Exploring Features of Non-Resonant Divertor Design at the Compact Toroidal Hybrid (CTH)

Non-resonant divertors (NRD) separate the confined plasma from surrounding material structures with the resulting boundary region rendered by a chaotic magnetic edge structure​. This edge structure is formed by interacting magnetic islands and is eventually guided to wall elements such that the field lines end in rigid strike lines. The Compact Toroidal Hybrid (CTH) can serve as a test-bed for NRD solutions. The background field coils and the ohmic current drive system of CTH are used to alter the rotational transform between 0.3 < ι < 0.75. We show the presence of the chaotic edge field line structure which evolves with current and is connected to the wall through stable trajectories that yield strike lines. These chaotic structures define the plasma edge transport towards the wall – an interplay that we aim to explore. We calculate strike point locations for the exiting plasma for multiple ohmic current values. The calculated strike point locations enable us to design and numerically test an instrumented divertor plate for CTH which can then be used to experimentally measure NRD resiliencies with respect to equilibrium changes. The test plate in CTH is designed to be movable along a radial axis and can act both as a limiter, when fully extended, and as a divertor when placed outside the LCFS. We present results from both field-line calculations and EMC3-EIRENE fluid Monte-Carlo simulations of the ​edge behavior in the vicinity of the CTH test plate for various configurations and different plate positions.