HBT-EP research program: 3D effects and control in a tokamak

The High Beta Tokamak – Extended Pulse (HBT-EP) research program aims to test control methods and validate simulation codes at low cost on a small-scale facility, qualifying concepts to be applied to larger facilities. Efforts are focused on natural and applied 3D features in tokamak plasmas, including (i) addressing the runaway electron problem relevant to tokamak disruptions via a Runaway Electron Mitigation Coil (REMC), (ii) measuring halo currents arising in disruptions and their effects, and (iii) applying fast feedback using innovative methods. The REMC is passively driven by the disruption-induced loop voltage to provide a large 3D perturbation for reducing confinement of high‐energy electrons. This is the first experimental demonstration of this concept, using a purpose-built 3D coil in vessel. Halo currents are measured using 72 diagnostic tiles, including 36 located in high-field-side regions where the plasma strongly impacts during disruptions. A convolutional neural network (CNN) implemented on a FPGA to measure MHD modes using solely optical measurements from high-speed videography has been applied in a feedback loop to control m/n=3/1 modes with a latency below 20 μs, on par with the magnetic sensor GPU-based control system. Simulations of sawtooth activity are conducted using NIMROD with non-axisymmetric wall resistivity to study effects of parameters and 3D features on sawtooth suppression. Upcoming plans for HBT-EP will also be presented.