X-Ray Temperatures from Laser-Induced Breakdown Plasmas in Air

Abstract

The soft x‐ray flux from laser sparks in air at a pressure near 330 Torr has been measured with a three‐channel absorption spectrometer. From these measurements plasma temperatures were inferred. For laser peak‐power levels from 300 to 600 MW and pulse durations from 10 to 25 nsec, the temperatures varied from 40 to 160 eV. Streak camera records were taken of the axial plasma growth. For the shortest laser pulses, the plasma temperatures and luminous front velocities were in near agreement with the predictions of the radiatively supported detonation wave model. As the laser pulse width increased so did the plasma velocity, but not the temperature. These higher‐velocity streak records also showed several slower‐moving luminous fronts behind the faster front. These slower fronts are believed to be detonation waves. Space‐ and time‐resolved x‐ray flux measurements were also made, from which the x rays were found to be associated with the highly luminous plasma and only occurred during laser heating of the plasma.