A combined mmwave and CO2 interferometer on the C-2W Jet plasma

International Atomic Energy Agency Nuclear Fusion Nucl. Fusion 58 (2018) 082011 (4pp) https://doi.org/10.1088/1741-4326/aab6c6
Experimental characterization of Alfvén modes in a field-reversed configuration plasma
R.M. Magee, T. Roche, M.C. Thompson, M. Tobin, M. Beall, B.H. Deng and S. Korepanov
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610, United States of America E-mail: RMagee@tae.com
Received 4 December 2017, revised 8 March 2018 Accepted for publication 15 March 2018 Published 29 June 2018
Abstract
High power neutral beam injection into the C-2U advanced field-reversed configuration (FRC) stimulates at least three distinct energetic particle modes, none of which have been linked to
a performance limitation. Here we present an experimental characterization of one mode in particular, a high frequency mode ( f > fci ) observed in the decaying phase of the FRC. The mode is reminiscent of the compressional Alfvén eigenmodes observed in spherical tokamaks: its frequency scales with the edge Alfvén velocity and it co-propagates with injected beam ions perpendicular to the magnetic field. There are multiple pieces of evidence that the mode resides outside of the FRC and therefore likely has little to no effect on the confinement properties of the FRC itself.
Keywords: Alfvén modes, energetic particle modes, field-reversed configuration (Some figures may appear in colour only in the online journal)
1. Introduction
The C-2U advanced field-reversed configuration (FRC) is a unique magnetic confinement device (figure 1(a)). It com- bines the high power, perpendicular neutral beam injection of magnetic mirror devices [1, 2] with the good confinement properties of closed-field line toroidal devices while main- taining the natural engineering advantages of a cylindrical vacuum vessel.
Approximately 10 MW of neutral beam injection (NBI) power into a 1 m3 FRC target plasma results in a large fast ion population that sustains the field-reversed configura- tion. Multiple measurements indicate that the fast ion beta approaches, and perhaps even surpasses, the thermal beta [3].
One might expect then that energetic particle modes (EPMs) play a significant role limiting fast ion accumulation or degrading thermal confinement, but no such detriments have yet been observed. This is likely due to the large orbits of the fast ions in the relatively modest magnetic field: machine- sized orbits allow fast ions to effectively ‘average over’ small- scale turbulence.
This is not to say that energetic particle modes are not observed. In fact, measurements of magnetic fluctuations at the edge of the plasma reveal that there are at least three (see figure 2): a low frequency, chirping mode; an ion cyclotron mode; and a high frequency Alfvénic mode.
The chirping mode and ion cyclotron mode will be dis- cussed in future publications. In this presentation, we focus on the Alfvén mode. We will use data from multiple diagnostics, including arrays of Mirnov probes located at the plasma periphery (figure 1(b)) [4, 5] and far infrared interferometry (FIR) [6] to characterize this mode in terms of frequency and its dependencies, mode number, radial localization, and onset criterion.
2. Background
Recent progress in the understanding of energetic particle modes has been primarily focused on toroidal devices, with open sys- tems and compact toroids largely neglected. For example, the list of known eigenmode species in toroidal devices has grown to over ten (e.g. Toroidal Alfvén Eigenmodes, Reversed-shear
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