Description
Parametric instabilities of electromagnetic (EM) waves in plasma are non-linear processes which involve coupling between an incident EM wave, an electrostatic plasma wave and a scattered EM wave. These interactions are known to occur in magnetic and inertial confinement fusion, laser-plasma interactions, and in radio wave interactions with the ionosphere. An understanding of these phenomena may inform the development of new energy injection methods into plasma, for use in spherical tokamaks for example; or used to avoid plasma conditions which give rise to these instabilities, where they lead to undesirable effects.
One such interaction is Raman Scattering (RS), in which energy is exchanged between incident and scattered EM waves and an electron plasma (Langmuir) wave. The dynamics of RS are investigated using the linear plasma apparatus at the University of Strathclyde. This apparatus operates in the inductive mode driven by an RF antenna at $14~\mathrm{MHz}$, producing He or Ar plasma with density $10^{15} ~\mathrm{m^{-3}} < n_e < 10^{16}~\mathrm{m^{-3}}$ with bulk temperature $<1~\mathrm{eV}$ at pressure $6.4~\mathrm{Pa}$. Plasma conditions are diagnosed using an RF-compensated Langmuir probe. For these plasma parameters, RS can be driven by powerful microwave sources. This experiment makes use of a travelling wave tube amplifier and a magnetron oscillator to launch counter propagating microwaves via Satoh horns into the plasma, with the microwave sources tuned such that the beat frequency is similar to the plasma frequency. Theoretical analysis suggests that the interaction between the counter propagating beams will generate a Langmuir wave propagating in the direction of the higher frequency EM wave. This wave can then interact with the incident waves to produce electromagnetic sidebands - evidence for which has been seen in some preliminary experiments. We will present the results of further experiments and comparison with theoretical predictions.