X-ray spectroscopic diagnostics of a copper plasma produced by a laser line focused onto a thin foil

Abstract

A 500-J, 2–6-ns, Nd laser was focused to a 100-μm-wide line on a thin (375–1500 Å) copper film of 4–23 mm length, supported by a 1500-Å thick Formvar substrate. Space-resolved 8–16-Å wavelength x-ray spectroscopy and pinhole photography were utilized. The main spectral features include intense neon-like Cu xx lines, and somewhat less intense Cu xix and xxi lines. Two types of axial structure are observed, one characterized by a finely (≲100 μm) corrugated structure, and the other by a mm scale irregular nonuniformity. The former could well be due to breakups caused by a filamentation instability. The latter appears to be associated with a mismatch between the laser pulse rise time and a characteristic (thickness-dependent) foil heating time, which makes it very sensitive to both laser-beam and copper-coating nonuniformities. The electron temperature in the Cu xx plasma, as estimated from intensity ratios of the 2p-nd (n≥4) transition lines, increases with copper thickness.