Design and characterisation of a helicon apparatus for investigations of parametric instabilities in magnetised plasma

Parametric instabilities [1] arise across many plasma physics environments including inertial [2] and magnetic [3] confinement fusion and in ionospheric plasmas [4] . A helicon [5] plasma source can provide a stable, controllable, low temperature and relatively tenuous plasma for periods of hours, making it suited to diagnosis by techniques such as probes and interferometry. Our apparatus operates in inductive and helicon modes driven by an RF antenna in the range of 3<f<30 MHz at powers <4 kW, which creates a noble gas plasma in a 1 m diameter, 3 m long stainless-steel vacuum chamber surrounded by electromagnets providing an axial B0<90 mT. Typically, a source like this would be expected to provide a plasma with 10 15 <n e <10 18 m -3 and T e <10 eV. For these plasma parameters, parametric instabilities such as Raman and Brillouin scattering can be triggered by readily available, powerful microwave sources such as magnetron oscillators and travelling-wave tube amplifiers and broadband pulses developed in dispersive pulse compressors. These sources will launch microwaves into the plasma via a pair of Satoh horns providing a Gaussian beam for microwave scattering experiments. The characterisation of the plasma using the RF-compensated Langmuir probes and complementary microwave interferometry diagnostic is presented.