Description
Understanding and controlling runaway electrons (REs) remains a critical challenge for future tokamak reactors. While REs are initially accelerated by strong electric fields, their maximum energy and confinement are strongly influenced by pitch-angle dynamics, which determine synchrotron damping and particle losses. Applied three-dimensional (3D) magnetic fields offer multiple pathways to modify these dynamics, either by directly perturbing magnetic flux surfaces or by enhancing phase-space transport through wave–particle–like interactions. The Tokamak à Configuration Variable (TCV), with its moderate size and flexible magnetic configuration, provides a unique laboratory for systematically exploring these effects.
Two complementary approaches to applied 3D fields are being developed on TCV. The first exploits controlled modification of the intrinsic toroidal field ripple. A hardware upgrade will allow the current in every other toroidal field coil to be reduced by diverting a controlled fraction of the current through parallel resistors. This configuration enables discharge-to-discharge scans of the ripple magnitude, providing a well-controlled means of studying resonant interactions between relativistic electrons and static magnetic field perturbations. Such interactions have been proposed as an effective mechanism for enhancing perpendicular momentum and limiting maximum RE energy, offering an experimentally accessible proxy for wave–particle effects on RE phase-space dynamics.
The second approach focuses on the development of a passive Runaway Electron Mitigation Coil (REMC). This non-axisymmetric coil is designed to couple inductively to the disruption-driven loop voltage during the current quench, generating strong n=1 magnetic perturbations without active power supplies. Extensive electromagnetic modeling using the ThinCurr code has guided the coil design, capturing induced currents in both the coil and surrounding conducting structures and highlighting the critical role of wall currents in determining the resulting 3D fields. The REMC is intended as a mitigation tool, aiming to disrupt RE confinement and prevent plateau formation.
Together, these developments position TCV to explore how distinct classes of applied 3D magnetic fields influence RE dynamics, spanning controlled phase-space studies and passive mitigation concepts relevant to future devices.