Gyrokinetic simulation of driftwave instability in field-reversed configuration

PHYSICS OF PLASMAS 23, 056111 (2016)
Gyrokinetic simulation of driftwave instability in field-reversed configuration
D. P. Fulton,1,2,a),b) C. K. Lau,2 L. Schmitz,1 I. Holod,2 Z. Lin,2 T. Tajima,1 M. W. Binderbauer,1 and TAE Team1
1Tri Alpha Energy, Inc., Rancho Santa Margarita, California 92688, USA 2University of California, Irvine, California 92697, USA
(Received 5 January 2016; accepted 15 April 2016; published online 4 May 2016)
Following the recent remarkable progress in magnetohydrodynamic (MHD) stability control in the C-2U advanced beam driven field-reversed configuration (FRC), turbulent transport has become one of the foremost obstacles on the path towards an FRC-based fusion reactor. Significant effort has been made to expand kinetic simulation capabilities in FRC magnetic geometry. The recently upgraded Gyrokinetic Toroidal Code (GTC) now accommodates realistic magnetic geometry from the C-2U experiment at Tri Alpha Energy, Inc. and is optimized to efficiently handle the FRC’s magnetic field line orientation. Initial electrostatic GTC simulations find that ion-scale instabilities are linearly stable in the FRC core for realistic pressure gradient drives. Estimated instability thresholds from linear GTC simulations are qualitatively consistent with critical gradients deter- mined from experimental Doppler backscattering fluctuation data, which also find ion scale modes to be depressed in the FRC core. Beyond GTC, A New Code (ANC) has been developed to accu- rately resolve the magnetic field separatrix and address the interaction between the core and scrape-off layer regions, which ultimately determines global plasma confinement in the FRC. The current status of ANC and future development targets are discussed. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4948285]
I. INTRODUCTION
A field-reversed configuration (FRC) is a compact toroid with magnetic field predominantly in the poloidal direction. Somewhat after the discovery of FRCs in the late 1950s,1,2 it was recognized that their characteristic high-b (ratio of plasma pressure to magnetic pressure) and compact size would be favorable to the design of a fusion reactor. Despite significant advances, maintaining macrostability remained a challenge throughout resurgence in research that took place in the 1970s and 1980s.3 Renewed interest in FRC research in the late 1990s onward leads to the development of new formation techniques and better theoretical understanding of macroscopic instabilities.4
The “advanced beam-driven” flavor of the FRC was originally conceived in 1993 by Rostoker et al.5,6 The salient feature of this concept is a dominant nonadiabatic fast ion population, which brings a number of stability benefits to the configuration. It was known at the time that nonadiabatic ions in tokamaks were relatively insensitive to microturbu- lence,7 likely due to the spatial averaging effect of ion Larmour radii comparable to the plasma size. It was hypothesized that a significant fast ion population, in an FRC, could also stiffen the plasma against some macroinst- abilities (e.g., tilt mode). Other suppression techniques, such as electric biasing schemes or additional magnetic windings, could be used to stabilize any remaining macroscopic modes. In practice, neutral beam injection (NBI) is used to provide
Note: Paper YI2 3, Bull. Am. Phys. Soc. 60, 391 (2015).
a)Invited speaker.
b)Author to whom correspondence should be addressed. Electronic mail:
dfulton@trialphaenergy.com
the fast ion population, which is the origin of the “advanced beam-driven” moniker.
The fast-ion conjecture was taken up by Tri Alpha Energy, Inc. (TAE), which began work on FRC experiments in the late 1990s. The C-2 experiment at TAE began in 2008, and among other goals, aimed to demonstrate that NBI could stabilize and sustain an FRC plasma.8 From 2008 to the pres- ent, remarkable progress in FRC plasma confinement in C-2 and its successor, C-2U, has brought the advanced beam- driven FRC concept to a point where microturbulent instabil- ity is one of the foremost physics obstacles.9–11
In this paper, we begin to investigate microturbulence in advanced beam-driven FRC via gyrokinetic simulations. Section II summarizes the state of recent progress at TAE and motivates kinetic turbulence simulation. The initial sim- ulation model is electrostatic and linear. Details of this simu- lation model are presented in Section III. Remarkably, ion-scale driftwave turbulence is found to be linearly stable in the FRC core. These results and others are presented in Section IV. Interpretation of results, possible stabilization mechanisms, and caveats are discussed in Section V. This work represents a first effort to characterize microturbulence in realistic FRC geometry, but model refinements and addi- tional physics are needed. Future work and direction of code development are discussed in Section VI.
II. MOTIVATIONS FOR MICROTURBULENCE SIMULATION IN FRC GEOMETRY
A. Status of advanced beam-driven FRC experiment at TAE
The C-2 experiment at TAE was designed as a testbed for advanced beam-driven FRC physics, with an ambitious
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