Georg Pretzler

Generating positrons with femtosecond-laser pulses

Utilizing a femtosecond table-top laser system, we have succeeded in converting via electron acceleration in a plasma channel, low-energy photons into antiparticles, namely positrons. The average intensity of this source of positrons is estimated to be equivalent to 2x10(8) Bq and it exhibits a very favorable scaling for higher laser intensities. The advent of positron production utilizing femtosecond laser pulses may be the forerunner to a table-top positron source appropriate for applications in material science, and fundamental physics research like positronium spectroscopy

GeV-scale electron acceleration in a gas-filled capillary discharge waveguide

We report experimental results on laser-driven electron acceleration with low divergence. The electron beam was generated by focussing 750 mJ, 42 fs laser pulses into a gas-filled capillary discharge waveguide at electron densities in the range between 10 18 and 10 19 cm 3 . Quasi-monoenergetic electron bunches with energies as high as 500 MeV have been detected, with features reaching up to 1 GeV, albeit with large shot-to-shot fluctuations. A more stable regime with higher bunch charge (20-45 pC) and less energy (200-300 MeV) could also be observed. The beam divergence and the pointing stability are around or below 1 mrad and 8 mrad, respectively. These findings are consistent with self-injection of electrons into a breaking plasma wave.

Proton radiography as an electromagnetic field and density perturbation diagnostic (invited)

Laser driven proton beams have been used to diagnose transient fields and density perturbations in laser produced plasmas. Grid deflectometry techniques have been applied to proton radiography to obtain precise measurements of proton beam angles caused by electromagnetic fields in laser produced plasmas. Application of proton radiography to laser driven implosions has demonstrated that density conditions in compressed media can be diagnosed with million electron volt protons. This data has shown that proton radiography can provide unique insight into transient electromagnetic fields in super critical density plasmas and provide a density perturbation diagnostics in compressed matter.

Generation of MeV electrons and positrons with femtosecond pulses from a table-top laser system

In experiments, the feasibility was demonstrated of generating multi-MeV electrons in a form of a collimated beam utilizing a table-top laser system delivering 200 fs pulses with PL=1.2 TW and 10 Hz capability. The method uses the process of relativistic self-channeling in a high-density gas jet producing electron densities in the range of 3×1019–6×1020 cm−3. In a thorough investigation, angularly resolved and absolutely calibrated electron spectra were measured and their dependence on the plasma density, laser intensity, and gas medium was studied. For the optimum electron density of ne=2×1020 cm−3 the effective temperature of the electron energy distribution and the channel length exhibit a maximum of 5 MeV and 400 μm respectively. The laser-energyto-MeV-electron efficiency is estimated to be 5%. In a second step, utilizing the multi-MeV electron beam anti-particles, namely positrons, were successfully generated in a 2 mm Pb converter. The average intensity of this new source of positrons is estimated t...

Novel method for characterizing relativistic electron beams in a harsh laser-plasma environment.

Particle pulses generated by laser-plasma interaction are characterized by ultrashort duration, high particle density, and sometimes a very strong accompanying electromagnetic pulse (EMP). Therefore, beam diagnostics different from those known from classical particle accelerators such as synchrotrons or linacs are required. Easy to use single-shot techniques are favored, which must be insensitive towards the EMP and associated stray light of all frequencies, taking into account the comparably low repetition rates and which, at the same time, allow for usage in very space-limited environments. Various measurement techniques are discussed here, and a space-saving method to determine several important properties of laser-generated electron bunches simultaneously is presented. The method is based on experimental results of electron-sensitive imaging plate stacks and combines these with Monte Carlo-type ray-tracing calculations, yielding a comprehensive picture of the properties of particle beams. The total charge, the energy spectrum, and the divergence can be derived simultaneously for a single bunch.

Correction of strong phase and amplitude modulations by two deformable mirrors in a multistaged Ti:sapphire laser

We describe a novel scheme consisting of two deformable bimorph mirrors that can free ultrashort laser pulses from simultaneously present strong wave-front distortions and intensity-profile modulations. This scheme is applied to the Max-Planck-Institut für Quantenoptik 10-TW Advanced Titanium-Sapphire Laser (ATLAS) facility. We demonstrate that with this scheme the focusability of the ATLAS pulses can be improved from 10(18) to 2x10(19) W/cm(2) without any penalty in recompression fidelity.

Study of Ne- and Ni-like x-ray lasers using the prepulse technique

Recent studies of lasing in Ne- and Ni-like ions on the Asterix IV iodine laser [H. Baumhacker et al. Appl. Phys. B 61, 325 (1995)] using the prepulse technique are reviewed. Experimental evidence shows that beam refraction is the main factor for the lack of lasing in low-Z elements, as well as the J=0−1 vs J=2−1 anomaly in Ne-like ion lasers when there is no prepulse. It is shown that the role of the prepulse in enhancing the J=0−1 lasing line in Ne-like ion is to produce a larger and more homogeneous plasma. The measurement of lasing on the J=0−1, 3p−3s transition in Ne-like Mn, V, Sc, Ca, K, Cl, S, and Si using the prepulse technique is reviewed. Wavelengths of these lasers range from 22 to 87 nm with gain lengths between 7 and 12. The drive energy for S was scaled down to 20 J. The experiment demonstrating the 12 nm lasing on the J=0−1, 4d−4p transition in Ni-like Sn is also reviewed.

A thermoluminescence detector-based few-channel spectrometer for simultaneous detection of electrons and photons from relativistic laser-produced plasmas

A new method was applied to simultaneously measure the absolute energy- and angle-dependent emission of electrons (500 keV to 20 MeV) and photons (50 keV to 2 MeV) emitted by laser-produced plasmas. For this purpose, a newly developed few-channel spectrometer based on thermoluminescence detectors was used. The device measures the curve of depth dose values in a stack of different materials. The deconvolution of electron and photon spectra from the depth dose curve was performed using a computing algorithm based on a Bayesian inference using Gibbs sampling. Several characteristics of the measured particle spectra were investigated: The electron distribution function of the electrons was found to be describable by Maxwellian distributions in energy. The hot electron temperatures obtained (between 1.1 and 1.7 MeV depending on target material and thickness) are in accordance with well-known scaling laws. The angular emission of the electrons was found to be highly anisotropic with a maximum in the direction of the laser reflection (region of the target normal and parallel to the target surface) for a thick target and an additional maximum in the forward direction of the laser for a thin target. Conversion efficiencies depending on the material and thickness of the target for the conversion of laser light energy to relativistic electrons and of electrons to photons were determined to be up to 10% and 1%, respectively.

MeV γ-ray yield from solid targets irradiated with fs-laser pulses

We have investigated the MeV bremsstrahlung which is emitted when fast electrons generated by the interaction of 200 mJ, 130 fs Ti:sapphire laser pulses with a preformed plasma penetrate into a solid target. Employing different targets the dependence of the γ-ray spectrum on the atomic number was studied. We detected single γ photons with an energy up to 2.5 MeV and found a maximum conversion efficiency of the laser energy into MeV bremsstrahlung of 4×10−6. Data analysis using a Monte Carlo code revealed a fast-electron temperature of 0.9 MeV.

High-intensity regime of x-ray generation from relativistic laser plasmas

We report experiments exhibiting specific features in generating hard x rays with femtosecond laser plasmas as relativistic intensities are approached. Copper foils are irradiated with 1-J/130-fs Ti:sapphire laser pulses, and the x rays are detected with spatial resolution. The results demonstrate a dramatic reduction in the x-ray-emitting spot size at intensities around 1019 W/cm2, and a corresponding increase in the x-ray flux density. These findings are explained in terms of forward acceleration of electrons due to relativistic effects.