Enhanced laser-driven proton acceleration via improved fast electron heating in a controlled pre-plasma

Leonida A Gizzi,Petra Koester,Daniele Palla,Gabriele Cristoforetti,Elisabetta Boella,Pablo J Bilbao,Dario Giove,Fernando Brandi,Federica Baffigi,Lorenzo Fulgentini,Paolo Tomassini,Luca Labate,Alberto Fazzi

doi:10.1038/s41598-021-93011-3

Abstract

The interaction of ultraintense laser pulses with solids is largely affected by the plasma gradient at the vacuum–solid interface, which modifies the absorption and ultimately, controls the energy distribution function of heated electrons. A micrometer scale-length plasma has been predicted to yield a significant enhancement of the energy and weight of the fast electron population and to play a major role in laser-driven proton acceleration with thin foils. We report on recent experimental results on proton acceleration from laser interaction with foil targets at ultra-relativistic intensities. We show a threefold increase of the proton cut-off energy when a micrometer scale-length pre-plasma is introduced by irradiation with a low energy femtosecond pre-pulse. Our realistic numerical simulations agree with the observed gain of the proton cut-off energy and confirm the role of stochastic heating of fast electrons in the enhancement of the accelerating sheath field.

Highlights

The interaction of ultraintense laser pulses with solids is largely affected by the plasma gradient at the vacuum–solid interface, which modifies the absorption and controls the energy distribution function of heated electrons
Ion acceleration driven by ultraintense lasers using Target Normal Sheath Acceleration (TNSA)[1] is establishing itself as a powerful technique to access relatively high energy ion beams in a compact and affordable layout
Practical exploitation of laser-driven ion acceleration relies heavily on the original TNSA configuration based on thin foil targets and optimized contrast without plasma mirror, possibly operating at the repetition rate required for applications like radiobiology[7,8], where high dose irradiation is needed for meaningful studies

Summary

Introduction

The interaction of ultraintense laser pulses with solids is largely affected by the plasma gradient at the vacuum–solid interface, which modifies the absorption and controls the energy distribution function of heated electrons. Practical exploitation of laser-driven ion acceleration relies heavily on the original TNSA configuration based on thin foil targets and optimized contrast without plasma mirror, possibly operating at the repetition rate required for applications like radiobiology[7,8], where high dose irradiation is needed for meaningful studies In this context, great attention is being dedicated to the control of accelerated ions, including energy cut-off, beam divergence, charge and emittance. In this intensity regime PIC simulations show that in the presence of a pre-plasma consisting of a 10 μm linear density ramp, the proton cut-off energy is expected to be approximately 8.5 MeV, while without the pre-plasma, the cut-off energy is 5 MeV, showing again an enhancement of approximately a factor 1.7 In spite of these theoretical predictions, to date, a clear experimental evidence of stochastic heating of fast electrons in proton acceleration measurements is still lacking. In most previous experiments, the generation of the required plasma gradient at the vacuum–solid interface is achieved with uncontrolled precursor laser irradiation due to Amplified Spontaneous Emission (ASE)[14] or nanosecond irradiation[15] that have unpredictable, detrimental effects on the target integrity

Methods

Results

Discussion

Conclusion