Constraints on conceptual design of diagnostics for the high magnetic field COMPASS-U tokamak with hot walls
Introduction
The project for the design and construction of the high magnetic field COMPASS-U tokamak with hot walls [1] (major radius R = 0.894 m, minor radius a = 0.27 m, plasma elongation 1.8, triangularity less than 0.6, toroidal magnetic field BT on axis 5 T, plasma current Ip = 2 MA for safety factor at the edge q95 = 2.4, 1 s long flat-top at full plasma parameters), started at IPP Prague in 2018. Unique features of this new device caused by a combination of its compactness, plasma parameters linked to a high magnetic field and strong auxiliary heating (4 MW NBI, 2 MW ECRH) and hot walls (300 °C) bring constraints and requirements, which make the development of necessary plasma diagnostics highly demanding. Among the most important features of COMPASS-U leading to consideration of new diagnostic designs there are: a high temperature of the vacuum vessel (VV); a high plasma density (up to 5*1020 m−3); a high heat flux density; cooled copper-based and vertically symmetric magnetic field coils; and finally, the proposed future use of the liquid metal divertor. As a consequence, the diagnostic designs will require dedicated solutions for all in-vessel components. Last but not least, liquid metals proposed to be used in the divertor introduce the question of the material compatibility, mainly at elevated temperatures, and their transport and re-deposition on in-vessel components, including those important for optical diagnostics. Maintenance of installed systems will be ensured using the human access to the vacuum vessel or using their redundancy. In the next sections, the main expected constraints influencing the conceptual design of individual diagnostic tools for COMPASS-U are reviewed and examples of possible solutions are indicated.
Section snippets
Hot vacuum vessel and in-vessel components
The COMPASS-U tokamak, being equipped with a conventional closed divertor made from a solid material, will be able to regulate the amount of wall-stored fuel particles and reach a high recycling regime via an elevated temperature of the VV [2]. Therefore, the VV is designed to withstand 500 °C and all removable parts (plasma facing components, diagnostics) should be able to operate up to 300 °C, being kept at a predefined temperature by an active heating system of the VV. Also, a low recycling
Summary and outlook
The construction of COMPASS-U, a high magnetic field tokamak with hot walls, brings many constraints on plasma diagnostics, which can be also met on either already present or on near future-planned fusion relevant devices. In our short review, the most important problems were named and possible ways, how they can be bridged, were indicated, including necessary future R&D.
Acknowledgments
This work has been carried out within the framework of the project COMPASS-U: Tokamak for cutting-edge fusion research (No. CZ.02.1.01/0.0/0.0/16_019/0000768) and co-funded from European structural and investment funds.
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