Two different methods have been employed to determine the plasma temperature in a laser-cluster fusion experiment on the Texas Petawatt laser. In the first, the temperature was derived from time-of-flight data of deuterium ions ejected from exploding ${\mathrm{D}}_2$ or ${\mathrm{CD}}_4$ clusters. In the second, the temperature was measured from the ratio of the rates of two different nuclear fusion reactions occurring in the plasma at the same time: $\mathrm{D}(d,{}^3\mathrm{He})n$ and ${}^3\mathrm{He}(d,p){}^4\mathrm{He}$. The temperatures determined by these two methods agree well, which indicates that (i) the ion energy distribution is not significantly distorted when ions travel in the disassembling plasma; (ii) the kinetic energy of deuterium ions, especially the “hottest part” responsible for nuclear fusion, is well described by a near-Maxwellian distribution.

• Received 25 February 2013

DOI:https://doi.org/10.1103/PhysRevLett.111.055002