Loop-voltage Tomography in Tokamaks Using Transient Synchrotron Radiation

The loop voltage in Tokamaks is particularly difficult to measure anywhere but at the plasma periphery. A brief, deliberate, perturbation of hot plasma electrons, however, produces a transient radiation response that is sensitive to this voltage. We investigate how such a radiation response can be used to diagnose the loop voltage. 1. I N T R O D U C T I O N AN EXAMIKATION of the transient, synchrotron radiation signal which arises from a deliberate, perturbation of hot Tokamak electrons. can be quite informative. The perturbation might be produced, for example, through brief heating of superthermal electrons by lower-hybrid waves. The plasma radiation response to this perturbation, in frequency-time space, forms a two-dimensional pattern that looks different under different plasma conditions. An example of this radiation pattern, R(w, t ) , is given in Fig. 1. The parameters to which this radiation is sensitive include the dc electric field E, the ion charge state ZeK, the angle of viewing with respect to the magnetic field 0, the density y1, and the precise velocity of the perturbed electrons. These parameters comprise a set of conditions under which the radiation response is observed. Through a comparison of the radiation patterns that would be produced with any parameter sets that might possibly explain the transient signal, the relative probabilities of the competing parameter sets can be evaluated. The deliberate heating or probing of the plasma to produce synchrotron radiation directly attributable to this probe has been the subject of previous work. A mathematical inversion of the two-dimensional transient synchrotron data to obtain a twodimensional electron momentum distribution function was described by FISCH (1 988), assuming as given, however, parameters describing the plasma. Of greater diagnostic interest is that the deliberately produced transient emissions cculd be employed to deduce various plasma parameters, as shown by FISCH and KRITZ (1989a). In these previous works, the dc electric field was assumed absent. A generalization of this work to include a dc parallel electric field (FISCH and KRITZ, 1989b) both expands the range of experiments for which the radiation can be used to deduce other parameters and allows us to deduce the electric field itself, something entirely unavailable otherwise. Typically less than 1 V min a Tokamak, this field is far too small to be inferred through atomic phenomena, and cannot be measured