Charge Exchange Recombination Research Articles

Deuterium charge exchange recombination spectroscopy from the top of the pedestal to the scrape off layer in H-mode plasmas

Recent completion of the thirty two channel main-ion (deuterium) charge exchange recombination spectroscopy (CER) diagnostic on DIII-D [J.L. Luxon, Nucl. Fusion 42 (2002) 614] enables detailed comparisons between impurity and main-ion temperature, density, and toroidal rotation. Sixteen sightlines cover the core of the plasma and another sixteen are densely packed towards the edge, providing high resolution measurements of the pedestal and steep gradient edge region of H-mode plasmas. The complexities of the Dα spectrum require fitting with a comprehensive model, as well as using iterative collisional radiative modeling to determine the underlying thermal deuterium ion properties. Large differences in the structure and magnitude of impurity (C6+) and main-ion (D+) toroidal rotation profiles are seen in the H-mode pedestal. Additionally the D+ temperature can be half the value of the C6+ temperature at the separatrix and shows more of a pedestal structure. Typically only the impurity properties are measured and the main-ion properties are either assumed to be the same, or inferred using neoclassical models, which require validation in the steep gradient region. These measured differences have implications for transport model validation, intrinsic rotation studies, pedestal stability, and the boundary conditions for scrape off layer and plasma material interactions studies.

Effect of wall light reflection in ITER diagnostics

The reflection of light from walls will result in parasitic signals for various optical diagnostics and can be a serious issue in ITER. In this study, we show recent progress in the assessment of the effects of wall reflections in ITER based on ray tracing simulation results. Four different diagnostics in ITER were chosen for the simulation, i.e. visible spectroscopy, infrared thermography, edge laser Thomson scattering, and charge exchange recombination spectroscopy.

Spatially resolved spectroscopic ion temperature measurements at plasma edge of the T-10 tokamak.

Spectroscopic diagnostics of the edge ion temperature were developed on the T-10 tokamak. Spatially resolved measurements of C5+ and other ionization states of carbon spectral line shapes are provided. Spectra were measured with high spectral resolution using 14 lines of sight in one poloidal section of the tokamak. Each measured line-integrated spectrum contains a combination of multiple local spectra with corresponding values of ion temperature. Modeling of spatial distribution of line emissivity and spectral line shapes along the lines of sight allows the reconstruction of the ion temperature profile on the basis of the closest match of measured and modeled spectra. The fine structure of spectral line, Zeeman effect, and apparatus function are taken into account during data processing. Obtained ion temperature profiles, Ti(r), at the plasma edge are in good agreement with ion temperature profiles measured by Charge eXchange Recombination Spectroscopy (CXRS) diagnostics of T-10. Use of the CXRS equipment for measurements of passive spectra can provide additional information on the temporal evolution of the edge ion temperature. Developed diagnostics provide necessary data for the research of geodesic acoustic modes, which are strongly dependent on plasma edge ion temperature.

Pedestal and Er profile evolution during an edge localized mode cycle at ASDEX Upgrade

The upgrade of the edge charge exchange recombination spectroscopy diagnostic at ASDEX Upgrade has enabled highly spatially resolved measurements of the impurity ion dynamics during an edge-localized mode cycle (ELM) with unprecedented temporal resolution, i.e. 65 μs. The increase of transport during an ELM induces a relaxation of the ion, electron edge gradients in impurity density and flows. Detailed characterization of the recovery of the edge temperature gradients reveals a difference in the ion and electron channel: the maximum ion temperature gradient is re-established on similar timescales as , which is faster than the recovery of . After the clamping of the maximum gradient, Ti and Te at the pedestal top continue to rise up to the next ELM while ne stays constant which means that the temperature pedestal and the resulting pedestal pressure widen until the next ELM. The edge radial electric field Er at the ELM crash is found to reduce to typical L-mode values and its maximum recovers to its pre-ELM conditions on a similar time scale as for ne and Ti. Within the uncertainties, the measurements of Er align with their neoclassical predictions for most of the ELM cycle, thus indicating that Er is dominated by collisional processes. However, between 2 and 4 ms after the ELM crash, other contributions to flow, e.g. zonal flows or ion orbit effects, could not be excluded within the uncertainties.

Radiation In-Port Cross-Talks for ITER Port Diagnostics

This paper emphasizes the need of estimation of the mutual influence, called “cross-talk,” for neutronic analyses of neighboring diagnostics systems shared by the same ITER port. Using examples of several diagnostic systems inserted inside the ITER Equatorial and Upper Port Plugs, we have demonstrated this mutual influence. Cross-talk effects have been shown by examining the radiation environment inside the port plug in terms of neutron energy spectra and Shut-Down Dose Rate (SDDR) inside the Port Interspace (PI) area. In-port cross-talk was investigated for the diagnostic systems deployed in two Equatorial Port Plugs (EPP) #17 and #8, and for the components of Upper Port Plug (UPP) #3. One example of in-port cross-talks is a gamma shadow effect of the Tritium and Deposit Monitor (TDM) shield block, which affects the SDDR inside the PI of EPP#17. Where the gamma radiation originated from the dominant radioactive sources of the irradiated structures of Core-Imaging X-ray Spectrometer (CIXS) is blocked by the TDM shield. Another example is an influence of neutron streaming along the Fast Ion Loss Detector (FILD) channel on the neutron energy spectra calculated in the Tangential Neutron Spectrometer (TNS) in EPP#8. For the example of UPP#3 with Charge eXchange Recombination Spectroscopy (CXRS-core), performed neutronic analysis identified excessive neutron streaming along the CXRS shutter, which must be reduced by further design iterations.

Radial impurity flux measuring method with plasma heating in general geometry

Devising a means to measure the radial impurity flux across the pedestal could be used to reduce impurity accumulation, if not prevent it while providing natural fueling, and thus improving fusion performance in tokamaks. We employ a novel solution procedure that takes advantage of the poloidal flow measurement to obtain the radial impurity flux directly from available diagnostics, such as charge exchange recombination spectroscopy and Thomson scattering. In the absence of our procedure, a computationally demanding kinetic calculation of the full bulk ion response would be required at finite aspect ratio for the flux surface shape of interest. The more general form of the model considered here permits large toroidal impurity flow on the order of the impurity thermal speed. Moreover, it allows plasma heating techniques to be employed to actively modify the poloidal variation of the potential to adjust the location of impurity accumulation and thereby alter the radial impurity flux.

Design of Ultra-Fast Charge eXchange Recombination Spectroscopy diagnostic on EAST tokamak

In this article, we present the design of a four-channel Ultra-Fast Charge eXchange Recombination Spectroscopy (UF-CXRS) diagnostic on the EAST tokamak. This diagnostic is based on the active charge exchange to measure ion temperature with a time resolution at the order of 1 μ s and a spatial resolution of 1 cm. The design of the main components is carefully discussed, including the dichroic mirror light path, the fiber, the spectrometer and the customized lens. The theoretical estimation of the emission photon flux has been carried out, proving the feasibility of the whole design.

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.

A fast edge charge exchange recombination spectroscopy system at the ASDEX Upgrade tokamak.

In this work, a new type of high through-put Czerny-Turner spectrometer has been developed which allows us to acquire multiple channels simultaneously with a repetition time on the order of 10 μs at different wavelengths. The spectrometer has been coupled to the edge charge exchange recombination system at ASDEX Upgrade which has been recently refurbished with new lines of sight. Construction features, calibration methods, and initial measurements obtained with the new setup will be presented.

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 forms of the Coulomb approximation as a useful source of atomic data for the spectroscopy of astrophysical and fusion plasmas

The Coulomb approximation (CA) has long been regarded as a useful tool for rapid estimates of line strengths, absorption oscillator strengths, and spontaneous transition probabilities of the lighter multi-electron atoms and ions, in situations where large quantities of atomic data are required for the analysis of spectroscopic measurements from a variety of plasma sources, in particular interesting stellar objects (e.g. white dwarf stars) and magnetically confined fusion plasmas. This applies especially in cases where the plasma is spatially inhomogeneous, and produces several ionisation stages of the same impurity element, emitting copious radiation in bound–bound transitions from cascade processes following charge-exchange recombination. While more advanced theoretical methods are routinely used by the specialist, the CA provides a very convenient method of checking atomic data chosen by the experimentalist from extensive compilations through the internet, or by the use of machine codes provided by others. The origins, advantages and shortcomings of the method are described and discussed, as well as convenient modifications thereof, which may readily be implemented for these purposes. Particular attention is paid to the choice of electron coupling of states in which the optical electron has a large orbital angular momentum The text is illustrated by numerous examples of application to spectra of practical interest from astrophysical and laboratory plasmas.

Determination of radial electric field from Pfirsch–Schlüter flows in the HSX stellarator

Inboard/outboard asymmetry in the impurity ion parallel flow is observed in the HSX stellarator using charge exchange recombination spectroscopy (CHERS). This observation shows the presence of counter-streaming Pfirsch–Schlüter flow predicted by neoclassical theory. The asymmetry of the flow is used to calculate the magnitude and direction of the radial electric field (Er), as well as the mean flow, using computed magnetic geometry factors. This method enables calculation of Er near the core of the HSX plasma where the Er obtained from the radial force balance equation has large uncertainties due to the relatively large width of the diagnostic neutral beam with respect to the plasma minor radius.

Radial localization of edge modes in Alcator C-Mod pedestals using optical diagnostics

Dedicated experiments in ion cyclotron range heated enhanced D-alpha (EDA) H-mode and I-mode plasmas have been performed on Alcator C-Mod to identify the location of edge fluctuations inside the pedestal and to determine their plasma frame phase velocity. For this purpose, measurements from gas puff imaging (GPI) and gas puff charge exchange recombination spectroscopy (GP-CXRS) have been collected using the same optical views. The data suggest that the EDA H-mode-specific quasi-coherent mode (QCM) is centered near the radial electric field (Er) well minimum and propagates along the ion diamagnetic drift direction in the plasma frame. The weakly coherent mode (WCM) and the geodesic acoustic mode observed in I-mode, on the other hand, are found to be located around the outer shear layer of the Er well. This results in a weak plasma frame phase velocity mostly along the electron diamagnetic drift direction for the WCM. The findings in these EDA H-mode plasmas differ from probe measurements in ohmic EDA H-mode (LaBombard et al 2014 Phys. Plasmas 21 056108), where the QCM was identified as an electron drift-wave located several mm outside the Er well minimum in a region of positive Er. To explore if instrumental effects of the optical diagnostics could be the cause of the difference, a synthetic diagnostic for GPI is introduced. This diagnostic reproduces amplitude ratios and relative radial shifts of the mode profiles determined from poloidally and toroidally oriented optics and, if instrumental effects related to GP-CXRS are also included, indicates that the measured location of the QCM and WCM relative to the Er well reported here is only weakly affected by instrumental effects.

CAD-Based Shielding Analysis for ITER Port Diagnostics

Radiation shielding analysis conducted in support of design development of the contemporary diagnostic systems integrated inside the ITER ports is relied on the use of CAD models. This paper presents the CAD-based MCNP Monte Carlo radiation transport and activation analyses for the Diagnostic Upper and Equatorial Port Plugs (UPP #3 and EPP #8, #17). The creation process of the complicated 3D MCNP models of the diagnostics systems was substantially accelerated by application of the CAD-to-MCNP converter programs MCAM and McCad. High performance computing resources of the Helios supercomputer allowed to speed-up the MCNP parallel transport calculations with the MPI/OpenMP interface. The found shielding solutions could be universal, reducing ports R&D costs. The shield block behind the Tritium and Deposit Monitor (TDM) optical box was added to study its influence on Shut-Down Dose Rate (SDDR) in Port Interspace (PI) of EPP#17. Influence of neutron streaming along the Lost Alpha Monitor (LAM) on the neutron energy spectra calculated in the Tangential Neutron Spectrometer (TNS) of EPP#8. For the UPP#3 with Charge eXchange Recombination Spectroscopy (CXRS-core), an excessive neutron streaming along the CXRS shutter, which should be prevented in further design iteration.

Initial operation of the NSTX-Upgrade real-time velocity diagnostic

A real-time velocity (RTV) diagnostic based on active charge-exchange recombination spectroscopy is now operational on the National Spherical Torus Experiment-Upgrade (NSTX-U) spherical torus (Menard et al 2012 Nucl. Fusion 52 083015). The system has been designed to supply plasma velocity data in real time to the NSTX-U plasma control system, as required for the implementation of toroidal rotation control. Measurements are available from four radii at a maximum sampling frequency of 5 kHz. Post-discharge analysis of RTV data provides additional information on ion temperature, toroidal velocity and density of carbon impurities. Examples of physics studies enabled by RTV measurements from initial operations of NSTX-U are discussed.

Edge rotation from momentum transport by neutrals

Due to their high cross field mobility, neutral atoms can have a strong effect on transport even at the low relative densities found inside the separatrix. We use a charge-exchange dominated model for the neutrals, coupled to neoclassical ions, to calculate momentum transport when it is dominated by the neutrals. We can then calculate self-consistently the radial electric field and predict the intrinsic rotation in an otherwise torque-free plasma. Using a numerical solver for the ion distribution to allow arbitrary collisionality, we investigate the effects of inverse aspect ratio and elongation on plasma rotation. We also calculate the rotation of a trace carbon impurity, to facilitate future comparison to experiments using charge exchange recombination spectroscopy diagnostics.

Modeling of helium transport and exhaust in the LHD edge

Experimental results from LHD show a reduction of helium concentration in the plasma with the introduction of a magnetic island on the m/n = 1/1 resonant surface in the plasma edge. Simulations of the plasma with and without the island are carried out with the coupled code EMC3-EIRENE and compared to charge exchange recombination spectroscopy measurements of ionized core helium, and visible spectroscopy measurements of edge neutral helium. The numerical simulations indicate that the experimental parameters lie in a high density regime where the impurity transport is dominated by the outward directed friction force. The EMC3-EIRENE simulations capture the reduction in helium transport well and indicate that: (1) the reduction in core helium is a result of increased outward transport caused by the magnetic island and an increased opening of the edge-surface layer to the divertor plates; (2) the dominant source of neutral helium is best modeled by recycled helium at the targets; and (3) ionized helium density profiles are best matched in the simulations when there is a large core helium source in addition to a smaller edge source.

Ion temperature and toroidal rotation in JET's low torque plasmas.

This paper reports on the procedure developed as the best method to provide an accurate and reliable estimation of the ion temperature Ti and the toroidal velocity vϕ from Charge-eXchange Recombination Spectroscopy (CXRS) data from intrinsic rotation experiments at the Joint European Torus with the carbon wall. The low impurity content observed in such plasmas, resulting in low active CXRS signal, alongside low Doppler shifts makes the determination of Ti and vϕ particularly difficult. The beam modulation method will be discussed along with the measures taken to increase photon statistics and minimise errors from the absolute calibration and magneto-hydro-dynamics effects that may impact the CXRS passive emission.

Fast-ion Dα spectrum diagnostic in the EAST.

In toroidal magnetic fusion devices, fast-ion D-alpha diagnostic (FIDA) is a powerful method to study the fast-ion feature. The fast-ion characteristics can be inferred from the Doppler shifted spectrum of Dα light according to charge exchange recombination process between fast ions and probe beam. Since conceptual design presented in the last HTPD conference, significant progress has been made to apply FIDA systems on the Experimental Advanced Superconducting Tokamak (EAST). Both co-current and counter-current neutral beam injectors are available, and each can deliver 2-4 MW beam power with 50-80 keV beam energy. Presently, two sets of high throughput spectrometer systems have been installed on EAST, allowing to capture passing and trapped fast-ion characteristics simultaneously, using Kaiser HoloSpec transmission grating spectrometer and Bunkoukeiki FLP-200 volume phase holographic spectrometer coupled with Princeton Instruments ProEM 1024B eXcelon and Andor DU-888 iXon3 1024 CCD camera, respectively. This paper will present the details of the hardware descriptions and experimental spectrum.