Vacuum ultraviolet impurity spectroscopy on the Alcator C-Mod tokamak

High-n Rydberg transition spectroscopy for heavy impurity transport studies in W7-X (invited)

Here, we present a novel spectroscopy approach to investigate impurity transport by analyzing line-radiation following high-n Rydberg transitions. While high-n Rydberg states of impurity ions are unlikely to be populated via impact excitation, they can be accessed by charge exchange (CX) reactions along the neutral beams in high-temperature plasmas. Hence, localized radiation of highly ionized impurities, free of passive contributions, can be observed at multiple wavelengths in the visible range. For the analysis and modeling of the observed Rydberg transitions, a technique for calculating effective emission coefficients is presented that can well reproduce the energy dependence seen in datasets available on the OPEN-ADAS database. By using the rate coefficients and comparing modeling results with the new high-n Rydberg CX measurements, impurity transport coefficients are determined with well-documented 2σ confidence intervals for the first time. This demonstrates that high-n Rydberg spectroscopy provides important constraints on the determination of impurity transport coefficients. By additionally considering Bolometer measurements, which provide constraints on the overall impurity emissivity and, therefore, impurity densities, error bars can be reduced even further.

Design of a multichannel vacuum ultraviolet spectroscopy system for SPARC

The design of a vacuum ultraviolet spectroscopy system has been performed to monitor and provide feedback for impurity control in SPARC. The spectrometer, covering a wavelength range of 10-2000 Å through a flat-field configuration with diffraction gratings, incorporates five survey lines of sight. This allows for comprehensive impurity analysis across the core and four divertor regions (inner/outer and upper/lower). Its compact modular design facilitates vertical stacking of each spectrometer unit, significantly reducing space in the tokamak hall, where a dedicated radiation shielding bunker will be built. Safety features include a secondary helium enclosure to mitigate tritium permeation risks during deuterium-tritium (D-T) operations and shielding within the beamlines for enhanced radiation protection. The silicon carbide mirror design for divertor observation ensures its survivability in the in-vessel environment of SPARC, validated by thermal and electromagnetic analysis. Signal modeling and data acquisition testing results show that an exposure time of a few milliseconds is appropriate considering photon flux reaching the detector, demonstrating the system's capability for discharge control that includes disruption avoidance.

Upgrade of vacuum ultraviolet spectroscopy system on J-TEXT tokamak

The vacuum ultraviolet (VUV) spectroscopy system on the Joint Texas Experimental Tokamak has been upgraded to achieve fast acquisition for the study of impurity transport in transient modulated experiments. In this upgrade, the previous high-energy charge-coupled device detector was replaced by a microchannel plate with a CsI-coated photocathode and P43 phosphor to transform the VUV light to visible light, which is then acquired by a high-speed electron-multiplying charge-coupled device. Two-stage focusing was achieved using a reference slit plate illuminated successively by a green light source and the Lyman series hydrogen spectral lines from the vacuum-conditioning plasma. The spatial resolution was evaluated as ∼4 mm based on the level of image blurring from the alignment plate. A response time of ∼2 ms was obtained with the ten-vertical-track setup.

Efficient design and verification of diagnostics for impurity transport experiments

Recent attempts to measure impurity transport in Alcator C-Mod using an x-ray imaging crystal spectrometer and laser blow-off impurity injector have failed to yield unique reconstructions of the transport coefficient profiles. This paper presents a fast, linearized model which was constructed to estimate diagnostic requirements for impurity transport experiments. The analysis shows that the spectroscopic diagnostics on Alcator C-Mod should be capable of inferring simple profiles of impurity diffusion D_Z and convection V_Z accurate to better than ±10% uncertainty, suggesting that the failure to infer unique D_Z and V_Z from experimental data is attributable to an inadequate analysis procedure rather than the result of insufficient diagnostics. Furthermore, the analysis reveals that even a modest spatial resolution can overcome a low time resolution. This approach can be adapted to design and verify diagnostics for transport experiments on any magnetic confinement device.

Responsivity calibration of the LoWEUS spectrometer

We performed an in situ calibration of the relative responsivity function of the Long-Wavelength Extreme Ultraviolet Spectrometer (LoWEUS), while operating on the Lithium Tokamak Experiment (LTX) at Princeton Plasma Physics Laboratory. The calibration was accomplished by measuring oxygen lines, which are typically present in LTX plasmas. The measured spectral line intensities of each oxygen charge state were then compared to the calculated emission strengths given in the CHIANTI atomic database. Normalizing the strongest line in each charge state to the CHIANTI predictions, we obtained the differences between the measured and predicted values for the relative strengths of the other lines of a given charge state. We find that a 3rd degree polynomial function provides a good fit to the data points. Our measurements show that the responsivity between about 120 and 300 Å varies by factor of ∼30.

Three new extreme ultraviolet spectrometers on NSTX-U for impurity monitoring

Three extreme ultraviolet (EUV) spectrometers have been mounted on the National Spherical Torus Experiment-Upgrade (NSTX-U). All three are flat-field grazing-incidence spectrometers and are dubbed X-ray and Extreme Ultraviolet Spectrometer (XEUS, 8-70 Å), Long-Wavelength Extreme Ultraviolet Spectrometer (LoWEUS, 190-440 Å), and Metal Monitor and Lithium Spectrometer Assembly (MonaLisa, 50-220 Å). XEUS and LoWEUS were previously implemented on NSTX to monitor impurities from low- to high-Z sources and to study impurity transport while MonaLisa is new and provides the system increased spectral coverage. The spectrometers will also be a critical diagnostic on the planned laser blow-off system for NSTX-U, which will be used for impurity edge and core ion transport studies, edge-transport code development, and benchmarking atomic physics codes.

A fast-time-response extreme ultraviolet spectrometer for measurement of impurity line emissions in the Experimental Advanced Superconducting Tokamak

A flat-field extreme ultraviolet (EUV) spectrometer working in the 20-500 Å wavelength range with fast time response has been newly developed to measure line emissions from highly ionized tungsten in the Experimental Advanced Superconducting Tokamak (EAST) with a tungsten divertor, while the monitoring of light and medium impurities is also an aim in the present development. A flat-field focal plane for spectral image detection is made by a laminar-type varied-line-spacing concave holographic grating with an angle of incidence of 87°. A back-illuminated charge-coupled device (CCD) with a total size of 26.6 × 6.6 mm(2) and pixel numbers of 1024 × 255 (26 × 26 μm(2)/pixel) is used for recording the focal image of spectral lines. An excellent spectral resolution of Δλ0 = 3-4 pixels, where Δλ0 is defined as full width at the foot position of a spectral line, is obtained at the 80-400 Å wavelength range after careful adjustment of the grating and CCD positions. The high signal readout rate of the CCD can improve the temporal resolution of time-resolved spectra when the CCD is operated in the full vertical binning mode. It is usually operated at 5 ms per frame. If the vertical size of the CCD is reduced with a narrow slit, the time response becomes faster. The high-time response in the spectral measurement therefore makes possible a variety of spectroscopic studies, e.g., impurity behavior in long pulse discharges with edge-localized mode bursts. An absolute intensity calibration of the EUV spectrometer is also carried out with a technique using the EUV bremsstrahlung continuum at 20-150 Å for quantitative data analysis. Thus, the high-time resolution tungsten spectra have been successfully observed with good spectral resolution using the present EUV spectrometer system. Typical tungsten spectra in the EUV wavelength range observed from EAST discharges are presented with absolute intensity and spectral identification.

Extended-range grazing-incidence spectrometer for high-resolution extreme ultraviolet measurements on an electron beam ion trap

A high-resolution grazing-incidence grating spectrometer has been implemented on the Livermore electron beam ion traps for performing very high-resolution measurements in the soft x-ray and extreme ultraviolet region spanning from below 10 Å to above 300 Å. The instrument operates without an entrance slit and focuses the light emitted by highly charged ions located in the roughly 50 μm wide electron beam onto a cryogenically cooled back-illuminated charge-coupled device detector. The measured line widths are below 0.025 Å above 100 Å, and the resolving power appears to be limited by the source size and Doppler broadening of the trapped ions. Comparisons with spectra obtained with existing grating spectrometers show an order of magnitude improvement in spectral resolution.

First measurements of highly ionized impurity emission distribution by grazing-incidence flat-field extreme ultraviolet spectrometer in HL-2A

A space-resolved grazing-incidence flat-field extreme ultraviolet (EUV) spectrometer has been developed in the HL-2A tokamak to measure vertical impurity emission profiles with simultaneous spectral, temporal, and spatial resolution. The spectrometer working in the wavelength range of 30-500 Å has been equipped with a gold-coated varied-line-spacing holographic grating with curvature of 5606 mm and a back illuminated charge-coupled device with size of 6.6 × 26.6 mm(2) (255 × 1024 pixels). A lower half of the HL-2A plasma with averaged minor radius of 40 cm is observed when the spectrometer with horizontal dispersion is placed at a distance of 7.5 m away from the plasma center. An excellent spatial resolution of 12 mm is achieved when a space-resolved slit with vertical width of 0.5 mm is adopted. The radial profiles of intrinsic impurities in several ionization stages have been measured with high throughput and extremely low stray light.

High-resolution time-resolved extreme ultraviolet spectroscopy on NSTX

We report on upgrades to the flat-field grazing-incidence grating spectrometers X-ray and Extreme Ultraviolet Spectrometer (XEUS) and Long-Wavelength Extreme Ultraviolet Spectrometer (LoWEUS), at the National Spherical Torus Experiment (NSTX) at the Princeton Plasma Physics Laboratory. XEUS employs a variable space grating with an average spacing of 2400 lines/mm and covers the 9-64 Å wavelength band, while LoWEUS has an average spacing of 1200 lines/mm and is positioned to monitor the 90-270 Å wavelength band. Both spectrometers have been upgraded with new cameras that achieve 12.5 ms time resolution. We demonstrate the new time resolution capability by showing the time evolution of iron in the NSTX plasma.

Rest-wavelength fiducials for the ITER core imaging x-ray spectrometer

Absolute wavelength references are needed to derive the plasma velocities from the Doppler shift of a given line emitted by a moving plasma. We show that such reference standards exist for the strongest x-ray line in neonlike W(64+), which has become the line of choice for the ITER (Latin "the way") core imaging x-ray spectrometer. Close-by standards are the Hf Lβ(3) line and the Ir Lα(2) line, which bracket the W(64+) line by ±30 eV; other standards are given by the Ir Lα(1) and Lα(2) lines and the Hf Lβ(1) and Lβ(2) lines, which bracket the W(64+) line by ±40 and ±160 eV, respectively. The reference standards can be produced by an x-ray tube built into the ITER spectrometer. We present spectra of the reference lines obtained with an x-ray microcalorimeter and compare them to spectra of the W(64+) line obtained both with an x-ray microcalorimeter and a crystal spectrometer.