Bulletin of the American Physical Society
69th Annual Gaseous Electronics Conference
Volume 61, Number 9
Monday–Friday, October 10–14, 2016; Bochum, Germany
Session TR1: Laser-Based Diagnostics IIFocus
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Chair: Edward Barnat, Sandia National Laboratories Room: 1 |
Thursday, October 13, 2016 4:00PM - 4:30PM |
TR1.00001: Measurements of the momentum flux from a low-temperature plasma to a surface Invited Speaker: Thomas Trottenberg The forces that low-temperature plasmas exert on surfaces in contact with the plasma have never been a significant topic. The reason might be the smallness of such forces and the expected difficulties in their measurement. Therefore, only in cases of special plasmas which were designed for the generation of directed momentum (in particular electric space propulsion), force measurements have been reported\footnote{G. Makrinich and A. Fruchtman, \textbf{Phys. Plasmas} 21, 023505 (2014)}. Recently, our group demonstrated that the forces related to plasma-wall interactions are experimentally accessible with some effort\footnote{Th. Trottenberg, Th. Richter, and H. Kersten, \textbf{Eur. Phys. J. D} 69, 91 (2015)}. This presentation overviews our experimental approaches in the design of force measuring probes and reports on recent measurements with probes that have been integrated into a plane wall. The observations and prospects for an application as novel plasma diagnostic\footnote{U. Czarnetzki and T. V. Tsankov, \textbf{Eur. Phys. J. D} 69, 236 (2015)} are discussed. [Preview Abstract] |
Thursday, October 13, 2016 4:30PM - 4:45PM |
TR1.00002: Thomson Scattering of Plasma Turbulence in PSI-2 Michael Hubeny, Bernd Schweer, Dirk Luggenh\"{o}lscher, Uwe Czarnetzki, Bernhard Unterberg Linear plasma devices are widely used to study fundamental plasma characteristics and to simulate particle and heat loads representing first wall/divertor conditions of fusion reactors. In high power discharges at PSI-2 the plasma edge exhibits turbulence with intermittent transport events. The combination of Thomson Scattering by a photon counting method and a fast framing CMOS camera in conjunction with conditional averaging gives access to the evolution of density and temperature profiles during transient plasma dynamics. Radial density and temperature profiles in Ar, D$_2$ , He and Ne discharges were measured and compared with existing diagnostics. In high power, low gas-feed Argon discharges the dominating m=1 rotation was found to correspond to a 20\% Te fluctuation amplitude around the temporal mean at the profile maxima. In the edge of D$_2$ discharges transients are selected by conditional averaging and a significant temperature increase was found in the edge of TS profiles upon ejection accompanied by a 20\% drop in bulk density. [Preview Abstract] |
Thursday, October 13, 2016 4:45PM - 5:00PM |
TR1.00003: Experimental determination of the wavelength of the instability developing over the gas-vacuum interface of a supersonic annular flow Tal Queller, Eyal Kroupp, David Naimark, Yitzhak Maron In the present experimental work the instability wavelength of the boundary between a supersonic annular gas puff flow, and the ambient vacuum was measured using two-dimensional interferometry. An annular De-Laval nozzle connected to a fast valve was used in order to create the supersonic flow. The interference pattern of the laser beam traversing the boundary was projected onto a large format CCD, resulting in a high spatial resolution (30 um). An ultra-fast mechanical shutter enabled the rapid exposure of the beam, resulting in a high temporal resolution (3 us). These resolutions were necessary in order to enable the observation of the quasi-periodic structure of the boundary. The fact that this structure was not observable when using lower resolutions, enables us to set an upper limit on the wavelength of the instability in the flow edge and a lower limit on the time scale in which this instability wavelength travels. The knowledge of these gas-puff flow parameters can be used for setting more realistically the initial boundary conditions in computer simulations that attempt to study, e.g., the implosion of a plasma cylinder and other processes developing around a gas-vacuum interface of a supersonic flow. [Preview Abstract] |
Thursday, October 13, 2016 5:00PM - 5:15PM |
TR1.00004: Polarization resolved electric field measurements on plasma bullets in N$_{\mathrm{2}}$ using four-wave mixing Marc van der Schans, Patrick Boehm, Sander Nijdam, Wilbert IJzerman, Uwe Czarnetzki Atmospheric pressure plasma jets generated by kHz AC or pulsed DC voltages typically consist of discrete guided ionization waves called plasma bullets. In this work, the electric field of plasma bullets generated in a pulsed DC jet with N$_{\mathrm{2}}$ as feed gas is investigated using the four-wave mixing method. In this diagnostic two laser beams, where one is Stokes shifted from the other, non-linearly interact with the N$_{\mathrm{2}}$ molecules and the bullet's electric field. As a result of the interaction a coherent anti-Stokes Raman scattered (CARS) beam and an infrared beam are generated from which the electric field can be determined. Compared to emission-based methods, this technique has the advantage of being able to also probe the electric field in regions around the plasma bullet where no photons are emitted. The four-wave mixing method and its analysis have been adapted to work with the non-uniform electric field of plasma bullets. In addition, an ex-situ calibration procedure using an electrode geometry different from the discharge geometry has been developed. An experimentally obtained radial profile of the axial electric field component of a plasma bullet in N$_{\mathrm{2}}$ is presented. The position of this profile is related to the location of the propagating bullet from temporally resolved images. [Preview Abstract] |
Thursday, October 13, 2016 5:15PM - 5:30PM |
TR1.00005: Stark spectroscopy of atomic hydrogen balmer-alpha line for electric field measurement in plasmas by saturation spectroscopy S. Nishiyama, K. Katayama, H. Nakano, M. Goto, K. Sasaki Detailed structures of electric fields in sheath and pre-sheath regions of various plasmas are interested from the viewpoint of basic plasma physics. Several researchers observed Stark spectra of Doppler-broadened Rydberg states to evaluate electric fields in plasmas; however, these measurements needed high-power, expensive tunable lasers. In this study, we carried out another Stark spectroscopy with a low-cost diode laser system. We applied saturation spectroscopy, which achieves a Doppler-free wavelength resolution, to observe the Stark spectrum of the Balmer-alpha line of atomic hydrogen in the sheath region of a low-pressure hydrogen plasma. The hydrogen plasma was generated in an ICP source which was driven by on-off modulated rf power at 20 kHz. A planar electrode was inserted into the plasma. Weak probe and intense pump laser beams were injected into the plasma from the counter directions in parallel to the electrode surface. The laser beams crossed with a small angle above the electrode. The observed fine-structure spectra showed shifts, deformations, and/or splits when varying the distance between the observation position and the electrode surface. The detection limit for the electric field was estimated to be several tens of V/cm. [Preview Abstract] |
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