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Abstract
Density fluctuations in the PDX tokamak have been analyzed with the scattering of 2 mm microwaves. The primary focus of the study has been the low frequency (< 1MHz), broad band spectrum fluctuations which are ubiquitous in tokamaks and have the characteristics of drift wave turbulence. The fluctuations typically have frequencies and wavelengths characterized by (DELTA)(omega) (TURN) (omega) (TURN) (omega)(,*(,e)) and K(,(PERP))(rho)(,i) of order 1, where (omega)(,*(,e)) is the electron diamagnetic drift frequency and (rho)(,i) is the ion Larmor radius. In PDX, the mean observed frequencies are about 50-100 kHz and the mean wavenumbers are k(,(PERP)) (TURN) 3 cm('-1). It is believed that these waves are a possible cause of the anomalous heat transport in tokamaks. Density fluctuations with a narrow frequency spectrum have also been observed. The latter include the quasi-coherent fluctuation observed in H-mode plasmas.
Several scaling studies of the broad band turbulence have been carried out. The frequency spectra, k(,(PERP)) spectra, and magnitude of the fluctuations have been measured and their variation with toroidal field, neutral beam heating power, plasma current, position, and confinement regime in PDX has been documented. Since the turbulence is a possible cause of anomalous heat transport in PDX and other tokamaks, the scaling of the turbulence has been compared with energy confinement scalings. In many cases, increased fluctuation level correlates with decreased energy confinement. In another case, the fluctuation level does not change as the confinement properties of the plasma change. The scaling of the experimental fluctuation levels has also been compared with various drift wave parameters. The fluctuation level scales as the mixing length limit, n/n (TURN) 1/k(,(PERP))L(,n), in some sets of data, but not in other sets.
In addition to the broad band frequency spectrum present on all shots, density fluctuations with a narrow frequency spectrum ((DELTA)(omega)/(omega) (TURN) 0.1) are observed in H-mode plasmas. These fluctuations typically have frequencies of 50-150 kHz. Signals are largest from scattering volumes with k(,(PERP)) in the range of 0.5 to 3.0 cm('-1).
