A new Langmuir probe structure using externally placed filters that can be tuned in the absence of plasma is proposed. The probe design and tuning procedure take into account especially the change in the probe's environment when plasma is turned on, thereby ensuring that the filters do not become detuned in the presence of plasma. Measurement of the RF voltage amplitudes in RF plasma using a calibrated capacitive probe gave, respectively, ≈15.7 V, ≈4.1 V, ≈2.1 V and ≈0.5 V at the fundamental frequency (≈13.56 MHz), the second, third and the fourth harmonics; based on these values a three-stage filter was built at the fundamental and the second and third harmonics. A complete analysis of the probe including its stray capacitance, RF equivalent circuit, filter and plasma impedance has been carried out, from which the maximum RF sheath voltage drop could be estimated as ≈0.27 V, ≈0.34 V and ≈1.30 V at the fundamental, the second harmonic and the third harmonic, respectively; the drop for the latter is somewhat large because of unexpectedly high loss in the filter components at the higher frequencies. I -V characteristics presented show that the floating potential of the probe decreases by ≈60 V, as the probe is detuned progressively from its tuned condition; also, the electron temperature increases from ≈1.7 to 3.5 eV with progressive detuning. It is worth noting here that although the method of calibrating the capacitive probe in this work is accurate for moderately high plasma densities (RF skin depth in plasma (= δ s ) small in comparison with the plasma dimension, L) its accuracy for lower densities (δ s ∼ L) is not too certain. Therefore, although the probe itself can be used at low plasma densities, verifying its efficacy for such cases could be difficult.

This paper investigates the mechanisms by which the helicon and associated Trivelpiece-Gould waves are absorbed in helicon discharges produced in conducting chamber; the experiments were based on a recent theory of damping and absorption of helicon modes in conducting waveguides (Ganguli et al 2007 Phys. Plasmas 14 113503). In particular, it was also planned to investigate the manner in which the absorbed energy is utilized for the production of warm electrons that are needed for ionization because helicon discharges are high density, low T e discharges and the tail of the bulk electron population may not have sufficient high-energy electrons. To this end, two separate regimes were considered. The first was a low pressure (≈0.2-0.3 mTorr), low magnetic field (≈16-20 G) regime where both wave absorption and warm electron production are shown to proceed through Landau damping. The second was a moderate pressure (≈10 mTorr), moderate magnetic field (≈60-65 G) regime, where both power absorption (which is collisional) and warm electron production proceed via high-energy electrons produced by acceleration of bulk electrons (from neighboring regions) across large potential gradients.

This paper presents a comprehensive overview of work on the helicon plasmas and also discusses various aspects of RF power deposition in such plasmas. Some of the work presented here is a review of earlier work on theoretical [A. Ganguli et al., Phys. Plasmas 14, 113503 (2007)] and experimental [A. Ganguli et al., Plasma Sources Sci. Technol. 20(1), 015021 (2011)] investigations on helicon plasmas in a conducting cylindrical waveguide for m = −1 mode. This work also presents an approach to investigate the mechanisms by which the helicon and associated Trivelpiece-Gould (TG) waves are responsible for RF power deposition in Helicon discharges. Experiment design adopts the recent theory of damping and absorption of Helicon modes in conducting waveguides [A. Ganguli et al., Phys. Plasmas 14, 113503 (2007)]. The effort has also been made to detect the warm electrons, which are necessary for ionization, because Helicon discharges are of high density, low Te discharges and the tail of the bulk electron population may not have sufficient high-energy electrons. Experimental set up also comprises of the mirror magnetic field. Measurements using RF compensated Langmuir probes [A. Ganguli et al., Plasma Sources Sci. Technol. 17, 015003 (2008)], B-dot probe and computations based on the theory shows that the warm electrons at low pressure (0.2–0.3 mTorr) Helicon discharges, are because of the Landau damping of TG waves. In collisional environment, at a pressure ≈10 mTorr, these high-energy electrons are due to the acceleration of bulk electrons from the neighboring regions across steep potential gradients possibly by the formation of double layers.

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 ≈ 1-2 × 10 −4 Torr; power ≤ 2.5 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.

Exact results are presented for the first time for the helicon (H) wave and associated Trivelpiece-Gould (TG) wave modes of a cold, uniform, lossy plasma column inside a conducting waveguide. Both Landau and collisional damping are considered. Contrary to expectation, it is found that even in cases of severe damping, the axial propagation constant is modified only marginally, and damping shows up mainly in the radial propagation constant of only one of the two waves (TG or H). A consequence of such damping is that the damped wave does not form a standing wave in the radial direction; it remains a traveling wave, decaying as it propagates inwards from the plasma surface. These traveling waves can be absorbed efficiently near the plasma surface if their radial wavelengths are short, as usually happens with the TG waves. Unlike previous work, however, where only damping of the TG waves has been reported, the present work also reports damping of H waves. An attempt has been made to understand the damping behavior of the different modes in terms of certain “reduced” dispersion relations obtained from a modified Appleton-Hartree equation derived for the present work.