ITER Core Research Articles

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.

On the numerical implementation of the Closest Point Projection algorithm in anisotropic elasto-plasticity with nonlinear mixed hardening

In finite element analysis, among the possible frameworks for stress-point integration algorithms in computational elastoplasticity, the implicit Closest Point Projection (CPP) algorithm is probably the most used one. The idea behind this algorithm is that all necessary variables, including the flow and hardening directions, are iteratively updated and enforced at the final solution. Therefore the algorithm is fully implicit and the final solution is independent of previous iterations. However, there are several possible implementations of the ideas behind the CPP algorithm. Even though asymptotic quadratic convergence may be obtained in all implementations if the algorithm is properly linearized, these different possibilities result in a different number of local iterations and in a different computational effort. Small stress-integration algorithms are frequently the only iterative core of large strain elastoplastic formulations. At the same time they are responsible for a large share of the overall computational time in finite element simulations and key in the overall robustness. In this work we present a new algorithm based on the ideas of the Closest Point Projection algorithm for anisotropic elastoplasticity with mixed hardening. We also compare our proposal with other possible implementations of the CPP algorithm for the same problem, namely the General CPP implementation and the Governing Parameter Method. We show that our proposal is in general more efficient.

Neutronics analysis for the ITER core imaging X-ray spectrometer

This paper presents new results of the MCNP neutronics analysis for the core imaging X-ray spectrometer (CIXS) system of the ITER Equatorial Port Plug #17 (EPP#17). Substantial radiation shielding design improvements of the CIXS system have been suggested as the outcomes of this analysis. These suggested improvements allow reaching two major goals: (1) radiation protection of the CIXS Port Interspace (PI) to provide personnel access for maintenance of the vacuum extension flange; (2) reduction of the neutron and gamma loads on the detectors to reduce the need for maintenance itself. By implementing the improvements in our models such as filling void spaces around the CIXS beams with boron carbide and inserting the tungsten collimators in the narrowed beam channel, we were able to reduce the Shut-Down Dose Rate (SDDR) in the PI by 100× from 2 mSv/h in the original CIXS design to 20 microSv/h. In case of non-changed MCNP geometry, the D1S method was applied in calculations of SDDR. To allow the maintenance access to the flange, a part of shielding was removed, therefore the R2Smesh methodology was applied instead of D1S. During the maintenance access of CIXS from the PI side, a screen plate was proposed to introduce behind which a worker receives much less SDDR.

LIDAR Thomson scattering for ITER core plasma revisited

The authors have become aware that the development of the hitherto planned time-of-flight Thomson scattering system for the ITER core plasma is not proceeding and that conventional Thomson scattering set-ups are being discussed as an alternative. In this paper, we want to point out the advantages of LIDAR and show how criticized details of the original design can be improved. We present a design of the front optics, which in neutronics terms closely resembles a layout already previously accepted. The presented design does not require Raman scattering calibration for the density measurement. Comparison with the JET Core LIDAR system and simulations at higher temperatures both show that with the new design the specified accuracy can be met with a 1–2 J laser. Current laser and detector technology is reviewed. A strategy for how to proceed is presented.

On the calibration of polarimetric Thomson scattering by Raman polarimetry

Polarimetric Thomson scattering (TS) is an alternative method for the analysis of Thomson scattering spectra in which the plasma temperature Te is determined from the depolarization of the TS radiation. This is a relativistic effect and therefore the technique is suitable only for very hot plasmas (Te > 10 keV) such as those of ITER. The practical implementation of polarimetric TS requires a method to calibrate the polarimetric response of the collection optics carrying the TS light to the detection system, and in particular to measure the additional depolarization of the TS radiation introduced by the plasma-exposed first mirror. Rotational Raman scattering of laser light from diatomic gases such as H2, D2, N2 and O2 can provide a radiation source of predictable intensity and polarization state from a well-defined volume inside the vacuum vessel and is therefore suitable for these calibrations. In this paper we discuss Raman polarimetry as a technique for the calibration of a hypothetical polarimetric TS system operating in the same conditions of the ITER core TS system and suggest two calibration methods for the measurement of the additional depolarization introduced by the plasma-exposed first mirror, and in general for calibrating the polarimetric response of the detection system.

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.

Thermal analysis for optimization of the optical duct of the ITER core CXRS diagnostics

The First Mirror (M1), as part of the ITER core CXRS diagnostics, is responsible for acquisition and transportation of the optical signal from the plasma to the corresponding spectrometer. The M1 is the most vulnerable component of this system working in severe conditions caused by its location in the direct view of the plasma. Based on the numerical analysis an optimized arrangement of baffles was found for the optical channel. It is shown that such measures like the deeper M1 positioning in the shielding, duct configuration and baffles implementation made it possible to reduce heat loads on the M1 by a factor of about 102–103.

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.

A dynamic homogenization model for pebble bed reactors

We have used a macro-stochastic model to provide a new homogenization paradigm for pebble bed core calculations. Our method results in an iterative core homogenization for which each homogenized region accounts for interactions between different pebble types and the neighboring regions. Hence, at convergence pebble depletion and power computation are done using the fine-group transport fluxes. We discuss the validation of the method and give a new and accurate estimation of the critical height for the HTR-10 international benchmark. Finally, we introduce a general formulation for the search for the equilibrium core with a multi-pass fuel management where the pebble recycling strategy is defined by a limit of the actual pebble burnup in the core, as opposed to a fixed number of passes through the core. We also extend this formulation to account for a radial loading distribution.

Network decomposition into fixed points of degree peeling

Degree peeling is used to study complex networks. It is a decomposition of the network into vertex groups of increasing minimum degree. However, the peeling value of a vertex is non-local in this context since it relies on the number of connections the vertex has to groups above it. We explore a different way to decompose a network into edge layers such that the local peeling value of the vertices on each layer does not depend on their non-local connections with the other layers. This corresponds to the decomposition of a graph into subgraphs that are invariant with respect to degree peeling, i.e., they are fixed points. We introduce a general method to partition the edges of an arbitrary graph into fixed points of degree peeling called the iterative edge core decomposition. Information from this decomposition is used to formulate a novel notion of vertex diversity based on Shannon’s entropy. We illustrate the usefulness of this decomposition on a variety of social networks including weighted graphs. Our method can be used as a preprocessing step for community detection and graph visualization.

Dual-laser calibration of Thomson scattering systems in ITER and RFX-mod

We first review the principles of the dual-laser calibration technique for measuring the relative sensitivities of the spectral channels in a Thomson scattering (TS) diagnostic system by detecting with the same spectrometer the spectra scattered by the same plasma volume from two laser pulses of different wavelengths. A new data analysis method is then introduced, based on the minimization of a single χ2 function, that provides a simpler and more convenient way to determine the measurement errors on the calibration coefficients. The new analysis method is used here to investigate the expected performances of this calibration technique in the core LIDAR TS system of ITER currently under design and in the conventional multipoint TS system of RFX-mod. By calculating the expected calibration errors for typical plasma scenarios we discuss the different possible choices of the calibration laser, the characteristics of the calibrating plasma and other system parameters with an impact on the application of the technique. For ITER core LIDAR TS, designed with Nd : YAG at 1064 nm as main laser, a ruby laser shows slightly better performances as a calibration laser compared with a second harmonic Nd : YAG and a calibration accuracy ∼1% can be achieved in a relatively small number of pairs of laser pulses. In RFX-mod the combination of a Nd : YAG and a Nd : YLF laser systems is the only viable choice, and we find that, in spite of the small difference between the two wavelengths (λ = 1064 nm and λ = 1053 nm, respectively), dual-laser calibration is still possible to the required accuracy with an affordable number of pairs of laser shots.

Analysis and modelling of semantic attacks in online social networks

The emergence of online social networks as an important media for communication and information dissemination during the last decade has also seen the increase in abuse of the media to spread misinformation, disinformation and propaganda. Detecting different types of semantic attacks possible in online social networks would require their accurate classification. Drawing similarities with other social computing systems like recommender systems, we propose a new taxonomy for semantic attacks in online social networks. We propose an algorithm which uses social network as a medium of social computing to analyse patterns of propagation of information and identify sources of disinformation in them. We construct a new information repropagation graph from social network data and carry out iterative core analysis of the graph to isolate possible contents of misinformation and user nodes which are involved in their propagation. We used seven different datasets obtained from Twitter to validate our results. We also propose an information propagation model for deliberate spread of false information in online social networks.

Status of the JET LIDAR Thomson scattering diagnostic

The LIDAR Thomson scattering concept was proposed in 1983and then implemented for the first time on the JET tokamak in 1987. A numberof modifications were performed and published in 1995, but since then nomajor changes were made for almost 15 years. In 2010 a refurbishment of thediagnostic was started, with as main goals to improve its performance and totest the potential of new detectors which are considered as candidates forITER. During the subsequent years a wide range of activities was performedaimed at increasing the diagnostic's light throughput, improvement of signalto noise ratio and amendment of the calibration procedures. Previously usedMA-2 detectors were replaced by fast GaAsP detectors with much higheraverage QE. After all the changes were implemented, a significantimprovement of the measured data was achieved. Statistical errors ofmeasured temperature and density were reduced by a factor of 2 or more,depending on plasma conditions, and comfortably surpassed the valuesrequested for ITER Core Thomson Scattering (10% for Te and 5% forne). Excellent agreement with other diagnostics (conventional HighResolution Thomson Scattering, ECE, Reflectometer) was achieved over a widerange of plasma conditions. It was demonstrated that together with long termreliability and modest access port requirements, LIDAR can providemeasurements of a quality similar to a conventional imaging ThomsonScattering instrument.

Availability Requirement for ITER Plasma Parameter Measurements

The diagnostics are critical systems for the operation and research program of ITER, as they are used to protect the components, to control the plasma discharge, to understand the behaviour of the plasma and to measure its performance. Representing more than 50 subsystems, they add an extra level of complexity and technical challenge to the project. In order to ensure that the overall ITER machine will be able to reach its ambitious availability objective, a Reliability, Availability, Maintainability and Inspectability (RAMI) approach has been defined and applied to the diagnostics as well as all the other ITER core systems.The purpose of this study is to estimate the inherent availability of the measurements groups used for machine protection and basic control in order to ensure that the ITER machine reaches its inherent availability objectives.

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.

Fluctuation BES measurements with the ITER core CXRS prototype spectrometer

The ITER core CXRS diagnostic system collects the light emitted from the interaction of the diagnostic neutral beam with the core plasma and guides it via a mirror labyrinth through the upper port plug #3 towards a fiber bundle, which then transmits the light into a set of spectrometers for spectral analysis. In order to test the accessibility of the special parameter range required for the ITER measurement, a prototype spectrometer was built and operated successfully at the TEXTOR tokamak. In addition to the He/Be, C/Ne and H/D/T regular spectral channels, a fluctuation beam emission spectroscopy (BES) system has been integrated to measure core MHD activity, and validate corresponding ITER simulation results. The fluctuation system can be operated as an alternative to the spectral BES measurement, and has 8 spatial channels sampled at 2MHz. In this paper, we present details of the fluctuation BES system and its interface to the ITER prototype spectrometer along with simulation and measurement results at TEXTOR. We show that the measurement fully confirms the simulation results on achievable photon current at the detector and on the signal to noise ratio.

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.