The structure of magnetic fluctuations in tokamaks: Observations in the TEXT tokamak

Abstract

The structure of the magnetic fluctuations in a tokamak has been determined from extensive measurements using a variety of probes outside the limiter in TEXT [Plasma Phys. Controlled Nucl. Fusion 27, 1335 (1985)]. The spectrum has been measured to 500 kHz, but little energy is present above 150 kHz. The spectrum ranges from low-frequency, low-m-number modes with high coherence to higher m values at higher frequencies, which have limited poloidal and temporal coherence but are specifically correlated with electrostatic fluctuations in the edge. Although these magnetic fluctuations are not directly significant for transport, they are a useful indication of edge turbulence. They are associated with turbulence only inside the limiter. The correlation length along field lines is long, and the phase variation of the correlated components suggests k∥/k⊥ ≅ 0.005. These magnetic signals are consistent with a modest modulation of the plasma resistivity in the edge as in resistivity-gradient-driven modes, but the magnetic fluctuation signal is not proportional to the applied E∥. Their response to many other perturbations is reported. © 1991 American Institute of Physics.