title: Diagnostic options for radiative divertor feedback control on NSTX-U.
abstract: A radiative divertor technique is used in present tokamak experiments and planned for ITER to mitigate high heat loads on divertor plasma-facing components (PFCs) to prevent excessive material erosion and thermal damage. In NSTX, a large spherical tokamak with lithium-coated graphite PFCs and high divertor heat flux (q(peak) ≤ 15 MW/m(2)), radiative divertor experiments have demonstrated a significant reduction of divertor peak heat flux simultaneously with good core H-mode confinement using pre-programmed D(2) or CD(4) gas injections. In this work diagnostic options for a new real-time feedback control system for active radiative divertor detachment control in NSTX-U, where steady-state peak divertor heat fluxes are projected to reach 20-30 MW/m(2), are discussed. Based on the NSTX divertor detachment measurements and analysis, the control diagnostic signals available for NSTX-U include divertor radiated power, neutral pressure, spectroscopic deuterium recombination signatures, infrared thermography of PFC surfaces, and thermoelectric scrape-off layer current. In addition, spectroscopic "security" monitoring of possible confinement or pedestal degradation is recommended. These signals would be implemented in a digital plasma control system to manage the divertor detachment process via an actuator (impurity gas seeding rate).
authors.affiliation: Lawrence Livermore National Laboratory, California, 94550, USA.
authors.forename: V A
authors.initials: VA
authors.lastname: Soukhanovskii
authors.forename: S P
authors.initials: SP
authors.lastname: Gerhardt
authors.forename: R
authors.initials: R
authors.lastname: Kaita
authors.forename: A G
authors.initials: AG
authors.lastname: McLean
authors.forename: R
authors.initials: R
authors.lastname: Raman