Active Charge Exchange Recombination Spectroscopy Research Articles

Publisher's Note: "Measurements of impurity ion temperature and velocity distributions via active charge-exchange recombination spectroscopy in C-2W" [Rev. Sci. Instrum. 92, 053512 (2021)

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Measurements of impurity ion temperature and velocity distributions via active charge-exchange recombination spectroscopy in C-2W.

In TAE Technologies' C-2W experiment, electrode biasing is utilized for boundary control of a field-reversed configuration (FRC) plasma embedded in a magnetic mirror. Understanding the underlying physics associated with FRC rotation, stabilization, and heating is crucial for improving machine performance. Impurity ion rotation and temperature are sensitive to biasing effects, and measurements of these quantities can provide insight into important plasma dynamics and overall effectiveness of the biasing system. To this end, a charge-exchange recombination spectroscopy (ChERS) diagnostic was developed and deployed to measure local impurity ion temperature and velocity in the confinement vessel of C-2W. The system utilizes a new diagnostic neutral beam (40 keV, 8.5A) and a fiber-coupled spectrometer with an image-intensified high-speed camera to measure beam-induced spectral line emission at multiple lines-of-sight. Design details and the first experimental results obtained with this new diagnostic are presented and discussed.

A new type of ionizer for charge-exchange particle analyzer.

The application of effective methods of charge-exchange neutral particle diagnostics to determine ion temperatures of plasmas in modern plasma installations is of great practical interest. In this paper, we propose a new type of ionizer based on ionization of neutral charge-exchange particles reflecting from metallic surfaces. A set of Ta plates is used as a reflecting surface converter. The experimental results of using the proposed ionizer on the T-10 tokamak installation are presented. The obtained ion temperature is compared with temperatures derived by other methods for ohmic plasma heating. It is shown that the determined ion temperature has an error of 10% and agrees well with values calculated by the Artsimovich formula and obtained by the active Charge eXchange Recombination Spectroscopy. The proposed analyzer for ion temperature determination has an enhanced sensitivity in comparison with other analyzers.

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.

Measurements of ion temperature and plasma hydrogenic composition by collective Thomson scattering in neutral beam heated discharges at TEXTOR

A method is developed to perform plasma composition and ion temperature measurements across the plasma minor radius in TEXTOR based on ion cyclotron structures in collective Thomson scattering spectra. By gradually moving the scattering volume, we obtain measurements across the outer midplane of the plasma. Results for the ion temperature are compared with ion temperatures measured by active charge-exchange recombination spectroscopy.

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.

A real-time velocity diagnostic for NSTX

A new system for fast measurements of the plasma toroidal velocity has been installed on the National Spherical Torus Experiment, NSTX [M. Ono et al., Nucl. Fusion 40, 557 (2000)]. The diagnostic, based on active charge-exchange recombination spectroscopy, can measure at up to six radial locations with maximum sampling rate of 5 kHz. The system is interfaced in real time with the NSTX plasma control system, in order to feed back on plasma velocity by means of actuators such as neutral beams and external coils. The paper describes the design criteria and implementation of the diagnostic. Examples from the initial tests of the system during neon glows are also discussed.

Investigating the possibility of a monitoring fast ion diagnostic for ITER

In burning plasma fusion devices, fast ion transport plays a central role in the performances of the machines. Moreover the losses of energetic particles might cause severe damages on plasma facing components. Therefore real time measurements of fast ion transport would provide valuable information for safe and reliable plasma operations. In this paper, we examine the feasibility of a monitoring system based on active charge exchange recombination spectroscopy making use of the 0.5 MeV/amu ITER heating neutral beams for detecting fast (4)He(+2) (alphas) particles in ITER plasmas. There are two time scales relevant to fast ion dynamics: the first is the slowing down time of the distribution function which is of the order of 1 s, and the second is the time scale of burstlike transport events such as collective Alfven mode excitations, which--for typical ITER plasma parameters--can be as low as 0.2-1 ms. To detect such fast events a broadband high-throughput spectrometer is needed, while for the reconstruction of the alpha velocity distribution function a higher resolution spectrometer and longer integration time are necessary. To monitor a spatial redistribution of fast particles due to the propagation of the instability, it is proposed to use a limited number of spatial channels, looking at the charge exchange He II spectra induced by the heating beams, whose energy matches the slowing down energies of fast particles. The proposal is to share the motional stark effect periscope on equatorial port 3 [A. Malaquias et al., Rev. Sci. Instrum. 75, 3393 (2004)] adding additional fibers and suitable instruments. A signal to noise ratio of 5 could be achieved with a spatial resolution of a/15 and a time resolution of 5 ms, in a broad spectral band of 100 A, corresponding to the spectral broadening of the line emitted by alpha particles with energies DeltaE < or = 1.5 MeV. Fast H and D ion populations created by heating neutral beam or ion cyclotron resonance heating are expected to produce significantly lower charge exchange signal levels and can only be monitored on substantially longer time scales as it is expected because of the strong energy difference with respect to the heating neutral beam and the consequently low charge exchange cross sections.

The NSTX fast-ion D-alpha diagnostic

A new diagnostic, aimed at energy-resolved measurements of the spatial and temporal dynamics of fast ions in NSTX plasmas, is described. It is based on active charge-exchange recombination spectroscopy. The fast-ion signal is inferred from light emitted in the wavelength range of the D(alpha) line by fast ions recombining with an injected neutral beam. Two complementary systems are operational. The first system, based on a spectrometer coupled to a charge coupled device detector, has 16 channels with space, time, and energy resolution of 5 cm, 10 ms, and 10 keV, respectively. The second system monitors the energy-integrated fast-ion signal on time scales of approximately 20 micros at three different radii. Signals are measured by a multianode photomultiplier tube. For both systems, each channel includes two paired views, intercepting and missing the neutral beam for a direct subtraction of the background signal not associated with fast ions. Examples of signals from the 2008 NSTX run are presented.

Toroidal plasma rotation in the TCV tokamak

The first toroidal rotation measurements in TCV ohmic L-mode plasmas with no external momentum injection are presented. The toroidal velocity profile of the fully stripped carbon species is measured by active Charge eXchange Recombination Spectroscopy with a temporal resolution of typically 90 ms and a spatial resolution of 2.5 cm, about 1/10 of the plasma radius. The observed carbon velocity is of the order of the deuterium diamagnetic drift velocity and up to 1/5 of the deuterium thermal velocity. It is directed opposite to plasma current in the electron diamagnetic toroidal drift direction. The profile reverses when reversing the plasma current. The angular velocity profile is flat, or hollow, inside the sawtooth inversion radius and decreases quasi linearly towards the plasma edge. By vertically shifting the plasma magnetic axis within the TCV vessel the plasma edge velocity profile was measured with high resolution. Such experiments confirm that, close to the limiter, the stationary rotation velocity is close to zero or somewhat positive (co-current directed). This suggests that the angular momentum is not driven from the plasma edge. The maximum carbon velocity scales as vϕ,Max [km s−1] = −12.5Ti/Ip [eV/kA] for a significant range of densities and values of the edge safety factor. Comparison with neoclassical predictions show that the TCV plasma rotation is mainly driven by radial electric fields, with a negligible contribution from the toroidal electric fields. The neoclassical theory of small toroidal rotation quantitatively and qualitatively disagrees with the experimental observation. An alternative empirical equation for the angular momentum flux, able to reproduce the measured stationary profile outside the inversion radius, is proposed.

Results of upgrade of the diagnostic neutral beam injector for the TCV tokamak

A diagnostic neutral beam injector has been developed at BINP for beam emission spectroscopy measurements in TCV tokamak, Lausanne, Switzerland. The beam has been commissioned in 1999. It operates with a beam energy of up to 50 keV, equivalent neutral beam current (for hydrogen) up to 1 A and pulse duration up to 2 s. Plasma in the ion source is produced by inductively coupled 4.6 MHz radio-frequency discharge. Ions are extracted and accelerated by a four-grid ion optical system with 163 circular 4 mm i.d. apertures. The beam is to provide local measurements of plasma ion temperature, velocity, and impurity densities through active charge exchange recombination spectroscopy. The beam parameters of the diagnostic injector enabled to carry out the measurements at plasma density up to 5×1019 m−3. In order to improve signal to noise ratio in the charge exchange recombination spectroscopy (CXRS) measurements and extend the operational density up to 1020 m−3, the diagnostic injector has been upgraded in 2002. As a result, the neutral beam fraction of full energy was increased from 0.5 to 1 equivalent A (eq A). This paper describes the results of the beam parameters measurements after upgrade.

Conceptual design of the charge exchange recombination spectroscopy diagnostic for ITER

Active charge exchange recombination spectroscopy (CXRS) is used in most of the present fusion experiments as a proven tool for local measurements of the main ions in the plasma [R. Isler, Plasma Phys. Controlled Fusion 36, 171 (1994)]. A comprehensive diagnostic coverage of intrinsic and injected impurities is essential for any self-consistent plasma simulation and prediction of plasma performance. In particular, for the assessment of local helium ash densities [M. von Hellermann et al., Plasma Phys. Controlled Fusion 35, 799 (1993)], CXRS will play a key role for future fusion devices such as ITER. However, it should be emphasized that any helium ash analysis is only viable in a fully diagnosed plasma, that is, all other ions need to be measured as well. Two fundamental limitations are considered in the following assessment. First is the detectability of a weak CX signal against a strong background of plasma continuum radiation. A second, equally important requirement is the accuracy with which local neutral beam densities can be established in order to derive absolute ion densities from the extracted active CX signals. The second problem can be approached either by experimental data from beam emission spectroscopy (M. von Hellerman et al., Conference on Advanced Diagnostics for Magnetic and Inertial Fusion Varenna, Sept. 2001), or calculated by beam attenuation codes using electron and both bulk and impurity ion density profiles. This report includes a description of the CXRS measurement concept, the experimental and optical scheme of measurements for ITER (S. Tugarinov et al., Conference on Advanced Diagnostics for Magnetic and Inertial Fusion, Varenna, Sept. 2001), and the main conclusions and recommendation that we have made from the results of a signal-to-noise (SN) value calculations. The main conclusions are the following: CXRS at ITER appears to be a viable diagnostic tool and the projected diagnostic neutral beam will provide adequate SN ratios for spectral analysis. A comprehensive approach to helium ash measurement, making use of a simultaneous measurement of the main low-Z impurities, will considerably increase the overall data consistency and will also significantly reduce the statistical error of each single impurity density determination. Three periscopes located in upper port for the intrinsic impurity measurements will also provide ion temperature, plasma rotation, and deuteron density profiles.

Derivation of local impurity transport quantities from soft-x radiation evolution during tracer injection at W7-AS

A frequently used method for determination of impurity transport coefficients is the analysis of the radial and temporal time evolution of the impurity density of injected tracer impurities. In cases when the impurity density cannot be measured directly, e.g., by active charge exchange recombination spectroscopy, the impurity density has to be reconstructed from impurity radiation as measured, e.g., by the soft-x camera. Due to the energy integrated information, assumption about the spatial ionization state distribution, e.g., coronal equilibrium, is necessary. This might not be valid in cases of large transport. Consequently, the transport quantities, derived under the preliminary assumption of radial coronal distribution, are used again in an iterative way to improve the density reconstruction in order to approach more realistic transport quantities. The procedure was applied to plasma discharges in the stellarator Wendelstein 7-AS.

Measurement and calculation of the radial electric field in the stellarator W7-AS

At the advanced stellarator W7-AS, active charge exchange recombination spectroscopy (CXRS) is used to measure the toroidal and poloidal impurity rotation velocity. The radial electric field is determined experimentally from the radial force balance equation. In parallel, is calculated by means of the neoclassical drift kinetic equation solver (DKES) transport code. Neoclassical calculation and measurement are, in general, in good agreement. A selection of discharges is presented in this paper which shows typical properties of at W7-AS, and which highlights the mutual relationship between and transport.

Charge exchange recombination spectroscopy on the stellarator W7-AS

In the advanced stellarator W7-AS, active charge exchange recombination spectroscopy CXRS is performed on intrinsic plasma impurities. Measurements are done on the doublet lines of the singly charged species of He II, B V, C VI, and N VII. A pulsed diagnostic neutral beam injector excites CX spectral lines in the visible or near UV region. By switching the beam off and on, the electron impact excited spectral light intensity from the cold plasma edge can be subtracted from the signal of interest. The spectral lines are observed after transmission by imaging mirrors and glass fibers by means of Czerny–Turner monochromators with high spectral resolution of down to 0.3 Å per spectral channel. A turnable mirror device allows for the observation at well defined spatial locations within the plasma. As detectors, photomultiplier arrays, and charge coupled device camera systems are used. By spectral measurement of the CX Doppler line intensity, line shift, and line broadening, we evaluate the impurity density and rotation velocity profile and the impurity ion temperature. These parameters provide the transport relevant radial electric field and the ion temperature profile. The radial ion particle and heat transport together with the ambipolar electric field play a decisive role for the optimization of the three-dimensional stellarator plasma.