Characterization of a helicon plasma source in low diverging magnetic fields

A detailed experimental characterization has been performed of a helicon mode in low nonuniform magnetic fields (B₀ < 5 mT) for pressures between 0.04 Pa < p0 < 0.4 Pa and rf powers between 50W < P0 < 400W, using a number of electrostatic probes as well as circuit measurements of the antenna and matching network system. The helicon mode is produced over a narrow range of magnetic field values, where a distinct density peak is formed that becomes broader (and higher) as the power or pressure is increased. The density peak is found to shift to larger magnetic fields for increasing powers or pressures, giving an almost linear relationship between the maximum density and the magnetic field at this maximum density, in agreement with helicon dispersion theory. The density within the peak (>10¹⁷ m^-3) is more than an order of magnitude larger than that before or after, and is associated with a corresponding peak in the measured antenna resistance, thus showing a larger percentage of the input power is deposited within the plasma.