Measurements of high energy density electrons via observation of Cherenkov radiation

Abstract

Direct measurement of extremely high energy density electrons created in ultraintense laser-plasma interactions is crucial issue for fast ignition. Recently Cherenkov radiation has been studied to obtain the energy distribution of electrons because the emission angle depends on the electron energy. However in the previous studies [F. Brandl et al., Europhys. Lett. 61, 632 (2003); M. Manclossi et al., Phys. Rev. Lett. 96, 125002 (2006)], the experimental configurations using a planar target raised issues of spatial overlapping among the light from the different energy electrons as well as from the other emissions, such as transition radiation. A novel prism shaped target is developed in which Cherenkov lights emitted from different energy electrons are spatially separated, realizing an absolute measurement of the energy spectrum by counting the light intensities in each observed position. The observed image clearly shows the horseshoe pattern as expected in fully three-dimensional ray-trace calculations, and the image is successfully converted into the electron spectrum inside the target. In addition, it is found from the blur of the outer edge of the Cherenkov pattern that the electrons have a small beam divergence. The calibrated energy spectrum well agrees with particle simulations.