Robustness of waves with a high phase velocity

Control of Ion Gyroscale Fluctuations via Electrostatic Biasing and Sheared E×B Flow in the C-2 Field Reversed Configuration
L. Schmitz1,a), E. Ruskov2,b), B.H. Deng3,c), M. Binderbauer3,d), T. Tajima2,3,e), H. Gota3,f), M. Tuszewski3,g), and the TAE Team
1University of California Los Angeles, Los Angeles, CA 90095, USA 2University of California, Irvine, Irvine, CA 92697, USA
3Tri Alpha Energy, Inc., P.O. Box 7010, Rancho Santa Margarita, CA 92688, USA
a)Corresponding author: lschmitz@trialphaenergy.com b)eruskov@uci.edu c)bdeng@trialphaenergy.com d)michl@trialphaenergy.com e)tajima@trialphaenergy.com f)hgota@trialphaenergy.com g)mgtu@trialphaenergy.com
Abstract. Control of radial particle and thermal transport is instrumental for achieving and sustaining well-confined high-β plasma in a Field-Reversed Configuration (FRC). Radial profiles of low frequency ion gyro-scale density fluctuations (0.5 ≤ kρs ≤ 40), consistent with drift- or drift-interchange modes, have been measured in the scrape-off layer (SOL) and core of the C-2 Field-Reversed Configuration (FRC), together with the toroidal ExB velocity. It is shown here that axial electrostatic SOL biasing controls and reduces gyro-scale density fluctuations, resulting in very low FRC core fluctuation levels. When the radial ExB flow shearing rate decreases below the turbulence decorrelation rate, fluctuation levels increase substantially, concomitantly with onset of the n=2 instability and rapid loss of diamagnetism. Low turbulence levels, improved energy/particle confinement and substantially increased FRC life times are achieved when ExB shear near the separatrix is maintained via axial SOL biasing using an annular washer gun.
INTRODUCTION
Field-Reversed configurations (FRCs [1,2]), characterized by high normalized kinetic pressure β, are of interest as a fusion reactor concept due to their axisymmetric geometry and potential for aneutronic fusion based on advanced fuels, such as the proton-boron fusion reaction (p-B11) [3]. FRCs also allow investigating the formation, stability, and kinetic properties of high β, high temperature, low collisionality plasmas in a laboratory environment. Compared to toroidal plama confinement concepts such as the tokamak [4] or stellarator [5], FRCs have intrinsically axisymmetric magnetic field configuration. Due to the larger ion Larmor orbits, FRCs are also expected to have favorable microstability properties, as recognized early by Rostoker and others [6,7,8]. However, similar to other plasma confinement concepts, anomalously large plasma resistivity, and radial particle and energy transport in excess of classical collisional transport have historically been observed in FRCs [9,10]. The increased radial losses have been attributed to microinstabilities, however the FRC life time in early experiments has often been too short to achieve transport-dominated states [1,2], and macroscopic MHD instabilities and micro-instabilities sometimes co- exist. Among the instabilities investigated in some detail are the lower hybrid drift instability (LHD), drift cyclotron instabilities, and density/temperature-gradient-driven drift waves [1,2]. So far, experimentally measured LHD fluctuation levels could not account for the observed radial transport level [11]. Unstable collisionless
The Physics of Plasma-Driven Accelerators and Accelerator-Driven Fusion
AIP Conf. Proc. 1721, 030002-1–030002-9; doi: 10.1063/1.4944018 © 2016 AIP Publishing LLC 978-0-7354-1368-9/$30.00
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