Fast magnetic fluctuation diagnostics for Alfvén eigenmode and magnetohydrodynamics studies at the Joint European Torus

Alfven eigenmodes and other magnetohydrodynamic phenomena have been studied in tokamak plasmas at the Joint European Torus (JET) using a new eight-channel, 4 s, 1 MHz, 12-bit data acquisition system (KC1F) in conjunction with the JET fast Mirnov magnetic fluctuation pickup coils. To this end, the JET magnetic pickup coils were calibrated in the range 30–460 kHz using a new remote calibration technique which accounts for the presence of the first few LRC circuit resonances. Signal processing software has been developed to implement the calibration via digital filtering. A data analysis program has been written which produces spectrograms of fluctuation amplitude and toroidal mode number versus frequency and time, both interactively and for automatic overnight analyses. Modes with amplitudes δB/B⩾10−8 and toroidal mode numbers |n|<32 are now routinely detected. Since KC1F data are now available for over 4000 JET discharges, a pulse-characterization database has been developed to help select pulses of intere...