Core Charge Exchange Recombination Spectroscopy Research Articles

Main design features of the Rh-based first mirror developed for the ITER CXRS core diagnostics

The ITER core charge exchange recombination spectroscopy, which contains five in-port optical mirrors, is intended to transfer the visible light emitted by interaction of the plasma with the diagnostic neutral beam to the corresponding spectrometers. The first mirror (M1) is a key and the most vulnerable component of the diagnostics. In order to provide the required mirror lifetime, maintainability and structural integrity, M1 is composed of special materials, i.e. it is made of a thin 1 mm single-crystal rhodium (ScRh) plate diffusion bonded to a specially matched tungsten-copper substrate. A rhodium nanocrystalline coating (NcRh) can be an option.The paper presents the evolution of the M1 design developed for the optical layout of 2018. M1 is adjustable and cleanable with about 100÷500 procedures of 60 MHz plasma discharge. The mirror design is supported by multifield thermal, electromagnetic and structural analyses and uses experimental data of R&Ds made by Forschungszentrum Juelich, Germany. The study confirms the working capacity of the developed mirror solution.

Geometrical effect in the measurement of the CXRS on EAST

Charge eXchange Recombination Spectroscopy (CXRS) systems have been successfully applied on EAST since 2014. However, geometrical effects will deteriorate the diagnostic locality and introduce uncertainties into the measurement. A simplified spatial average model for the average of effective integral paths and an upgraded simulation of spectra (SOS) code are both used to evaluate these geometrical effects. The results show that spatial averaging effect with one beam can introduce up to ˜10% relative error in the ion temperature and velocity measurement at ITB region. Simultaneous injection of two beams can produce ˜20% uncertainties in core CXRS with single gaussian description of the spectra. To reduce the error, the spatial averaged position ρav calculated from the simplified model and a double gaussians fitting with a constrained intensity ratio model are applied in the CXRS data analysis, and the fitting results show a good consistency with the input profiles.

First results of upgraded beam emission spectrometer on EAST

The beam emission spectroscopy (BES) system that shares the collection optics with the core charge exchange recombination spectroscopy (cCXRS) has recently been upgraded on EAST. The enhanced system aims at the simultaneous measurements of the neutral beam emission from the co-current and counter-current neutral beam injection (NBI) as well as the active charge exchange line (Dα, n = 3→2, 656.1 nm) from the interaction of the main ions and the injected neutral beam. The operating spectral band of the system is 656.1 ± 6 nm, with respect to the maximum Doppler shift and stark splitting induced by the highest energy of 80 keV for deuterium beam. Strong edge Dα line is effectively suppressed by one curved chromate strip (˜3% transmission and ˜83 μm width) on the fused silica. Main ion Dα emission and beam emission spectra are acquired simultaneously without any saturation of the signal on the detector. In this paper, an overview of the upgraded spectrometer will be shown and the first experimental results are presented.

Evaluation of impurity densities from charge exchange recombination spectroscopy measurements at ASDEX Upgrade

At ASDEX upgrade (AUG) a new framework for the evaluation of impurity densities based on measurements from charge exchange recombination spectroscopy (CXRS) diagnostics has been developed. The charge exchange impurity concentration analysis code, or CHICA, can perform these calculations for all of the beam-based CXRS diagnostics at AUG and is equipped with the atomic data for all of the regularly measured charge exchange spectral lines (He, Li, B, C, N, O, and Ne). CHICA includes four different methods for the evaluation of the neutral density populations, which feature different implementations and contain varying levels of sophistication. These methods have been thoroughly benchmarked against one another, enabling the important processes for the evaluation of neutral densities to be identified as well as the neutral populations that are most critical to the accurate interpretation of the measured CXRS intensities. For the AUG neutral beams, charge exchange with the ground state of the first energy component is typically the dominant contribution to the measured CXRS intensities, but emission from reactions with the n = 2 beam halo population can contribute up to 35% to the total signal and must be included in the analysis. Neglect of this population leads to incorrect magnitudes and incorrect profile shapes of the calculated impurity density profiles. The edge lines of sight (LOS) of the core CXRS diagnostics at AUG intersect the edge pedestal inside of the neutral beam volume. Therefore, the impurity density is not constant along the LOS, complicating the interpretation of the measured intensities. Within CHICA a forward model for the edge impurity densities has been implemented, enabling the reconstruction of accurate edge profiles.

Assessment and Mitigation of Wall Light Reflection in ITER by Ray Tracing

In ITER, reflection of photons on vacuum vessel will make parasitic signals (stray light) for optical diagnostics. In this study, to estimate and mitigate the effect of the stray light in ITER in a systematic manner, a ray transfer matrix was constructed based on ray tracing calculations for a divertor impurity monitor and charge-exchange recombination spectroscopy (CXRS). It is shown that the allocation of the sources around the strike point and the X-point, where the emission is strong, is important for the model used to build the transfer matrix to effectively mitigate the stray light. The origin of the stray light for the core CXRS is investigated, and a case study to subtract the stray light is shown.

Measurement of the complete core plasma flow across the LOC–SOC transition at ASDEX Upgrade

A newly installed core charge exchange recombination spectroscopy (CXRS) diagnostic at ASDEX Upgrade (AUG) enables the evaluation of the core poloidal rotation () through the inboard-outboard asymmetry of the toroidal rotation with an accuracy of 0.5 to 1 km . Using this technique, the total plasma flow has been measured in Ohmic L-mode plasmas across the transition from the linear to saturated ohmic confinement (LOC–SOC) regimes. The core poloidal rotation of the plasma around mid-radius is found to be always in the ion diamagnetic direction, in disagreement with neoclassical (NC) predictions. The edge rotation is found to be electron-directed and consistent with NC codes. This measurement provides as well the missing ingredient to evaluate the core velocity () from data only, which can then be compared to measurements of the perpendicular velocity of the turbulent fluctuations () to gain information on the turbulent phase velocity (). The non neoclassical from CXRS leads to good agreement between and indicating that is small and at similar values as found with gyrokinetic simulations. Moreover, the data shows a shift of in the ion-diamagnetic direction at the edge after the transition from LOC to SOC consistent with a change in the dominant turbulence regime. The upgrade of the core CXRS system provides as well a deeper insight into the intrinsic rotation. This paper shows that the reversal of the core toroidal rotation occurs clearly after the LOC–SOC transition and concomitant with the peaking of the electron density.

Extensions to the charge exchange recombination spectroscopy diagnostic suite at ASDEX Upgrade.

A new core charge exchange recombination spectroscopy diagnostic has been installed in the ASDEX Upgrade tokamak that is capable of measuring the impurity ion temperature, toroidal rotation, and density on both the low field side (LFS) and high field side (HFS) of the plasma. The new system features 48 lines-of-sight (LOS) with a radial resolution that varies from ±2 cm on the LFS down to ±0.75 cm on the HFS and has sufficient signal to run routinely at 10 ms and for special circumstances down to 2.5 ms integration time. The LFS-HFS ion temperature profiles provide an additional constraint on the magnetic equilibrium reconstruction, and the toroidal rotation frequency profiles are of sufficiently high quality that information on the poloidal velocity can be extracted from the LFS-HFS asymmetry. The diagnostic LOS are coupled to two flexible-wavelength spectrometers such that complete LFS-HFS profiles from two separate impurities can be imaged simultaneously, albeit with reduced radial coverage. More frequently, the systems measure the same impurity providing very detailed information on the chosen species. Care has been taken to calibrate the systems as accurately as possible and to include in the data analysis any effects that could lead to spurious temperatures or rotations.

Specific design and structural issues of single crystalline first mirrors for diagnostics

The first mirrors of ITER diagnostic systems are the most vulnerable ones since they are directed to the plasma and are subjected to erosion and intensive impurity deposition. To keep their optical performance and maintainability, single crystalline molybdenum and rhodium have been considered as mirror materials, subject to intensive investigations including R&Ds and mirror cleaning studies.The paper presents specific design and structural issues of the first mirror of 300mm in the size considered for the ITER core charge exchange recombination spectroscopy (cCXRS). Such a large mirror can be assumed as a generic one for solid middle (size) mirrors (up to 100mm in diameter) and for composed large mirrors (diameter >150mm) that are potentially made of an assembly of smaller pieces.The main design aspects like the first mirror material, the cooling capability of passively and actively cooled mirrors and the mechanical behavior of rhodium and molybdenum mirrors composed of single crystals pieces (i.e. plates and tiles) have been addressed and studied. The mirror design is supported by extensive structural and multi-field analysis.

ITER core CXRS diagnostic: Assessment of different optical designs with respect to neutronics criteria

The Charge Exchange Recombination Spectroscopy (CXRS) diagnostic aims to measure emission lines of impurity isotopes in the ITER plasma in order to quantify several parameters like the composition of the plasma (density of helium, deuterium or tritium), the ion temperature or rotation velocities. The core plasma CXRS shall be installed in one of the ITER Upper Port Plugs (UPP #3). Currently, four different optical layouts are being assessed with respect to the optical performance, engineering feasibility, cost, maintenance especially with respect to remote handling and the performance of their neutron radiation shielding.This work is devoted to the neutronic analysis performed in support for the design of ITER CXRS-core Diagnostic System, presently under development by the IC3 Consortium (FZJ, KIT, BME, Wigner RCP, TU/Eindhoven, FOM-DIFFER, CCFE, CIEMAT, Optimal Optik). Results of the neutronic analyses are presented showing the differences between the four different designs with respect to several nuclear responses such as neutron fluxes around the upper port plug and in the port interspace, maps of nuclear heating around the UPP including the toroidal/poloidal field coils as well as the vacuum vessel. Furthermore, radiation damage maps were calculated covering large areas of the upper port plug and of its environment. The results indicate the viability of one of the preferred designs from the neutronic point of view but also show the potential for improvements.

Status of the R&D activities to the design of an ITER core CXRS diagnostic system

The CXRS (Charge-eXchange Recombination Spectroscopy) diagnostic for the core plasma of ITER will be designed to provide observation of the dedicated diagnostic beam (DNB) over a wide radial range, roughly from a normalised radius r/a=0.7 to close to the plasma axis. The collected light will be transported through the Upper Port Plug #3 (UPP3) to a bundle of fibres and ultimately to a set of remote spectrometers. The design is particularly challenging in view of the ITER environment of particle, heat and neutron fluxes, temperature cycles, electromagnetic loads, vibrations, expected material degradation and fatigue, constraints against tritium penetration, integration in the plug and limited opportunities for maintenance. Moreover, a high performance (étendue×transmission, dynamic range) is expected for the port plug system since the beam attenuation is large and the background light omnipresent, especially in terms of bremsstrahlung, line radiation and reflections. The present contribution will give an overview of the current status and activities which deal with the core CXRS system, summarising the investigations which have taken place before entering the actual development and design phase.

Design overview of the ITER core CXRS fast shutter and manufacturing implications during the detailed design work

At first a detailed fast shutter design was finalized for the ITER core charge exchange recombination spectroscopy (CXRS) diagnostic. The shutter has approximately 70kg of mass and a length of 2.1m. It operates in fractions of a second (0.7s) protecting critical optical components against degradation and providing means of calibration for the optical system. The shutter structure is driven by a bidirectional frictionless helium actuator, with forces and axial strokes of 3.4kN and 2mm respectively. The shutter structure consists of: (a) two blades made of CuCrZr and stainless steel, calibration surfaces (currently Al2O3) on the top and on the bottom a protective TZM (Mo–0.5Ti–0.08Zr) screens, (b) two arms interconnected that form one cooling circuit including the blades, (c) a bumper system to limit the arms movement, and (d) a support. A description of these components and their functions are given in this paper, followed by some issues, and their corresponding solutions or ongoing investigations, encountered during the design work. Detailed manufacturing drawings have been developed as the deliverable final product of this design stage, and are used in the prototyping phase which includes testing, numerical benchmarking, and validation of the shutter concept.

Major aspects of the design of a first mirror for the ITER core CXRS diagnostics

The ITER core charge exchange recombination spectroscopy diagnostics (cCXRS) occupies the vacuum vessel upper port #3 and includes, in its generic version, the following in-vessel components: an optical mirror system, a shutter, the diagnostic first wall and the neutron shielding block. The most vulnerable diagnostic mirror is obviously the first one (M1) directly observing the plasma. The M1 reference option is made of a single crystalline molybdenum (ScMo).The paper indicates major aspects influencing the first mirror design and identifies the most reasonable and reliable concept for cCXRS M1 at present. The applicability of the option presented is determined by many reasons, and especially, by the ITER generic upper port plug and its customization flexibility.The largest dimension of the mirror polished face is ∼300mm. Such large ScMo workpieces are currently not available on the market. The mirror should be designed as an assembly of several ScMo pieces joined together.The M1 design is supported by multifield thermal, electromagnetic and structural analyses. The performed study confirms the feasibility of the proposed solutions. At the same time, the paper indicates numerous technological issues of the M1 unit to be solved in future.

Thermal and hydraulic performance of the helium-operated shutter protecting the first mirror of the ITER diagnostics

The ITER core charge exchange recombination spectroscopy (core CXRS) is a diagnostic system designed to collect the light emitted from interaction of the Diagnostic Neutral Beam (DNB) with the core plasma and guides it via a mirror labyrinth through the Upper Port Plug toward a fiber bundle, which then transmits the light to a set of spectrometers for spectral analysis. The first mirror (M1), being the most important part of the optical system which defines workability of the entire CXRS, works in direct view of the plasma. The latter strongly impacts the mirror causing its surface erosion and deposition with degradation of the optical capability. The fast shutter (FS) is supposed to play the main role in reducing the time of the first mirror exposition to the plasma. The performed work is focused on the detailed thermal and hydraulic analyses of the main components of the M1 protecting device (blades, arms and actuator) and its cooling and pneumatic systems. The performed analysis confirms, in general, the workability of the helium-operated shutter and verifies the pneumatic actuation concept proposed for the M1 protecting device.

Retractable tube design issues in ITER CXRS UPP #3

The retractable tube is located in the upper port plug #3 of ITER core charge exchange recombination spectroscopy (cCXRS), where it is surrounded by a set of mirrors, diagnostic shielding module (DSM), outer shell, cleaning device and shutter. Accordingly to the recent port plug concept the tube is an optional unit that carries the cleaning system of the first mirror. The cleaning device shall be a compact instrument at the tube head that influences the tube diameter primarily. Independently of the cleaning device the tube cooling channels and its proper support concept has to be developed.Various manufacturing methods are investigated for the proper layout of the tube cooling structure. They are the gun-drilled deep holes, the solid state bonded sheets forming cooling channels between the welded layers, ‘tube-in-tube’ approach. Most of them were analyzed in view of their thermal conditions.The tube support system design is a complex task, that has to take into account several boundary conditions and limitations. The tube must be compatible with the assumed ITER generic installation procedure, withstand the electromagnetic (EM) and thermal loads. Furthermore, the tube has to be exchanged several times during its lifetime.The paper summarizes the tube cooling layout possibilities and the assumptions on its support concepts.

Design and testing of secondary mirrors for the core CXRS diagnostic system in ITER

The core Charge Exchange Recombination Spectroscopy (core CXRS) diagnostic system for ITER is projected with 7 consecutive in-vacuum mirrors. Based on overall system stability, maintenance and lifetime considerations, ITEr grade Stainless Steel 316L(N) was chosen as first candidate for the substrate of the mirrors two to seven. A robust design with remote handling friendly bolted interface has been devised. It features uniformly distributed internal temperature control with minimal welds. The design, simulation results and plans for a prototype are discussed.Mirrors 4–7 are planned with a multilayer dielectric coating at preferably >98% reflectivity. The ability to manufacture such a coating on SS316L(N)-substrate which works reliably under ITER conditions is investigated. A 64 λ/4 layer coating of TiO2 and SiO2 was chosen for initial investigations. Adhesion tests were carried out and testing under ITER-conditions is under preparation. The paper covers the coating, sample manufacturing and status of the testing.

Fast shutter concepts for the new ITER core CXRS upper port plug baseline considering the actuator located inside and outside the port plug

For the ITER core charge exchange recombination spectroscopy (CXRS) upper port plug #3 (UPP) diagnostics, a fast shutter is necessary for the first optical mirrors. The shutter concept for the reference CXRS layout has been well developed and studied focusing on the structure dynamic behavior. This paper presents two shutter concepts for the new baseline layout. Concept-1 is similar to the reference concept, including the helium actuator attached to the diagnostic shielding module (DSM) in the primary vacuum. Concept-2 considers the actuator outside the primary vacuum. It is attached to the back side of the UPP and connected to the shutting mechanism with a push–pull rod. Assembly and disassembly, actuator location, and actuation type are key issues for concept assessment. For a feasibility study, two different leak cases for the shutter concepts have also been analyzed. Concept-1 requires a specialized actuator design compatible with the in-vessel conditions. The actuator size is limited by the lack of available space in the DSM. This option needs complex procedures to access the actuator for maintenance. Concept-2 allows the use of standard pneumatic/hydraulic actuators and other industrial components. Space allocation is needed inside the UPP to accommodate the push–pull rod system. Concept-2 allows independent maintenance procedures for the actuator(s) and the shutting mechanism. It was generally concluded that both options are suitable for the new baseline layout, and are to be studied in detail.

Approaches to multifield numerical analysis for components of the ITER core CXRS upper port plug diagnostics

The core charge exchange recombination spectroscopy (cCXRS) upper port plug diagnostic system serves to control and optimize plasma performance. The system contains a labyrinth of optical mirrors delivering the visible light from the plasma to the end spectrometers. Harsh ITER near plasma environment, resulting in high thermal fluxes and severe electromagnetic (EM) loads, dictates a necessity of a robust design for the plug components.The structural response of the plug components to combined loading coming from different sources has to be calculated and analyzed. The global ANSYS EM model featuring the reference cCXRS layout has been upgraded to be in line with the recent ITER generic upper port plug. Impact of different parameters on the plug EM loading was studied. The dynamic structural behavior of the plug (new baseline layout) under the EM forces has been analyzed. The plug lateral deflection is about 9mm and does not exceed the ITER limit of 12mm.The cooling and supporting system of the cCXRS first mirrors, located near the plasma, have to cope with significant EM loads and high heat fluxes. The engineering ‘express’ approach, that was recently effectively used to calculate the eddy currents and EM forces for the cCXRS fast shutter, has been further validated for the mirrors. The paper outlines approaches to a multifield analysis of the first mirror.

A high etendue spectrometer suitable for core charge eXchange recombination spectroscopy on ITER

A feasibility study for the use of core charge exchange recombination spectroscopy on ITER has shown that accurate measurements on the helium ash require a spectrometer with a high etendue of 1mm(2)sr to comply with the measurement requirements [S. Tugarinov et al., Rev. Sci. Instrum. 74, 2075 (2003)]. To this purpose such an instrument has been developed consisting of three separate wavelength channels (to measure simultaneously He/Be, C/Ne, and H/D/T together with the Doppler shifted direct emission of the diagnostic neutral beam, the beam emission (BES) signal), combining high dispersion (0.02 nm/pixel), sufficient resolution (0.2 nm), high efficiency (55%), and extended wavelength range (14 nm) at high etendue. The combined measurement of the BES along the same sightline within a third wavelength range provides the possibility for in situ calibration of the charge eXchange recombination spectroscopy signals. In addition, the option is included to use the same instrument for measurements of the fast fluctuations of the beam emission intensity up to 2 MHz, with the aim to study MHD activity.

Feasibility of non-thermal helium measurements with charge exchange spectroscopy on ITER

The use of active charge exchange recombination spectroscopy (CXRS) as a diagnostic for fusion-produced alpha particles on ITER is constrained by the signal-to-noise ratio, which is determined by the intensity of the line of interest, the optical throughput of the diagnostic, the neutral beam penetration, and the intensity of bremsstrahlung radiation. The CX spectral line for fast ions has been modelled together with the expected background emission and we present the signal-to-noise ratios calculated as a function of the diagnostic design parameters. Combining the CXRS data from both the heating and the diagnostic neutral beams on ITER, information on fast ions with energies up to 1 MeV can be obtained for the parameters of the ITER core CXRS diagnostic design. To achieve this, energy binning of the signal is used (100 keV bins or larger), in order to improve the signal-to-noise ratio, with a time resolution of 2 s. The time resolution of the measurement can be improved using a higher throughput spectrometer, but this is ultimately limited by the amount of light from the neutral beam that can be collected. Despite the challenges and the fact that the results are not as optimistic as previously assumed, it is concluded that useful information on fast helium density profiles can be obtained using CXRS on ITER.

Alignment Principles for ITER In-Vessel Diagnostic Mirrors

Optical mirrors inside the ITER torus vacuum are required by many of the ITER port-based diagnostic systems. As an example, the core charge exchange recombination spectroscopy (core CXRS) concept is based on a mirror labyrinth of seven consecutive in-vessel (exposed to the torus vacuum) mirrors. For this system, alignment of up to three mirrors during assembly might be needed. Because of the unique and demanding environment in the ITER diagnostic port plugs, particularly in the first meter from the plasma, existing designs of adjustable mirror holders cannot be used without modification. The wish to allow the realignment of the mirrors during maintenance of the diagnostic systems, if possible by remote handling only, adds to the complexity. This paper discusses mirror alignment principles in the light of the requirements by ITER diagnostics. The core CXRS system is used as example throughout this paper. Loads and requirements for in-vessel diagnostic mirrors are looked into. Three basic groups of alignment principles are assessed for applicability in close proximity to the ITER plasma. The strength and weaknesses of the different alignment principles are discussed, and a conclusion about their general applicability in ITER is given.