Advanced measurement concepts for heavy ion beam probes

The feasibility of measuring fluctuating magnetic fields in a tokamak using a heavy ion beam probe has been examined using the RENTOR tokamak and a 30 keV ion beam probe. A beam of particles passing through the plasma is deflected toroidally by the poloidal magnetic field generated by the plasma current. The toroidal velocity of the particle can then be related to the plasma flux function. The RENTOR experiment measures the displacement of the beam rather than its velocity change by recording the current collected by two toroidally displaced plates. Fluctuations in this signal can be compared with simultaneous B loop measurements. Rough estimates of the field variations are made and estimates of the beam probe's ability to detect magnetic fluctuations presented. The authors have also been investigating the algorithms used to process beam probe fluctuation data. An analysis technique that processes data in the time domain has been shown to produce better frequency resolution and smaller variances in the calculation of the phase between density and potential, and the phase between two density or two potential signals at different locations. Comparisons between this analysis technique and the standard frequency domain analysis techniques used by most plasma physicists are presented. Computer generated artificial data, and data from the heavy ion beam probe experiment on TEXT are used to illustrate the differences in the two techniques. The implications for the interpretation of the instabilities and transport estimates are discussed