Real-time optical plasma edge detection and position control on MAST

For some years the plasma edge at the midplane has been detected on the MAST spherical tokamak (ST) using a linear CCD camera with a Dalpha bandpass filter. The pixel data are stored during shots and post-processed to reduce reflections and compute the plasma edge position by Abel inversion. This work has now been extended to compute the edge position in real-time, and to use this in the MAST Plasma Control System (PCS) for position control as a better alternative to the existing simple magnetic reconstruction. Possible further developments include real-time optical H-mode detection, and the use of the computed edge position as a constraint for real-time EFIT when the ongoing port to MAST PCS is complete.The camera controller is a PC running RTAI real-time Linux. As each new frame comes in from the camera, it is stored in shared memory by direct memory access (DMA) and the processor is sent an interrupt. The interrupt handler computes the edge position and sends it to PCS with a status flag. If the computed position is flagged as invalid, PCS switches to magnetic reconstruction in a way that avoids step changes in control output. The shared memory can be accessed by separate display and archiving programs during or after shots. This use of DMA, interrupt handler, and shared memory on a PC under real-time Linux is a versatile design pattern with many potential control and data acquisition applications.