Evidence of electron trapping in a helicon discharge

Summary form only given, as follows. The mechanisms responsible for efficient plasma production in helicon wave sources are investigated experimentally. Measurements taken in the large volume helicon source WOMBAT (Waves On Magnetised Beams And Turbulence) show that helicon waves are excited in an argon discharge by a double half-turn antenna over a broad range of wavelengths between 0.1 and 0.6 m. It is found that a strong density peak occurs well downstream from the antenna at a magnetic field of 50 gauss, and corresponding wave phase velocity (v/sub /spl phi// of 3/spl times/10/sup 6/ m/s. It is argued that the observed density peak is the result of local ionisation. A simple numerical model of the electron trapping process in the presence of an electrostatic wave shows that the ionisation rate is most sensitive to perturbations in the electron distribution function caused by waves with v(equal to the threshold velocity for ionisation of argon, 2.35/spl times/10/sup 6/ m/s, which is close to the optimum v/sub /spl phi// of 3/spl times/10/sup 6/ m/s found in experiments. It is also demonstrated that the electron trapping effect causes pulses in the ionisation rate that propagate at the wave phase. A similar effect on the excitation rate of gaseous species in the plasma is found experimentally using an in situ optical probe to observe Ar II emission as a function of position within the source, and time resolved to a fraction of an rf cycle. In this experiment, pulses in the Ar II emission are observed to propagate away from the antenna synchronised with the wave phase. New measurements of phase resonant optical emission during the ´Blue Mode´ (a high density helicon wave mode that exhibits intense ArII emission along the source axis) show that a correlation exists between locations of strong optical emission and wave amplitude.