CURRENT STATUS OF ITER HARD X-RAY MONITOR AND THE CHALLENGING OF INFERRING THE RUNAWAY ELECTRON PARAMETERS Yong Liu1, Santosh Pandya2, Patryk Nowak Vel Nowakowski3, Miklos Palankai4, Jeno Kadi4, Raphael Tieulent1, Dariusz Makowski3 1 ITER Organization, Route de Vinon-sur-Verdon, CS 90046, 13067 St. Paul Lez Durance Cedex, France 2 Institute for Plasma Research, Gandhinagar India 3 Department of Microelectronics and Computer Science, Lodz University of Technology, Lodz 93-005, Poland 4 GEMS Engineering Kft, Hungary Abstract: The Hard X-Ray Monitor (HXRM) system is one of the diagnostics that have primary role in the measurement of Runaway Electrons (REs) in ITER. It has been proposed for the ITER Hydrogen/Helium phase (non-nuclear phase) of ITER plasma operations to monitor Hard X-Ray (HXR) emission generated due to REs interacting with plasma particles and in-vessel components (thick target bremsstrahlung emission). The HXRM system records and provides time resolved HXR counts/signals and energy spectra during different phases of a plasma discharge. The ITER HXRM system is a 2-detector system at a single toroidal location. Each channel of the HXRM system is composed of the in-vessel subsystem and the ex-vessel subsystem. The in-vessel subsystem consists of the scintillator-crystal and its housing, the in-vessel optical fibre bundle and the upstream lens assembly, while the ex-vessel subsystem consists of the downstream lens assembly, the ex-vessel fibre bundle, the PMT box with the Photomultiplier tubes (PMTs) and the electronics inside it, and the instrumentation and control (I&C) cubicle that is not shown in the below figure. For each channel, two PMTs are there to measure respectively the spectra and the integration of the spectra. Up to now, all the components that are locating inside the Port Plug have been manufactured, delivered, and tested. The ex-vessel components are currently under finalization. Per the planning, all the ex-vessel components (excluding I&C) will be delivered by the end of 2026, and the I&C will be delivered in 2027. Due to the design constraints and challenges in ITER, the configuration for ITER HXRM system is different with the HXRM system in the conventional tokamaks. The scintillator and the PMT are decoupled, and the fibre bundles and relay lenses are utilized to transmit the ultraviolet (UV) photons from the scintillator to the PMT. This configuration is the baseline configuration used in modern scintillation dosimetry, but it has never been used in tokamaks. With this configuration, the overall system efficiency is as low as 6.09E-04 for the counting-mode PMT and 7.35E-05 for the current-mode PMT. Due to the low efficiency, the spectral resolution is compromised. Consequently, it needs to be assessed how it impacts the system performance especially the maximum energy of the REs. Meanwhile, a prototype has been developed over the past years to better understand the system performance and to facilitate the development of I&C system. Earlier this year, the ITER HXRM prototype system has been tested in ADITYA-U.