Arrays of edge magnetic coils and statistical analysis techniques have been used to investigate the magnetic fluctuation structure in the HBTX-1A reversed field pinch. The superficially random fluctuations can in fact be attributed almost entirely to global modes with poloidal mode number m = 0 and 1, provided account is taken of toroidal distortion of the modes. A toroidal array of coils discloses a broad spectrum of toroidal mode numbers with peak at Inl ~ 10 and significant variation with time and frequency. Cross-correlation establishes that | n | ~ 10 corresponds to m = 1, a helical mode resonant inside the reversal surface, and also shows the presence of m = 0, n ~ 0. The time-scales of the fluctuation indicate that the instabilities are probably resistive in character, and the mode amplitudes are such that island overlap and magnetic field ergodization should occur. The energy confinement time due to stochastic transport, estimated from the measured fluctuations, is consistent with that observed experimentally.

An insertable magnetic probe has been used to investigate the internal structure of the magnetic fluctuations in the HBTXlA 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 2 0 kHz) the dominant internal fluctuations are to be associated with a band of global IM = 1, In1 -8 resistive modes resonant inside the reversal surface. Although non-linear 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 (A, = 3 cm) which is peaked in the central regions of the discharge. At high frequencies ( 1 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.

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