Control and acquisition for MAST Thomson scattering diagnostics

Abstract

The MAST (mega-amp spherical tokamak) Thomson scattering (TS) diagnostics have been radically upgraded and expanded. Eight 30 Hz 1.6 J Nd:YAG lasers have been combined to produce a sampling rate of 240 Hz. The scattered signals are acquired by two spectrometer systems: core and edge. The core system has been built anew: collection optics, polychromators, digitizers, and control computers. It allows measurement of electron temperature and density at 130 spatial points with ∼10 mm resolution across the plasma. The Nd:YAG scattered light signals are registered in 650 channels as polychromator outputs; each channel is registered on two ADCs: at 1 GHz rate in a short interval around each laser pulse and at 100 kHz for background data. The fast ADCs are combined in 26 data acquisition units. Each unit is assembled in a 6 U PXI chassis with embedded controller and six 4-channel 1 GHz ADC cards. Some chassis contain a 96-channel slow ADC card with Ethernet control. The Ruby TS has been rebuilt with a new spectrometer and CCD camera to provide higher spatial resolution – 512 points; the laser has been modified to add double pulse capability. A new control and acquisition system has been developed; it has modular design allowing flexibility and seamless expansion. The system supports event-triggered and real-time operation (will be added in a later stage). A smart trigger device has been developed for TS timing and synchronisation. It provides complex pulse sequences for laser firing with resynchronisation on a number of digital and analogue inputs including plasma events. This device also triggers TS acquisition. The system is integrated by a TS master process running on the dedicated computer; it is represented as a standard MAST data acquisition unit. The Ruby TS is also implemented as a standard MAST unit linked with the Nd:YAG TS by MAST system services.