Non-Maxwellian electron distributions resulting from direct laser acceleration in near-critical plasmas

The irradiation of few-nm-thick targets by a finite-contrast high-intensity short-pulse laser results in a strong pre-expansion of these targets at the arrival time of the main pulse.The targets decompress to near and lower than critical densities with plasmas extending over few micrometers,i.e.multiple wavelengths.The interaction of the main pulse with such a highly localized but inhomogeneous target leads to the generation of a short channel and further self-focusing of the laser beam.Experiments at the Glass Hybrid OPCPA Scaled Test-bed(GHOST)laser system at University of Texas,Austin using such targets measured non-Maxwellian,peaked electron distribution with large bunch charge and high electron density in the laser propagation direction.These results are reproduced in 2D PIC simulations using the EPOCH code,identifying direct laser acceleration(DLA)[1]as the responsible mechanism.This is the first time that DLA has been observed to produce peaked spectra as opposed to broad,Maxwellian spectra observed in earlier experiments[2].This high-density electrons have potential applications as injector beams for a further wakefield acceleration stage as well as for pump-probe applications.

Volumetric changes in the focal areas of seismic events corresponding to destress blasting

The typical development of total volumetric change in the focal areas of seismic events,corresponding to destress blasting,is characterized as an explosive phase followed by an implosive phase and with alternating additional phases following on from that.In a few cases,a non-typical development of volumetric change was identified,where the first phase was implosive and the second phase,explosive.This development is mainly typical for induced seismic events recorded during mining,not for destress blasting.Seismic events were recorded during longwall mining in the Czech part of the Upper Silesian Coal Basin,where the destress blasting technique is used as a rockburst prevention active measure.Kinematic source processes in the focal areas of selected seismic events were analyzed by the seismic moment tensor inversion method,as well as by studying geomechanical rock mass conditions at the localities of the seismic events.The main goal of the analysis was to attempt to identify the reasons for non-typical development of volumetric changes in these cases.Volumetric changes were analyzed for seismic events with energy greater than 104 J,recorded in the period of time from 1993 to 2009(1109 events).80%(891)of the recorded seismic events were induced seismic events that were registered during longwall mining and 20%(218)corresponded to destress blasting events.Research shows that the main reason for the non-typical development of volumetric changes in the focal areas of seismic events is an association with destress blasting in the rock mass,which is very close to rock mass overstressing.The detonation of explosives in boreholes,which would dominate the first phase of volumetric changes,probably obscured stress release in the rock mass,as manifested in the first implosion phase of the volumetric changes in this case.

Target fabrication for laser-ion acceleration research at the Technological Laboratory of the LMU Munich

The Technological Laboratory of LMU Munich supplies various types of solid-state target for laser plasma experiments at the Centre for Advanced Laser Applications in Garching.Our main focus here is on the production of free-standing,thin foil targets,such as diamond-likecarbon foils,carbon nanotube foams(CNFs),plastic,and gold foils.The presented methods comprise cathodic arc deposition for DLC targets,chemical vapor deposition for CNFs,a droplet and spin-coating process for plastic foil production,as well as physical vapor deposition that has been optimized to provide ultrathin gold foils and tailored sacrifice layers.This paper reviews our current capabilities,which are a result of a close collaboration between target production processes and experiment,using high-power chirped pulse amplification laser systems over the past eight years.

Ion-bunch energy acoustic tracing by modulation of the depth-dose curve

Characterizing exact energy density distributions for laser-accelerated ion bunches in a medium is challenging due to very high beam intensities and the electro-magnetic pulse emitted in the laser-plasma interaction.Ion-bunch energy acoustic tracing allows for reconstructing the spatial energy density from the ionoacoustic wave generated upon impact in water.We have extended this approach to tracing ionoacoustic modulations of broad energy distributions by introducing thin foils in the water reservoir to shape the acoustic waves at distinct points along the depth-dose curve.Here,we present first simulation studies of this new detector and reconstruction approach,which provides an online read-out of the deposited energy with depth within the centimeter range behind the ion source of state-of-the-art laser-plasma-based accelerators.

Three-dimensional acoustic monitoring of laser-accelerated protons in the focus of a pulsed-power solenoid lens

The acoustic pulse emitted from the Bragg peak of a laser-accelerated proton bunch focused into water has recently enabled the reconstruction of the bunch energy distribution.By adding three ultrasonic transducers and implementing a fast data analysis of the filtered raw signals,I-BEAT(Ion-Bunch Energy Acoustic Tracing)3D now provides the mean bunch energy and absolute lateral bunch position in real-time and for individual bunches.Relative changes in energy spread and lateral bunch size can also be monitored.Our experiments at DRACO with proton bunch energies between 10 and 30 MeV reveal sub-MeV and sub-mm resolution.In addition to this 3D bunch information,the signal strength correlates also with the absolute bunch particle number.

Optimization of relativistic laser–ion acceleration

Experiments have shown that the ion energy obtained by laser–ion acceleration can be optimized by choosing either the appropriate pulse duration or the appropriate target thickness. We demonstrate that this behavior can be described either by the target normal sheath acceleration model of Schreiber et al. or by the radiation pressure acceleration model of Bulanov and coworkers. The starting point of our considerations is that the essential property of a laser system for ion acceleration is its pulse energy and not its intensity. Maybe surprisingly we show that higher ion energies can be reached with reduced intensities.