Charge Exchange Spectroscopy Research Articles

Measurement of small island characteristics using high resolution ECE and CER at DIII-D

Abstract The measurements using the high resolution electron cyclotron emission radiometry and the charge exchange and recombination spectroscopy are processed using analytic formulas to allow for the detection of islands as small as 1.9 cm. In contrast to large, saturated magnetic islands which are relatively well understood to be governed by the loss of bootstrap current inside the island, small islands are less well understood due to the difficulty of their accurate measurement in tokamaks. Here, ‘small’ islands are islands comparable in size to the ion banana width, which can be as small as 0.8 cm at DIII-D. The new measurement methods allow for the detection of small island widths when the predicted increase of mode frequency to match the Doppler shifted ion diamagnetic frequency is observed. Therefore, for the first time, the mode frequency increase can be unambiguously associated to the acceleration of the magnetic island propagation. Such association allows for a further development and validation of the much-debated theory of ion polarization currents, which is thought to govern the small island growth.

Kinetic profile inference with outlier detection using support vector machine regression and Gaussian process regression

Abstract We propose an outlier-resilient Gaussian process regression (GPR) model supported by support vector machine regression (SVMR) for kinetic profile inference. GPR, being a non-parametric regression using Bayesian statistics, has advantages in that it imposes no constraints on profile shapes and can be readily used to integrate different kinds of diagnostics, while it is vulnerable to the presence of even a single outlier among a measured dataset. As an outlier classifier, an optimized SVMR is developed based only on the measurements. Hyper-parameters of the developed GPR model with informative prior distributions are treated in two different ways, i.e. maximum a posteriori (MAP) estimator and marginalization using a Markov Chain Monte Carlo sampler. Our SVMR-supported GPR model is applied to infer ion temperature Ti profiles using measured data from the KSTAR charge exchange spectroscopy system. The GPR-inferred Ti profiles with and without an outlier are compared and show prominent improvement when the outlier is removed by the SVMR. Ti profiles inferred with the MAP estimator and the marginalization scheme are compared. They are noticeably different when observation uncertainties are not small enough, and the marginalization scheme generally provides a smoother profile.

Visible core spectroscopy at Wendelstein 7-X.

This paper presents an overview of recent hardware extensions and data analysis developments to the Wendelstein 7-X visible core spectroscopy systems. These include upgrades to prepare the in-vessel components for long-pulse operation, nine additional spectrometers, a new line of sight array for passive spectroscopy, and a coherence imaging charge exchange spectroscopy diagnostic. Progress in data analysis includes ion temperatures and densities from multiple impurity species, a statistical comparison with x-ray crystal spectrometer measurements, neutral density measurements from thermal passive Balmer-alpha emission, and a Bayesian analysis of active hydrogen emission, which is able to infer electron density and main ion temperature profiles.

The CXSFIT spectral fitting code: Past, present and future.

Magnetically confined plasma experiments generate a wealth of spectroscopic data. The first step toward extracting physical parameters is to fit a spectral model to the often complex spectra. The CXSFIT (Charge eXchange Spectroscopy FITting) spectral fitting code was originally developed for fitting charge exchange spectra on JET from the late 1980s onward and has been further developed over decades to keep up with the needs of the users. The primary use is to efficiently fit a large number of spectra with many constrained Gaussian spectral lines of which the physical parameters can be coupled in a user-friendly manner. More recent additions to the code include time-dependent couplings between parameters, flexible background subtraction, and a non-linear coupling scheme between fit parameters. The latter was a pre-requisite for implementing Zeeman and motional Stark effect multiplets in the library of spectral features. The ability to save and replay "fit recipes," even when multiple iterations are required, has ensured the traceability of the results and is one of the keys to the longevity and success of the code. The code is also in use on other tokamaks (AUG, ST-40) and to fit data from other spectroscopic diagnostics on JET. In this paper, we document the current capabilities and philosophy behind the structure of the code, including some of the algorithms used to calculate spectral features numerically efficiently. We also provide an outline of how CXSFIT could be transferred into a framework that would be able to meet the spectral fitting requirements of future devices, such as ITER.

Charge Exchange Spectroscopy of Multiply Charged Erbium Ions

The origin of heavier elements than iron is still under discussion, and recent studies suggest that the contribution of the r-process in neutron star mergers is dominant. Future modeling of such processes will require a huge amount of spectroscopic data on multiply charged ions of heavy elements. However, these experimental data are extremely scarce for heavy elements. In this work, we have performed the measurements of charge exchange spectroscopy for multiply charged Er ions in the visible light range. We report observed emission lines from multiply charged Er ions and their identification based on theoretical estimates.

Direct observation of mass-dependent collisionless energy transfer via Landau and transit-time damping

The energy transfer from wave to particle occurs in collisionless plasma through the interaction between particle and wave, associated with the deformation of ion velocity space from Maxwell-Boltzmann distribution. Here we show the direct observation of mass-dependent collisionless energy transfer via Landau and transit-time damping in a laboratory plasma. The Landau and transit-time damping are confirmed by the bipolar velocity-space signature of the ion velocity distribution function, measured by fast charge exchange spectroscopy with a time resolution less than ion-ion collision time. The excellent agreement between the resonant phase velocity evaluated from the bipolar velocity-space signature and the wave’s phase velocity, estimated from the frequency of the magnetohydrodynamics oscillation measured with the plasma displacement is clear evidence for the Landau damping. The energy transfer from solitary wave to fully ionized carbon impurity ions is larger than that of bulk ions 2-3 times due to heavier mass.

The measurement of carbon density profile using charge exchange spectroscopy in KSTAR

The first measurement of the impurity density profile via charge exchange spectroscopy (CES) has been successfully achieved in KSTAR. Since the neutral beam density profile is essential for the measurement, the neutral beam penetration code that was originally developed for the Alcator C-mod tokamak has been optimized for the KSTAR experimental environment. The method of the impurity density measurement by the KSTAR CES system is introduced and the sensitivity analysis of various physical parameters, such as the effective charge in the estimation of the impurity density, is performed to examine the validation of the method. This method has been applied to measure the C6+ density profile affected by the resonant magnetic perturbations (RMPs), which is mainly used to suppress the edge localized modes (ELMs) in KSTAR. The dynamics of the C6+ density profile represent that the C6+ density decreases immediately after the application of RMPs but recovers soon during the ELM-suppressed phase.

Charge exchange spectroscopy using spatial heterodyne spectrometer in the large helical device.

In this study, the use of a spatial heterodyne spectrometer (SHS) to measure the toroidal flow velocity (Vf) and the ion temperature (TC6+ ) of the C6+ impurity ion by charge exchange spectroscopy was explored. The instrumental width (IW) of the SHS (aperture size = 16.77 mm2, etendue = 2.9867 mm2sr) was extrapolated to be 0.09nm, which is half of the 0.17nm IW extrapolated for a conventionally used dispersive spectrometer (DS) (aperture size = 2.6 mm2, etendue = 0.2605 mm2sr). The resulting Vf and TC6+ measurements were found to be in good agreement with those measured using the DS.

Kinetic equilibrium reconstruction and the impact on stability analysis of KSTAR plasmas

High fidelity kinetic plasma equilibrium reconstructions are an essential requirement for accurate stability and disruption prediction analyses to support continuous operation of high beta tokamak plasmas. The present kinetic equilibrium reconstructions of plasmas in the KSTAR device include plasma density and temperature profiles from Thomson scattering and ion temperature from charge exchange spectroscopy diagnostics, and allowance for fast particle pressure. In addition, up to 25 channels of motional Stark effect diagnostic data are used to constrain the magnetic field pitch angle profile in the plasma to produce a reliable computation of the safety factor, q, profile. H-mode plasmas exhibit clear pedestal characteristics in the reconstructed pressure profile compared to internal transport barrier or L-mode plasmas. The plasma configuration and vertical position of inner strike points are validated by CCD and infrared camera images. Ideal and resistive magnetohydrodynamic (MHD) stability analyses using the DCON and resistive DCON codes utilize these kinetic equilibrium reconstructions to compare to experimental plasma stability. Equilibria with sufficiently low convergence error can provide reliable computation of ideal and resistive MHD stability analysis.

Studies of the halo effect in the charge exchange recombination spectroscopy edge measurements at ITER

Active charge exchange spectroscopy will be used for measuring the ion temperature, density and rotation velocity of the ITER plasma. Because of the halo effect, the measurement accuracy can get worse. In this paper, the active spectroscopy measurement errors caused by the halo effect are estimated for ITER. The estimates were performed using the Simulation of Spectra (SOS) code. According to the simulation results, the halo effect decreases the density measurement accuracies by less than 4% and 1% for the impurity and bulk ions, respectively, and the measurement errors for the temperature and velocity are less than 1%. It was also obtained, that the halo effect can decrease the active spectroscopy spatial resolution by 1.5 and 2.5 cm for impurities and bulk deuterium atoms, respectively. The measurement errors were also estimated for determining the deuterium-to-tritium concentration ratio.

Measurements of radial profile of isotope density ratio using bulk charge exchange spectroscopy.

A bulk charge exchange spectroscopy (BCXS) system using a grism (grating prism) spectrometer has been applied to measure the profile of the deuterium (D) fraction in deuterium and hydrogen (H) mixture plasma in the Large Helical Device. The observed spectrum can be fitted with four Gaussian functions successfully by reduction of free parameters for the least-squares fit. The plasma flow velocity and ion temperature profile measured by charge exchange spectroscopy using carbon impurity are used for estimation of the wavelength shift of hot components to reduce the free parameter. The ion temperature is used to estimate the apparent wavelength shift due to the energy dependent emission cross section only and is not used to set the Doppler width for H and D in the fitting. The sensitivity of the evaluated D fraction on the velocity is increased for a higher D fraction. The error of the D fraction is calculated from the error in the fitted parameter and sensitivity on the velocity of the hot component. The difference in the profile and time trace of the D fraction with D pellet and H pellet injection was observed clearly by BCXS using a grism spectrometer.

Development of an ultrafast charge exchange spectroscopy system on the KSTAR tokamak.

An ultrafast charge exchange spectroscopy (UFCES) system has been designed for measuring fluctuations in ion temperature and toroidal rotation velocity. The UFCES on the KSTAR tokamak is a powerful tool for investigating plasma instabilities and long-wavelength turbulence related to ion temperature gradient and flow. The UFCES system is designed to measure the C VI line (n = 8 → 7, λ0 = 529.05nm) from the charge exchange reaction between a deuterium-heating neutral beam and the intrinsic carbon impurity in KSTAR. The ion temperature and toroidal rotation velocity at two radial positions will be observed simultaneously with UFCES. The key difference between the UFCES system and conventional charge exchange spectrometers is the application of high-throughput collection optics, a high-efficiency transmission grating combined with prisms, and a high-speed detector. We use a comprehensive spectrum simulation code with input parameters of KSTAR's plasmas and a neutral beam injection system to estimate the performance of the designed UFCES system. The results simulated with the code show that the diagnostic achieves a turbulence-relevant time resolution of 10 µs with a high enough signal-to-noise ratio. Furthermore, a preliminary test is performed using a complementary metal-oxide semiconductor camera and Ne spectral lamp to confirm the linear dispersion and curvature radius.

Characteristics of plasma parameters and turbulence in the isotope-mixing and the non-mixing states in hydrogen–deuterium mixture plasmas in the large helical device

Characteristics of plasma parameters and turbulence in the isotope-mixing and the non-mixing states in hydrogen-deuterium mixture plasmas in the large helical device are discussed. The isotope mixing state is characterized by the uniform isotope ratio profile regardless of the location of the particle source of each species in the isotope mixture plasma. The isotope non-mixing state is identified by the non-uniform isotope ratio profile measured with bulk charge exchange spectroscopy when the beam fueling isotope species differs from the recycling isotope species. The effect of collisionality, ratio, sign of density gradient on transition between isotope mixing and non-mixing is discussed. The plasma parameters preferable for the non-mixing state are found to be lower collisionality, higher , and negative or zero density gradient (peaked or flat density profile). The time scale of transition from non-mixing to mixing is evaluated by the hydrogen and deuterium pellet injection near the plasma edge and is found to be less than 5 ms, which is much shorter than the particle confinement time. The strong correlation between isotope mixing and turbulence characteristics is observed. This strong correlation suggests the change in turbulence is a strong candidate for the mechanism causing the transition between uniform and non-uniform isotope density ratio profiles.

First impurity powder injection experiments in LHD

Abstract Injection of impurities in the form of sub-millimeter powder grains is performed for the first time in the Large Helical Device (LHD) plasma, employing the Impurity Powder Dropper (IPD) (Nagy et al., 2018), developed and built by PPPL. Controlled amounts of boron (B) and boron nitride (BN) powder are injected into the helical plasma. Visible camera imaging, UV and charge exchange spectroscopy measurements show that the injected impurities effectively penetrate into the plasma in two different magnetic configurations. The prompt effects of the impurities on the plasma are characterized as the injection rate is scanned. The injected impurities provide a supplemental electron source, causing the plasma density to increase, together with the radiated power. Beneficial effects on the confined plasma temperature are observed at low plasma densities, due to an increased efficiency in NBI power absorption. For n e , a v 1 0 19 m − 3 the powder grains penetrate deeper into the plasma, as they can be less effectively deflected by the plasma flow in the divertor leg, which they have to cross first as they are injected from the top of the machine. In this case, the created B ions are observed to move outwards from UV spectroscopy and charge exchange measurements, due to the outwards direction of the radial electric field. This makes low density plasmas a better candidate for powder boronization techniques.

Cross Comparisons of X-Ray Imaging Crystal Spectrometer and Charge Exchange Spectroscopy from KSTAR

Toroidal rotation and ion temperature validation studies utilizing an X-ray imaging crystal spectrometer (XICS) and charge exchange spectroscopy (CES) are continuously investigated from previous experiments as part of an International Tokamak Physics Activity Diagnostics Working Group Joint Experimental Activity. Core toroidal rotation and ion temperature measurements of different impurity species from XICS and CES diagnostics have shown good agreement again with various plasma discharges from KSTAR. The experimental results from the XICS diagnostic show detailed toroidal rotation behavior due to improved time resolution.

Two-Gaussian Fitting Method for Charge Exchange Spectroscopy on Measurements of Ion Temperature and Toroidal Plasma Flow Velocity in the KSTAR Tokamak

Charge exchange spectroscopy has been widely used in fusion devices to measure ion temperature, and toroidal and poloidal flow velocities of plasma. For local measurement, especially in the core region of the plasma, the spectrum emitted by the charge exchange reaction between the main plasma ions or impurity ions and the intentionally injected neutral beam should be analyzed so that parameters can be accurately deduced. Since the line-integrated spectrum signal through the line of sight of the diagnostic optics usually contains an unnecessary overlapped spectrum signal, referred to as the background signal, that typically originates from the plasma boundary region, a beam modulation technique is commonly applied to separate the background signal from the measured spectrum. Recently, it has been demonstrated in the KSTAR tokamak that a two-Gaussian fitting (TGF) method can be applied to analyze the spectrum and deduce plasma ion temperature and toroidal rotation velocity profiles of reasonable accuracy without beam modulation. It has been realized that the measurement result by the TGF method can be alternatively used to investigate plasma transport dynamics when beam modulation is prohibited to avoid any possible disturbance inhibiting robust plasma control and stable operation of the neutral beam injection system.

Analysis of the inter-species power balance in JET plasmas

Most auxiliary heating methods provide heating to more than one particle species (electrons, ions, impurities) in a fusion plasma. This can lead to substantial temperature differences between species, depending on conditions such as heating power to the different species and collisionality, with temperature differences between species limited by inter-species thermal equipartition and transport. The analysis of the steady-state electron-ion and impurity-ion power balances presented in this paper are used for consistency-checking experimental ion and electron temperature measurements and for inferring the main ion temperature from measured impurity temperatures. As ion temperature measurements by charge exchange spectroscopy (CXS) based on impurity ions have become more difficult and time-consuming since the installation of the ITER-like wall (ILW) with Be and W PFC’s, knowing the maximum sustainable temperature difference between ions and electrons, |Ti − Te| allows rejecting erroneous measurements. It also obviates the need for an ion temperature measurement, if an electron temperature measurement is available and |Ti − Te| cannot be larger than the combined errors of the underlying measurements. A power balance analysis is also required for estimating the errors of the ion and electron heat fluxes prior to any species-resolved transport analysis. The ion-impurity temperature differences are usually found to be small due to strong thermal equipartition between ion species. However, they can approach 10% in JET-ILW low density, high power discharges, such the ones under development for a future JET deuterium–tritium campaign (Joffrin et al 2019 Nucl. Fusion). This has a generally small, but not always negligible effect on the calculation of fusion reaction rates, which depend on main ion temperatures. An important outcome of this analysis is that temperature differences between impurity species are always much smaller than between the impurities and hydrogenic species and can usually be neglected. The paper presents two methods for calculating the impurity-to-main ion temperature ratio. Finally, this analysis leads to a method for the reconstruction ion temperature profiles from ion temperature data available at only one or a small number of spatial locations.

Charge exchange UV-Visible and EUV spectroscopy of multiply charged heavy ions

SynopsisTo provide spectroscopic data of multiply charged heavy ions related not only the development of EUV light and soft X-ray sources and also optical measurements of neutron star mergers, charge exchange spectroscopy was performed using an ECR ion source with two spectrometers in EUV and UV-Visible regions.

Validating the ASCOT modelling of NBI fast ions in Wendelstein 7-X stellarator

The first fast ion experiments in Wendelstein 7-X were performed in 2018. They are one of the first steps in demonstrating the optimised fast ion confinement of the stellarator. The fast ions were produced with a neutral beam injection (NBI) system and detected with infrared cameras (IR), a fast ion loss detector (FILD), fast ion charge exchange spectroscopy (FIDA), and post-mortem analysis of plasma facing components. The fast ion distribution function in the plasma and at the wall is being modelled with the ASCOT suite of codes. They calculate the ionisation of the injected neutrals and the consecutive slowing down process of the fast ions. The primary output of the code is the multidimensional fast ion distribution function within the plasma and the distribution of particle hit locations and velocities on the wall. Synthetic measurements based on ASCOT output are compared to experimental results to assess the validity of the modelling. This contribution presents an overview of the various fast ion measurements in 2018 and the current modelling status. The validation and data-analysis is on-going, but the wall load IR modelling already yield results that match with the experiments.

Asymmetry of parallel flow on the Large Helical Device

An asymmetric parallel return flow, which modifies the parallel component of the flow, is expected to meet the zero divergence of the flow on a flux surface based on the common neoclassical theory for torus plasma. The full flow structure is measured by charge exchange spectroscopy on the Large Helical Device. Inboard/outboard asymmetry of the parallel flow is observed according to the full flow profile measurement. Flow asymmetry is considered to be induced by the Pfirsch–Schlüter flow closely associated with the radial electric field. A linear relationship between the integrated flow asymmetry and the electric potential difference is obtained in different magnetic fields and configurations. A model based upon the incompressibility of the flow is applied to acquire a geometric factor hB, which only connects to the magnetic configuration from the experiment. The asymmetric component of the parallel flow measured is compared with the asymmetric component of parallel flow calculated in the incompressibility conditions of flow on the magnetic flux surface. The measured asymmetric flow is consistent with the calculation in plasma with a small toroidal torque input in the inward shifted configuration. However, the measured asymmetric flow is significantly smaller than that calculated for plasma with a large toroidal torque or in the outward shifted configuration. One possible explanation for this variation could be radial transport due to anomalous perpendicular viscosity as well as strongly poloidally asymmetric radial flow.