S. K. Karkari

The temporal evolution in plasma potential during laser photo-detachment used to diagnose electronegative plasma

A floating emissive probe is applied in conjunction with pulsed laser photo-detachment of O− ions to enable measurement of the dynamic evolution in a plasma potential resulting from the presence of photoelectrons in a 13.56 MHz inductive radio-frequency oxygen discharge. The emissive probe emits thermionic electrons, allowing it to reach a saturation potential which is characterized as the local space potential of the plasma. After the photo-detachment pulse, the local space plasma potential in the illuminated region shoots up to a higher positive value and then relaxes to equilibrium in microsecond time scales. Using the relaxation time of the space potential, the negative ion temperature of O− is estimated over a 10–50 mTorr range and is found to be in the 0.19–0.03 eV range. The negative ion temperature measured by this method is found to be lower than that calculated from the time evolution in electron density resulting from photo-detachment which is independently measured using a resonance hairpin probe.

Properties of a differentially pumped constricted hollow anode plasma source

Uniform high density plasma sources are important for homogeneous deposition of thin films on material surfaces. For plasma heating by energetic neutral beams in magnetically confined fusion devices, uniform plasma density in front of the extraction grids of negative ion beam source is highly desirable. In this paper we investigate the properties of a direct current plasma source based on electrostatic confinement of secondary electrons between adjacent parallel plate cathodes while the constricted anode is a differentially pumped narrow cylindrical tube whose outer surface is insulated from the plasma. The discharge is operated using a dc power supply which provides maximum rated current of 1.5 A at 3.0 kV peak voltage. Uniform plasma density is obtained outside the parallel plates which can be extended over arbitrary area by simply increasing the size and number of the cathode plates. The plasma near the constricted anode is characterized by an intense luminous glow whose brightness depends inversely on the surface area of the anode. The bulk plasma has spatially distinct electron density and electron temperature regions. The electron temperature between the cathode plates is almost 2–3 times higher than outside the plates. The region around the anode shows a steep rise in electron density and electron temperature. The discharge current is found to be oscillating in the range 250–400 kHz which is attributed due to the instability of the anode glow. The frequency of the oscillation depends on the applied power, pressure and the separation between the cathode plates. A qualitative discussion is presented which explains the various characteristic properties of the discharge.

Probing negative ion density and temperature using a resonance hairpin probe

The sheath around the cylindrical pins of a resonance hairpin probe is modulated by applying a train of negative pulse voltages with respect to the grounded chamber. This phenomenon leads to the creation of a conjugate dielectric due to the electron-free sheath around the probe surface and the ambient plasma outside the sheath region. Synchronous measurement of electron density, ne, with respect to pulse waveform finds an overshoot in ne during the withdrawal of the negative pulses in the electronegative oxygen plasma. The physical reason behind this observation is presented together with a qualitative model for interpreting the density and temperature of negative ions based on this method.

Electro-negative plasma diagnostic using pulse bias hairpin probe

Summary form only given. Electro-negative plasmas are widely used in plasma processing, ion source for neutral beam heating in fusion devices, plasma etching and ion Hall thrusters. Presence of negative ions principally modifies the sheath thereby affecting the direct transport of charged particle toward the wall or substrate. In some scenarios they lead to discharge instabilities. They are also responsible for various plasma chemistries. While conventional method of measuring negative ion is based on laser photodetachment in conjunction with Langmuir probe, recently we have demonstrated application of a hairpin probe with laser photodetachment for measuring negative ion density and temperature in an ICP Oxygen discharge1. This paper presents the novel technique in which pulsed bias hairpin is used against pulsed laser photodetachment for the measurement of negative ions. The principle remains the same except that a strong transient negative potential of the order of few 100 Volts allows to discriminate electrons and negative ions responding to the probe at distinct time scales. This results in observing peak electron density immediately post stimulation of the applied negative pulse. Some preliminary results are presented in the oxygen ICP discharge and results are compared with those obtained by hairpin probe assisted laser photo-detachment.

Transient properties of anodic glow in constricted anode plasma source

Summary form only given. Anodic glow is an interesting physical phenomenon which is observed in front of the small electrode biased positive than the plasma potential. These anodic glows, sometimes referred as fireballs have generated some renewed attention due to observation of plasma instabilities (non-chaotic) in the electrode current and launching of electrostatic plasma waves. The present study deals with the investigations on the origin and control of these plasma instabilities associated with the formation of anodic glow.

Studies of electronegative Ar/O 2 discharge in a constricted hollow anode plasma source using dual probe technique

Uniform high density electronegative plasmas find important application for plasma heating by neutral beams in fusion devices and in material processing. In this work we present a constricted hollow anode plasma source (CHAPS) for the investigation of electronegative Ar/O2 discharge. The source consists of a series of equidistant stainless steel parallel plates acting as cathode with a small stainless steel tube as the anode. The plasma comprise of highly uniform density outside the cathode plates and an intense glow near the anode1. The negative oxygen ion fraction is determined by measuring the electron density by hairpin probe and ion density using a planer Langmuir probe. The negative ion density in the bulk plasma is typically 1016 m−3 and increases monotonically as a function of pressure and power. The electronegativity of the discharge is typically close to 0.5 and remains constant with the applied power. Whereas the negative ion density in the anode region varies from 1016 m−3 to 1017 m−3 with electronegativity of 3.0 near the edge of the anodic glow and decreases to 1.0 with the distance from the anode glow.

Introducing hairpin probe for electron density measurement in a KAMABOKO-III negative ion source

Neutral beam injection based on a negative ion source is one of the most promising candidates for heating and current drive in future fusion reactors. The physical and chemical processes involved in the filter field region of an ion sources are especially important to investigate, as this is the region where the negative ions are mainly generated and extracted. The externally applied magnetic/filter field around 0.012T cools the high-energy electrons in order to avoid collisional destruction of the negative ions. The electron density (ne) measurements using conventional diagnostic techniques such as Langmuir Probe (LP) and Optical Emission Spectroscopy becomes complicated in presence of negative ions as well as magnetic field and gives a qualitative result. For accurate det ermination of ne in such complex plasma, the floating resonance hairpin probe1 (HP) is applied in KAMABOKO-III negative ion source, at the MANTIS test bed in CEA Cadarache, France. The technique is based on measuring the plasma permittivity using a U-shaped microwave resonating structure whose relative shift of characteristics frequency in plasma from that in vacuum directly gives the ne without rely on any other plasma parameter. However, in the presence of magnetic field the plasma density becomes anisotropic and thus the HP gives an effective ne measurement within its spatial resolution. The measured ne is compared with the positive ion density measured by planar LP. The ne is studied as a function operating source parameters such as arc power, pressure, applied magnetic field and the plasma grid bias exclusively in the filter field region. The effect of probe orientation to the B-field on the resonant properties is also studied under given plasma conditions. The performance of the HP shows that the technique is more reliable, sensitive and accurate diagnostic tool for such ion source.

Study of resonant properties of hairpin probe for high-density operation

A well-defined sharp drop in the amplitude of the reflected signal characterizes the resonance signal of the hairpin probe (HP) when the frequency is varied over a given range typically of the order of electron plasma frequency. A quantity that defines the ratio of width of the resonance signal, Δf (fupper - flower) to the central frequency, fcentral is the quality/Q factor of the HP. For lower Q it is difficult to resolve the fcentral against the background noise1. The factors governing the Q are the losses of the energy stored in the resonator due to the resistive heating of the probe that depends on the probes temperature and the specific resistivity of the wire material as shown in figure. The coupling between the adjacent loop antenna and the hairpin also determines the shape of the resonance peak. In the plasma, the losses are further enhanced due to resistive heating of the oscillating electrons influenced by the electric field between the probe tips and the presence of external magnetic field. In this work, we present an analytical model based on the transmission line theory of the hairpin resonator circuit that explains the physical factors responsible for the observation of broad resonance peaks.

Observation of Electron Density Oscillations in Confined Plasma with Two Radio-Frequency Capacitive Sheath

Summary form only given. The spatial electron density oscillation in a symmetric, two radio-frequencies, 27.12 MHz and 1.937 MHz, confined- capacitive-coupled discharge has been measured using a floating hairpin resonance probe. By measuring ne in a space and phase-resolved manner, we observe oscillations of bulk electron density at both (1.937 MHz and 27.12 MHz) drive frequencies. With the probe placed within the region of the maximum low-frequency sheath extent, the expulsion of the electrons by the large low-frequency voltage is observed. When the probe is placed in the opposing sheath, the phase of the electron expulsion is shifted by half-cycle as expected. Near the mid-plane of the parallel plate electrodes, the plasma density oscillates twice per low-frequency cycle. The observed electron density oscillations are attributed to the expansion and the collapse of the radio-frequency sheaths, resulting in the spatial electron density variation between the discharge electrodes. The 27.12 MHz oscillation in electron density is observed to be higher in magnitude during the phase when 1.937 MHz sheath voltage has maximum in amplitude.