Real-Time Processing System for the JET Hard X-Ray and Gamma-Ray Profile Monitor Enhancement

The Joint European Torus (JET) is currently undertaking an enhancement program, in which one of the objectives is to test relevant diagnostics for the International Thermonuclear Experimental Reactor (ITER), the reference for the next generation of fusion experiments. One of the challenges in ITER is the provision of real-time data analysis and compression capabilities, to sustain the expected long duration discharges and the high acquisition rates achieved by recent data acquisition systems. Foreseeing this real-time requirement, a new system was developed and installed at JET for the gamma-ray and hard X-ray profile monitor diagnostic. The new system, which is connected to 19 CsI(Tl) photodiodes in order to obtain the line-integrated profiles of the gamma-ray and hard X-ray emissions, was designed to overcome the data acquisition limitations of the present Fast Electron Bremsstrahlung diagnostic (FEB), while exploiting the required real-time features. This paper presents the real-time processing architecture for the JET gamma-ray and hard X-ray profile monitor. The system hardware, based on the Advanced Telecommunication Computer Architecture (ATCA) standard, includes reconfigurable digitizer modules with embedded Field Programmable Gate Array (FPGA) devices capable of acquiring and simultaneously processing data in real-time from the 19 detectors. A suitable algorithm was developed and implemented in the FPGAs, which are able to deliver the corresponding energy of the acquired pulses, and its associated occurrence time. The real-time processed data is sent periodically, during the discharge, through the JET real-time Asynchronous Transfer Mode (ATM) network, and stored in the JET scientific databases at the end of the pulse. Publishing the processed data in the ATM network enables it to be used for machine control purposes (e.g. the information about the line-integrated emissions of the hard X-rays in real time can be used to determine the lower hybrid current drive deposition before the main heating phase). Additionally, the real-time processed data is used for local calibration, using embedded radioactive sources to build in real-time the 19 channels spectra. The acquired raw data is also stored in the digitizer modules' local memory and retrieved after the pulse to the JET database, where it can be post-processed offline to validate the real-time algorithms. The interface between the ATCA digitizers, the JET Control and Data Acquisition System (CODAS) and the JET real-time network is provided by the Multithreaded Application Real-Time executor (MARTe). From the experimental results it was concluded that it is possible to measure in real-time the line-integrals of both hard X-ray and gamma-ray emissions, covering energy range from ~200keV to 8 MeV. This allows us to meet two of the major milestones: the ability to process and supply high volume data rates in real-time over a wide spectrum energy range.
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