Small, high frequency probe for internal magnetic field measurements in high temperature plasmas

Abstract

In previous experiments on high temperature (>50 eV), high density (> 1020 m-3) plasmas such as the field-reversed configuration (FRC), it has not been possible to obtain direct information of the internal field structure in a nondestructive way. The probe surface would vaporize due to high electron thermal transport as well as ablate due to high energy ion bombardment. To minimize these processes, the smallest possible probes made from materials with the longest thermal time to melting were constructed and tested. In order to measure fast magnetic field changes (∼several MHz), as well as not influence the FRC internal electric fields, the probe wall material was constructed from a nonconducting material. Of several insulating materials tested, beryllia was the only material that was found to be suitable. The probe wall consisted of a 0.3-m-long 2-mm-diam beryllia tube bored out to 1.5 mm. Inside the small bore, a "chain" probe of 24 loops was constructed out of 50-μm-diam magnet wire. The two axis probe measured axial and azimuthal FRC magnetic fields as small as a few gauss with centimeter resolution and a frequency response of 1 MHz or better. With the probe inserted, no changes in FRC confinement or behavior were observed over the entire 1 ms lifetime of the discharge. © 2001 American Institute of Physics.