Laboratory study of anti-Stokes Langmuir waves in a magnetized plasma

Abstract

Summary form only given, as follows. Anti-Stokes Langmuir waves were observed in our ionospheric plasma heating experiments at Arecibo, Puerto Rico. They are referred to as frequency upshifted Langmuir waves excited by injected high power HF waves. The frequency shifts are found to be inversely proportional to the HF pump wave frequencies, and of the magnitudes of the lower hybrid wave frequencies in the ionosphere. Kuo and Lee (1992) suggest that these anti-Stokes Langmuir waves are caused by the nonlinear scattering of HF wave-produced Langmuir waves off lightning induced lower hybrid waves. Laboratory experiments with a large toroidal plasma machine known as the Versatile Toroidal Facility (VTF) have been conducted to investigate this nonlinear scattering process. In VTF, a background plasma is created by a Taylor discharge device which has a peak plasma frequency greater than the electron gyro-frequency by a factor of 3-5. This factor has been chosen in order to simulate the ionospheric plasma environment. Overdense heating of the VTF plasma by injected microwaves can generate Langmuir waves and ion acoustic waves via the decay instability. These Langmuir waves may then be scattered by lower hybrid turbulence existing in the background VTF plasmas to produce the anti-Stokes modes. Preliminary results of our laboratory experiments are presented and compared with results of field experiments and theory.