Laser-produced MeV electrons and hard X-ray spectroscopic diagnostics

A new spectroscopic technique for the measurement of the sizes of hard X-ray sources produced by the irradiation of solid-density targets by intense laser radiation is discussed. The technique is based on the source broadening of K shell spectral lines from targets irradiated by intense picosecond laser pulses. The spectra are recorded by a modified Cauchois type spectrometer, where the detector is placed far behind the Rowland circle where source broadening dominates instrumental resolution. The laser irradiation with focused intensity greater than 1018 W/cm2 produces relativistic electrons that propagate from the focal spot into the surrounding target material with mm range. The energetic electrons produce 1 s electron ionization and K shell radiation with picosecond duration that can be utilized for transient radiography of dense objects including evolving dense plasmas. However, the hard X-ray source has mm lateral size when extended targets are utilized while a much smaller source size (of order 10 μm) is necessary for high-resolution point projection radiography. The lateral source size can be greatly reduced by using targets with limited aspect to the radiography object such as thin foils and wires, but the brightness of these sources is greatly reduced compared to thick planar targets. Studies indicate that the electron range and source size can also be reduced by utilizing an electrically resistive target material such as teflon. In this case the electron propagation from the focal spot is inhibited by a weak return current and incomplete space-charge neutralization. These experimental results are important not only for producing a small hard X-ray source for picosecond radiography but also for reducing the lateral propagation of energetic electrons that can be detrimental to fast-ignition fusion.