X-ray emission from a CO2-laser-produced plasma

Abstract

A 0.3-GW, 60-ns TEA CO2 laser has been used to create plasma from a solid carbon target. Studies of the x-ray emission from the plasma at 4×1011 W/cm2 indicate the existence of a high-energy electron component with an equivalent temperature of ∼ 2 keV.

The interaction of CO2laser radiation with various solid targets

“…In the case of the Nd-glass laserplasma interaction, for which the aforementioned models were primarily developed, this occurs at a relatively high temperature ( T , -1 keV, Chase et ul 1973), whereas for the CO2-laser-produced plasma the corresponding temperature is much less (Te -100 eV) because of the lower density. In the limiting case of a fully conduction-dominated plasma, simple steady state modelling (Dyer et al 1974a) predicts a scaling of Te N (52/11 which is fairly close to that determined for the heavy targets (Cu, Ag, Pb). It should be pointed out, however, that the model takes no account of ionization energy which, with the heavy targets, may be a serious omission.…”

“…From measurements using charge-collector probes and a range of solid targets, the temperature-flux density scaling has been found to be lower than that predicted by a simple self-regulating (Te N (5419) or deflagration wave (TO -(5213) heating model. This relatively weak scaling, which has been observed in other experiments using CO2 lasers (Dick and Pepin 1975), is probably explicable in terms of thermal conduction broadening of the plasma which occurs when the scale length for conduction approaches or exceeds the characteristic laser spot size (Dyer et al 1974a). In the case of the Nd-glass laserplasma interaction, for which the aforementioned models were primarily developed, this occurs at a relatively high temperature ( T , -1 keV, Chase et ul 1973), whereas for the CO2-laser-produced plasma the corresponding temperature is much less (Te -100 eV) because of the lower density.…”

Non-linear interaction of intense CO2radiation with dense plasma

“…Note that k|| is the component of the wave vector parallel to the electric field which is normal to the density gradient. On the other hand the Brillouin backscatter, involving as it does only phonons and photons, is mostly influenced by the non-uniform expansion velocity effect on the sound speed which is characterized by the flow velocity gradient scale length L u = [(l/u o )(du/dx)]: 1 For our experiment we estimate L to be 100 /zm. x fcnA T Although from Fig.l one might conclude that all processes are important, when inhomogeneity is included one sees from Table I that, for a flux of 10 13 W/cm 2 or less, the only parametric processes which are at all likely to play a role in the experiment are the plasmon-phonon instability, the aperiodic instability, the stimulated Brillouin backscatter and perhaps the two-plasmon decay.…”

“…To the curves of Fig.l one may readily apply the inhomogeneity corrections of Table I when the relevant scale lengths are known. Since the parameteric processes which involve as a decay product an electron plasma wave depend sensitively on density, the relevant length in the inhomogeneous threshold formula is the density gradient scale length, L n =[(l/n)(dn/dx)]" 1 . The plasma phonon decay instability and the aperiodic instability thresholds are the least strongly affected by the density gradient since in the interaction geometry for the most unstable case, the excited electrostatic waves propagate normal to the density gradient and thus tend to stay in the interaction region.…”

“…In Fig. 19 we show a typical spectrum scattered perpendicularly to both the wave vector and the electric field (S scattering) of the incident laser beam (direction (1) in Fig. 18).…”