Comparison of drift wave models with fluctuation data from the interior of the TEXT tokamak

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Abstract
Experimental evidence for and against drift waves as the origin of the observed fluctuations and anomalous transport in the plasma interior is reviewed. Fluctuation spectra observed by far‐infrared (FIR) scattering and a heavy ion beam probe (HIBP) are compared. The FIR system observes broad S(k,ω), which are spatially resolved at large k and readily identified with electron drift waves. At higher densities a clear ion feature, which may be associated with ion temperature gradient (ITG) modes, appears as well. A quasicoherent feature in the drift wave range of phase velocities is also found at the inner midplane. But interior HIBP measurements using a thallium beam have, by a two‐point correlation method, measured wave numbers far too small (i.e., phase velocities far too high) to be those of drift waves of either variety. Some recent measurements with a cesium beam have produced phase velocities more closely in accord with drift waves at low frequencies. It is suggested that a new mode may be present, which is not drift wavelike. However, the phase velocity and coherence of the HIBP data exhibit qualitative features that suggest possible instrumental effects. Several of these are investigated, which individually do not appear to reconcile the data with a pure drift wave model. They are (1) sample volume size, (2) common mode effects such as cross‐talk or path integrals of beam attenuation, and (3) two‐stream instabilities. Reconciliation of these measurements is important to transport studies. That is, calculated particle and energy fluxes depend sensitively on the frequency and wave‐number spectrum employed. The theoretical considerations about two‐point correlations and possible instrumental effects are also relevant to other diagnostics using this technique.