Magnetic fluctuations in the reversed field pinch
Arrays of edge magnetic coils and an insertable magnetic probe have been used to study the behaviour of the magnetic fluctuations in the HBTX1A Reversed Field Pinch. EDGE COILS: In the sustainment phase of the discharge poloidal arrays of edge coils show that the superficially random fluctuations can be attributed almost entirely to global modes of poloidal mode number m ≈ 0 and 1 provided account is taken of the toroidal distortion of these instabilities. A toroidal array of edge coils discloses a broad spectrum of toroidal mode numbers with a peak at |n| ≈ 10 and significant variation with time and frequency. Cross correlation between signals from poloidal and toroidal edge coil arrays establishes that the |n| ≈ 10 is m = 1, a set of helical modes resonant inside the reversal surface and also shows the presence of m = 0, |n| ≈ 0. Timescales of the measured fluctuations indicate that the instabilities are probably resistive in character and mode amplitudes are such that island overlap and magnetic field ergodization should occur. The energy confinement time due to stochastic transport, estimated directly from the measured fluctuations, is consistent with that experimentally observed. Studies of the edge magnetic fluctuations have been applied to discharges of differing conditions and in the termination and current set-up phases. Results show that, although systematic trends in the amplitude of the fluctuations occur, mode numbers and frequencies appear invarient with respect to changes in plasma current and filling pressure. At high values of [theta] an |n| ≈ 3 mode becomes of equal significance to the m = 1, |n| ≈ 10 modes. Estimates of the safety factor indicate that, although the observed timescale of this mode would label it resistive, it is not resonant. The structure of the global fluctuations in the current set-up phase appears very similar to that during sustainment, although the amplitude is higher. In the termination phase the fluctuations show several differences in the frequency and mode numbers. However, after reversal is lost, the observed frequencies correspond to resistive timescales rather than the Alfven timescale expected for ideal modes. INSERTABLE PROBE: A statistical method for determining the radial amplitude distributions of instabilities is presented. This is used to analyse probe data from which it is possible to distinguish three types of instability. At low frequencies (4 20 kHz) the dominant internal fluctuations are to be associated with the global m = 1, |n| ≈ 10 resistive modes seen by the edge coils. These modes possess a radial structure in agreement with that predicted by a linear tearing mode stability analysis of the measured equilibrium. At similar amplitudes to these modes there is also a short correlation component ([lambda]r = 3 cm) which is peaked in the central regions of the discharge. At high frequencies ( > 30 kHz) this local turbulence dominates over the global modes. Finally, at about the peak power of the dominant global modes and with a similar frequency dependence, an m = 1 mode with some ideal characteristics is observed. Stability calculations show that ideal modes that are either destabilised by a resistive shell or whose growth rates are reduced by a resistive liner would have the same radial structure and timescales as this mode.