Analysis of internal magnetic fluctuations in the HBTX1A reversed field pinch

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Abstract
An insertable magnetic probe has been used to investigate the internal structure of the magnetic fluctuations in the HBTX1A reversed field pinch. A statistical method for determining the radial amplitude distributions of instabilities is discussed in some detail. This is used to analyse the experimental data from which it is possible to distinguish three types of instability. At low frequencies (4-20 kHz) the dominant internal fluctuations are found to be associated with a band of global resistive modes resonant inside the reversal surface. Although nonlinear processes are taking place, 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 a short correlation length component ( Lambda r=3 cm) which is peaked in the central regions of the discharge. At high frequencies (>30 kHz) this local activity dominates over the global fluctuations. Finally, at about 1/4 the peak power of the dominant global instabilities and with a similar frequency dependence, a second m=1 mode is observed. Stability calculations show that ideal modes whose growth rates are controlled by a resistive wall would have timescales similar to this mode.