Hot-electron production and resonance absorption of laser light in the shock-front targets

Laser targets, created by fast optical breakdown of shock fronts produced in an electrothermal shock tube filled with D2 or H2, have been used for studies of resonance absorption of CO2 laser light. In these targets, the critical-surface density-gradient vector always points along the shock-tube axis, leading to reproducible hot-electron emission by plasma wave breaking. The angular distribution and energy spectrum of this emission have been studied. For 200 psec risetime laser pulses, the measured scaling of hot-electron temperature with laser pulse energy agrees with predictions of models of profile steepening by the ponderomotive force of the laser light. For 20 psec risetime laser pulses, the results are consistent with a model in which the detected electron emission is dominated by a ’’chirped’’ monoenergetic burst lasting only 1 to 2 psec. In this model, the electron energy spectrum is due to the integrated temporal variation of the enery of the burst, and the hot-electron temperature is practically independent of laser pulse energy.