Abstract. Operation of helicon discharges at magnetic fields B 0 below 100 G is of interest for plasma etching and deposition reactors if high ion flux can be maintained with reduced field requirements. The theory of coupled helicon and Trivelpiece-Gould modes is summarized for uniform B 0 . Initial results from two experiments are reported. The first has a single 5 cm diameter tube with B 0 = 0-100 G injecting plasma into a field-free region. The second contains a two-dimensional array of seven such tubes covering a large area. Densities and density profiles are measured for various fields, RF powers and gas pressures. The highest density generally occurs at zero field. Because of the non-uniformity in B 0 , direct comparison with theory cannot yet be made.
Known for their ability to produce high densities at low power, helicon discharges have found many uses. However, it has been discovered that the plasma density saturates, and even falls for light ion gases, as the magnetic field is increased. Detailed measurements of fluctuations in plasma density reveal the onset of a strong, low-frequency electrostatic instability. This onset correlates well with density saturation and is predicted from a linear theory.
Plasma densities produced by half-and full-wavelength (HW and FW) helical antennae in helicon discharges are compared. It is found that HW antennae are more efficient than FW ones in producing plasma downstream from the antenna. The measured wave amplitudes and the apparent importance of downstream ionization do not agree with computations.
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