C. Brabetz

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.

Optimization of plasma mirror reflectivity and optical quality using double laser pulses

We measure a record 96 ±2.5% specularly reflected energy fraction from an interaction with a plasma mirror (PM) surface preionized by a controlled prepulse and find that the optical quality is dependent on the inter pulse time delay. Simulations show that the main pulse reflected energy is a strong function of plasma density scale length, which increases with the time delay and reaches a peak reflectivity for a scale length of 0.3 μm, which is achieved here for a pulse separation time of 3 ps. It is found that the incident laser quasi near field intensity distribution leads to nonuniformities in this plasma expansion and consequent critical surface position distribution. The PM optical quality is found to be governed by the resultant perturbations in the critical surface position, which become larger with inter pulse time delay.

Multi-pulse enhanced laser ion acceleration using plasma half cavity targets

We report on a plasma half cavity target design for laser driven ion acceleration that enhances the laser to proton energy conversion efficiency and has been found to modify the low energy region of the proton spectrum. The target design utilizes the high fraction of laser energy reflected from an ionized surface and refocuses it such that a double pulse interaction is attained. We report on numerical simulations and experimental results demonstrating that conversion efficiencies can be doubled, compared to planar foil interactions, when the secondary pulse is delivered within picoseconds of the primary pulse.

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.

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:

Far-field characteristics of a petawatt-class laser using plasma mirrors

We propose and demonstrate an experimental setup capable of handling many 10s of Joules, allowing for the direct characterization of the focal spot of a petawatt-class laser after a plasma mirror. On the one hand we observed that the focal spot shape of the laser is qualitatively not affected by the mirror, even at high working intensities. On the other hand the Strehl ratio of the beam is largely reduced at high intensities because of scattering on the expending plasma. Together with the measurement of the mirror reflectivity, we could define the optimal working condition of the mirror.

Plasma cavity enhanced ion acceleration

Summary form only given. Laser driven ion acceleration is particularly interesting due to its many potential applications, including (isochoric) heating of matter which has been proposed as an attractive method for heating nuclear fuel in fusion reactions. In theory ignition is predicted to be possible with currently achievable proton temperatures, however conversion efficiencies of laser energy to protons must be increased beyond the few percent so far routinely achieved to upwards of ten percent for this to be a feasible concept1.