Confinement studies in Culham stellarators with Ohmic heating currents

In Cleo apparatus, tokamak and Ohmically-heated stellarator plasmas have been directly compared for gas currents of 10-25kA and Bsub(phi)=12.7kG. The vacuum poloidal field due to the helical winding has a considerable influence on the containment properties. In particular, for the same current the electron energy content is twice as great in the stellarator as in the tokamak. In the Torso appratus the confinement has been studied as a function of drift parameter xi identical vsub(d)/vsub(e). For xi<=0.02, when the plasma is collisonal, the confinement scales in accordance with the predictions of classical theory for the Pfirsich-Schlueter regime. Electrostatic density fluctuations, apparently current-driven modes, increase in amplitude with xi and for xi>0.02 the confinement becomes increasingly determined by turbulent diffusion due to these modes. (author)