Compact Toroid Injection Fueling in a Large Field-Reversed Configuration

PHYSICS OF PLASMAS 24, 082512 (2017) Drift-wave stability in the field-reversed configuration
C. K. Lau,1,a) D. P. Fulton,2 I. Holod,1 Z. Lin,1,b) M. Binderbauer,2 T. Tajima,1,2 and L. Schmitz3
1University of California, Irvine, California 92697, USA
2Tri Alpha Energy, Inc., Rancho Santa Margarita, California 92688, USA 3University of California, Los Angeles, California 90095, USA
(Received 10 May 2017; accepted 29 June 2017; published online 2 August 2017)
Gyrokinetic simulations of C-2-like field-reversed configuration (FRC) find that electrostatic drift- waves are locally stable in the core. The stabilization mechanisms include finite Larmor radius effects, magnetic well (negative grad-B), and fast electron short circuit effects. In the scrape-off layer (SOL), collisionless electrostatic drift-waves in the ion-to-electron-scale are destabilized by electron temperature gradients due to the resonance with locally barely trapped electrons. Collisions can sup- press this instability, but a collisional drift-wave instability still exists at realistic pressure gradients. Simulation results are in qualitative agreement with C-2 FRC experiments. In particular, the lack of ion-scale instability in the core is not inconsistent with experimental measurements of a fluctuation spectrum showing a depression at ion-scales. The pressure gradient thresholds for the SOL instability from simulations are also consistent with the critical gradient behavior observed in experiments. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4993630]
I. INTRODUCTION
A field-reversed configuration (FRC) is an elongated prolate compact toroid (CT) with purely poloidal magnetic fields. The FRC consists of two regions separated by a sepa- ratrix: an inner, closed field-line core region and an outer, open field-line scrape-off layer (SOL) region. Research inter- est in the FRC persists because of potential reactor benefits: (1) the FRC is a plasma with b (the ratio of plasma pressure to magnetic energy density) near unity, which suggests less magnetic energy investment and less cyclotron radiation than low b approaches such as the tokamak; (2) the compact nature of the plasma simplifies the construction of the device hull and external magnetic field coils; (3) engineering is also aided by the SOL, which naturally connects to the divertor; and (4) the lack of toroidal magnetic fields radically changes the magnetic topology and the consequential stability of the plasma, as detailed below.
It was suggested by Rostoker et al.1,2 that adding a sig- nificant energetic ion population via neutral beam injection (NBI) would improve FRC macro-stability while preserving the FRC’s favorable transport properties1,3–7 due to the large ion Larmor radius relative to the plasma size.5,8 In 2008, Tri Alpha Energy, Inc. (TAE), launched a campaign on the FRC experiment, C-2.9 With the use of NBI, electron gun biasing, and magnetic end plugs, the C-2 experiments have succeeded in suppressing the major MHD instabilities, the rotational (n 1⁄4 2), wobble, and tilt10 (n 1⁄4 1) modes (where n is the toroidal mode number), and increasing FRC confinement times to the order of several milliseconds.10–12 This achieve- ment has made the FRC reach the transport-limited regime.
In early experiments of FRCs, besides the fact that many FRCs may not have reached the transport-limited
a)Electronic mail: calvin.lau@uci.edu b)Electronic mail: zhihongl@uci.edu
regime, the transport studies have showed relatively short confinement times. In these experiments, particle,13,14 flux,15 and energy confinement were well identified as anomalous. Possible electrostatic micro-instabilities have been investi- gated,16–19 with the lower hybrid drift instability (LHDI) theo- retically identified as the most linearly unstable. However, experiments found that the LHDI saturates at levels two orders of magnitude below oft-predicted values.16 Electromagnetic modes such as the electron temperature gradient driven electro- magnetic micro-tearing modes may also be present in FRCs20 but have not been studied in detail. Confinement is signifi- cantly affected by radial diffusion through the edge,21,22 where particles move from the closed field-lines of the core to the open field-lines of the SOL. A number of analytical studies have been made of classical transport in simple equilibria23–26 and using quasi-steady 1-D plasma profiles.27–29 Numerical models of transport have been made to include more details using both simple 1-D and 2-D equilibria.13,15,30–33
However, once the FRC plasmas in C-2 and C-2U10–12,34 clearly reached the transport-limited regime with sufficient remedies of macro-instabilities mentioned earlier, the trans- port times have been found to lengthen considerably12 and show markedly different properties of fluctuations.35 In these FRC shots, the Q1D fluid transport code,36 based on the CFRX code,37 has been developed and employed for transport analysis of C-2 plasma conditions.
Schmitz et al.35 found that, while the plasma in the SOL shows robust fluctuations driven by micro-instabilities, the level of fluctuations in the FRC core is less than in the SOL by 1–2 orders of magnitude. The level of fluctuations is reduced when the neutral beam injection commences. An appropriately applied end voltage bias can further reduce the level of fluctua- tions. These are strong indications that fluctuations are strongly dependent on the plasma’s density, temperature, presence of large orbit particles, and presence of plasma shear flows, which may influence stability properties of micro-instabilities. Taking
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24, 082512-1 Published by AIP Publishing.