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P: Plasmaphysik

P 4: Diagnostics II

P 4.7: Vortrag

Dienstag, 3. April 2001, 18:10–18:25, 2032

Thomson Scattering from Optical-Field Ionized Plasmas — •Claus Michael Jost and Ernst Fill — Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching bei München

Using Thomson scattering of laser light we have studied optical-field ionized Hydrogen and Deuterium plasmas with respect to electron density, electron temperature and ion temperature. The plasmas were created by focusing 130 fs, 790 nm pulses of our 10 Hz Ti:sapphire laser system ATLAS at intensities of up to 3×10¹⁶ W/cm² into a low-pressure (10−100 mbar) gas volume. A frequency-doubled, weaker pulse following the pump pulse with a delay of 1.5 ps probed the plasma. The measurements were performed for linear and elliptic polarization of the ionizing laser light. Comparing Hydrogen and Deuterium we find no significant difference in the electron temperatures, however, the ion temperature of Deuterium is significantly lower than that of Hydrogen. The measured ion temperatures are consistent with an enhancement of ionization in a distinct range of internuclear separation as predicted within the framework of electron localization in a double-quantum-well molecular model.

Surprisingly the electron temperatures for H₂ as well as for D₂ are found to be quite insensitive to the polarization of the ionizing laser pulse. From standard theory for optical-field ionization we would expect much higher electron temperatures for elliptic than for linear laser polarization. But for Deuterium the electron temperature for elliptic polarization is found to be even slightly lower than for linear polarization. Also in case of Hydrogen we find that the electron temperature is almost the same for linear and elliptic polarization. With increasing particle density ionization defocusing of the laser beam and heating of the electrons due to inverse bremsstrahlung gain in significance. In addition an increasing blueshift and a broadening of the input laser spectrum with pressure is observed.