[en] Emissive and Langmuir probe techniques have been used to obtain two-dimensional (2D) spatial maps of the plasma potential V_p, electric field E, and ion trajectories in a pulsed bipolar magnetron discharge. The magnetron was pulsed at a frequency of 100 kHz, with a 50% duty cycle and operated at an argon pressure of 0.74 Pa. The pulse wave form was characterized by three distinct phases: the 'overshoot', 'reverse', and 'on' phases. In the 'on' phase of the pulse, when the cathode voltage is driven to -670 V, the 2D spatial distribution of V_p has a similar form to that in dc magnetron, with significant axial and radial electric fields in the bulk plasma, accelerating ions to the sheath edge above the cathode racetrack region. During the 'overshoot' phase (duration 200 ns), V_p is raised to values greater than +330 V, more than 100 V above the cathode potential, with E pointing away from the target. In the 'reverse' phase V_p has a value of +45 V at all measured positions, 2 V more positive than the target potential. In this phase there is no electric field present in the plasma. In the bulk of the plasma, the results from Langmuir probe and the emissive probe are in good agreement, however, in one particular region of the plasma outside the radius of the cathode, the emissive probe measurements are consistently more positive (up to 45 V in the 'on' time). This discrepancy is discussed in terms of the different frequency response of the probes and their perturbation of the plasma. A simple circuit model of the plasma-probe system has been proposed to explain our results. A brief discussion of the effect of the changing plasma potential distribution on the operation of the magnetron is given

[en] Full text: A direct current (DC) plasma source consisting of hollow cathode geometry and a constricted anode is presented. The effect of a hollow cathode geometry on radial density distribution of a magnetized plasma column has been studied in a low-pressure (approximately 1.4 Pa) argon discharge. The plasma column is characterized using Langmuir probe and the radial density distribution for two different “inside” profiles of a hollow cathode is discussed. Probe measurement indicates that a conical-profile hollow cathode produces a plasma column with centrally peaked plasma density whereas a cylindrical-profile hollow cathode forms plasma column with off-centred density peak. Thus overall dynamics of perpendicular and oblique cathode sheaths behind the sustenance of magnetized plasma column has been discussed. (author)

[en] Using an emissive probe, the temporal evolution of plasma potential V_p in front of an electrically isolated substrate in an asymmetric pulsed dc magnetron has been determined. The discharge pulsing frequency was 100 kHz, with a 50% duty cycle. Through a scheme of externally biasing the emissive probe, it was found that the time response of the probe could be improved greatly, and a resolution of 20 ns was achieved. This good response revealed that V_p is highly modulated by the transient cathode potential, following it closely and varying from a value just above the ground potential in the pulse 'on' phase, up to a value of +277 V during the positive overshoot in the 'reverse' pulse phase. During the whole pulse cycle, V_p was found to remain above the most positive surface in the discharge. The results confirm our previous prediction for V_p, based on energy-resolved mass spectrometry [Bradley et al., Plasma Sources Sci. Technol. 11, 165 (2002)], which indicated that ions must be created at high positive plasma potentials. However, measurements here show that the substrate floating potential V_f is also strongly modulated and the difference V_p-V_f, which determines the ion bombarding energy, always remains below 40 V during steady phases of the discharge throughout the pulse cycle

[en] A circular planar probe of diameter 4 mm has been used to determine the ratio of the electron E x B drift speed V_d to the thermal speed V_th in the bulk plasma of a magnetron discharge. It is found that when the probe is orientated into the E x B drift, with its surface parallel to the local B-field, significantly higher electron saturation currents are detected than when the probe is orientated away from the drift (i.e. 180 degrees from this orientation). Using the electron and ion saturation current measurements and a simple model of the anisotropic plasma-probe system, which assumes both Hall and collisional cross-field transport, V_d/V_th has been determined at different positions in the bulk plasma. The maximum in V_d/V_th is found to be about 0.14 (corresponding to V_d=1.10⁵ ms^-1) and the measured distribution of drift current agrees well with data that was found in a previous study, in which V_d was determined from the knowledge of E and B, namely V_d=E x B/B², and with V_th determined from electron temperature measurements, i.e. V_th=(2kT_e/m_e)^1/2. (copyright 2004 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

[en] Full text: The APPEL (Applied Plasma Physics Experiments in Linear Device) is an experimental system designed to carry out basic plasma physics experiments as well as serve as a test-bed for experimenting various plasma facings components interaction with strongly magnetized plasma. This versatile device consists of 16 large electromagnet coils weighing up to 700 kg each, which are made from CTC (continuously transposed conductors), wound in double pancake configuration. Each electromagnet coil is made by sandwiching two double pancakes between 10 mm thick stainless steel plates to obtain 100 turns/magnet. The individual coil has 52 cm internal diameter and outer diameter is around 110 cm. The stainless steel plates provide the necessary strength to the magnet while its surface acts as radiator to dissipate heat. The individual electromagnet can be operated continuously to produce peak axial magnetic field in excess of 0.1 T by passing 750 A for 600 s. All 16 magnets in linear configuration produce peak magnetic field in excess of 0.5 T by passing 750 A for 600 s. In this set-up different magnetic field profile can be generated by optimizing the current using two high current DC power supplies as well as configuration of the coils. The paper presents the electromagnets field simulation performed by finite element analysis (FEA) using Comsol Multiphysics and ANSYS to obtain tailored magnetic field profile in linear, magnetic cusp and mirror configurations achieved in APPEL device. In-house magnetic field measurements carried out for the APPEL magnet and experimental validation of the FEA results. The heat loads and stresses on the coils have been calculated for steady state operation of the APPEL device. (author)

[en] Full text: Capacitive discharges are created in the near field regions of ICRF antennas and thus power coupling by these antennas depend on the sheaths around them. Magnetization of ions in the plasma around these antennas also affects the power coupling into the plasma with major implications in ICRF antenna’s in tokamaks. A capacitive discharge is designed to study power coupling in such plasmas in a linear device. A symmetric capacitively coupled helium discharge is created by three cylindrical electrodes placed at specific axial positions in a linear chamber in presence of axial magnetic fields. Axial magnetic field is strong enough to magnetize helium ions with their cyclotron radius smaller than, the cylindrical electrode radius. In this study, power measurement in conjunction with detail circuit analysis of magnetized capacitive sheaths has been performed to determine the plasma impedance. Plasma impedance can reveal many important aspects of the power coupling into the plasma such as the mode of discharge, power coupling to individual species (ions and electrons) and conditions of electron series resonance all of which are modified extensively in presence of magnetic field. The obtained impedance characteristics along with power measurements are qualitatively discussed to understand the effect of magnetization of ions on the discharge. (author)

[en] This paper describes the methodology for processing Ampere-Volts (I-V) characteristics of the Langmuir probe in magnetized plasma using graphical programming language based on LabVIEW. Computing the plasma parameters from I-V characteristic involves several steps that include signal processing, interpolation, linear and non-linear curve fitting based on physical models, finding the derivatives of the experimental curve and determining the zero-crossing of the probe current as a function of the applied voltage. These operations are practically tedious to perform manually causing systematic errors in output parameters. To overcome this challenge, software is developed to analyze the planar Langmuir probe characteristics in magnetized plasma. The software allows simultaneous display of different plasma parameters that helps to verify the consistency of the analyzed plasma parameters with the standard probe theory. Using this software, plasma parameters are obtained in a linear plasma device and its characteristics are discussed

[en] The article presents an experimental investigation on Langmuir probe measurements in a magnetized plasma column which exhibits two-temperature electron populations. It is a known fact that probe I(U) traces follow the usual exponential law if the measurements are performed with a reference electrode in good contact with plasma; which is usually a grounded discharge electrode. However in the present case, as the grounded probe reference is not a part of the discharge circuit the resulting I(U) analysis is not straightforward. It is found that owing to the high impedance between bulk plasma and probe reference, the probe measurement results in lower values of electron saturation current as compared to the ideal scenario. An appropriate correction is thus required to account actual electron saturation current and thereby to extract subsequent plasma parameters. Therefore, a simple analysis technique has been proposed to interpret probe I(U) traces resulting from such magneto-plasma devices, where reference to the probe is in partial/ poor contact with the bulk plasma.

[en] The plasma electron density n_e in a symmetric confined capacitive-coupled plasma processing tool containing Ar/O₂/C₄F₈ gas mixtures is studied as a function of two, combined radio frequency (2 MHz+27 MHz) powers. For measuring n_e we have used a floating hairpin resonance probe. The results show a linear increase in n_e with 27 MHz power. Also the density is higher with an increase in 2 MHz power, in contrast with published particle-in-cell simulation results in argon where the plasma density decreased with increases in low frequency voltage, for fixed high frequency current [P. C. Boyle et al., J. Phys. D 37, 697 (2004)]. Analyzing the relative phase between radio frequency current and voltage, we observe slightly lower 2 MHz phase shifts at higher 2 MHz voltage, which is attributed to an increase in the real component of the current through the sheath. This is possible due to the increase in secondary electron emissions arising from ion bombardment, which is favored by an increase in 2 MHz voltage. We therefore conclude that the secondary electrons could play an important role in the discharge process

[en] Full text: The cross-field transport of electrons/ions across magnetic field is fundamentally important as it determines the characteristics of plasma wetted area in the scrape of layer region and particle confinement in magnetically confined plasma devices. The electrically biased objects in the edge region inside tokamaks as well as in linear plasma devices are known to influence the dynamics of charge particles. The external electrodes in the magnetized column can introduce long range electric fields in the plasma column. This leads to either excitation or suppression of the instabilities responsible for such transport. In this paper we present experimental results on radial plasma characteristics obtained of a cylindrical plasma column produced in a linear device. The magnetized plasma column at one end is terminated with conducting electrodes which are deliberately biased with respect to the plasma. The nature of the long range perturbation during application of electric bias on the electrodes have been investigated using electric probes and its impact on the radial characteristics have been qualitatively explained. (author)