MOTIONAL STARK-EFFECT MEASUREMENTS OF THE LOCAL ICRH INDUCED DIAMAGNETISM IN JET PLASMAS

Measurements of the toroidal magnetic field inside the JET tokamak pla sma by means of the motional Stark effect are reported. In the vicinit y of the resonance layer ion cyclotron resonance heating (ICRH) genera tes a population of energetic ions. For the first time, the diamagneti c change in the toroidal magnetic field due to these ICRH fast ions in combination with a pressure increase of the thermal population has be en measured. A magnetic field decrease of up to 4% has been observed. Oscillations of the toroidal magnetic field caused by sawtooth oscilla tions of the thermal plasma pressure have been resolved. First results of the local beta increase due to ICRH and the slowing-down time of t he fast particles are presented.