Laser-driven ion acceleration via target normal sheath acceleration in the relativistic transparency regime

Abstract

We present an experimental study investigating laser-driven proton acceleration via target normal sheath acceleration (TNSA) over a target thickness range spanning the typical TNSA-dominant regime (∼1 μm) down to below the onset of relativistic laser-transparency (<40 nm). This is done with a single target material in the form of freely adjustable films of liquid crystals along with high contrast (via plasma mirror) laser interaction (∼2.65 J, 30 fs, I 1 10 21 >´W cm −2 ). Thickness dependent maximum proton energies scale well with TNSA models down to the thinnest targets, while those under ∼40 nm indicate the influence of relativistic transparency on TNSA, observed via differences in light transmission, maximum proton energy, and proton beam spatial profile. Oblique laser incidence (45°) allowed the fielding of numerous diagnostics to determine the interaction quality and details: ion energy and spatial distribution was measured along the laser axis and both front and rear target normal directions; these along with reflected and transmitted light measurements on-shot verify TNSA as dominant during high contrast interaction, even for ultra-thin targets. Additionally, 3D particle-incell simulations qualitatively support the experimental observations of target-normal-directed proton acceleration from ultra-thin films.

On-shot characterization of single plasma mirror temporal contrast improvement

“…The effect of debris produced at one of the substrates and deposited on the second and its impact on the contrast enhancement performance would certainly have to be investigated over the course of several shots, which is a straightforward measurement with SRSI-ETE. Figure 4 displays the proton acceleration results with ultra-thin liquid crystal film targets under oblique incidence (details of the experiment and the full target thickness scan can be found in [6]) in the form of a target focus scan. The target position was monitored before every shot with a scattered light imaging system that was referenced to § The laser intensity at the position of the pick-off mirror for contrast measurement is slightly lower (40% as deduced from the recorded near field images) than in the center of the beam.…”

Characterization of Accumulated B-Integral of Regenerative Amplifier Based CPA Systems

“…High-power laser systems are powerful instruments enabling a variety of applications in the fields of high energy density physics and relativistic laser plasma physics, with the prominent example of compact laser plasma accelerators [1][2][3]. In particular, the acceleration of protons to multi 10 MeV energies requires full control over interaction conditions and thus excellent temporal pulse contrast [4][5][6][7][8][9][10]. This key property of any high-power laser chain characterizes the temporal pulse profile relative to the peak intensity on ns to fs time scales [11].…”

Generation of focusing ion beams by magnetized electron sheath acceleration

“…Following its initial demonstration 20 – 22 , non-magnetized sheath-based ion acceleration has been extensively studied 23 , including in configurations compatible with experimental magnetic field generation platforms. Improvements in the ion source characteristics, including efforts to generate a focusing ion beam 24 – 26 and increase the energy 27 – 29 , are additionally desirable for applications including isochoric heating 30 and ion fast ignition 31 . It is therefore advantageous to elucidate the mechanism via which applied magnetic fields can beneficially alter sheath-based ion acceleration, particularly in the context of realistic magnetic field strengths.…”