Bulletin of the American Physical Society
63rd Annual Gaseous Electronics Conference and 7th International Conference on Reactive Plasmas
Volume 55, Number 7
Monday–Friday, October 4–8, 2010; Paris, France
Session NR1: Plasma Diagnostics I: Microwave |
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Chair: Nick Braithwaite, Open University, UK Room: 151 |
Thursday, October 7, 2010 10:30AM - 11:00AM |
NR1.00001: The Multipole Resonance Probe -- Concept, Theory, Experiments Invited Speaker: Plasma diagnostics is a highly developed science. Of the many available techniques, however, only few are suitable for an industrial setting. To be useful for the supervision or control of technical plasmas, a diagnostic must be i) robust and stable, ii) insensitive against perturbation by the process, iii) itself not perturbing to the process, iv) clearly and easily interpretable without the need of calibration, v) compliant with the requirements of process integration, and, last not least, vi) economical in terms of investment, footprint, and maintenance. Plasma resonance spectroscopy -- exploiting the natural ability of a plasma to resonate on or near the electron plasma frequency -- is believed to provide a good basis for such an ``industry compatible'' plasma diagnostics. The idea is time-honored but has found renewed interest in recent years: An electric probe is used to feed a high frequency signal into the plasma and the response is evaluated with the help of a mathematical model. Once $\omega_{\rm pe}$ is known, one can calculate the electron density as $n_{\rm pe}=\omega_{\rm pe}^2\epsilon_0 m_{\rm e}/e^2=1.24 f_{\rm GHz}^2 \times 10^{10} {\rm cm}^{-3}$. The contribution will describe the general idea of active plasma resonance spectroscopy and introduce a mathematical formalism for its analysis. It will then focus on a novel realization, the so-called multipole resonance probe MRP, where the excited resonances can be mathematically classified and the connection between the probe response and the desired electron density can be evaluated analytically. The current state of the MRP project will be described, including the outcome results of first experiments. [Preview Abstract] |
Thursday, October 7, 2010 11:00AM - 11:15AM |
NR1.00002: RF Probe Theory R.F. Fernsler, D.N. Walker, D.D. Blackwell, D.R. Boris Langmuir probes are the preferred method for diagnosing plasmas, but interpreting the dc data is difficult. In this work we show that the ac probe impedance Z is easier to interpret and provides more information because it contains real and imaginary parts and depends on both the applied frequency f and dc bias V. At the bulk plasma frequency, Re(Z) peaks while Im(Z) equals zero. These two conditions uniquely determine the bulk plasma electron density. In addition, the electron density within the sheath can be computed from the variation in Re(Z) with f. At much lower frequencies, Re(Z) becomes independent of f and approaches dV/dI$_{e}$, where I$_{e}$ is the dc electron current. The variation in Re(Z) with V can be used to determine the plasma potential and the electron energy distribution and temperature. Compared with Langmuir probes, the rf method requires one less derivative and provides more information and multiple checks. Experimental results are presented for spherical probes, cylindrical probes, and a pair of closely spaced parallel plates in broad and narrow plasmas with densities ranging from 10$^{7}$ to 10$^{11}$ cm$^{-3}$. The parallel-plate scheme is especially useful in magnetized plasmas. [Preview Abstract] |
Thursday, October 7, 2010 11:15AM - 11:30AM |
NR1.00003: Spatial and temporal evolution of electron density and plasma potential by resonance hairpin probe and emissive probe during pulsed laser photo-detachment of negative ions N. Sirse, M.A. Mujawar, J. Conway, M.M. Turner, S.K. Karkari Laser~photo-detachment is the most commonly used technique for measuring negative ion parameters. In this study we measured the electron density perturbation using a resonance hairpin probe and plasma potential using floating emissive probe along the path and outside the laser beam. Experiment is performed in a 13.56 MHz inductive RF discharge for various ranges of power and pressures. It is found that the plasma potential rises instantaneously for confining the photo-detached electrons and decays at a rate dictated by the diffusion of negative ions from the adjacent layer. This is found to be consistent with the spatial and temporal evolution of electron density. [Preview Abstract] |
Thursday, October 7, 2010 11:30AM - 11:45AM |
NR1.00004: Effect of strong external magnetic field on the properties of resonance hairpin probe G.S. Gogna, S.K. Karkari, M.M. Turner The hairpin probe is a well known technique for measuring the plasma electron density. It is characterized by a sharp resonance signal at a particular frequency which depends on the plasma permittivity surrounding the resonator pins. The signal quality is found to be adversely affected due to the e-n collisions and by radiation losses in the plasma. In presence of strong magnetic field above 0.1 T, these losses are enhanced due to strong interaction with the $E \times B$ field along the magnetic flux tubes. We systematically investigated the effects of the probe orientation with respect to the external B-field on the signal quality and electron density. The results are compared with the positive ion density n$_{+}$ obtained by a slit-shaped planar Langmuir probe positioned at the end of the flux tube. At B = 0.07 T, the n$_{e}$ is found to be higher as compare to n$_{+}$ due to gradient in the B-field along the flux tube which shows strong dependencies with the probe orientation. The relationship between n$_{e}$/n$_{+}$ ratio is established with the probe orientation (0-360$^{\circ})$ which accounts for the non-uniform spatial electric field distribution around the resonator pins. [Preview Abstract] |
Thursday, October 7, 2010 11:45AM - 12:15PM |
NR1.00005: Wave-Cutoff Method with High Time Resolution Invited Speaker: Wave-cutoff method is a diagnostic tool to measure electron density and electron temperature. Most of the diagnostic tools need a few seconds to measure the plasma parameters. In this presentation, a fast measurement method will be newly introduced. A wave-cutoff probe system consists of two antennas and a network analyzer. The network analyzer provides the transmission spectrum and the reflection spectrum by frequency sweeping. The plasma parameters such as electron density and electron temperature are obtained through these spectra. The frequency sweeping time, the time resolution of the wave-cutoff method, is about 1 second. A short pulse with a broad band spectrum of a few GHz is used with an oscilloscope to acquire the spectra data in a short time. The data acquisition time can be reduced with this method. In this work, the plasma parameter measurement methods, Langmuir probe, pulsed wave-cutoff method and frequency sweeping wave-cutoff method, are compared. The measurement results are well matched. The real time resolution is less than 1 micro second. The pulsed wave-cutoff technique is found to be very useful in pulsed plasma and tokamak edge plasma measurement. \\[4pt] In collaboration with Byung-Keun Na and Kwang-Ho You, Department of Physics, Korea Advanced Institute of Science and Technology; and Chi-Kyu Choi, Department of Physics, Cheju National University. [Preview Abstract] |
Thursday, October 7, 2010 12:15PM - 12:30PM |
NR1.00006: Physics on Cut-off Probe and its Application ShinJae You, DaeWoong Kim, Junghyung Kim, ByungKun Na, Yonghyun Shin Although the cut-off probe, a precise measurement method for the electron density, is widely used in the industry, the physics on the wave spectrum of the cut-off is not understood yet, only cut-off point frequency containing the information of electron density has been analyzed well. This paper analyzes the microwave frequency spectrum of the cut-off probe to see the physics behind using both microwave field simulation (CST Microwave Studio) and simplified circuit simulation. The result shows that the circuit model well reproduces the cut-off wave spectrum especially in the low frequency regime where the wavelength of the driving frequency is larger than the characteristic length and reveals the physics of transmission characteristics with frequency as resonances between vacuum, plasma and sheath. Furthermore, by controlling the time domain in solver of the microwave simulator, the cut-off like transmission peaks above the cut-off frequency which has been believed as cavity effect is verified as chamber geometry effect. The result of this paper can be used as the basis for the improvement of cut-off probe. [Preview Abstract] |
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