ICRF Discharge Production for Ion Cyclotron Wall Conditioning on JET

ABSTRACT. Discharge wall conditioning is an effective tool to improve plasma performance by (i) reducing the generation of plasma impurities liberated from the wall and (ii) controlling the recycling of hydrogenic fluxes. On ITER discharge wall conditioning will be employed as well for (iii) mitigating the tritium inventory build-up, for which one relies mostly on the removal of tritium-rich co-deposited layers. Ion cyclotron wall conditioning (ICWC) is a well-studied discharge wall conditioning technique having the advantage over Glow Discharge Conditioning (GDC) that it is applicable in the presence of magnetic fields. The ICWC mode of operation is included in the functional requirements of the ITER ion cyclotron resonance heating and current drive system, and is envisaged for use between ITER plasma pulses, in the presence of the toroidal magnetic field. Ion Cyclotron Range of Frequencies (ICRF) plasma production employing ICRH&CD antennas designed for Fast Waves excitation is studied extensively on JET in the frame of fuel removal experiments by isotopic exchange aiming at the development of ICWC scenarios for ITER. This paper compares isotopic exchange efficiencies of JET ICWC discharges produced at ITER half and full field conditions for the JET carbon (C) and ITER like wall (ILW). ICWC on the ILW is found to be more efficient providing cleaner plasma faster, and has as significant advantage compared to the C-wall: an improved ratio of retained discharge gas to removed fuel, mitigating permanent retention during conditioning. A close to complete isotopic change over of the JET-ILW by D2-ICWC alone, evidenced by sampling the plasma isotopic ratio in tokamak discharges, was achieved within 630s of cumulated ICWC discharge time. The accessible reservoir by H2-ICWC at ITER half field conditions on the JET-ILW preloaded by D2 tokamak operation is larger than 7.3 × 1022 hydrogenic atoms. Conditioning efficiency optimization, ICRF discharge initiation and the characterization of the ICWC particle flux on the PFC are briefly addressed.
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