Helicon Sources: Why they work

A helicon source is a cylindrical ceramic tube containing a gas in the milliTorr region immersed in an axial magnet field surrounded at some region by an antenna fed with rf around 10 MHz. As the power is increased it will show a variety of modes characterised by changes in the plasma density. Low power produces a spatially uniform capacitive discharge with the electric field heating the electrons: at higher power the discharge can show a jump (with hysteresis) into an inductive mode when the skin depth enters the plasma that5 produces an annular plasma: powers around a kilowatt produce centrally peaked plasmas of densities $\sim $10$^{12}$ cm$^{-3}$ where optical measurements show bursts of electrons traveling with the helicon velocity, consistent with acceleration by the E$_{z}$ fields of the m=1 mode. At higher densities, when the plasma enters the ``blue mode'', the coulomb mean free path becomes sufficiently small that the plasma becomes resistive and the helicon damps linearly. Other electron heating proposals are discussed and compared to experiment.