Magnetic flux surface mapping in the Proto-Cleo Stellarator

The magnetic flux surface topology of the University of Wisconsin Proto-Cleo Stellarator has been reexamined experimentally in order to verify the presumed magnetic structure in the device. The experiment utilized an electron beam injected parallel to the 1.2 kG magnetic field. The beam was detected by a probe which could be scanned through the containment volume. The experimentally measured passes of the electron beam are subsequently compared with numerical data obtained from the drift surfaces traced out by a simulated beam. This is basically the same technique as used for a similar experiment performed on the Interchangeable Module Stellarator (IMS), a few months earlier. The authors have mapped four flux surfaces and obtain relatively good agreement between the experiment and the theoretical prediction. It is important to note that the magnetic structure depends sensitively on the current ratio in the toroidal and helical coils. Actual measurement of this current ratio showed that the existing ratio equals 1.48 instead of the previous ratio of 1.33. This latter ratio leads to a theoretical rotational transform at the edge of 0.6, while the authors found a lower iota value of about 0.5, being consistent with the larger current ratio