Motional Stark effect measurements of the local ICRH induced diamagnetism in JET plasmas

Measurements of the toroidal magnetic field inside the JET tokamak plasma by means of the motional Stark effect are reported. In the vicinity of the resonance layer ion-cyclotron-resonance-heating (ICRH) generates a population of energetic ions. For the first time the diamagnetic change in the toroidal magnetic field due to these ICRH fast ions has been measured. A magnetic field decrease up to 4% has been observed. Details of the measurement of the motional Stark effect, the magnitude of which is given by the Lorentz electric field experienced by the neutral beam are described. The deduction of the toroidal magnetic field from the Stark wavelength splitting is discussed, in particular the involved accuracy which is necessary to resolve toroidal field effects of the order of 0.5%. The evolution of the thermal plasma pressure - measured independently -combined with the PION-T code simulation of the fast ion pressure agree within an order of magnitude with the measured toroidal field changes and indicate that the contribution of the anisotropic pressure from fast particles of the total pressure increases towards the plasma centre. First applications are presented, including β-increase due to ICRH and the slowing down time of the ICRH fast ions. (author) 4 refs., 5 figs