First physics results from the Wisconsin HTS Axisymmetric Mirror (WHAM)

WHAM aims to confine and stabilize an axisymmetric mirror plasma using 17 T, 55 mm diameter bore HTS mirror coils and with its recent first plasmas set a new world record

for steady field applied to a magnetic confinement fusion experiment. The WHAM device is now operating with an array of high power heating and control systems, including a 500 kW, 110 GHz gyrotron, a 1 MW, 25 kV neutral beam injection system, and multi-megawatt radially controlled plasma biasing actuator for stirring the plasma to provide vortex stabilization of MHD interchange, as well as a substantial base-line diagnostic set that will be described. One major accomplishment of the first campaign was to (1) the commissioning of the CFS supplied HTS magnets to full field operation, and (2) to demonstrate robust and reliable operation of the heating systems and diagnostic set in the presence of the high magnetic field. In the first experimental campaign the vacuum field was entirely due to the two HTS magnets corresponding to a mirror ratio of 70. The application of up to 400 kW of ECH, limited to 10 ms pulse lengths, demonstrated robust target plasma formation achieving average line densities ~ 3 and in separate plasmas a midplane averaged beta of > 5%. These ECH plasmas show clear evidence of both a cold high density plasma and a low density, long-lived hot electron plasma (with hard x-rays of energy > 100 keV observed) that appear to modify MHD stability (both positively and negatively). Preliminary data analysis indicates high m-number flute modes with real frequencies in the 40-60 kHz range suggestive of hot electron interchange in low ion temperature plasmas. Biasing of the tungsten limiter modifies MHD activity and by optimizing the bias has led to an increased plasma stored energy by 50% (and can also be used to drive MHD and make confinement worse). Neutral beam injection has successfully been used to fuel the plasma by injecting into ECH target plasmas, but at present the lack of wall conditioning

in this first campaign indicates that charge exchange of the fast ions on residual and recycled neutral particles is limiting the build-up of fast ion pressure and controlling the overall confinement.