1 - Nuclear Sciences and Applications - IAEA

542 ASHRAF et al.
experiments on FTI [1] and Dili [2] showed that significant heating can be
achieved at line average densities (ne) up to more than double the value
at which this X-mode left-hand cut-off is present on axis but this is not
observed in the present experiments. In addition, HFS launch requires
fc|| 7^ 0 for effective absorption. This leads to damping of the rf wave
on tail electrons and is of interest for down-shifted resonance heating [3].
Absorption can only occur where the resonance condition is satisfied, that
is,
— = 7(1 - nfli cos 0) (2)
where 7 = (1 — v 2 /c 2 )~"i,n = ck/u!,fi\\ = v\\jc and 9 is the angle between k
and B. With appropriate values of 7, /?|| and cos 0,(2) can be satisfied for
wc/u >1, the down-shifted resonance condition. In previous down-shifted
resonance experiments (in WT2 [4] and PLT [5] for example) power was
absorbed under conditions where a substantial distortion of the electron
distribution was present, due either to lower hybrid heating or operation
under slide-away conditions. In BITE the distribution was close to thermal.
2 EXPERIMENTAL ARRANGEMENT
The rf power was launched from the HFS from three antennas [6] designed
to overcome the problem of breakdown at the point where the cyclotron
resonance occurs in the waveguide. The mirrors used to direct the rf power
towards the magnetic axis could be rotated so that the launch angle 0 with
respect to the toroidal field could be varied between nominally parallel and
antiparallel values. A residual rf power detector, placed 135° toroidally from
the antennas, was used to estimate the fraction of the power absorbed by
the plasma. The experiments were made in H2, D2 and He plasmas. The
latter provided good density control in the presence of ECRH and allowed
tokamak operation at electron densities well above the X mode cut-off value,
even at plasma currents of 100 kA.
3 EXPERIMENTAL RESULTS
Doppler Shifted Absorption
The extent of Doppler shifted absorption depends on the electron temperature
(Te) since the absorption occurs when the condition (2) is satisfied and
there are an adequate number of electrons present at that point with the
required V||. In addition, the absorption coefficient is inversely proportional