Standardizing high electron temperature measurement comparisons: a method for cross-diagnostic and cross-machine analysis

In tokamaks, measuring electron temperatures in the plasma core may be quite challenging, especially when they exceed 6-8 keV [1]. Discrepancies are detected between the values measured by different diagnostics, such as Thomson Scattering (TS) and Electron Cyclotron Emission (ECE), which are expected to agree. Accurate and reliable determination of electron temperature in high-temperature scenarios, is crucial for the development of future reactors like ITER and Demo (with ITER's core plasma expected to have an electron temperature of about 25 keV) [2], as well as for the Chinese Fusion Engineering Test Reactor (CFETR) [3]), in which discrepancies in electron temperature measurements could be even more pronounced. Resolving this diagnostic issue is crucial because it implies a deep understanding of important aspects of plasma physics in the core and beyond. Recently, further studies on this topic have yielded substantial results and clarified several aspects [9,10,11,12]. Current research focuses on the possible causes of the local non-Maxwellian shape of the electron energy distribution function, which is at the root of these discrepancies [1,4,5,6,7,8,9,10,11,12]. Ongoing research by various groups working on magnetic confinement machines worldwide aims to address this long-standing issue within the framework of an ITPA (International Tokamak Physics Activity) initiative. This paper proposes a method to compare data collected by ECE and TS, which is based upon previously developed techniques for analyzing JET data [1], and is implemented in a dedicated Python code, to tackle issues identified in recent years, while also ensuring the output is comparable across different machines. The goal is to perform comparisons under consistent conditions, irrespective of machine-specific factors such as dimensions, fields, and coordinates. The methods developed and the corresponding implementations in a code, addresses several critical aspects, including the positions of measurements, i.e. the plasma position relative to the diagnostics' lines of sight (LoS), the involved volumes, and other controls to ensure uniformity of results during multi-shot analyses. These controls encompass acquisition rate, data interpolation, and the equilibrium reconstruction codes employed, with the objective of obtaining the best possible comparison between the two diagnostics.