EXPERIMENTAL-STUDY OF ULTRA-LOW Q DISCHARGES IN THE LINEAR EXTRAP L1-DEVICE

Linear pinch discharges with combined octupole and longitudinal magnetic fields are experimentally studied in the Extrap L1 device. Plasma currents are around I(p) = 10 kA, plasma temperatures are up to T(e) = 50 eV and plasma densities are of the order of n = 5 x 10(21) m-3. The plasma equilibria are in the ultra-low q (ULQ) regime corresponding to operation with plasma currents in excess of the Kruskal-Shafranov stability limit (q < 1). The plasma current exhibits the typical time behavior seen in toroidal ULQ experiments; the unstable setting up phase and the step-wise decay with current levels corresponding to q-values in windows between rational values. Longitudinal plasma current generated by radial plasma diffusion is seen, with amplitudes up to 30% of the externally driven current during the initial phase of the discharge. The effect of the octupole magnetic field on the ULQ confinement is investigated. The plasma temperature increases by more than a factor of two, for the optimum octupole rod current (I(v) = I(p)), compared to the case without octupole field. A plasma current limitation for stable operation corresponding to q > 1/2 is observed, except for low axial magnetic field strength. In the low axial field regime, the octupole field alone provides sufficient stabilization for operation with q < 1/2. Plasma density and temperature both increase linearly with applied axial magnetic field. The density shows a strong, approximately exponential, dependence on discharge voltage.