TFTR prototype electrostatic-calorimeter probe head

Power loading of the limiters in TFTR is a major concern because of problems of impurity generation and thermal stresses. To diagnose this edge plasma we have prepared a prototype electrostatic-calorimeter probe and tested it on the PDX and PLT tokamaks. The probe provides time-resolved measurements of heat flux Q and Langmuir (I-V) characteristics on each of its opposing active elements. The present design, employing carbon shields and tantalum elements, is able to withstand =5 kW/cm² for = 10 ms or 1 kW/cm² for 500 ms and provides 5–10 ms time resolution of heat flux. This allows simultaneous estimation of time-resolved radial profiles of n_e T_e and T_i. By fitting the probe with narrow slit apertures and rotating the probe head, the device provides discrimination between particles having different values of Larmor radius and/or ratios of perpendicular to parallel velocities. Calculations required to interpret the results of probe head rotation are also discussed. This information is of interest in the study of fast ion losses during neutral beam, ICRF, and LH auxiliary heating.