A heavy ion beam probe for the Madison Symmetric Torus (MST)

Summary form only given. Initial design studies of a heavy ion beam probe (HIBP) for MST indicate that a system built around a 100-200 kV accelerator, such as the Rensselaer HIBP used previously on the Advanced Toroidal Facility (ATF) at ORNL, is appropriate for this application. There are presently three such systems available for application: the 160 kV ATF HIBP, the 500 kV TEXT HIBP, and the 2 MeV TEXT Upgrade HIBP. Design activities are now under way to assess the capabilities of the 2 MeV system on DIII-D and are planned to consider the 500 kV system for HSX. All three systems can utilize a variety of probing ion species typically, cesium (Cs/sup +/) or thallium (Tl/sup +/). The TEXT and ATF HIBP systems have proven to be very effective in obtaining measurements of electric potential profiles, /spl phi/(r), fluctuating electric potential /spl phi/(r,t) and fluctuating electron density n/sub c/(r,t). Based on TEXT and ATF results and the geometry of the ion trajectories we have found work best on MST, an MST HIBP should have a broadband potential sensitivity of about 2 V rms. The electric field calculated from a potential profile is estimated to be accurate to within a few volts per meter. Measurements of the space potential relative to the vacuum vessel wall are the most difficult type of potential measurements to make, since there is always the possibility of an offset due to a variety of experimental errors. However, absolute accuracy of the ATF potential profiles was generally 100 volts or less.