Abstract
The absorption of fast Alfvén waves (FW) by ion cyclotron harmonic damping in the range of harmonics from fourth to eighth is studied theoretically and with experiments in the DIII‐D tokamak. A formula for linear ion cyclotron absorption on Maxwellian ion species 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 eighth harmonic if the fast ion beta and the background plasma density are both high enough. 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. 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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