Local Thermodynamic Equilibrium Conditions in Superhigh-Pressure Helium Plasmas Produced by Laser Action

Abstract

Breakdown experiments in superhigh-pressure gases 1 irradiated by a focused Q-switched laser show a dependence of threshold electric field versus initial gas pressure in the rather large interval of 20 to 2000 atm. These curves present electric-field minima, which approximately agree with the values predicted by the wellknown theory of electron-impact ionization. In other kinds of experiments performed on laserproduced plasmas, 2 it has been shown that local thermodynamic equilibrium (LTE) conditions may be approximately attained in time intervals as short as the laser-pulse duration (-65-nsec half-width), working at energy density levels of 1000 J/cm 2 (power density ^1.5xl0 10 W/cm 2 ). Measurements made at that laser energy density show that the plasma electron temperature, the gas temperature, and the blackbody radiation temperature converge to the same limit.This Letter presents a theoretical analysis, attempting to define a unique local temperature T for electrons, ions, neutrals, and radiation, and to deduce a relation between threshold breakdown power, temperature, and pressure that may account for the experimental curves. 1 Different mechanisms have been proposed to explain the gas heating: the radiation-supported shock-wave process 3 ' 4 ; the traveling ionization-15 H. R. Schlossberg and A. Javan, Phys. Rev. 150, 267 (1966). 16 M. S. Feld and A. Javan, Phys. Rev. 177, 540 (1969).

Étude des conditions du seuil de claquage de l'argon a haute pression sous l'effet du rayonnement d'un laser

“…De 151,5 à 2 040 atm les Tt tendent à se rapprocher de Tm2 et atteignent presque Tm2 à très haute pression. Le tableau IV résume l'ensemble des calculs ainsi que la figure 4. Ces résultats peuvent être comparés à ceux obtenus pour l'hélium [9] dans la même gamme de pression. A cette différence près que le minimum de claquage de l'hélium est beaucoup plus aplati [1] que dans le cas de l'argon pour un même accroissement de champ électrique de part et d'autre de ce minimum.…”

Nonlocal thermodynamic equilibrium in laser-sustained plasmas

“…No determination of kinetic temperatures was performed either. In general it is thought that LTE does not exist in high power pulsed plasmas, except at super high pressure [Rachman and Bassani, 1969]. Tonon, et al [1971] concluded that electron and ion kinetic temperatures are the same in a plasma produced by a giant laser pulse on a solid target.…”