Q dependence of magnetic turbulence in a tokamak

Low-frequency (ω≪ω_ci) radial magnetic field turbulence has been measured over the full minor radius for discharges in the Tokapole II tokamak [Nucl. Fusion 9, 1509 (1979)], and scaled over the range of edge safety factor 0.6≤q_a ≤ 5.0. It was found that q _a served as a control parameter for both the magnetic fluctuation amplitude and the global confinement time. As q_a is reduced from 5 to 0.6 the turbulence level increases by a factor of 50 while the confinement time decreases by more than a factor of 10. At q_a ≤ 1, the full fluctuation amplitude is roughly large enough to account for the global confinement using simple estimates of collisionless stochastic magnetic transport. At high q_a, the turbulence is too small to account for transport using these estimates. Frequency spectra have been obtained from 10 to 400 kHz. For all q_a most of the fluctuation power appears below 100 kHz. This low-frequency structure changes as the safety factor is varied. Although broadband in frequency, the long radial coherence lengths at these frequencies indicate global modes. For f≥ 100 kHz, the spectra become featureless, with a power-law frequency decay. Radial, poloidal, and parallel correlation analyses have been done as well. The parallel correlation length is estimated to be 150 ± 10 cm. Radial and poloidal coherence lengths are of the order of the machine dimensions for frequencies below 100 kHz for all q_a. For f≥ 100 kHz, radial coherence lengths are 0.5-2.5 cm. Poloidal mode number spectral information is obtained from two-point phase shift measurements, radial decay of fluctuations beyond the separatrix, and poloidal coherence measurements. The m spectrum broadens as q_a decreases. This might explain the increase in fluctuation amplitudes as q_a decreases. It is suggested that the turbulence for all discharges is due to the same underlying cause or set of causes, and that the differences result from geometric effects associated with the variation in q_a.