High-density plasma production using a slotted helical antenna at high microwave power

Abstract

This paper presents results of high-density plasma production experiments carried out using an optimally designed and excited slotted helical antenna. The slotted helical antenna was excited by linearly polarized, right-hand polarized (RHP) and left-hand polarized (LHP) CW microwaves (f = 2.45 GHz) with power up to 2.5 kW. Two different discharges - the resonant and the non-resonant discharges - were studied. In the resonant discharges the magnetic field at the edge of the helical antenna was equal to the ECR field, so that a resonant coupling of the electrons and the microwaves could occur. In non-resonant discharges the value of the magnetic field was greater than the resonant field throughout the mirror. It was seen that the linearly polarized and RHP waves can produce about 100% ionization (at pressures Torr; power kW) for both resonant and non-resonant discharges. In most cases an enhancement over the neutral particle density could be observed due to improved confinement. The resonant discharge using LHP waves also yielded similar results as obtained using the RHP and the linearly polarized waves. This result is somewhat unexpected since the LHP waves are not expected to undergo resonance at electron cyclotron resonance. The non-resonant discharge using LHP waves was found to be very weak (underdense). Finally, an attempt has been made to understand the unexpected nature of the resonant discharges using LHP waves, in terms of the modes of plasma loaded helices and waveguides.