Internal magnetic field measurements in a translating field-reversed configuration

Magnetic field probes have been employed to study the internal field structure of Field-Reversed Configurations (FRCs) translating past the probes in the FRX-C/T device. Internal closed flux surfaces can be studied in this manner with minimal perturbation because of the rapid transit of the plasma (translational velocity v/sub z/ approx. 10 cm/..mu..s). Data have been taken using a low-field (5 kG), 5-mtorr-D/sub 2/ gas-puff mode of operation in the FRC source coil which yields an initial plasma density of approx. 1 x 10/sup 15/ cm/sup -3/ and x/sub s/ approx. 0.04. FRCs translate from the approx. 25 cm radius source coil into a 20 cm radius metal translation vessel. Two translation conditions are studied: (1) translation into a 4 kG guide field (matched guide-field case), resulting in similar plasma parameters but with x/sub s/ approx. .45, and (2) translation into a 1 kG guide field (reduced guide-field case), resulting in expansion of the FRC to conditions of density approx. 3 x 10/sup 14/, external field B/sub 0/ approx. 2 kG and x/sub s/ approx. 0.7. The expected reversed B/sub z/ structure is observed in both cases. However, the field measurements indicate a possible sideways offset of the FRC from themore » machine axis in the matched case. There is also evidence of island structure in the reduced guide-field case. Fluctuating levels of B/sub theta/ are ovserved with amplitudes less than or equal to B/sub 0//3 in both cases. Field measurements on the FRC symmetry axis in the reduced guide-field case indicate ..beta.. on the separatrix of ..beta../sub s/ approx. = 0.3 (indexed to the external field) has been achieved. This decrease of ..beta../sub s/ with increased x/sub s/ is expected, and desirable for improved plasma confinement.« less