Multimode observations and 3D magnetic control of the boundary of a tokamak plasma

Abstract

We present high-resolution detection and control of the 3D magnetic boundary in the High Beta Tokamak-Extended Pulse (HBT-EP) device. Measurements of non-axisymmetric radial and poloidal fields are made using 216 magnetic sensors positioned near the plasma surface. Control of 3D fields is accomplished using 40 independent saddle coils attached to the passive stabilizing wall. The control coils are energized with high-power solid-state amplifiers, and massively parallel, high-throughput feedback control experiments are performed using low-latency connections between PCI Express analogue input and output modules and a graphics processing unit. The time evolution of unstable and saturated wall-stabilized external kink modes are studied with and without applying magnetic perturbations using the control coils. The 3D dynamic structure of the magnetic field surrounding the plasma is determined through biorthogonal decomposition using the full set of magnetic sensors without the need to fit either a Fourier or a model-based basis. Naturally occurring external kinks are composed of multiple independent helical modes. Smooth transitions between dominant poloidal mode numbers are observed for simultaneous n = 1 and n = 2 modes as the edge safety factor changes. Relative amplitudes of coexistent m/n = 3/1 and 6/2 modes depend on the plasma's major radius and edge safety factor. When stationary 3/1 magnetic perturbations are applied, the resonant response can be linear, saturated, or disruptive, depending upon the perturbation amplitude and the edge safety factor; increased plasma–wall interactions from the perturbed plasma are proposed as a saturation mechanism. Initial feedback experiments have used 40 sensors and 40 control coils, producing mode amplification or suppression, and acceleration or deceleration depending on the feedback phase angle.

Active and passive kink mode studies in a tokamak with a movable ferromagnetic walla)

“…Coherent structures in the sensor signals can be studied, in detail, using biorthogonal decomposition 33 (BD), as is routinely done for HBT-EP data. 17,19 BD analysis of kink modes in these plasmas yields spatial eigenfunctions and temporal amplitudes 19 as seen in Figures 2(d)-2(f). Rotating modes in HBT-EP often have a period of moderate growth followed by saturation, then fast growth leading to a disruption, as seen in Figure 2(f).…”

“…15,17 Over 200 pickup coils measure local poloidal and radial magnetic fields $1 cm from the plasma surface. Ferritic components were installed in order to explore MHD stability properties of plasmas with a nearby ferromagnetic wall.…”

“…14 This paper extends previous studies by describing the effects of movable ferromagnetic material installed near the plasma surface in the High Beta Tokamak-Extended Pulse (HBT-EP) experiment. [15][16][17] With newly installed nearby ferromagnetic material, high frequency (xs w ) 1) rotating kink modes grow faster, resonant field amplification is larger, and disruptions occur at smaller imposed error fields than observed in discharges with the ferritic material withdrawn from the plasma. Despite the plasma's higher sensitivity to non-axisymmetric magnetic fields, HBT-EP's magnetic feedback system 18 remains effective at suppressing rotating kink modes.…”

The structure of global conservation laws in Galerkin plasma models

“…The present work adapts and extends these innovations for plasmas, enabling a wealth of advanced modeling and control machinery. The POD is already used extensively for interpreting plasma physics data across a range of parameter regimes [22][23][24][25][26]. For clarity of presentation and robust connection with the Galerkin literature in fluid mechanics, we present results for MHD models which are at most quadratic in nonlinearity.…”

Numerical studies and metric development for validation of magnetohydrodynamic models on the HIT-SI experimenta)

“…Additionally, BD has been leveraged to define semi-automatic and quantitative metrics 6 for validating MHD models to experimental data. 5 BD is already used for analyzing experimental data from large diagnostic arrays 8,9 and simulation results 19 within a single data set. Application of Biorthogonal Decomposition to HIT-SI data had been described previously.…”