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

Lieselotte Obst,Uwe Helbig,João Branco,Uwe Hübner,René Gebhardt,Jongjin B Kim,Christian Rödel,Lev Kazak ,M Gauthier ,R Mishra ,J Tiggesbäumker ,Mathias Siebold ,Michael Bußmann ,S H Glenzer ,Frederico Fiúza ,C B Curry ,Stephan Kraft,T Kluge ,U Schramm ,Axel Huebl,Hans-Peter Schlenvoigt,Steffen Wolter,Tim Ziegler,Markus Loeser,Martin Rehwald,T E Cowan ,J Metzkes ,Stefan Bock,Florian‐Emanuel Brack ,Arie Irman,S Göde ,Karl Zeil

doi:10.1038/s41598-017-10589-3

Abstract

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 109 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.

Highlights

Cryogenic targets, as have been deployed in inertial confinement fusion experiments and related warm dense matter studies[14], promise to fulfill most of the stated requirements
We report on the first experimental demonstration of the acceleration of pure proton beams from a continuous hydrogen jet at optimized Target Normal Sheath Acceleration (TNSA) conditions leading to a higher proton energy and beam quality
Ion detectors consisted of two Thomson parabolas (TP) that were aligned along the drive laser axis and under 45°, and radio-chromic film (RCF) stacks to diagnose the proton beam profile

Summary

Introduction

As have been deployed in inertial confinement fusion experiments and related warm dense matter studies[14], promise to fulfill most of the stated requirements. A broad data set was collected allowing for an extensive statistical evaluation of the proton beam performance and correlation of the latter to the on-shot positioning of the jet with respect to the drive laser focus, implemented by means of probe beams in two perpendicular axes Both target geometries deliver typical TNSA-like proton beams with an angular emission distribution that resembles those obtained from wire targets[23,24,25] with exponential energy spectra terminating in cut-off energies that reach 20 MeV. The proton acceleration performance, regarding maximum proton energy and angular emission pattern, is expected to be influenced by both the local target geometry, i.e. the surface curvature, and the confinement of the sheath electrons due to the strongly limited lateral size of the hydrogen jet. We show that the planar jet’s dimensions lead to higher proton numbers in laser forward direction as compared to the cylindrical case, making it the favorable hydrogen jet geometry for potential applications of the generated proton beams

Methods

Results

Discussion

Conclusion