Concept of the bolometry diagnostics design for COMPASS-Upgrade

Abstract

The COMPASS-Upgrade tokamak, being designed as a medium-sized tokamak, operating with a hot first wall, allows for the study of DEMO-relevant plasma exhaust physics, crucial for future reactors. Bolometry diagnostics for COMPASS-U, consisting of metal foil bolometers and AXUV diodes, are proposed to measure spatially- and time-resolved radiation losses. Metal foil bolometers supply the absolute value of radiation power, whereas the AXUV diodes can observe fast phenomena such as MHD activity. Coverage of the whole poloidal cross-section by bolometers’ cameras allows tomography reconstruction of the local plasma emissivity. The metallic foil bolometry system will be based on sensors with a gold absorber on a silicon nitride substrate with a platinum resistor. Special modifications, e.g., channel separation, will be applied to the detectors to fulfil the requirements. Due to the high temperature, effective thermal shielding and cooling are essential to reduce the risk of damage to the detectors as well as reducing noise in the measured signal. The pin-hole cameras spatial configuration was optimized to provide the best performance under the given engineering constraints. The position of the cameras strongly depends on the space available behind the plasma-facing components (PFC). The proposed layout allows for an efficient observation of the confined plasma as well as that of the divertor region.

Introduction

The COMPASS-Upgrade will be a medium size tokamak developed for a flexible operation at extremely high heat fluxes of order of 100 MW/m². The COMPASS-U will provide an operation with a strong magnetic field up to 5 T, plasma current up to 2 MA, and high densities up to 3∙10²⁰ m³. Additionally, the tokamak will have its metallic first wall maintained at high temperatures (200 °C – 500 °C) during a high-performance operation, being able to modify recycling and/or to handle liquid metals when tested such divertor modules. Due to these unique capabilities, the COMPASS-U will allow for the study of reactor-relevant plasma exhaust physics, high-temperature operation, and advanced confinement scenarios providing a promising outlook for ITER and DEMO. The upper and lower closed divertors will provide the capability of studying different divertor configurations (single null, double null, snowflake, etc.) with extreme heat fluxes [1].

Power losses, which are an essential part of the power balance in plasma, are connected to the conductive losses at the first wall, electromagnetic radiation and nuclear reactions. The plasma radiation is a parameter carrying valuable information about plasma performance and its connected physics-based processes. It is important to monitor the plasma radiation especially during disruptions and other instabilities. In the COMPASS-U, the radiation power will be observed by the bolometry system consisting of both the metallic foil bolometers and AXUV diodes. The metallic foil bolometers will provide the absolutely calibrated measurement of the power radiated by the plasma. In the COMPASS-U, four-channel bolometers with gold absorber on silicon nitride (Si₃N₄) and platinum resistor [2] will be used. The sensors with a 6 μm absorber are sensitive to electromagnetic radiation of a range from 2 eV to 10 keV as well as to fast neutrals. A loss of measurement efficiency is observed at low energies due to reflections from the gold surface. The carbon coating on the sensors can significantly improve the sensitivity below 2 eV [3]. The use of carbon in the COMPASS-U vacuum vessel is forbidden for all plasma facing components but the thin coating applied on the cameras should not impact the operation, and therefore an exception might be obtained. The gold on the Si₃N₄ bolometers is radiation hard and allows for a relatively quick measurement (time resolution ∼ ms). The most prominent advantage of this type of detector is the possibility of self-calibration between shots and the presence of the reference sensor, which makes the detector insensitive to the temperature fluctuations in the tokamak environment. Use of the detectors with specifications tested for ITER [4], e.g., specially prepared ceramic housing without connectors influenced on the silicon plate, increases the recommended operational temperatures up to 250 °C. At temperatures higher than 300 °C drifts, and failures are observed for detectors with Si₃N₄ substrate [2,4].

The AXUV diodes system implemented as fast bolometry (time resolution ∼ 1 μs) will be partially restored from the AXUV diagnostic at the COMPASS [5] as it will use the photodiode array AXUV20ELG, containing 20 channels in a small space (32 × 10 × 7.5 mm). These n-on-p diodes can measure radiation from 1.1 eV, but approximately constant responsivity is only for energies above 200 eV. The spectral sensitivity in the UV region is connected to the passivation layer, and it is the main reason limiting AXUV diodes application as true bolometers. Nevertheless, correction factors for the diodes can be estimated with the use of metallic foil bolometers. For the AXUV diode, the maximum baking temperature is also 200 °C, but the operating temperature is below 80 °C [6]. Spatially resolved, line-integrated measurement by both types of detectors allows for the 2D emissivity profile reconstruction. The metallic foil bolometers will focus on slower phenomena such as a detachment regime. The AXUV diodes will monitor mainly fast events, e.g. MHD activity and disruptions.

The diagnostics field of view will cover the entire plasma volume in one poloidal plane. The additional AXUV diode cameras will be placed at two other toroidal positions for monitoring toroidal asymmetries. The systems are divided into a confined plasma part and a divertor part. Both parts separately need to provide the reconstruction of the radiation profile in the area of interest. The measurement will be performed during the entire shot. The plasma studies require the performance of the highest possible quality, which is limited by technical constraints. Degradation of AXUV diodes at all toroidal positions will be monitored using an external calibration tool.