Faraday Cup Measurements of the Energy Spectrum of Laser-Accelerated Protons

Several experiments at the Nevada Terawatt Facility (NTF) study the interaction of laser-created plasmas with large external magnetic fields. Examples include a ldquosolar windrdquo experiment that studies the development of a shock when an ablation plasma flow interacts with a strong magnetic field, and an isochoric heating experiment, in which the effect of a confining external magnetic field on target heating will be investigated. The plasmas can be created with one of our two multiterawatt laser systems, namely, Tomcat (up to 10 TW) and Leopard (up to 100 TW). Analyzing the experiments requires a thorough understanding of the initial conditions of the laser plasma. The measurements performed with Faraday cups yield the hot-electron temperature, which is characteristic for electrons that directly absorb the laser energy. Energy spectrum measurements have been carried out with Tomcat pulses (1057 nm) with up to 4-J pulse energy and 4 ps/1 ps duration. For an optimized focal spot size, a hot-electron temperature of more than 2.6 keV was measured, which is consistent with a laser intensity between 10¹⁵ and 10¹⁶ W/cm².