JET-P(90)44

ICRH Produced H-Modes in the JET Tokamak

H-modes produced by the ion-cyclotron resonace heating (JCRH) in a double-null Xpoint open-divertor configuration in the JET tokamak are studied where the ICRH antennas, operated in the dipole (0, ) phasing, are located on the low-field side of the tokamak. The beryllium evaporation on the nickel antenna screen and the first wall of the tokamak have played a crucial role in achieving H-modes with ICRH alone. These H-modes have a duration of up to 1.5s with an ICRH power level of up to 12MW. In these discharges, all characteristics typical of neutral-beam injection heating H-modes are found and the global energy confinement time approaches two-times the Goldston L-mode prediction. For most of their duration, ICRH Hmodes are free from ELMs (edge localized modes). In addition, the monster sawtooth (long sawtooth free period) feature of ICRH is maintained during H-modes leading to central electron temperature (Te0) of 10keV nearly twice the value of central ion temperature (Ti0). In NBI heating H-modes, the Te0 is generally lower than the Ti0, but the combined ICRH + NBI heating H-modes allow both Te0 and Ti0 close to 10keV simultaneously. The elevated Te0 by ICRH in the combined heating increases the slowing down time of beam ions and increases the D-D reaction rate and the neutron output. ICRH H-modes often occur as a two-step transition and the antenna plasma coupling resistance (Rc) also decreases in two-steps. Theoretical values of Rc agree well with experimental values during the H-phase.
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JETP90044 1.38 Mb