A. Ganguli

Theory of high-frequency guided waves in a plasma-loaded waveguide

The theory of high-frequency guided waves in a plasma-loaded waveguide is presented. The formulation is based on kinetic theory. The theory includes the effects of finite electron/ion temperatures and radial gradients in the plasma density and particle temperatures. The inclusion of gradients into the formalism results in three, coupled, second-order, ordinary differential equations in the plasma fields. These equations show a singularity on the plasma axis (r=0); regularization of the field equations at the singular point leads to constraints on the plasma fields that yield the necessary boundary conditions at r=0. Using the vacuum solutions, and the boundary conditions at r=0 and the plasma–vacuum interface, the field equations are solved as a two-point boundary-value problem. The numerical methods used are also discussed. Solutions are presented in the form of dispersion curves (plots of vphase vs ω/Ωe) and radial profiles of the field polarizations for the m=±1 modes near electron cyclotron resonance ...

A new structure for RF-compensated Langmuir probes with external filters tunable in the absence of plasma

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 probes 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 (= ?s) small in comparison with the plasma dimension, L) its accuracy for lower densities (?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.

Feed optimization for the slotted line antenna for high-density plasma production

Investigations on the optimization of feed structures for exciting the slotted line antenna for high-density plasma production are presented. Each feed structure used (except the direct feed) excites a preferred component of the wave electric/magnetic field. It is seen that the efficacy of plasma production using the different feeds depends directly on the relative importance of the field components (which the feeds excite) for the flow wave mode of the antenna. The optimal feed is shown to be a dipole antenna, oriented so as to excite the radial component of the electric field within the slotted line structure. The plasma characterization results as a function of the input microwave power and the magnetic field in the antenna region are also presented and discussed. The ability of the antenna to maintain high-density plasmas well away from electron-cyclotron resonance is demonstrated. >

Investigation of absorption mechanisms in helicon discharges in conducting waveguides

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 Te 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.

Warm electrons are responsible for helicon plasma production

The present work discusses an overview of RF power absorption in helicon plasmas. Analysis suggests that the warm electron population is accountable for the argon plasma ionization in helicon plasma. Along with the experiment, there is an observation of a potential structure, identified as a double layer. However, unlike the other helicon double layer, the present experiment shows that the double layer does not appear at the magnetic field diffusion region; it is present in the linear magnetic field region.

Understanding helicon plasmas

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 populatio...

Investigation of microwave plasmas produced in a mirror machine using ordinary-mode polarization

This paper presents results of plasma production in a mirror machine with the microwaves being coupled from a side port, in the ordinary-mode polarization. In this method of coupling, contamination due to the launching antenna is avoided; also, there is very little window degradation since plasma bombardment on the side port is negligible. The coupling of the microwaves to the plasma in this method relies on the existence of certain surface waves that exist at low values of the magnetic field (below that for electron cyclotron resonance) and have a dominant Ez component. The existence of such surface waves is predicted by a recent theory (Ganguli A et al 1998 Phys. Plasmas 5 1178). The plasma characterization at microwave power levels of about 200 W, indicates moderately overdense plasmas (density 1011 cm-3), with fairly uniform radial and axial profiles. From the data, the mechanism of plasma production and confinement in the mirror has been analysed. To gain insight into the nature of the waves in the plasma-loaded waveguide, the wavelengths of the waves within the plasma and the radial profiles of the wave electric field were measured. The results are compared with the theory of Ganguli et al for wave identification.

Helicon wave modes, their damping and absorption in lossy plasma loaded conducting waveguide

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 beha...

Evidence of current free double layer in high density helicon discharge

This paper investigates the formation of double layer (DL) in helicon plasmas. In the experiment, argon plasma production is using the excitation of m = −1 helicon mode with magnetic mirror field with high mirror ratio of ∼1:1.7. We have specifically used the radio frequency compensated Langmuir probe (LP) to measure the relevant plasma parameters simultaneously so as to investigate the details about the plasma production. The DL, which consists of both warm and bulk populations towards higher potential region and only dense bulk plasmas towards the lower potential region downstream the antenna, is present in the transition region. LP measurements also show an abrupt fall of density along with a potential drop of about 20 V and e ΔVpk Te≈12 within a few cm. The potential drop is equal to the difference of the electron temperatures between the two plasma regions forming the DL, which is present in the plateau region of mirror, unlike in several prior studies on the DL formation in the region of strong grad...

Experimental investigation of current free double layers in helicon plasmas

The paper presents investigations of current free double layer (CFDL) that forms in helicon plasmas. In contrast to the other work reporting on the same subject, in the present investigations the double layer (DL) forms in a mirror-like magnetic field topology. The RF compensated Langmuir probe measurements show multiple DLs, which are in connection with, the abrupt fall of densities along with potential drop of about 24 V and 18 V. The DLs strengths e ΔVpk Te are about 9.5 and 6, and the corresponding widths are about 6 and 5 D lengths. The potential drop is nearly equal to the thermal anisotropies between the two plasma regions forming the DL, which is present in the plateau region of mirror, unlike the earlier studies on the DL formation in the region of strong gradients in the magnetic field. Also, it presents a qualitative discussion on the mechanism of DL formation.