Hard x-ray (HXR) spectroscopy is applied for diagnostics of runaway electrons in nuclear fusion reactors. The scintillation counter is one of the most commonly used types of detectors for HXR spectroscopy. It consists of a detector that emits light when excited by HXR radiation (scintillator) directly coupled to a PMT (Photomultiplier Tube) that converts light pulses into an electrical signal. This type of detector is commonly used in existing tokamaks, such as Joint European Torus (JET), Experimental Advanced Superconducting Tokamak (EAST), Compact Assembly (COMPASS), and Axially Symmetric Divertor Experiment (ASDEX-U). In all these cases, the scintillator is directly coupled to the PMT to provide the best possible light transmission efficiency. The Hard X-ray Monitor (HXRM) is one among the first plasma diagnostic systems at ITER that provides information about the energy distribution of runaway electrons inside a tokamak by HXR spectroscopy. This system also uses a scintillator and a PMT as a detector. Due to the heavy shielding of the blanket modules, vacuum vessel, and port-plugs, it is not possible to assemble the scintillator outside the tokamak vacuum vessel. The PMT detector cannot be installed in the close vicinity of the tokamak due to either the significant magnetic field or temperature. A possible solution is to decouple the scintillator from the PMT and place the PMT inside the port-cell. Light pulses will be transmitted to the PMT via a 12 m long optical fiber bundle. Evaluation of the optical transmission was carried out to assess the performance of the HXR monitor and verify possible problems related to the PMT pulse discrimination under low light conditions.
Machine protection is a core task of real-time image diagnostics aiming for steady-state operation in nuclear fusion devices. The paper evaluates the applicability of the newest low-power NVIDIA Jetson Xavier NX platform for image plasma diagnostics. This embedded NVIDIA Tegra System-on-a-Chip (SoC) integrates a Graphics Processing Unit (GPU) and Central Processing Unit (CPU) on a single chip. The hardware differences and features compared to the previous NVIDIA Jetson TX2 are signified. Implemented algorithms detect thermal events in real-time, utilising the high parallelism provided by the embedded General-Purpose computing on Graphics Processing Units (GPGPU). The performance and accuracy are evaluated on the experimental data from the Wendelstein 7-X (W7-X) stellarator. Strike-line and reflection events are primarily investigated, yet benchmarks for overload hotspots, surface layers and visualisation algorithms are also included. Their detection might allow for automating real-time risk evaluation incorporated in the divertor protection system in W7-X. For the first time, the paper demonstrates the feasibility of complex real-time image processing in nuclear fusion applications on low-power embedded devices. Moreover, GPU-accelerated reference processing pipelines yielding higher accuracy compared to the literature results are proposed, and remarkable performance improvement resulting from the upgrade to the Xavier NX platform is attained.
A pair of a scintillator and a Photomultiplier Tube (PMT) is often used as a Hard X-Ray (HXR) radiation detector in existing tokamaks such as JET, EAST, COMPASS or ASDEX-U. Future nuclear fusion reactors such as ITER or DEMO will use more powerful magnets and confine a larger volume of hot plasma. Placement of the detectors used for plasma diagnostic will be constrained by high temperatures, magnetic fields and ionizing radiation present near the tokamak vessel. It might be necessary to move detectors away from tokamak to a safer location. As a result, it might generate problems with pulse discrimination and transmission of the signal. In such scenario, a new approach to the design of plasma diagnostic systems such as HXR spectroscopy is required. In case of the ITER tokamak, sensitive electronics such as Field-Programmable Gate Arrays (FPGA) cannot be installed close to the reactor. The signal from the PMT will be transmitted via an over 100 m long cable to a digitizer located in the diagnostic building. Along the way, electromagnetic interferences can couple into the cable. Additional measures have to be taken to reduce the influence of noise on the signal. The paper presents issues with signal transmission in HXR diagnostic systems and includes a discussion on the methodology of PMT signal transmission in the conditions of the future tokamaks. A proposal of guidelines for selection of the signal chain components and design of a dedicated PMT amplifier is part of this paper.
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