New digital waveform reference for ECRH on FTU

Abstract

The Electron Cyclotron Radio Frequency Heating (ECRH) system is mainly used to control the temperature profile of the plasma in a tokamak through sharp focusing of the RF power for bulk heating. Some experiments on tokamak devices have shown that Electron Cyclotron Current Drive (ECCD) can reduce the growth of rotating magnetic islands, thus controlling magnetohydrodynamic (MHD) resistive instabilities in steady-state operation. On Frascati Tokamak Upgrade (FTU), 1.6 MW 140 GHz Electron Cyclotron (EC) waves are injected using four gyrotrons, each emitting 0.4 MW for 0.5 s. A crucial point of the whole system is the fast control of the RF power output through a Digital Waveform Reference (DWR). This paper describes the new implementation of the DWR on FTU with a PXI-embedded controller and two PXI DAC boards by National Instruments. DWR control code integrates synthesized waveforms and graphic display, validity test on reference outputs, fully hardware-driven analog output, network process communication with ECRH plant PLC and the FTU Supervisor Control System. The validity test on the working parameters takes into account hardware constraints, system configuration and tetrode dissipation. The off-line calculations select the reference voltage using a wide choice of pulse shapes, while the real-time analog output is fully hardware-driven by the FTU central timing and trigger system. Because of the FIFO and DMA transfer data of the PXI DAC board, the 1 MHz update rate of the reference output has been successfully tested, largely matching the system requirements (up to 500 kHz). The high reliability shown by the present DWR gives us the chance to implement a 50 kHz closed-loop feedback system to control the growth of the plasma instabilities by EC waves.