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Thermal Plasma and Fast Ion Transport in Electrostatic Turbulence in the Large Plasma Devicea) Shu Zhou, W

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Thermal Plasma and Fast Ion Transport in Electrostatic Turbulence in the Large Plasma Devicea) Shu Zhou, W Thermal plasma and fast ion transport in electrostatic turbulence in the large plasma devicea) Shu Zhou, W. W. Heidbrink, H. Boehmer, R. McWilliams, T. A. Carter, S. Vincena, S. K. P. Tripathi, and B. Van Compernolle Citation: Physics of Plasmas (1994-present) 19, 055904 (2012); doi: 10.1063/1.3695341 View online: http://dx.doi.org/10.1063/1.3695341 View Table of Contents: http://scitation.aip.org/content/aip/journal/pop/19/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Nonlinear instability in simulations of Large Plasma Device turbulencea) Phys. Plasmas 20, 055704 (2013); 10.1063/1.4805084 Turbulence and transport suppression scaling with flow shear on the Large Plasma Devicea) Phys. Plasmas 20, 055907 (2013); 10.1063/1.4804637 Gyrokinetic turbulent transport simulation of a high ion temperature plasma in large helical device experiment Phys. Plasmas 19, 042504 (2012); 10.1063/1.4704568 Dependence of fast-ion transport on the nature of the turbulence in the Large Plasma Device Phys. Plasmas 18, 082104 (2011); 10.1063/1.3622203 Turbulent transport of fast ions in the Large Plasma Device Phys. Plasmas 17, 092103 (2010); 10.1063/1.3486532 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.2.19.100 On: Thu, 01 Jan 2015 06:18:10 PHYSICS OF PLASMAS 19, 055904 (2012) Thermal plasma and fast ion transport in electrostatic turbulence in the large plasma devicea) Shu Zhou,1,b) W. W. Heidbrink,1 H. Boehmer,1 R. McWilliams,1 T. A. Carter,2 S. Vincena,2 S. K. P. Tripathi,2 and B. Van Compernolle2 1Department of Physics and Astronomy, University of California, Irvine, California 92697, USA 2Department of Physics and Astronomy, University of California, Los Angeles, California 90095, USA (Received 30 November 2011; accepted 1 March 2012; published online 28 March 2012) The transport of thermal plasma and fast ions in electrostatic microturbulence is studied. Strong density and potential fluctuations (dn=n d/=kTe 0.5, f 5–50 kHz) are observed in the large plasma device (LAPD) [W. Gekelman, H. Pfister, Z. Lucky et al., Rev. Sci. Instrum. 62, 2875 (1991)] in density gradient regions produced by obstacles with slab or cylindrical geometry. Wave characteristics and the associated plasma transport are modified by driving sheared E Â B drift through biasing the obstacle and by modification of the axial magnetic fields (Bz) and the plasma species. Cross-field plasma transport is suppressed with small bias and large Bz and is enhanced with large bias and small Bz. The transition in thermal plasma confinement is well explained by the cross-phase between density and potential fluctuations. Large gyroradius lithium fast ion beam (qfast/qs 10) orbits through the turbulent region. Scans with a collimated analyzer give detailed profiles of the fast ion spatial-temporal distribution. Fast-ion transport decreases rapidly with increasing fast-ion energy and gyroradius. Background waves with different scale lengths also alter the fast ion transport. Experimental results agree well with gyro-averaging theory. When the fast ion interacts with the wave for most of a wave period, a transition from super-diffusive to sub-diffusive transport is observed, as predicted by diffusion theory. Besides turbulent-wave-induced fast-ion transport, the static radial electric field (Er) from biasing the obstacle leads to drift of the fast-ion beam centroid. The drift and broadening of the beam due to static Er are evaluated both analytically and numerically. Simulation results indicate that the Er induced transport is predominately convective. VC 2012 American Institute of Physics.[http://dx.doi.org/10.1063/1.3695341] I. INTRODUCTION well confined in electrostatic microturbulence (see, e.g., Ref. 10 and references therein). But anomalous transport at Plasma transport in turbulent waves is very important small E /T has been reported.11–17 It is widely accepted both in fusion experiments and in space physics. Specifi- fast i that the reduction of the energetic ion transport in microtur- cally, reduction of the cross-field transport by modification bulence is due to the averaging effect of its large gyro and of the turbulence at the plasma edge has attracted growing drift orbit over the turbulent structures with a scale length of interest since the discovery of the so-called high confinement thermal ion gyro-radius (q ). Many theoretical and simula- mode1 (H-mode) in magnetic confinement devices. Many i tion works (e. g., Refs. 18 and 19) have been performed to experiments and theories attribute the reduction in plasma model this gyro-averaging effect, but experimental results transport to the development of velocity shear at the plasma are rarely reported. edge. The sheared flow in plasmas can be spontaneously gen- Two experiments have been performed at the large erated by nonlinear interaction of the edge turbulence (zonal plasma device20 (LAPD) to study the transport of both ther- flows, see Ref. 2 and references therein) or induced by radial mal plasma and fast ions in electrostatic turbulent waves. An electric fields (E Â B drift, see Refs. 3 and 4, and references obstacle is inserted in the main plasma column to partially therein). It is demonstrated5 that the E Â B drift induced by block the plasma source and induces a sharp gradient in external bias can also trigger an H-mode like state in plasma. plasma density. Biasing the obstacle induces sheared E Â B The mechanism of this confinement transition has been flow at the plasma edge. The turbulent waves are driven by further studied in experiments.6–9 both density gradient and sheared flow. The plasma transport Another important topic in turbulent transport is the and confinement are effectively modified by control parame- transport of energetic ions in microturbulence. Energetic ters such as the bias voltage on the obstacle (V ) and ions are defined as ions with much higher energy than the bias strength of the axial magnetic field (B ). The confinement of plasma thermal ions. Experimental results reported in toka- z the plasma density is either improved or weakened depend- maks indicates that, in the high energy regime (the ratio of ing on the control parameters. The transition in density fast ion energy to thermal ion energy E /T 10), ions are fast i confinement is found to correlate with the phase overlap between fluctuating density and potential structures. a)Paper GI3 3, Bull. Am. Phys. Soc. 56, 95 (2011). The turbulent waves also serve as the background wave 21 b)Invited speaker. fields for the fast-ion transport studies. A fast-ion source is 1070-664X/2012/19(5)/055904/8/$30.00 19, 055904-1 VC 2012 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.2.19.100 On: Thu, 01 Jan 2015 06:18:10 055904-2 Zhou et al. Phys. Plasmas 19, 055904 (2012) inserted in the LAPD as a test-particle beam, and the electro- experiment are helium and neon. Typical parameters during static fluctuations cause the beam width to broaden. Fast-ion the active discharge in a helium plasma (at a fill pressure of À4 12 À3 transport in electrostatic waves with various spatial-temporal 2 Â 10 Torr) are ne 2.5 Â 10 cm ,Te 5 eV, and characteristics is documented. The energy scaling of the Ti 1 eV. The experimental setup is shown schematically in transport and the scaling on the structure size and correlation Fig. 1. A copper, annular obstacle with inner diameter of length of the background waves are consistent with theoreti- 12 cm and outer diameter of 20 cm is inserted upstream (port cal predictions. 29), concentrically with the plasma column. Once in place, This paper summarizes the main results reported in the obstacle blocks the plasma-forming primary electrons earlier publications about the thermal22 and fast-ion23,24 parallel to the magnetic field and significantly decreases the transport in these LAPD experiments. It also includes new downstream plasma density. A steep density gradient, along material on a possible source of uncertainty in the fast-ion with large density fluctuations, is observed on the inner transport experiment: the effect of the static electric field on edge region of the annular depletion. The annular obstacle is the fast ion beam. The experimental setup is introduced either floating or positively biased relative to the LAPD in Sec. II. The results of the thermal plasma and fast-ion anode. The obstacle shaft and the LAPD anode are con- transport in electrostatic turbulent waves are briefly nected by a bias circuit that contains a capacitor bank and an reviewed in Secs. III and IV. Section V discusses the effect insulated gate bipolar transistor (IGBT) switch. A pulsed of the bias-induced static electric field on the measured drift bias with 3 ms duration time can be imposed on the obsta- and broadening of the fast-ion beam. Conclusions are drawn cle during the steady portion of the LAPD discharge. The in Sec. VI. biasing voltage is up to 250 V, and it establishes radial elec- tric fields and spins the edge of the 12 cm-diameter plasma column through the E Â B drift. Measurements of plasma II. EXPERIMENTAL SETUP density, floating potential, temperature, and their fluctuations The experiments reported in this paper are performed in are made using Langmuir probes.25 the upgraded LAPD at the University of California, Los The lithium ion gun21 is inserted into the LAPD plasma Angeles (UCLA).
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