Abstract
The absorption of fast Alfvén waves (FW) by ion cyclotron harmonic damping in the range of harmonics from 4th to 8th is studied theoretically and with experiments in the DIII-D tokamak. A formula for linear ion cyclotron absorption on ions with an arbitrary distribution function which is symmetric about the magnetic field is used to estimate the single-pass damping for various cases of experimental interest. It is found that damping on fast ions from neutral beam injection can be significant even at the 8th harmonic if the fast ion beta, the beam injection energy and the background plasma density are high enough and the beam injection geometry is appropriate. The predictions are tested in several L-mode experiments in DIII-D with FW power at 60 MHz and at 116 MHz. It is found that 4th and 5th harmonic absorption of the 60 MHz power on the beam ions can be quite strong, but 8th harmonic absorption of the 116 MHz power appears to be weaker than expected. The linear modelling predicts a strong dependence of the 8th harmonic absorption on the initial pitch-angle of the injected beam, which is not observed in the experiment. Possible explanations of the discrepancy are discussed.
Highlights
A prerequisite for efficient fast wave current drive (FWCD) is that direct electron damping of the fast wave in the core of the plasma must dominate all other absorption mechanisms
Using the simpler ‘effective Maxwellian’ model embodied in equation (2), in which we use the effective temperature for the beam derived from ONETWO, Teff ≡ (2/3)Wf /nf in which Wf is the stored energy density in the fast ions, we find similar results: ∼18% single pass absorption at 4th harmonic and negligible 8th harmonic absorption
We have presented a new theoretical form for the linear ion cyclotron harmonic damping of fast Alfven waves which, though it is equivalent to existing formulations (e.g. [4]), has physical meaning more assigned to each term
Summary
A prerequisite for efficient fast wave current drive (FWCD) is that direct electron damping of the fast wave in the core of the plasma must dominate all other absorption mechanisms. As is well known [4, 5], the fraction of the wave power absorbed by ion cyclotron damping as the fast wave propagates through a particular cyclotron harmonic layer in a slightly inhomogeneous magnetic field depends on the density of the absorbing ion species and the ratio of th√e wavelength of the fast wave 2π/k⊥ to the gyroradius ρs = (κTs/ms)/(2πfcs) of the absorbing ion species s.
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