S. Busold

Laser-driven ion acceleration with hollow laser beams

The laser-driven acceleration of protons from thin foils irradiated by hollow high-intensity laser beams in the regime of target normal sheath acceleration (TNSA) is reported for the first time. The use of hollow beams aims at reducing the initial emission solid angle of the TNSA source, due to a flattening of the electron sheath at the target rear side. The experiments were conducted at the PHELIX laser facility at the GSI Helmholtzzentrum fur Schwerionenforschung GmbH with laser intensities in the range from 1018 W cm−2 to 1020 W cm−2. We observed an average reduction of the half opening angle by (3.07±0.42)° or (13.2±2.0)% when the targets have a thickness between 12 μm and 14 μm. In addition, the highest proton energies were achieved with the hollow laser beam in comparison to the typical Gaussian focal spot.

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.

Experimental discrimination of ion stopping models near the Bragg peak in highly ionized matter

The energy deposition of ions in dense plasmas is a key process in inertial confinement fusion that determines the α-particle heating expected to trigger a burn wave in the hydrogen pellet and resulting in high thermonuclear gain. However, measurements of ion stopping in plasmas are scarce and mostly restricted to high ion velocities where theory agrees with the data. Here, we report experimental data at low projectile velocities near the Bragg peak, where the stopping force reaches its maximum. This parameter range features the largest theoretical uncertainties and conclusive data are missing until today. The precision of our measurements, combined with a reliable knowledge of the plasma parameters, allows to disprove several standard models for the stopping power for beam velocities typically encountered in inertial fusion. On the other hand, our data support theories that include a detailed treatment of strong ion-electron collisions.

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.

Temperature measurement of hohlraum radiation for energy loss experiments in indirectly laser heated carbon plasma

For ion energy loss measurements in plasmas with near solid densities, an indirect laser heating scheme for carbon foils has been developed at GSI Helmholtzzentrum für Schwerionenforschung GmbH (Darmstadt, Germany). To achieve an electron density of 10^{22}cm^{3} and an electron temperature of 10-30eV, two carbon foils with an areal density of 100μg/cm^{2} heated in a double-hohlraum configuration have been chosen. In this paper we present the results of temperature measurements of both primary and secondary hohlraums for two different hohlraum designs. They were heated by the PHELIX laser with a wavelength of 527nm and an energy of 150J in 1.5ns. For this purpose the temperature has been investigated by an x-ray streak camera with a transmission grating as the dispersive element.

Ion energy loss in plasma beyond the linear interaction regime

W. Cayzac 1, A. Ortner2, V. Bagnoud3,4, M.M. Basko5, S. Bedacht 2, A. Blǎzevíc3,4, S. Busold3,4, O. Deppert 2, M. Ehret2, S. Faik10, A. Frank4, D.O. Gericke6, L. Hallo7, J. Helfrich2, E. Kjartansson2, A. Knetsch8, D. Kraus9, G. Malka1, K. Pepitone7, T. Rienecker 10, G. Schaumann 2, D. Schumacher 3, An. Tauschwitz 10, J. Vorberger 12, F. Wagner 2, and M. Roth2 1Univ. Bordeaux-CEA-CNRS CELIA UMR 5107; 2Technical University of Darmstadt; 3GSI; 4Helmholtz institute Jena;5KIAM Moscow; 6University of Warwick; 7CEA/CESTA; 8University of Hamburg & CFEL;9University of California; 10University of Frankfurt;11HIC for FAIR; 12MPI for physics of complex systems

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].

Image plate characterization and absolute calibration to low kilo-electron-volt electrons

We report on the characterization of an image plate and its absolute calibration to electrons in the low keV energy range (1-30 keV). In our case, an Agfa MD4.0 without protection layer was used in combination with a Fuji FLA7000 scanner. The calibration data are compared to other published data and a consistent picture of the sensitivity of image plates to electrons is obtained, which suggests a validity of the obtained calibration up to 100 keV.

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:

Recent Advances in Proton Acceleration and Beam Shaping

A report on recent-developments will be given with focus on experiments to control and combine laser-accelerated ion-beams with beam-transport structures and new targets and results using geometries for ion-driven fast-ignition and the generation of warm-dense-matter.