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

Summary form only given. Several experiments at 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-terrawatt laser systems, Tomcat (up to 10 TW), and Leopard (up to 100 TW). The laser absorption on target can be characterized by analyzing the energy spectrum of protons emitted from the target surface, which are either from a contamination layer on the target or from the target itself. 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 is indirectly heated by the hot electrons). The energy spectrum measurements have been carried out with Tomcat pulses with Lip to 4 J pulse energy and 1 ps duration. For an optimized focal spot size, a hot electron temperature of up to 4.3 keV and a cold electron temperature of 0.8 keV were measured, corresponding to an absorbed laser intensity of 10¹⁶ W/cm². In the near future, when the high-energy laser Leopard will be fully operational, intensities of up to 10¹⁹W/cm² will be used in our experiment.