PMT Signal Transmission for Hard X-Ray Diagnostics of Future Tokamaks

Abstract 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.