Small toroidal field experiments in RECE-Christa

Earlier experiments in the RECE-Christa device gave first indications of the expected MHD stability, in particular tilt stability, of ''mixed-CT'' configurations in which ring currents are carried by large orbit fast electrons as well as the cold plasma. Such rings were produced by inducing plasma currents in field-reversing electron rings using an on-axis transformer coil. Both currents are in the range of 5-10 kA. In order to investigate the transition to true ''mixed-CT'' configurations without axial current, the ring experiments are presently being extended to smaller values of the toroidal field Btheta. Two parallel approaches are used: The toroidal field is crowbarred at peak, and during the slow ( about5 msec) crowbar decay, a 2.5 MeV, 40 kA electron beam is injected and a ring is trapped. Using a steady gas fill, stable electron rings were obtained. Ring trapping in a puffed gas cloud proved more difficult; however, at Btheta/Bz = 0.5, rings were obtained which decayed smoothly over 250 μsec, consistent with collisional decay. The toroidal field is crowbarred late in time after it has started to swing back towards zero. Rings are trapped while the toroidal field is peaking and stably translated about 15 ring radii into a region of low density gas, as in normal operation. Then the toroidal field is allowed to swing down to 20-90% of its initial value before being crowbarred. While the toroidal field is decreasing, an on-axis transformer coil is pulsed in order to induce plasma currents in the ring. In this case, toroidal flux in the ring plasma can be maintained during the transition to low external toroidal fields. Preliminary observations do not find the losses earlier observed by Meger