A NOVEL SEEDED PELLET TECHNIQUE TO MEASURE THE CURRENT DENSITY PROFILE NEAR AND WITHIN THE Q=1 SURFACE IN TOKAMAKS

THE PERFORMANCE CHARACTERISTICS OF MODERN TOKAMAK DISCHARGES CAN BE DOMINATED BY THE SAWTOOTH MODE, WHOSE BEHAVIOR IS INTIMATELY TIED TO THE DETAILED CURRENT DENSITY PROFILE IN THE VICINITY OF THE Q=1 FLUX SURFACE. THE FURTHER STUDY AND UNRAVELING OF THE DETAILED PLASMA PHYSICS INVOLVED IN THIS INSTABILITY URGENTLY REQUIRE NEW TECHNIQUES THAT GIVE A RELIABLE, DIRECT MEASURE OF THE CURRENT DENSITY PROFILE AND THE RESULTING SAFETY FACTOR PROFILE WITH HIGH SPATIAL RESOLUTION IN THE REGION NEAR AND WITHIN THE CRITICAL Q=1 FLUX SURFACE. A PROMISING TECHNIQUE HAS RECENTLY BEEN EXPLORED AT THE TOKAMAK FUSION TEST REACTOR, IN WHICH CIGAR-SHAPED IMAGES OF THE ABLATION CLOUDS OF IONIZED STATES OF IMPURITY PELLETS INJECTED INTO THE PLASMA ARE OBSERVED. BECAUSE THESE IONS ARE CLOSELY TIED TO THE MAGNETIC FIELD LINES, THE TILT ANGLE OF THIS ABLATION TRAIL DIRECTLY REVEALS THE LOCAL PITCH OF THE TOTAL MAGNETIC FIELD, AND BY INFERENCE, THE LOCAL Q VALUES AT POINTS ALONG THE PELLET'S TRAJECTORY. A NOVEL EXTENSION OF THIS MEASUREMENT TECHNIQUE THAT WILL GREATLY ENHANCE ITS ACCURACY AND SPATIAL RESOLUTION IS PLANNED, THEREBY YIELDING MORE PRECISE DATA ON THE SAFETY FACTOR AND CURRENT DENSITY PROFILES WITHIN AND NEAR THE Q=1 SURFACE. BY FOLLOWING THE TRAIL OF IONIZED ABLATANT FROM INJECTED IMPURITY PELLETS AS IT SPREADS FROM THE PELLET ALONG FIELD LINES FROM THE INJECTOR PORT TO ANOTHER PORT SOME DISTANCE AROUND THE TORUS, THE ADVANTAGES OF A LONG LEVER ARM ARE SEEN TO ALLOW THE ROTATIONAL TRANSFORM TO BE MEASURED WITH UNPRECEDENTED ACCURACY. THIS PELLET WOULD CONSIST OF A LOW Z OUTER LAYER ENCAPSULATING A HIGHER Z INNER CORE, OR "SEED". THE HIGHER Z SEED MATERIAL WOULD PROVIDE THE ABLATANT CONTRAIL TO BE MEASURED. THE FEASIBILITY OF THIS CONCEPT WILL BE DETERMINED DURING PHASE I.