Characterization of an inverted geometry helicon plasma source

Summary form only given. Helicon plasmas continue to be useful as a hot, dense source with low magnetic field requirements. Since the implementation of helicon waves in a low pressure gas by Boswell, there have been multiple advances in this field. To date, no single theory has completely explained the coupling mechanism of the waves to the plasma, although Trivelpiece-Gould (TG) modes remain a strong candidate. The concept of an inverted geometry helicon antenna is proposed here, comprised of a standard helicon antenna, covered by a dielectric, and placed within the vacuum chamber. RF energy that is normally radiated outward from the antenna will now also couple to the plasma, for additional heating and better efficiency. An internal antenna is also not constrained to the minimum radius of the vacuum chamber, thus antennas can be made smaller to determine what, if any, critical radius exists for plasma excitation. Smaller antennas also mean more tightly wound magnetic field coils that can deliver the same field for less current. The primary antenna type to be used is a Nagoya type III, although others are used for various experiments. With such an arrangement, it is now possible to make measurements of the plasma at an arbitrarily close distance to the antenna. This setup is in contrast to conventional antennas which surround a dielectric cylinder where the plasma, as well as the diagnostics are housed. To characterize the plasma, an RF-compensated Langmuir probe is employed to make measurements in both radial and axial directions. The radial measurements extend beyond the antenna region for comparison with conventional helicon plasma sources. To explore the method of wave-plasma coupling, mobile B-dot and J-dot probes measure the field shape in the plasma region as well as very close to the antenna itself. In addition to the above diagnostics, optical emission spectroscopy is used to observe visual changes in plasma intensity for mode jumps, as well as to measure- line ratios