Zonal flow measurements using a heavy ion beam probe

Zonal flows are of keen interest in efforts to understand transport in magnetically confined plasma. Of the well-developed diagnostics, the heavy ion beam probe, HIBP, is most suited to measure the radial electric field, $E_r,$ associated with the flows in present medium to large devices. This paper discusses why the HIBP is capable of measuring $E_r$ and how to design a HIBP to optimize zonal flow measurements. The TEXT HIBP is used as an example of a typical system. The NSTX spherical torus is used in a design study for future work with emphasis on zonal flow measurements. The key diagnostic considerations are (1) sample volume size, (2) sample volume orientation, and (3) the ability to rapidly scan the sample volume in the radial direction. The measurement of principal interest here is $E_r$ but there is additional information in a nonlinear analysis of the fluctuations.