Overview of C-2 field-reversed configuration experiment plasma diagnosticsa) Rev. Sci. Instrum. 85, 11D836 (2014); 10.1063/1.4884616 Density fluctuation measurements by far-forward collective scattering in the MST reversed-field pincha) Rev. Sci. Instrum. 83, 10E302 (2012);A new high performance field reversed configuration operating regime in the C-2 devicea)Conventional field-reversed configurations (FRCs), high-beta, prolate compact toroids embedded in poloidal magnetic fields, face notable stability and confinement concerns. These can be ameliorated by various control techniques, such as introducing a significant fast ion population. Indeed, adding neutral beam injection into the FRC over the past half-decade has contributed to striking improvements in confinement and stability. Further, the addition of electrically biased plasma guns at the ends, magnetic end plugs, and advanced surface conditioning led to dramatic reductions in turbulence-driven losses and greatly improved stability. Together, these enabled the build-up of a well-confined and dominant fast-ion population. Under such conditions, highly reproducible, macroscopically stable hot FRCs (with total plasma temperature of $1 keV) with record lifetimes were achieved. These accomplishments point to the prospect of advanced, beam-driven FRCs as an intriguing path toward fusion reactors. This paper reviews key results and presents context for further interpretation. V C 2015 AIP Publishing LLC. [http://dx.

Tri Alpha Energy's experimental program has demonstrated reliable field-reversed configuration (FRC) formation and sustainment, driven by fast ions via high-power neutral-beam (NB) injection. The world's largest compact-toroid device, C-2U, was upgraded from C-2 with the following key system upgrades: increased total NB input power from ~4 MW (20 keV hydrogen) to 10+ MW (15 keV hydrogen) with tilted injection angle; enhanced edge-biasing capability inside of each end divertor for boundary/stability control. C-2U experiments with those upgraded systems have successfully demonstrated dramatic improvements in FRC performance and achieved sustainment of advanced beam-driven FRCs with a macroscopically stable and hot plasma state for up to 5+ ms. Plasma diamagnetism in the best discharges has reached record lifetimes of over 11 ms, timescales twice as long as C-2. The C-2U plasma performance, including the sustainment feature, has a strong correlation with NB pulse duration, with the diamagnetism persisting even several milliseconds after NB termination due to the accumulated fast-ion population by NB injection. Power balance analysis shows substantial improvements in equilibrium and transport parameters, whereby electron energy confinement time strongly correlates with electron temperature; i.e. the confinement time in C-2U scales strongly with a positive power of T e .

Abstract. The High Beta Tokamak-Extended Pulse (HBT-EP) magnetohydrodynamic (MHD) mode control research program is studying ITER relevant internal modular feedback control coil configurations and their impact on kink mode rigidity, advanced digital control algorithms, and the effects of plasma rotation and three dimensional magnetic fields on MHD mode stability. A new segmented adjustable conducting wall has been installed on HBT-EP made up of 20 independent, movable, wall shell segments instrumented with 3 distinct sets of 40 saddle coils totaling 120 in-vessel modular feedback control coils. Each internal coil set has been designed with varying toroidal angular coil coverage of 5 • , 10 • , and 15 • , spanning the toroidal angle range of an ITER port plug based internal coil to test Resistive Wall Mode (RWM) interaction and multimode MHD plasma response to such highly localized control fields. In addition, we have implemented 336 new poloidal and radial magnetic sensors to quantify the applied three dimensional fields of our control coils along with the observed plasma response. This paper describes the design and implementation of the new control shell incorporating these control and sensor coils on HBT-EP, and the research program plan on the upgraded HBT-EP to understand how best to optimize the use of modular feedback coils to control instability growth near the ideal wall stabilization limit, answer critical questions about the role of plasma rotation in active control of the RWM and the Ferritic Resistive Wall Mode (FRWM), and to improve the performance of MHD control systems used in fusion experiments and future burning plasma systems.

Feedback control has become a crucial tool in the research on magnetic confinement of plasmas for achieving controlled nuclear fusion. We present the first experimental results from a novel feedback control system that, for the first time, employs a graphics processing unit (GPU) for microsecond-latency, real-time control computations. The system was tested on the HBT-EP tokamak using an adaptive control algorithm for control of rotating magnetic perturbations. The algorithm assumes that perturbations of known shape are rotating rigidly, but dynamically derives and updates the rotation frequency to improve phase and gain accuracy of the control signals. Experiments were set up to control four rotating n = 1 perturbations at different poloidal angles. The perturbations are treated as coupled in frequency but independent of amplitude and phase, so that the system effectively controls a helical n = 1 perturbation with unknown poloidal spectrum. The control system suppresses the amplitude of the dominant 8 kHz mode by up to 60%. Deviation from the optimal feedback phase combines suppression with a speed up or slow down of the mode rotation frequency. The feedback performance is found to exceed previous results obtained with an FPGA-and Kalman-filter based control system without requiring any tuning of system model parameters.

The detailed measurements of the 3D plasma response to applied external magnetic perturbations in the presence of a rotating external kink are presented, and compared with the predictions of a single-helicity linear model of kink mode dynamics. The modular control coils of the High Beta Tokamak-Extended Pulse (HBT-EP) device are used to apply resonant m/n = 3/1 magnetic perturbations to wall-stabilized tokamak plasmas with a pre-existing rotating 3/1 kink mode. The plasma response is measured in high-resolution with the extensive magnetic diagnostic set of the HBT-EP device. The spatial structures of both the naturally rotating kink mode and the externally driven response are independently measured and observed to be identical, while the temporal dynamics are consistent with the independent evolution and superposition of the two modes. This leads to the observation of a characteristic change in 3D field dynamics as a function of the applied field amplitude. This amplitude dependence is found to be different for poloidal and radial fields. The measured 3D response is compared to and shown to be consistent with the predictions of the linear single-helicity model in the “high-dissipation” regime, as reported previously [M. E. Mauel et al., Nucl. Fusion 45, 285 (2005)].