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 “solar wind” 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 multi-terawatt laser systems, Tomcat (up to 10TW) and Leopard (up to 100TW). Analyzing the experiments requires a thorough understanding of the initial conditions of the laser plasma. The laser absorption on target can be characterized by analyzing the energy spectrum of proton emitted from the target surface. The measurements, performed with Faraday cups, yield both the hot electron temperature (characteristic for electrons which directly absorb the laser energy) and the cold electron temperature (characteristic for the bulk of the target electrons, which are indirectly heated by the hot electrons). Energy spectrum measurements have been carried out with Tomcat pulses with up to 4J pulse energy and 4ps/1ps duration. For an optimized focal spot size, a hot electron temperature of more than 2.6keV was measured, corresponding to an absorbed laser intensity of 10¹⁶ W/cm².