A PASSIVE OPTICAL MONITOR FOR TOKAMAK PLASMA INHOMOGENEITIES

A NOVEL, INEXPENSIVE, AND PASSIVE OPTICAL DIAGNOSTIC FOR MEASUREMENT OF THE SCALE SIZE AND FLOW VELOCITY OF TOKAMAK PLASMA INJOMOGENEITIES HAS BEEN DEVELOPED. THE DIAGNOSTIC IS BASED ON AN EQUAL-PATH, LATERAL SHEARING INTERFEROMETER AND CAN BE ANLAYZED, IN PRINCIPLE, AS A FORM OF DOPPLER VELOCIMETER. THIS SYSTEM REQUIRES ONLY INCOHERENT LIGHT EMISSION FROM THE PLASMA ITSELF FOR OPERATION. THE DIAGNOSTIC CAN BE ADJUSTED TO RESPOND PRIMARILY TO INHOMOGENEITIES BELOW A GIVEN SCALE SIZE, MOVING ALONG A GIVEN AXIS, WITHIN THE LIMITED FOCAL VOLUME OF THE COLLECTION OPTICS. THE INFORMATION OBTAINED IS THUS LOCALIZED IN THREE DIMENSIONS AND IS NOT A LINE-OF-SIGHT AVERAGE. THE RESULTING SIGNAL IS DIGITIZED AT MHZ FREQUENCIES FOR THE DURATION OF A TOKAMAK RUN AND ANALYZED LATER WITH A FAST FOURIER TRANSFORM. THE PEAK FREQUENCY OF THE SIGNAL IN A DATA SEGMENT, COMBINED WITH THE SPATIAL SCALE SIZE SETTING AND THE ORIENTATION OF THE SENSING AXIS, DETERMINES THE FLOW VELOCITY OF THE PLASMA INHOMOGENEITIES IN THAT TIME INTERVAL. MULTIPLE SENSING HEADS CAN BE COMBINED TO PROVIDE INFORMATION ON THE SPATIAL SCALE SIZE SPECTRUM. SCALE SIZES FROM A FRACTION OF A MILLIMETER UP TO ONE CM CAN BE RESOLVED ALONG WITH FREQUENCIES OF UP TO SEVERAL MHZ. IN PHASE I THE RESOLUTION AND SIGNAL TO NOISE EXPECTED IN BOTH THE CORE AND EDGES OF THE ALCATOR C-MOD TOKAMAK PLASMA WILL BE CALCULATED. A NEW ACHROMATIC OPTICAL INSTRUMENT USING RONCHI GRATINGS INSTEAD OF A SHEARING INTERFEROMETER TO OBSERVE TWO SCALE SIZES SIMULTANEOUSLY WILL BE FABRICATED AND USED ON THE C-MOD PLASMA. FINALLY, A PRELIMINARY DESIGN WILL BE COMPLETED FOR A MORE ADVANCED INSTRUMENT TO BE FABRICATED AND USED IN PHASE II.