Extrap mode confinement in Extrap T1-U

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 (less than or equal to 15 eV) and low currents (less than or equal to 20 kA) at high loop voltages less than or equal to 1.4 kV, leading to energy confinement times of only a few mu s or Alfven times. The discharge length is typically of the order of greater than 100 Alfven times. Imposing a weak axial field (q(a) less than or equal to 0.15 at peak current) the temperature could be increased to less than or equal to 40 eV at toroidal currents less than or equal to 60 kA. Energy confinement times up to 15 Alfven times (approximate to 10 mu s) 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.