Charge exchange recombination spectroscopy measurements of DIII-D poloidal rotation with poloidal asymmetry in angular rotation.

Sixteen new tangential views for the charge exchange recombination (CER) spectroscopy diagnostic at DIII-D were installed in 2019 on the high-field side (HFS) of the tokamak with the main goal being the measurement of main-ion (deuterium) poloidal rotation. Eight of the new views are connected to spectrometers, which view the main-ion spectrum, adding main-ion measurements where there were previously none, and another eight new views increased the spatial resolution of existing impurity (carbon) measurements on the HFS. When combined with the existing low-field side measurements, measurements at two locations on flux surfaces out to a normalized minor radius of ≈0.6 are possible. The new tangential views have been used to measure the deuterium poloidal rotation directly for the first time using the Poloidal Asymmetry in Angular Rotation (PAAR) method. These new measurements enable further testing of the validity of neoclassical poloidal rotation predictions. Separate measurements of the radial electric field can be made for an impurity ion and the main-ion by combining the PAAR measurements with additional CER measurements of toroidal rotation, temperature, and density. These independent measurements of the radial electric field agree reasonably well.

Synthetic diagnostic for assessing spatial averaging of charge exchange recombination spectroscopy measurements.

A synthetic charge exchange recombination spectroscopy diagnostic based on the FIDASIM modeling suite has been created for the DIII-D tokamak. This synthetic diagnostic assumes that the ions have Maxwellian distribution functions on each flux surface and models emission from charge exchange events between the beam neutrals and a fully ionized impurity. This work was motivated by the observation of non-Gaussian spectra that may be caused by spatial averaging, atomic physics, or non-Maxwellian distribution functions. Measurements of non-Gaussian spectra commonly observed in the high confinement mode pedestal and in plasmas with large core gradients are compared to the synthetic diagnostic. Spatial averaging alone cannot account for the observations in these two cases, opening up the possibility of there being other causes such as non-Maxwellian distribution functions. The synthetic diagnostic has also been used to resolve a long-standing issue: it is shown that the lower temperatures measured by using vertical view chords relative to tangential view chords are due to increased spatial averaging for vertical views due to the DIII-D neutral beams being approximately twice as tall as they are wide.

Active spectroscopy measurements of the deuterium temperature, rotation, and density from the core to scrape off layer on the DIII-D tokamak (invited).

Main-ion charge exchange recombination spectroscopy (MICER) uses the neutral beam induced D α spectrum to measure the local deuterium ion (D+) temperature, rotation, and density, as well as parameters related to the neutral beams, fast ions, and magnetic field. An edge MICER system consisting of 16 densely packed chords was recently installed on DIII-D, extending the MICER technique from the core to the pedestal and steep gradient region of H-mode plasmas where the D+ and commonly measured impurity ion properties can differ significantly. A combination of iterative collisional radiative modeling techniques and greatly accelerated spectral fitting allowed the extension of this diagnostic technique to the plasma edge where the steep gradients introduce significant diagnostic challenges. The importance of including the fast ion D α emission in the fit to the spectrum for the edge system is investigated showing that it is typically not important except for cases which can have significant fast ion fractions near the plasma edge such as QH-mode. Example profiles from an Ohmic L-mode and a high power ITER baseline case show large differences in the toroidal rotation of the two species near the separatrix including a strong co-current D+ edge rotation. The measurements and analysis demonstrate the state of the art in active spectroscopy and integrated modeling for diagnosing fusion plasmas and the importance of direct main ion measurements.

Using motional Stark splitting of D α emission to constrain MHD equilibrium analysis in DIII-D plasmas.

We report tests of an alternate technique for constraining MHD equilibrium analysis in tokamak plasmas using internal magnetic field measurements based on | B | measurements from the motional Stark splitting of Dα spectral lines emitted by a neutral heating beam (MSE-LS). We compare results using MSE-LS with those of the standard equilibrium analysis technique based on line polarization of the Dα emission (MSE-LP). An alternative to MSE-LP is needed in future devices such as ITER where MSE-LP will be difficult due to a plasma-induced coating of the first optical element. The tests utilized data from 10 DIII-D shots with 7 MSE-LS and 14 MSE-LP views covering a range of radii along the outer midplane of the plasma. Seven MSE-LS measurements can contribute significantly to the equilibrium reconstruction of pressure and q profiles using both synthetic and experimental DIII-D MSE-LS data. For example, 7 MSE-LS plus seven MSE-LP measurements give a fit quality that is as good as the same cases with 14 MSE-LP measurements. Analyzing synthetic data for 14 MSE-LS measurements shows significant improvement in fitting quality over the case with 7 MSE-LS locations.

Relative intensity calibration of the DIII-D charge-exchange recombination spectroscopy system using neutral beam injection into gas.

A new calibration method for the DIII-D charge-exchange spectroscopy system produces a smoother impurity density profile compared to previous techniques, improving the accuracy of the impurity density profile reconstruction. The relative intensity calibration between the chords of the DIII-D charge-exchange recombination spectroscopy system is performed by firing neutral beams into the evacuated vacuum vessel pre-filled with neutral gas. Relative calibration is required in order to account for uncertainty in the 3D geometry of the neutral beam. Previous methods using helium gas have been improved by using xenon, which emits an emission line close to the commonly used carbon wavelength 5290.5 Å, as well as improved timing of the gas injection, inclusion of variations in the vessel pressure, and timing of neutral beam injection. Photoemission spectra recorded by 112 sightlines viewing 6 neutral beams are compared and used to form a relative calibration factor for each sightline. This relative calibration is shown to improve the quality of the measured ion density profile.

Quasistationary Plasma Predator-Prey System of Coupled Turbulence, Drive, and Sheared E×B Flow During High Performance DIII-D Tokamak Discharges.

A new, long-lived limit cycle oscillation (LCO) regime has been observed in the edge of near zero torque high performance DIII-D tokamak plasma discharges. These LCOs are localized and composed of density turbulence, gradient drives, and E×B velocity shear damping (E and B are the local radial electric and total magnetic fields). Density turbulence sequentially acts as a predator (via turbulence transport) of profile gradients and a prey (via shear suppression) to the E×B velocity shear. Reported here for the first time is a unique spatiotemporal variation of the local E×B velocity, which is found to be essential for the existence of this system. The LCO system is quasistationary, existing from 3 to 12 plasma energy confinement times (∼30-900 LCO cycles) limited by hardware constraints. This plasma system appears to contribute strongly to the edge transport in these high performance and transient-free plasmas, as evident from oscillations in transport relevant edge parameters at LCO time scale.

High resolution main-ion charge exchange spectroscopy in the DIII-D H-mode pedestal.

A new high spatial resolution main-ion (deuterium) charge-exchange spectroscopy system covering the tokamak boundary region has been installed on the DIII-D tokamak. Sixteen new edge main-ion charge-exchange recombination sightlines have been combined with nineteen impurity sightlines in a tangentially viewing geometry on the DIII-D midplane with an interleaving design that achieves 8 mm inter-channel radial resolution for detailed profiles of main-ion temperature, velocity, charge-exchange emission, and neutral beam emission. At the plasma boundary, we find a strong enhancement of the main-ion toroidal velocity that exceeds the impurity velocity by a factor of two. The unique combination of experimentally measured main-ion and impurity profiles provides a powerful quasi-neutrality constraint for reconstruction of tokamak H-mode pedestals.

Measurement of deuterium density profiles in the H-mode steep gradient region using charge exchange recombination spectroscopy on DIII-D.

Recent completion of a thirty two channel main-ion (deuterium) charge exchange recombination spectroscopy (CER) diagnostic on the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] enables detailed comparisons between impurity and main-ion temperature, density, and toroidal rotation. In a H-mode DIII-D discharge, these new measurement capabilities are used to provide the deuterium density profile, demonstrate the importance of profile alignment between Thomson scattering and CER diagnostics, and aid in determining the electron temperature at the separatrix. Sixteen sightlines cover the core of the plasma and another sixteen are densely packed towards the plasma edge, providing high resolution measurements across the pedestal and steep gradient region in H-mode plasmas. Extracting useful physical quantities such as deuterium density is challenging due to multiple photoemission processes. These challenges are overcome using a detailed fitting model and by forward modeling the photoemission using the FIDASIM code, which implements a comprehensive collisional radiative model.

Improved edge charge exchange recombination spectroscopy in DIII-D.

The charge exchange recombination spectroscopy diagnostic on the DIII-D tokamak has been upgraded with the addition of more high radial resolution view chords near the edge of the plasma (r/a > 0.8). The additional views are diagnosed with the same number of spectrometers by placing fiber optics side-by-side at the spectrometer entrance with a precise separation that avoids wavelength shifted crosstalk without the use of bandpass filters. The new views improve measurement of edge impurity parameters in steep gradient, H-mode plasmas with many different shapes. The number of edge view chords with 8 mm radial separation has increased from 16 to 38. New fused silica fibers have improved light throughput and clarify the observation of non-Gaussian spectra that suggest the ion distribution function can be non-Maxwellian in low collisionality plasmas.

Spatial calibration of a tokamak neutral beam diagnostic using in situ neutral beam emission.

Neutral beam injection is used in tokamaks to heat, apply torque, drive non-inductive current, and diagnose plasmas. Neutral beam diagnostics need accurate spatial calibrations to benefit from the measurement localization provided by the neutral beam. A new technique has been developed that uses in situ measurements of neutral beam emission to determine the spatial location of the beam and the associated diagnostic views. This technique was developed to improve the charge exchange recombination (CER) diagnostic at the DIII-D tokamak and uses measurements of the Doppler shift and Stark splitting of neutral beam emission made by that diagnostic. These measurements contain information about the geometric relation between the diagnostic views and the neutral beams when they are injecting power. This information is combined with standard spatial calibration measurements to create an integrated spatial calibration that provides a more complete description of the neutral beam-CER system. The integrated spatial calibration results are very similar to the standard calibration results and derived quantities from CER measurements are unchanged within their measurement errors. The methods developed to perform the integrated spatial calibration could be useful for tokamaks with limited physical access.

High speed measurements of neutral beam turn-on and impact of beam modulation on measurements of ion density.

Modulation of neutral beams on tokamaks is performed routinely, enabling background rejection for active spectroscopic diagnostics, and control of injected power and torque. We find that there exists an anomalous initial transient in the beam neutrals delivered to the tokamak that is not accounted for by the accelerator voltage and power supply current. Measurements of the charge-exchange and beam photoemission on the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] at high speed (200 µs) reveal that the energy of the beam neutrals is constant, but the density of beam neutrals displays dramatic variation in the first 2-3 ms following beam turn-on. The impact of this beam density variation on inferred ion densities and impurity transport is presented, with suggested means to correct for the anomalous transient.

A method for determining poloidal rotation from poloidal asymmetry in toroidal rotation (invited).

A new diagnostic has been developed on DIII-D that determines the impurity poloidal rotation from the poloidal asymmetry in the toroidal angular rotation velocity. This asymmetry is measured with recently added tangential charge exchange viewchords on the high-field side of the tokamak midplane. Measurements are made on co- and counter-current neutral beams, allowing the charge exchange cross section effect to be measured and eliminating the need for atomic physics calculations. The diagnostic implementation on DIII-D restricts the measurement range to the core (r/a < 0.6) where, relative to measurements made with the vertical charge exchange system, the spatial resolution is improved. Significant physics results have been obtained with this new diagnostic; for example, poloidal rotation measurements that significantly exceed neoclassical predictions.

Access to a new plasma edge state with high density and pressures using the quiescent H mode.

A path to a new high performance regime has been discovered in tokamaks that could improve the attractiveness of a fusion reactor. Experiments on DIII-D using a quiescent H-mode edge have navigated a valley of improved edge peeling-ballooning stability that opens up with strong plasma shaping at high density, leading to a doubling of the edge pressure over the standard H mode with edge localized modes at these parameters. The thermal energy confinement time increases as a result of both the increased pedestal height and improvements in the core transport and reduced low-k turbulence. Calculations of the pedestal height and width as a function of density using constraints imposed by peeling-ballooning and kinetic-ballooning theory are in quantitative agreement with the measurements.

Observation of a critical gradient threshold for electron temperature fluctuations in the DIII-D Tokamak.

A critical gradient threshold has been observed for the first time in a systematic, controlled experiment for a locally measured turbulent quantity in the core of a confined high-temperature plasma. In an experiment in the DIII-D tokamak where L(T(e))(-1) = |∇T(e)|/T(e) and toroidal rotation were varied, long wavelength (k(θ)ρ(s) ≲ 0.4) electron temperature fluctuations exhibit a threshold in L(T(e))(-1): below, they change little; above, they steadily increase. The increase in δT(e)/T(e) is concurrent with increased electron heat flux and transport stiffness. Observations were insensitive to rotation. Accumulated evidence strongly enforces the identification of the experimentally observed threshold with ∇T(e)-driven trapped electron mode turbulence.

Calculation of impurity poloidal rotation from measured poloidal asymmetries in the toroidal rotation of a tokamak plasma.

To improve poloidal rotation measurement capabilities on the DIII-D tokamak, new chords for the charge exchange recombination spectroscopy (CER) diagnostic have been installed. CER is a common method for measuring impurity rotation in tokamak plasmas. These new chords make measurements on the high-field side of the plasma. They are designed so that they can measure toroidal rotation without the need for the calculation of atomic physics corrections. Asymmetry between toroidal rotation on the high- and low-field sides of the plasma is used to calculate poloidal rotation. Results for the main impurity in the plasma are shown and compared with a neoclassical calculation of poloidal rotation.

Active spectroscopic measurements of the bulk deuterium properties in the DIII-D tokamak (invited).

The neutral-beam induced D(α) emission spectrum contains a wealth of information such as deuterium ion temperature, toroidal rotation, density, beam emission intensity, beam neutral density, and local magnetic field strength magnitude |B| from the Stark-split beam emission spectrum, and fast-ion D(α) emission (FIDA) proportional to the beam-injected fast ion density. A comprehensive spectral fitting routine which accounts for all photoemission processes is employed for the spectral analysis. Interpretation of the measurements to determine physically relevant plasma parameters is assisted by the use of an optimized viewing geometry and forward modeling of the emission spectra using a Monte-Carlo 3D simulation code.

ECE-imaging of the H-mode pedestal (invited).

A synthetic diagnostic has been developed that reproduces the highly structured electron cyclotron emission (ECE) spectrum radiated from the edge region of H-mode discharges. The modeled dependence on local perturbations of the equilibrium plasma pressure allows for interpretation of ECE data for diagnosis of local quantities. Forward modeling of the diagnostic response in this region allows for improved mapping of the observed fluctuations to flux surfaces within the plasma, allowing for the poloidal mode number of coherent structures to be resolved. In addition, other spectral features that are dependent on both T(e) and n(e) contain information about pedestal structure and the electron energy distribution of localized phenomena, such as edge filaments arising during edge-localized mode (ELM) activity.

Role of zonal flow predator-prey oscillations in triggering the transition to H-mode confinement.

Direct evidence of zonal flow (ZF) predator-prey oscillations and the synergistic roles of ZF- and equilibrium E×B flow shear in triggering the low- to high-confinement (L- to H-mode) transition in the DIII-D tokamak is presented. Periodic turbulence suppression is first observed in a narrow layer at and just inside the separatrix when the shearing rate transiently exceeds the turbulence decorrelation rate. The final transition to H mode with sustained turbulence and transport reduction is controlled by equilibrium E×B shear due to the increasing ion pressure gradient.

High-frequency coherent edge fluctuations in a high-pedestal-pressure quiescent H-mode plasma.

A set of high frequency coherent (HFC) modes (f=80-250 kHz) is observed with beam emission spectroscopy measurements of density fluctuations in the pedestal of a strongly shaped quiescent H-mode plasma on DIII-D, with characteristics predicted for kinetic ballooning modes (KBM): propagation in the ion-diamagnetic drift direction; a frequency near 0.2-0.3 times the ion-diamagnetic frequency; inferred toroidal mode numbers of n∼10-25; poloidal wave numbers of k(θ)∼0.17-0.4 cm(-1); and high measured decorrelation rates (τ(c)(-1)∼ω(s)∼0.5×10(6) s(-1)). Their appearance correlates with saturation of the pedestal pressure.

Experimental investigation of the role of fluid turbulent stresses and edge plasma flows for intrinsic rotation generation in DIII-D H-mode plasmas.

The first measurements of turbulent stresses and flows inside the separatrix of a tokamak H-mode plasma are reported, using a reciprocating multitip Langmuir probe at the DIII-D tokamak. A strong co-current rotation layer at the separatrix is found to precede intrinsic rotation development in the core. The measured fluid turbulent stresses transport toroidal momentum outward against the velocity gradient and thus try to sustain the edge layer. However, large kinetic stresses must exist to explain the net inward momentum transport leading to co-current core plasma rotation. The importance of such kinetic stresses is corroborated by the success of a simple orbit loss model, representing a purely kinetic mechanism, in the prediction of features of the edge corotation layer.