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.
An extensive series of experiments in the Extrap mode has been carried out on Extrap T1-U in an attempt to obtain hot, collisionless discharges. Using no external axial magnetic field, performance has been limited to low temperatures ( 15 eV) and low currents ( 20 kA) at high loop voltages 1.4 kV, leading to energy confinement times of only a few µs or Alfvén times. The discharge length is typically of the order of greater than 100 Alfvén times.Imposing a weak axial field (q(a) 0.15 at peak current) the temperature could be increased to 40 eV at toroidal currents 60 kA. Energy confinement times up to 15 Alfvén times (≈10 µs) were then obtained.The poor confinement is attributed to the weak axial magnetic field, generating strong instability-induced fluctuations and related radial transport in the core plasma. A turbulence core plasma transport model is suggested, based on theoretical evidence of linear instability in the kinetic regime.Self-generation of axial magnetic field is observed when the magnetic X-points, generated by the plasma current and an external octupole field, approach or pass beyond the wall.Edge transport appears to be dominated by line radiation and heat conduction to the wall along open field lines.
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