A nuclear-reaction-based method for probing the nonthermal ion energy spectrum in high energy density laboratory plasmas

The nuclear reactions in a plasma system with particle distributions deviated from the Maxwellian are proved to have some unique characteristics, in particular, in their reaction product energy spectra. Based on this, a new nuclear-reaction-based method for probing the nonthermal ion energy spectrum in high-energy-density (HED) laboratory plasmas is proposed, where the energy spectrum of the nonthermal ion high-energy tails can be accurately evaluated through analysis from the spread and peak of the product energy spectrum. The principle of this diagnostic method is theoretically derived and verified by two-dimensional particle-in-cell simulations that self-consistently includes the nuclear reaction calculations. As an example, our simulations demonstrate clearly how this method is applied for probing the nonthermal high-energy protons produced in the HED magnetic reconnection experiment, where a small ratio of boron element is dopped in the laser-ablated hydrocarbon target and the proton-boron (pB) reaction is chosen as the referenced nuclear reaction. The simulations also show that the pB reaction rate is increased by four orders of magnitude and the peak of the energy spectrum of the generated alpha particles shift significantly towards the high-energy range due to the nonthermal protons accelerated from the reconnections.