Preliminary study of a visible, high spatial resolution spectrometer for DEMO divertor survey

Abstract

Developments towards DEMO Diagnostic and Control (D&C) system conceptual design are based on a subset of ITER mature diagnostic systems, whose eligibility for DEMO has been endorsed by their robustness, long lifetime expectancy and feasible remote maintenance [1]. They are devoted to ensure the machine operation in compliance with safety requirements and high availability. In particular, the evolution of divertor spectroscopic measurements on fusion experiments has demonstrated their potential as a control method for divertor protection via detachment control [2, 3] (near ultraviolet, 300-400 nm) and monitoring of the plasma-wall interaction (visible range, 400-700 nm) [4]. These characteristics make this method one of the leading candidates for DEMO detachment and radiation control power. In line with the application of a system engineering approach [5], initial assessments of design and feasibility of a VIS high spatial resolution spectrometer for the DEMO divertor survey based on early DEMO control requirements are presented and discussed. The proposed system is located at the equatorial port and it is composed of 3 oblique lines of sights (LoS), 9 toroidal mirrors, 6 plane mirrors and 6 spectrometers examining the outer, inner and X-point divertor region, optimized for the monitoring of chord-integrated NUV/VIS signals under parallel divertor plasmas observation. The wavelengths of interest, spatial resolution and main integration issues are reported. K: Plasma diagnostics -interferometry, spectroscopy and imaging; Plasma diagnosticscharged-particle spectroscopy 1Corresponding author.

Demonstration of a Sparse Sensor Placement Technique to the Limited Diagnostic Set in a Fusion Power Plant

“…The amount of diagnostics is limited to maximize the volume of the blanket, so to have as little holes as possible. Also, the diagnostic port cannot be too close to the divertor region, because the heat and neutron fluxes are too high [13]. Especially the neutron flux is a large issue with optical components, as these high energy neutrons can damage these components rather quickly if they are not properly shielded.…”

“…The planned spectroscopic system in DEMO differs from those on existing tokamaks. Most notably, it has only a few lines-of-sight and different geometry, looking at the divertor leg from the midplane with a strong toroidal component [13]. Given the major differences between the existing spectroscopic system and the DEMO layout and the price of creating a new spectroscopic system, it is relevant to develop a strategy to validate the spectroscopic layout on contemporary machines.…”

“…The currently planned lines-of-sight on DEMO are defined in [13]. These cannot just be scaled to TCV proportions and implemented directly, since DEMO will have a different geometry compared to TCV.…”

“…Front tracking is performed in real-time on 2D images obtained from the multispectral imaging system MANTIS [9]. Such a multispectral imaging system cannot, in its current form, be used for detachment control in DEMO, because the high neutron flux will quickly destroy the sensitive optical components [13]. Therefore, the current plan for DEMO is to use spectroscopy which can be properly shielded [12,13].…”

Neutronics Simulations for DEMO Diagnostics

“…Most sub-systems are designed to be integrated in equatorial port (EP) plugs dedicated to diagnostics (five or six EPs are foreseen), in some cases with additional lines of sight in the upper ports (UPs) if there is space reserved for diagnostics in the UPs. These include spectroscopy diagnostics [7], neutron/gamma cameras [8,9], radiated power and soft Xray intensity [10] and IR polarimetry/interferometry [11], with the eventual addition of collective Thomson scattering [12], still under study. For diagnostics that require access to the plasma from several poloidal positions, such as MW reflectometry [13] and ECE [14], the diagnostics slim cassette (DSC) concept has been developed [15][16][17] as a modular approach compatible with the remote handling operations of the breeding blanket (BB).…”