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

In previous experiments on high temperature (> 50 eV), high density (> 10(2 0) m(-3)) plasmas such as the field-reversed configuration (FRC), it has no t been possible to obtain direct information of the internal field structur e in a nondestructive way. The probe surface would vaporize due to high ele ctron thermal transport as well as ablate due to high energy ion bombardmen t. To minimize these processes, the smallest possible probes made from mate rials with the longest thermal time to melting were constructed and tested. In order to measure fast magnetic field changes (similar to several MHz), as well as not influence the FRC internal electric fields, the probe wall m aterial was constructed from a nonconducting material. Of several insulatin g materials tested, beryllia was the only material that was found to be sui table. The probe wall consisted of a 0.3-m-long 2-mm-diam beryllia tube bor ed out to 1.5 mm. Inside the small bore, a "chain" probe of 24 loops was co nstructed out of 50-mum-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 inse rted, no changes in FRC confinement or behavior were observed over the enti re 1 ms lifetime of the discharge. (C) 2001 American Institute of Physics.