Laser-driven ion acceleration from relativistically transparent nanotargets

B M Hegelich ,Jörg Schreiber ,Joel Blakeney ,Hongling Wu ,C Wang ,Rahul Shah ,Daniel Jung ,D Habs ,Alexander R Meadows ,L Yin ,D C Gautier ,Lindsay Fuller ,K Allinger ,I Pomerantz ,G Dyer ,E Mccary ,B J Albright ,Rainer Hörlein ,E Gaul ,S Palaniyappan ,T Ditmire ,S Letzring ,Juan Fernández

doi:10.1088/1367-2630/15/8/085015

Abstract

Here we present experimental results on laser-driven ion acceleration from relativistically transparent, overdense plasmas in the break-out afterburner (BOA) regime. Experiments were preformed at the Trident ultra-high contrast laser facility at Los Alamos National Laboratory, and at the Texas Petawatt laser facility, located in the University of Texas at Austin. It is shown that when the target becomes relativistically transparent to the laser, an epoch of dramatic acceleration of ions occurs that lasts until the electron density in the expanding target reduces to the critical density in the non-relativistic limit. For given laser parameters, the optimal target thickness yielding the highest maximum ion energy is one in which this time window for ion acceleration overlaps with the intensity peak of the laser pulse. A simple analytic model of relativistically induced transparency is presented for plasma expansion at the time-evolving sound speed, from which these times may be estimated. The maximum ion energy attainable is controlled by the finite acceleration volume and time over which the BOA acts.

Highlights

We present experimental results on laser-driven ion acceleration from relativistically transparent, overdense plasmas in the break-out afterburner (BOA) regime
Radiation pressure acceleration (RPA) [17,18,19,20] is one proposed mechanism, and while we were able to show a first onset of RPA experimentally [21], the most promising ‘light sail’ regime [22] of RPA requires very high laser intensities (>1022 W cm−2) and short pulses (
While the underlying physics in these regimes differ, they share features distinguishing them from TNSA: in TNSA, ion acceleration is decoupled from the laser by the thick (> μm) target [10]

Summary

Experimental setup and results

The laser parameters were held nominally constant, while the target thickness was systematically varied to span the full range of the BOA regime. To realize these conditions experimentally, two advances were necessary: ultra-thin, nm-scale, free-standing robust targets and sufficiently high laser contrast to prevent premature ionization and expansion of the target before the peak of the pulse. To realize the second condition, extraordinarily high contrast laser pulses are required. At the Trident laser facility [38], we realized these ultra-high contrast conditions by using a new cleaning system based on an optical parametric pre-pulse eliminator [39, 40]. On the TPW experiments we used a double plasma mirror (PM) setup [43] to improve the contrast on-target, a method that we used on our earlier experiments on Trident [44]

Texas Petawatt experiments

Trident experiments

Analytical model

Findings

Conclusions