Magnetic Flux-Surface mapping using an electron beam diagnostic at the HSX Stellarator

In stellarators, small uncertainties of the alignment of the magnetic field coils can cause a significant degradation of confinement properties due to the formation of stochastic regions or magnetic islands. Field-line mapping is therefore used to check for possible deviations from the expected magnetic field structure. At the Helically Symmetric eXperiment (HSX), initial field-line mapping has been performed during the commission of the device [1], but no further checks have been performed for more than 20 years. To revisit this important topic, a novel electron gun, i.e., a biased and heated filament, has been designed, built, and commissioned on a table-top toroidal plasma device. After successful tests, the electron gun will be installed at HSX together with a movable phosphor probe that can be swept vertically to detect the produced electron beam. First experimental results obtained with the new field-line mapping system will be presented and compared with predictions from a field-line following code.

[1] J. Talmadge, V. Sakaguchi, F. Anderson, D. Anderson, A. F. Almagri, (2001). Physics of Plasmas. 8. 5165- 5170