The low-field density peak in helicon discharges

Although the density n in helicon discharges increases monotonically with magnetic field B for B larger than a few hundred gauss, as expected from theory, a pronounced density peak is often observed at B∼50 G or below. A peak in antenna loading is indeed found in computations using a fluid code as long as reflections from an endplate are taken into account. Various tests show that this peak is caused by constructive interference from the reflected wave. This effect can be used in the design of compact helicon plasma injectors. In addition, it can be the cause of density enhancements previously observed using cusped magnetic fields or aperture limiters