelerator technology based on the expertise of laser, plasma and accelerator physicists. Our collaboration has installed a laser-driven multi-MeV ion beamline at the GSI Helmholtzzentrum fur Schwerionenforschung delivering intense proton bunches in the subnanosecond regime. We investigate possible applications for this beamline, especially in this report we focus on the imaging capabilities. We report on our proton beam homogenization and on first imaging results of a solid target.

Chemical-vapor deposited ultra-fast diamond detectors for temporal measurements of ion bunches

This article reports on the development of thin diamond detectors and their characterization for their application in temporal profile measurements of subnanosecond ion bunches. Two types of diamonds were used: a 20 μm thin polycrystalline chemical vapor deposited (CVD) diamond and a membrane with a thickness of (5 ± 1) μm etched out of a single crystal (sc) CVD diamond. The combination of a small detector electrode and an impedance matched signal outlet leads to excellent time response properties with a signal pulse resolution (FWHM) of τ = (113 ± 11) ps. Such a fast diamond detector is a perfect device for the time of flight measurements of MeV ions with bunch durations in the subnanosecond regime. The scCVD diamond membrane detector was successfully implemented within the framework of the laser ion generation handling and transport project, in which ion beams are accelerated via a laser-driven source and shaped with conventional accelerator technology. The detector was used to measure subnanosecond proton bunches with an intensity of 108 protons per bunch.

Upgrade of GSI's laser-driven ion beamline at Z6

The LIGHT beamline The German national collaboration ”LIGHT” (Laser Ion Generation, Handling and Transport, [1]) has implemented a worldwide unique laser-driven proton beamline at GSI. Compact acceleration up to nearly 30 MeV proton energies is possible from the novel plasma source via the TNSA mechanism, which is driven by the PHELIX 100 TW laser beam. Therefore, at the Z6 experimental area laser intensities of up to 5×1019 W/cm are accessible. A pulsed highfield solenoid then provides for the necessary beam collimation and energy selection [2] and typically protons with an energy between 8 and 10 MeV are chosen. Furthermore, a radiofrequency (rf) cavity is implemented at 2 m distance to the source for phase rotation of the created single bunch, which shows a typical energy spread of around 20% (FWHM around central energy) and high particle numbers of up to 10 . Energy compression of the bunch below 3% was demonstrated in an experimental run in 2013 [3].

A laser-driven proton beamline at GSI

The LIGHT beamline. Laser-based ion acceleration became an extensively investigated field of research during the last 15 years. Within several micrometers particles are accelerated to MeV energies. The main drawback for many applications is their continuous exponential energy spectrum and large divergence angle from source. The exploration of proper beam shaping and transport is the major goal of the LIGHT collaboration [1], for which an experimental test beamline has been built at GSI. This LIGHT beamline at GSI is located at the Z6 area within the experimental hall. The PHELIX 100 TW laser beamline is currently capable of delivering up to 15 J of laser energy in a 650 fs short pulse on target, focused to intensities exceeding 10 W/cm within the Z6 target chamber. Protons could be accelerated via the TNSA mechanism to maximum energies of 28.4 MeV and propagated through a pulsed high-field solenoid with a field strength up to 9 T, which is used to select a specific energy interval from the continuous initial spectrum via chromatic focusing. A large capture efficiency of 34% of the initial protons within a selected energy interval (ΔE=(10±0.5)MeV) was measured [2]. The protons are weakly focused to a 15×15 mm spot at 3 m distance to the source, containing particle numbers >10 in a single 8 ns short bunch. The energy spread of the bunch is (18±3)% and the central part of the bunch can be described by a Gaussian-like distribution: