Abstract
The effects of a short plasma density scale length on laser-driven proton acceleration from foil targets is investigated by heating and driving expansion of a large area of the target rear surface. The maximum proton energy, proton flux and the divergence of the proton beam are all measured to decrease with increasing extent of the plasma expansion. Even for a small plasma scale length of the order of the laser wavelength (∼1 µm), a significant effect on the generated proton beam is evident; a substantial decrease in the number of protons over a wide spectral range is measured. A combination of radiation-hydrodynamic and particle-in-cell simulations provide insight into the underlying physics. The results provide new understanding of the importance of even a small plasma density gradient, with implications for applications that require efficient laser energy conversion to ions, such as proton-driven fast-ignition of compressed fusion fuel.
Highlights
We have shown via experimental and numerical methods that small scale length plasma gradients (Ls = 1 − 3 μm) on the rear side of a foil target can have a significant impact on the beam of laser-accelerated protons, especially when induced over the full area of the proton source
The maximum proton energy, laser-to-proton energy conversion efficiency and divergence of the proton beam are all observed to decrease with increasing Ls. 2D PIC simulations show similar results to those obtained experimentally, and indicate the change in beam properties with Ls is due to a reduction in the magnitude of the sheath field and a change in the field profile
These results, obtained by heating a large area of the target rear, extend previous investigations for which a reduction in maximum proton energy and proton numbers at high energy only were reported when heating a small region of the target rear surface [21, 22]
Summary
Over the past two decades [1] This has been driven both by exploration of the underpinning physics and by applications of the unique properties of the beams of energetic ions, including for isochoric heating of matter [2], radiographic density probing of materials with micrometre accuracy [3] and to probe highly transient electric and magnetic fields in plasmas with picosecond resolution [4]. Ultrathin foils can expand such that the combination of the decreasing peak electron density and increasing relativistic electron mass leads to the target becoming relativistically transparent [10, 11] to the laser light This can result in additional electron heating over the expanded plasma, enhancing the TNSA field and ion acceleration, in what is termed the break-out afterburner scheme [12, 13]. The results show the importance of a sharp density gradient for the maximum ion energy, but for the overall laser-to-ion energy conversion efficiency, and for applications for which that is important, such as proton fast ignition [7] and radioisotope generation [25]
Sign-in/Register to view highlights & summary 
Accepted Version (
Free)
Published Version
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call