The effect of aspect ratio on magnetic field fluctuations in reversed-field pinches is investigated using a three-dimensional magnetohydrodynamic code. Configurations with aspect ratios of 1.1, 2.2, and 4.4 are modeled. The results are extrapolated to aspect ratio 8.8 for comparison with the Extrap T1 experiment [Nucl. Fusion 34, 427 (1994)]. It is found that the average modal amplitudes decrease with aspect ratio. However, the spectrum broadens correspondingly, resulting in negligible effect on the magnetic fluctuation level. The computed spectrum dynamics are found to be in good agreement with experimental observations on the T1 experiment. Quantitative evaluations of the field line stochasticity indicate no dependence of the mean magnetic field diffusion rate on aspect ratio.

New insight in the nonlinear and quasilinear dynamics of kink-tearing modes in reversed-field pinches (RFP’s) is obtained by simultaneous measurements of the evolution of current profiles and associated m=1 helical mode spectra in the Extrap T1 RFP [Nucl. Fusion 34, 427 (1994)]. Results concerning near single helicity states, m=0 mediated m=1 spectral broadening and cascade, together with the associated relaxation dynamics, are reported. The role of phase locking, between the nonlinearly interacting modes, in the pinch dynamics is also discussed.

Experiments have been performed on the high aspect ratio, high current density Extrap-Tl reversed field pinch to study the response in confinement properties to variations in plasma current and pinch parameter. The study includes measurements of energy and particle confinement times as well as radiated power, impurity concentrations and magnetic fluctuations. The ratio of Spitzer to total input power in these experiments is varied over the wide range 0.4 to 0.8. The operational fie value is found to be primarily a function of plasma current and pinch parameter and scales only weakly with density since an increase in density is found to be accompanied by a decrease in temperature. Changes in TJne, however, directly affect the dynamo activity. At high TJn,, the scaling of the energy confinement time is governed by the enhancement of the dynamo activity, which offsets the decrease in Spitzer input power with temperature. On the other hand, the particle confinement time is not deteriorated by enhanced dynamo activity. Instead, particle confinement is degraded at a high fraction of Spitzer input power. In this limit, convection of thermal particles can account for a major part of the total energy loss.

Edge electrostatic fluctuations, in the Extrap T1 reversed-field pinch [Nucl. Fusion 34, 427 (1994)], are observed to be correlated to internal tearing mode activity. Bispectral analysis of the edge electrostatic fluctuations shows the occurrence of nonlinear coupling between the low frequency internal tearing-mode-related activity and the high frequency, external, electrostatic fluctuations. In addition, the fluctuation levels of both the edge electrostatic fluctuations and the internal tearing modes have comparable scaling with plasma current. These results suggest that suppression of the internal tearing mode activity may decrease the edge electrostatic fluctuations and the related particle loss in the reversed-field pinch configuration.

Investigations of the temperatures of the in situ impurity ions C 4+ , C 2+ , O 4+ , O 3+ and O 2+ have been performed at the Extrap-T1 reversed-field pinch (RFP) via measurements of the Doppler broadening of emission lines in the near-UV wavelength region. Ion temperatures of the high ionization stages are found to be of the order of the electron temperature.This means that ion heating can be connected with RFP dynamo activity giving rise to the non-Spitzer part of the power input. Power input to the ions of the order of the non-Spitzer part of the total power input is required to explain the high ion temperatures. MHD effects could be responsible for this heating. We present scaling of the observed C 4+ ion temperatures with plasma current I φ , electron density n e , pinch parameter and magnetic fluctuations. We also present an empirical scaling law in which the ion temperature can be expressed as T i ∝ I φ × (I φ /n e ) × 4 . Keeping constant, this gives a constant ion beta poloidal β θ,i = 2µ 0 p /B θ (a) 2 .