Real-time DSP-based shape determination and plasma position control in the ISTTOK tokamak

We have developed a digital signal processor-based system for real-time control of the ISTTOK plasma position based on a low-cost digital signal processor (DSP) board. A novel plasma shape reproduction method was developed using a Cauchy-condition expansion applied to the vacuum region around the plasma. This method showed to be best suited than classic methods such as Legendre–Fourier expansion (LFE) or current filaments (CF), particularly in small tokamaks with passive stabilizer conductors where the presence of strong eddy currents can cause significant errors in magnetic field sensors. We compare the results of the boundary reconstruction method with the measurements from a microwave interferometer diagnostic. We present an implementation in the DSP system that allows the real time control of the plasma position with a required 1 ms period.