Bulletin of the American Physical Society
2005 58th Gaseous Electronics Conference
Sunday–Thursday, October 16–20, 2005; San Jose, California
Session PW2: Plasma Sheaths |
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Chair: B. Graham, Queen's University-Belfast, UK Room: Doubletree Hotel Cedar |
Wednesday, October 19, 2005 8:00AM - 8:15AM |
PW2.00001: The plasma-sheath transition in the Tonks-Langmuir model Karl-Ulrich Riemann The plasma-sheath matching problem in the hydrodynamic plane Tonks-Langmuir model has attracted considerable interest during the last few years. It is complicated not only by the singular structure of the asymptotic ($\lambda_D/L\to 0$) plasma and sheath solutions but also by a coupling with the eigenvalue problem originating from the plasma balance. Due to these difficulties the existence of a matched asymptotic expression uniformly valid from the plasma core to the wall is widely questioned. The issue is clarified both analytically and numerically by the explicit construction of a matched asymptotic expression and comparison with exact solutions. Accounting for a shift in the ionization eigenvalue, the approximations obtained by consistent matching show an excellent agreement with numerical potential curves. The singularities of the asymptotic components are reflected by small discontinuities in the derivatives that vanish in the limit $\lambda_D/L\to 0$. Finally an outlook is given to the kinetic analysis of the same problem. [Preview Abstract] |
Wednesday, October 19, 2005 8:15AM - 8:30AM |
PW2.00002: Electron sheaths in low-pressure weakly-collisional plasma N. Hershkowitz, S. Baalrud, B. Longmier The plasma potential in anode sheaths can be negative going measured from the anode (electron sheaths) or positive going (ion sheaths). In weakly-collisional low-pressure plasma, electron sheaths are normally only present near small probes when they are biased more positive than the plasma potential or at electron emitting surfaces. Electron sheaths created along the axis of a large positively biased plate (diameter = 10 cm) located in a multi-dipole argon plasma are described here. Experimental data show non-ambipolar electron loss in which the electron sheath collects almost all electrons produced by ionization if sufficient loss area is provided for ions at the chamber walls. Measurements of the plasma potential with emissive probes and Langmuir probes show potential dips form at the sheath edges to limit electron loss and increase ion energy. The use of electron sheaths for extracting electron beams from both dc and rf plasma will also be discussed. [Preview Abstract] |
Wednesday, October 19, 2005 8:30AM - 9:00AM |
PW2.00003: Measurement of Electric Fields in Radio Frequency Argon Discharges Invited Speaker: We present a summary of our measurements of electric fields present in the sheath region formed between an electrode and a radio frequency generated argon plasma. Experimental calibrations for several Stark shifted argon Rydberg states are presented. Desired field resolution and field range are shown to be dependant on the choice of Rydberg level probed. For example, the 11d[1/2] level can be used for fields exceeding 5000 V/cm while the behavior of Rydberg states around 38n push detection sensitivities down to the V/cm level. Both spatial and temporal maps of the electric fields are presented for sheaths formed around several electrode configurations. Electrode configurations studied range from complex topologies present on a powered electrode to thin metallic wires (Langmuir probes) present in the bulk of the discharge. Discussion will be offered on the effect of the electrode structure on the resulting field structure as well as the resulting behavior of the discharge around the electrode. When possible, comparison between the measured sheath structure and expected sheath structure will be made. This work was supported by the Division of Material Sciences, BES, Office of Science, U. S. Department of Energy and Sandia National Laboratories, a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, October 19, 2005 9:00AM - 9:15AM |
PW2.00004: Measurements of the Sheath Capacitance using a Tunnel Diode Oscillator in Inductively Coupled Plasmas SeJin Oh, MinHyong Lee, PyungWoo Lee, ChinWook Chung A tunnel diode oscillation method is one of techniques to measure the sheath capacitance in plasmas. [ B .M Oliver et. al., J . Physics. E. 5, 718 ] Using this method, we investigated the sheath capacitance of a ring probe in an Inductively Coupled Plasmas (ICPs). The effects of the RF fluctuation and the noise on the sheath capacitance were considered. We measured the change in the tunnel diode oscillation frequency to acquire the sheath capacitance of the probe. The probe was biased with probe potentials ranging from 0V to -45V. The result agreed well with that calculated from a Child- Langmuir theory when the probe was highly negative biased with respect to the probe floating potential. However, it was found that the measured sheath capacitance increases rapidly than that from the Child-Langmuir theory as the bias potential increases. This is understood by the Bohm sheath theory. [Preview Abstract] |
Wednesday, October 19, 2005 9:15AM - 9:30AM |
PW2.00005: Investigation of anodes for high intensity discharge lamps Oliver Langenscheidt, Lars Dabringhausen, Stefan Lichtenberg, Marco Redwitz, Juergen Mentel, Peter Awakowicz To optimize the electrodes of high intensity discharge (HID) lamps a detailed physical understanding of the interaction between the arc column and the cathode and anode has to be achieved. In the Bochumer model lamp the anodic behaviour of electrodes for high intensity discharge lamps was characterized by pyrometric, electric and spectroscopic measurements. The lamp was operated with currents between 0.5A to 10A in pure noble gases at pressures up to 1MPa and pure or doped tungsten electrodes of different sizes. The temperature and power losses of the anode were determined by pyrometric measurements, the anode fall by Langmiur-probe measurements. Spatially resolved spectroscopic measurements yield the electron temperature and density in front of the anode. It is found that the plasma in front of the anode consist of a contraction zone with enhanced power input and an anodic boundary layer which converts thermal power into electrical power. The temperature of the anode shows a stronger current dependency than the cathodic tip temperature. In AC operation it is possible to pre-heat the electrode during the anodic phase and therefore to influence the subsequent type of cathodic arc attachment. [Preview Abstract] |
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