Fast Wave Current Drive and Direct Electron Heating in JET ITB Plasmas

T. Hellsten, T. Laxåback, Tuomas Tala, et al.

    Research output: Chapter in Book/Report/Conference proceedingConference article in proceedingsScientific

    Abstract

    Experiments with Fast Wave Current Drive, FWCD, and heating have been carried out in JET Internal Transport Barrier (ITB) discharges with strongly reversed magnetic shear. In order to maximize the current drive efficiency and increase the electron damping, and at the same time modifying the current profile in the transport barrier, hot low density ITB plasmas with strongly reversed magnetic shear, close to current hole, were created with Lower Hybrid Current Drive. It was difficult to strongly modify the central plasma current, even though the calculated current drive efficiency in terms of ampere per watts absorbed by the electrons was fairly high, 0.07A/W, because of: the strongly inductive nature of the plasma current due to the high electric conductivity; the interplay between the fast wave driven current and the bootstrap current, which, due to the dependence of the bootstrap current on the poloidal magnetic field, decreases the bootstrap current as the driven current increases; and parasitic absorption of the waves that decreased the power absorbed by the electrons. The power absorbed by the electrons was measured with a power modulation technique and the associated fast wave current drive calculated. Current diffusion simulations using the JETTO transport code, assuming neoclassical resistivity, were then carried out to calculate what changes to the plasma current profile could be expected from the current drive. The simulations showed a much slower response to the current drive compared to the measured central current densities suggesting a faster current penetration in the experiments than expected from neoclassical theory. Whereas the direct electron heating by fast magnetosonic waves using dipole spectra has proven to be an effective method to heat electrons in high-temperature ITB plasmas, even for a single pass damping of only a few percent, the heating in FWCD experiments with + 90° and - 90° antenna phasings were, for similar single pass damping as for the dipole, strongly degraded by parasitic losses, and with a heating efficiency of about half that of the dipole. Observations supporting that the losses are primarily caused by the presence of rectified RF-sheath potentials come from the large differences in performance and in Beryllium-II and Carbon-IV line radiation intensities between the dipole and ±90° phasings.
    Original languageEnglish
    Title of host publicationFusion Energy 2006
    Subtitle of host publicationProceedings of the 21st IAEA Conference
    PublisherInternational Atomic Energy Agency IAEA
    ISBN (Electronic)92-0-100907-0
    Publication statusPublished - 2006
    MoE publication typeB3 Non-refereed article in conference proceedings
    Event21st IAEA Fusion Energy Conference - Chengdu, China
    Duration: 16 Oct 200621 Oct 2006

    Publication series

    SeriesIAEA Conference Proceedings
    NumberIAEA-CN-149

    Conference

    Conference21st IAEA Fusion Energy Conference
    CountryChina
    CityChengdu
    Period16/10/0621/10/06

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