Enhanced neutral depletion in a static helium helicon discharge

Abstract

Laser-induced fluorescence (LIF) measurements of plasma opacity are used as a novel diagnostic to determine the absolute density of a metastable state of neutral helium atoms in a helicon plasma. The absorption scale length at a wavelength of 587.725 nm (vacuum) is determined from measurements of fluorescence intensity as a function of distance along the laser path. With a collisional–radiative model of the state populations, the absolute ground state neutral helium density is estimated from the metastable state density measurement. This paper expands upon previous work through measurements of neutral density, temperature and flow at different radial positions. The measured neutral density decreases by two orders of magnitude from the edge of the plasma to the axis of the plasma source. When the helicon source is operated in a static mode (i.e. no active gas pumping) the on-axis neutral density decreases by 69% from the pumping case and the on-axis plasma density increases by 42%; yielding an ionization fraction of approximately 90%.