Background pressure effects on MeV protons accelerated via relativistically intense laser-plasma interactions

Abstract

We present how chamber background pressure affects energetic proton acceleration from an ultra-intense laser incident on a thin liquid target. A high-repetition-rate (100 Hz), 3.5 mJ laser with peak intensity of $$8 \times 10^{18}\,\text {Wcm}^{-2}$$ 8 × 10 18 Wcm - 2 impinged on a 450 nm sheet of flowing liquid ethylene glycol. For these parameters, we experimentally demonstrate a threshold in laser-to-proton conversion efficiency at background pressures $$< 8\,\text {Torr}$$ < 8 Torr , wherein the overall energy in ions $$>1\,\text {MeV}$$ > 1 MeV increases by an order of magnitude. Proton acceleration becomes increasingly efficient at lower background pressures and laser-to-proton conversion efficiency approaches a constant as the vacuum pressure decreases. We present two-dimensional particle-in-cell simulations and a charge neutralization model to support our experimental findings. Our experiment demonstrates that high vacuum is not required for energetic ion acceleration, which relaxes target debris requirements and facilitates applications of high-repetition rate laser-based proton accelerators.

Low divergent MeV-class proton beam with micrometer source size driven by a few-cycle laser pulse

“…So far, high-energy, multi Joule level laser systems, operating under single-shot mode have been used to drive these experiments 1 , 2 . Recently, a different regime of generating high-quality particle beams has been explored with modest energy (10’s mJ), few-cycle laser systems 30 – 34 . By focusing a few-cycle, mJ laser pulse to the diffraction limit, relativistic intensities 35 or the so-called lambda-cube regime 36 can be achieved.…”

Detailed characterization of kHz-rate laser-driven fusion at a thin liquid sheet with a neutron detection suite

“…The 107-cm diameter stainless steel chamber is brought to a final vacuum of approximately 1 Torr, which is limited by the vapor pressure of heavy water and the tendency of the liquid target to freeze. This is below the approximately 7-Torr threshold for ion acceleration noted by Snyder et al [ 30 ] . This pressure is measured at the edge of the chamber; it is expected that the pressure in the vicinity of the target may be significantly higher than the stated 1 Torr.…”

“…[28]. Earlier, this system was also used in ion acceleration experiments described by Morrison et al [ 29 ] and Snyder et al [ 30 ] . Section 2.2 provides a brief review of the underlying nuclear physics of the tabletop laser-induced fusion, and Section 2.3 explains the principles of operation of the neutron detectors.

Figure 1 Simplified overhead view of the target chamber and surrounding detectors.

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Versatile tape-drive target for high-repetition-rate laser-driven proton acceleration

“…This is important, as transparency of the targets leads to a substantial change in the interaction and acceleration process. Vacuum compatibility of a tape target drive system and the tape itself can be achieved by careful selection of materials, while room temperature or cryogenic liquid jets often require differential pumping and/or high-speed cryo-pumping to maintain adequate interaction chamber pressure [ 24 ] .…”