Cryogenic Liquid Jets for High Repetition Rate Discovery Science

This protocol presents a detailed procedure for the operation of continuous, micron-sized cryogenic cylindrical and planar liquid jets. When operated as ...

Helmholtz-Zentrum Dresden-Rossendorf

Technische Universität Dresden

We report on recent experimental results deploying a continuous cryogenic hydrogen jet as a debris-free, renewable laser-driven source of pure proton beams generated at the 150 TW ultrashort pulse laser Draco. Efficient proton acceleration reaching cut-off energies of up to 20 MeV with particle numbers exceeding 10 particles per MeV per steradian is demonstrated, showing for the first time that the acceleration performance is comparable to solid foil targets with thicknesses in the micrometer range. Two different target geometries are presented and their proton beam deliverance characterized: cylindrical (∅ 5 μm) and planar (20 μm × 2 μm). In both cases typical Target Normal Sheath Acceleration emission patterns with exponential proton energy spectra are detected. Significantly higher proton numbers in laser-forward direction are observed when deploying the planar jet as compared to the cylindrical jet case. This is confirmed by two-dimensional Particle-in-Cell (2D3V PIC) simulations, which demonstrate that the planar jet proves favorable as its geometry leads to more optimized acceleration conditions.

Efficient laser-driven proton acceleration from cylindrical and planar cryogenic hydrogen jets.

Extreme field gradients intrinsic to relativistic laser-interactions with thin solid targets enable compact MeV proton accelerators with unique bunch characteristics. Yet, direct control of the proton beam profile is usually not possible. Here we present a readily applicable all-optical approach to imprint detailed spatial information from the driving laser pulse onto the proton bunch. In a series of experiments, counter-intuitively, the spatial profile of the energetic proton bunch was found to exhibit identical structures as the fraction of the laser pulse passing around a target of limited size. Such information transfer between the laser pulse and the naturally delayed proton bunch is attributed to the formation of quasi-static electric fields in the beam path by ionization of residual gas. Essentially acting as a programmable memory, these fields provide access to a higher level of proton beam manipulation.

All-optical structuring of laser-driven proton beam profiles.

We report on a proton acceleration experiment in which high-intensity laser pulses with a wavelength of 0.4 μm and with varying temporal intensity contrast have been used to irradiate water droplets of 20 μm diameter. Such droplets are a reliable and easy-to-implement type of target for proton acceleration experiments with the potential to be used at very high repetition rates. We have investigated the influence of the laser's angle of incidence by moving the droplet along the laser polarization axis. This position, which is coupled with the angle of incidence, has a crucial impact on the maximum proton energy. Central irradiation leads to an inefficient coupling of the laser energy into hot electrons, resulting in a low maximum proton energy. The introduction of a controlled pre-pulse produces an enhancement of hot electron generation in this geometry and therefore higher proton energies. However, two-dimensional particle-in-cell simulations support our experimental results confirming, that even slightly higher proton energies are achieved under grazing laser incidence when no additional pre-plasma is present. Illuminating a droplet under grazing incidence generates a stream of hot electrons that flows along the droplet's surface due to self-generated electric and magnetic fields and ultimately generates a strong electric field responsible for proton acceleration. The interaction conditions were monitored with the help of an ultra-short optical probe laser, with which the plasma expansion could be observed.

Martin Rehwald

Helmholtz-Zentrum Dresden-Rossendorf

Cryogenic Liquid Jets for High Repetition Rate Discovery Science