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Radially resolved hydrogenic isotope fraction measurement capabilities have been developed for DIII-D using the main-ion charge exchange recombination (MICER) spectroscopy system in preparation for mixed hydrogen and deuterium experiments. Constraints on the hydrogenic ion temperatures and velocities based on measurements of the impurity ion properties are required to accurately fit the spectrum. Corrections for cross sectional distortions, spatial smearing due to the halo, and a neoclassical offset between the impurity and hydrogenic toroidal rotation are applied to the constraints prior to fitting the MICER spectrum. Extensive atomic physics calculations have been performed using the FIDASIM code, which has recently been improved to allow simulations using mixtures of hydrogenic species. These results demonstrate that for the same plasma parameters, the Dα emission is 20%–30% brighter than Hα due to differences in rate coefficients associated with the different ion thermal velocities for the same temperature and therefore must be taken into consideration when calculating absolute densities. However, despite these differences, the absolute error when estimating the hydrogen isotope fraction [nH/(nH + nD)] by using the Hα radiance fraction [LHα/(LHα + LDα)] is typically less than 5% due to the way the fraction is formed, making the radiance fraction a reasonably accurate estimate of the isotope fraction for most cases.

In DIII-D, large currents flowing into the divertor floor during edge-localized modes (ELMs) have been measured by an array of shunt current resistors before an increase of heat flux is measured by IR thermography. The diagnostic consists of 40 tiles distributed in five concentric circles in the lower divertor with sampling rates range between 50 and 500 kHz. Typically, the current measured by a single tile during an ELM can reach 500 A. This amounts to 5–25 kA flowing in the divertor tiles. The temporal evolution of the ELM currents shows a first phase with large amplitude oscillations, occurring before the heat flux increase measured by infrared thermography at the same location, lasting between 0.05 ms and 0.3 ms. A second phase follows where the time evolution of the divertor current mimics the evolution of the divertor heat flux. These currents could affect the plasma edge stability in the nonlinear ELM phase and provide a mechanism leading to explosive growth of edge stochasticity, the need of which and existence is predicted in contemporary nonlinear ELM simulations.

Measurements of the E × B toroidal angular velocity, ω E × B = E r / R B θ ( E r is the radial electric field, B θ is the poloidal magnetic field), are made using the Doppler back-scattering (DBS) and charge-exchange recombination (CER) spectroscopy diagnostics. DBS uses the Doppler shift of wavenumber-resolved density fluctuations while CER uses the Doppler shift of impurity emission lines to independently measure plasma parameters for calculating the local radial electric field. DBS and CER profiles of ω E × B as a function of normalized toroidal flux (ρ) are compared at various levels of neutral beam applied torque on the plasma. Under standard neoclassical theory ω E × B is a flux surface quantity, making it appropriate to compare across diagnostics. DBS and CER generally show good agreement when comparing ω E × B profiles at different levels of neutral beam injection-applied torque. Furthermore, the DBS values have close to the same precision as CER values when averaged over a similar time-scale and effects, such as prompt-torque are considered. DBS is able to observe the rapid ( < 10 ms) modification of the E r profile by the diagnostic neutral beam ‘blips’. This modification is most pronounced when the blip applies a large relative change in torque on the plasma. Overall, these results could have implications on transport analysis and suggests using DBS and CER in conjunction to constrain values of the E × B-shear (sometimes called γ E × B ).

New studies identify the critical parameters and physics governing disruptive neoclassical tearing mode (NTM) onset. An m/n = 2/1 mode in DIII-D that begins to grow robustly after a seeding event (edge localized mode ELM or sawtooth precursor and crash) causes the mode rotation to drop close to the plasma’s E r = 0 rest frame; this condition opens the stabilizing ion-polarization current ‘gate’ and destabilizes an otherwise marginally stable NTM. Our new experimental and theoretical insights and novel toroidal theory-based modeling are benchmarked and scalable to ITER and other future experiments. The nominal ITER rotation at q = 2 is found to be stabilizing (‘gate closed’) except for MHD-induced transients that could ‘open the gate’. Extrapolating from the DIII-D ITER baseline scenario (IBS) discharges, MHD transients are much more likely to destabilize problematic robustly growing 2/1 NTMs in ITER; this makes predictions of seeding and control of both ELMs and sawteeth imperative for more than just minimizing divertor pulsed-heat loading.

Experiments on DIII-D show that resonant magnetic perturbations (RMPs) reduce the impurity confinement time in the H-mode pedestal below that of naturally ELMy plasmas. STRAHL modeling of discharges with aluminum laser blow off (LBO) injection show increased diffusion in the pedestal of H-mode discharges with RMPs. The increased diffusion from the RMPs provides improved impurity control compared to the naturally ELMy discharges and prevents the buildup of impurities near the edge of the plasma. Tungsten LBO injection into discharges with and without RMPs led to striking differences in the evolution of the discharge. The naturally ELMy discharge became ELM free and showed an increase in edge radiation, leading to the eventual radiative collapse of the plasma. Injecting a similar number of W particles into discharges with RMPs, either with or without ELMs, led to a measurable accumulation of core W but did not significantly impact the evolution of the discharge. These results indicate that RMPs are beneficial for controlling the buildup of impurities in the pedestal region of the plasma.

Edge localized modes (ELMs) are triggered using deuterium pellets injected into plasmas with ITER-relevant low collisionality pedestals, and the resulting peak ELM energy fluence is reduced by approximately 25%–50% relative to natural ELMs destabilized at similar pedestal pressures. Cryogenically frozen deuterium pellets are injected from the low-field side of the DIII-D tokamak at frequencies lower than the natural ELM frequency, and heat flux is measured by infrared cameras. Ideal MHD pedestal stability calculations show that without pellet injection, these low collisionality pedestals were limited by their current density (peeling-limited) rather than their pressure gradient (ballooning-limited). ELM triggering success correlates strongly with pellet mass, consistent with the theory that a large pressure perturbation is required to trigger an ELM in low collisionality discharges that are far from the ballooning stability boundary. For sufficiently large pellets, both instantaneous and time-integrated ELM energy deposition measured by infrared cameras is reduced with respect to naturally occurring ELMs at the inner strike point, which is the position where it is largest for natural ELMs. Energy fluence at the outer strike point is less effected. Cameras observing both heat flux and D-alpha emission often find significant toroidally asymmetric striations in the outboard far scrape-off layer resulting from ELMs that are triggered by pellets. Toroidal asymmetries at the inner strike point are similar between natural and pellet-triggered ELMs, suggesting that the reduction in peak heat flux and total fluence at that location is robust for the conditions reported here.

An optimized pedestal regime called the super-H (SH) mode is leveraged to couple a fusion relevant core plasma with a high density scrape-off layer appropriate for realistic reactor power exhaust solutions. Recent DIII-D experiments have expanded the operating space of the SH regime using advanced control algorithms and investigated optimization of impurity seeding, deuterium gas puffing, and 3D magnetic perturbations. Simultaneous real-time control of the pedestal density and radiated power with in-vessel coils and nitrogen seeding enable optimal coupled divertor and pedestal conditions. Four case studies are analysed with varied levels of radiated power in the divertor volume ranging from 0 (no seeding) to 8.5 MW radiated from carbon and nitrogen emission. Plasmas with a 4.5 MW radiated power target establish a radiative mantle, leading to divertor temperatures of ∼16 eV while maintaining SH-mode, and with only marginal impact on the pedestal and core performance. Increased levels of N2 seeding with a 7.5 MW radiated power target facilitate detachment onset and divertor temperatures 2.5{\tau }_{\text{E}}$?> > 2.5 τ E . Finally, a 8.5 MW radiated power target leads to partial detachment, which is so far associated with the loss of access to SH-mode pedestal conditions.

Negative-ion neutral-beam injection (NNBI) is an important source of heating and current drive for next-step fusion devices where the injected energy can range from hundreds of keV to 1 MeV. Few diagnostics are suitable for phase-space resolved measurements of fast ions with energy in excess of 100 keV. A study to assess the feasibility of fast-ion deuterium-alpha (FIDA) spectroscopy to diagnose high-energy ions produced by NNBI is presented. Case studies with the Large Helical Device (LHD) and JT-60SA illustrate possible solutions for the measurement. The distribution function of fast ions produced by NNBI is calculated for both devices, and the FIDA spectrum is predicted by synthetic diagnostic simulation. Results with 180 keV NNBI in LHD show that, with a judicious choice of viewing geometry, the FIDA intensity is comparable to that obtained with the existing FIDA system. The measurement is more challenging with the 500 keV NNBI in JT-60SA. Simulations predict the FIDA intensity to be about 1% of the background bremsstrahlung, which is small compared to existing FIDA implementations with positive neutral-beam injection where signal levels are an order of magnitude larger. The sampling time required to extract the small FIDA signal is determined using a probabilistic approach. Results indicate that long averaging periods, from ones to tens of seconds, are needed to resolve the FIDA signal in JT-60SA. These long averaging times are suitable in long-pulse (∼100 s), steady-state devices like JT-60SA where an important measurement objective is the spatial profile of the slowing-down distribution of fast ions.

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.

Laser blow-off injections of aluminum and tungsten have been performed on the DIII-D tokamak to investigate the variation of impurity transport in a set of dedicated ion and electron heating scans with a fixed value of the external torque. The particle transport is quantified via the Bayesian inference method, which, constrained by a combination of a charge exchange recombination spectroscopy, soft x-ray measurements, and vacuum ultraviolet spectroscopy provides a detailed uncertainty quantification of transport coefficients. Contrasting discharge phases with a dominant electron and ion heating reveal a threefold drop in the impurity confinement time and order of magnitude increase in midradius impurity diffusion, when additional electron heating is applied. Furthermore, the calculated stationary aluminum density profiles reverse from peaked in electron heated to hollow in the ion heated case, following a similar trend to electron and carbon density. Comparable values of a core diffusion have been observed for W and Al ions, while differences in the propagation dynamics of these impurities are attributed to pedestal and edge transport. Modeling of the core transport with non-linear gyrokinetics code CGYRO [J. Candy and E. Belly, J. Comput. Phys. 324, 73 (2016)], significantly underpredicts the magnitude of the variation in Al transport. Diffusion increases three-times steeper with additional electron heat flux, and 10-times lower diffusion is observed in ion heated case than predicted by the modeling. The CGYRO model quantitatively matches the increase in the Al diffusion when approaching the linear threshold for the transition from the ion temperature gradient to trapped electron mode.

Recent experiments on DIII-D have shown it is possible to create and sustain wide pedestal quiescent H-mode (QH-mode) plasmas with zero net torque from neutral beam injection (NBI) for the full discharge duration. Wide pedestal QH-mode has many of the features of the previously investigated QH-mode while having the advantage of increased edge pedestal pressure and energy confinement time. Both QH-mode variants operate without edge localized modes. Accordingly, these discharges demonstrate that significant input torque is not essential to the exploitation of wide pedestal QH-mode in future devices that are expected to have small or non-existent NBI torque. Developing operating conditions that allowed net zero torque access to wide pedestal QH-mode required implementing several techniques to avoid locked modes including minimizing intrinsic error fields, avoiding large sawteeth, and driving toroidal rotation via neoclassical toroidal viscosity.

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.

Experiments at the DIII-D tokamak have used dimensionless parameter scans to investigate the dependencies of intrinsic torque and momentum transport in order to inform a prediction of the rotation profile in ITER. Measurements of intrinsic torque profiles and momentum confinement time in dimensionless parameter scans of normalized gyroradius and collisionality are used to predict the amount of intrinsic rotation in the pedestal of ITER. Additional scans of Te/Ti and safety factor are used to determine the accuracy of momentum flux predictions of the quasi-linear gyrokinetic code TGLF. In these scans, applications of modulated torque are used to measure the incremental momentum diffusivity, and results are consistent with the E×B shear suppression of turbulent transport. These incremental transport measurements are also compared with the TGLF results. In order to form a prediction of the rotation profile for ITER, the pedestal prediction is used as a boundary condition to a simulation that uses TGLF to determine the transport in the core of the plasma. The predicted rotation is ≈20 krad/s in the core, lower than in many current tokamak operating scenarios. TGLF predictions show that this rotation is still significant enough to have a strong effect on confinement via E×B shear.

The dependence of intrinsic torque and momentum confinement time on normalized gyroradius ( ρ*) and collisionality ( ν*) has been measured in the DIII-D tokamak. The intrinsic torque normalized to temperature is found to have ρ* and ν* dependencies of ρ*−1.5±0.8 and ν*0.26±0.04. This dependence on ρ* is unexpectedly favorable (increasing as ρ* decreases). The choice of normalization is important, and the implications are discussed. The unexpected dependence on ρ* is found to be robust, despite some uncertainty in the choice of normalization. The dependence of momentum confinement on ρ* does not clearly demonstrate Bohm or gyro-Bohm like scaling, and a weaker dependence on ν* is found. The calculations required to use these dependencies to determine the intrinsic torque in future tokamaks such as ITER are presented, and the importance of the normalization is explained. Based on the currently available information, the intrinsic torque predicted for ITER is 33 N m, comparable to the expected torque available from neutral beam injection. The expected average intrinsic rotation associated with this intrinsic torque is small compared to current tokamaks, but it may still aid stability and performance in ITER.

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.

Direct measurements of deuterium main-ion toroidal rotation spanning the linear ohmic to saturated ohmic confinement (LOC-SOC) regime and with additional electron cyclotron heating (ECH) are presented and compared with the more commonly measured impurity (carbon) ion rotation in DIII-D. Main ions carry the bulk of the plasma toroidal momentum, and hence, the shape of the main-ion rotation is more relevant to the study of angular momentum transport in tokamaks. Both in the LOC regime and with ECH, the main-ion toroidal rotation frequency is flat across the profile from the sawtooth region to the plasma separatrix. However, the impurity rotation profile possesses a rotation gradient, with the rotation frequency being lower near the plasma edge, implying a momentum pinch or negative residual stress inferred from the impurity rotation that differs from the main-ion rotation. In the SOC regime, both the main-ion and impurity rotation profiles develop a deeply hollow feature near the midradius while maintaining the offset in the edge rotation, both implying a positive core residual stress. In the radial region where the rotation gradient changes most dramatically, turbulence measurements show that density fluctuations near the trapped electron mode (TEM) scale are higher when the rotation profile is flat and drop significantly when the plasma density is raised and the rotation profile hollows, consistent with instabilities damped by collisions. Linear initial value gyrokinetic simulations with GYRO indicate that the transition from LOC-SOC in DIII-D occurs as TEMs are replaced by ion temperature gradient (ITG) driven modes from the outer radii inwards as the plasma collisionality increases, Zeff decreases, and the power flow through the ion channel progressively increases due to the electron-ion energy exchange. Gyrofluid modeling with trap gyro-Landau fluid (TGLF) successfully reproduces the plasma profiles at key times in the discharge and in time dependent simulations with predictive TRANSP. TGLF indicates that in the LOC and SOC regimes as well as with ECH, subdominant modes are present and that the plasma is not in a pure TEM or ITG binary state, but rather a more subtle mixed state. Predictions of the main-ion rotation profiles are performed with global nonlinear gyrokinetic simulations using GTS and reveal that the flat rotation is due to oscillatory variation of the turbulent residual stress across the profile, whereas the deeply hollow rotation profile is due to a larger-scale, dipole-like stress profile. In these cases, the predicted and observed main-ion rotation profile is consistent with the balance of turbulent residual stress and momentum diffusion.Direct measurements of deuterium main-ion toroidal rotation spanning the linear ohmic to saturated ohmic confinement (LOC-SOC) regime and with additional electron cyclotron heating (ECH) are presented and compared with the more commonly measured impurity (carbon) ion rotation in DIII-D. Main ions carry the bulk of the plasma toroidal momentum, and hence, the shape of the main-ion rotation is more relevant to the study of angular momentum transport in tokamaks. Both in the LOC regime and with ECH, the main-ion toroidal rotation frequency is flat across the profile from the sawtooth region to the plasma separatrix. However, the impurity rotation profile possesses a rotation gradient, with the rotation frequency being lower near the plasma edge, implying a momentum pinch or negative residual stress inferred from the impurity rotation that differs from the main-ion rotation. In the SOC regime, both the main-ion and impurity rotation profiles develop a deeply hollow feature near the midradius while maintaining...

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.

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.

© 2015 IAEA, Vienna. In this paper we show that changing from an ion temperature gradient (ITG) to a trapped electron mode (TEM) dominant turbulence regime (based on linear gyrokinetic simulations) results experimentally in a strong density pump-out (defined as a reduction in line-averaged density) in low collisionality, low power H-mode plasmas. We vary the turbulence drive by changing the heating from predominantly ion heated using neutral beam injection to electron heated using electron cyclotron heating, which changes the ratio and the temperature gradients. Perturbed gas puff experiments show an increase in transport outside , through a strong increase in the perturbed diffusion coefficient and a decrease in the inward pinch. Linear gyrokinetic simulations with TGLF show an increase in the particle flux outside the mid-radius. In conjunction an increase in intermediate-scale length density fluctuations is observed, which indicates an increase in turbulence intensity at typical TEM wavelengths. However, although the experimental changes in particle transport agree with a change from ITG to TEM turbulence regimes, we do not observe a reduction in the core rotation at mid-radius, nor a rotation reversal.

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.

We present the first localized measurements of long and intermediate wavelength turbulent density fluctuations ( n) and long wavelength turbulent electron temperature fluctuations ( Te) modified by m/n=2/1 Neoclassical Tearing Mode (NTM) islands (m and n are the poloidal and toroidal mode numbers, respectively). These long and intermediate wavelengths correspond to the expected Ion Temperature Gradient and Trapped Electron Mode scales, respectively. Two regimes have been observed when tracking n during NTM evolution: (1) small islands are characterized by a steep Te radial profile and turbulence levels comparable to those of the background; (2) large islands have a flat Te profile and reduced turbulence level at the O-point. Radially outside the large island, the Te profile is steeper and the turbulence level increased compared to the no or small island case. Reduced turbulence at the O-point compared to the X-point leads to a 15% modulation of n2 across the island that is nearly in phase with the Te ...

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.

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.

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.

A newly installed Lithium Granule Injector (LGI) was used to pace edge localized modes (ELM) in DIII-D. ELM pacing efficiency was studied injecting lithium granules of nominal diameter 0.3–0.9 mm, speed of 50–120 m s−1 and average injection rates up to 100 Hz for 0.9 mm granules and up to 700 Hz for 0.3 mm granules. The efficiency of ELM triggering was found to depend strongly on size of the injected granules, with triggering efficiency close to 100% obtained with 0.9 mm diameter granules, lower with smaller sizes, and weakly depending on granule velocity. Robust ELM pacing was demonstrated in ITER-like plasmas for the entire shot length, at ELM frequencies 3–5 times larger than the ‘natural’ ELM frequency observed in reference discharges. Within the range of ELM frequencies obtained, the peak ELM heat flux at the outer strike point was reduced with increasing pacing frequency. The peak heat flux reduction at the inner strike point appears to saturate at high pacing frequency. Lithium was found in the plasma core, with a concurrent reduction of metallic impurities and carbon. Overall, high frequency ELM pacing using the lithium granule injection appears to be compatible with both H-mode energy confinement and attractive H-mode pedestal characteristics, but further assessment is needed to determine whether the projected heat flux reduction required for ITER can be met.

The neutral-beam induced Dαemission 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 Dαemission (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. © 2012 American Institute of Physics.

Charge exchange spectroscopy is one of the standard plasma diagnostic techniques used in tokamak research to determine ion temperature, rotation speed, particle density, and radial electric field. Configuring a charge coupled device (CCD) camera to serve as a detector in such a system requires a trade-off between the competing desires to detect light from as many independent spatial views as possible while still obtaining the best possible time resolution. High time resolution is essential, for example, for studying transient phenomena such as edge localized modes. By installing a mask in front of a camera with a 1024 × 1024 pixel CCD chip, we are able to acquire spectra from eight separate views while still achieving a minimum time resolution of 0.2 ms. The mask separates the light from the eight spectra, preventing spatial and temporal cross talk. A key part of the design was devising a compact translation stage which attaches to the front of the camera and allows adjustment of the position of the mask openings relative to the CCD surface. The stage is thin enough to fit into the restricted space between the CCD camera and the spectrometer endplate.

A small depression is created in a straight optical fiber taper to form a local probe suitable for studying closely spaced, planar microphotonic devices. The tension of the "dimpled" taper controls the probe-sample interaction length and the level of noise present during coupling measurements. Practical demonstrations with high-Q silicon microcavities include testing a dense array of undercut microdisks (maximum Q = 3.3x10^6) and a planar microring (Q = 4.8x10^6)., 8 pages, 5 figures, for high-res version see http://copilot.caltech.edu/publications/index.htm

While operating a magnetic fusion device in H-mode has many advantages, care must be taken to understand and control the release of energy during the H-L back transition, as the extra energy stored within the H-mode transport barrier will have the potential to cause damage to material components of a large future tokamak such as ITER. Examining a scenario where the H-L back transition sequence begins before the E × B shearing layer decays on its own, we identify a long-lived precursor mode that is tied to the events of the H-L sequence and we develop a robust control strategy for ensuring gradual release of energy during the transition sequence. Back transitions in this scenario commonly begin with a rapid relaxation of the pedestal, which was previously shown to be inconsistent with ideal peeling-ballooning instability as the trigger [Eldon et al., Phys. Plasmas 22, 052109 (2015)], despite being otherwise similar to a large type-I Edge Localized Mode (ELM). This so-called transient occurs when the E × B ...

In this paper we show that resonant magnetic perturbations (RMPs) affect the L- to H-mode power threshold. We find that during the L-mode phase, RMPs cause the particle pinch to reverse from traditionally inward to outward. As a result, the density at the plasma edge increases, while the density in the plasma core is reduced. Linear stability calculations indicate that the plasma transitions from an ion temperature gradient (ITG) to trapped electron mode (TEM) regime at the plasma edge. If the applied RMP current is below the threshold for penetration and island formation, we find that the changes in the edge radial electric field are minimal, while the carbon toroidal rotation brakes over the whole minor radius. Once the RMP field penetrates and the screening plasma response dissappears, the spin-up of the toroidal rotation at the plasma edge results in a positive radial electric field inside the separatrix.

Impurity transport in the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] is investigated in stationary high confinement (H-mode) regimes without edge localized modes (ELMs). In plasmas maintained by resonant magnetic perturbation (RMP), ELM-suppression, and QH-mode, the confinement time of fluorine (Z = 9) is equivalent to that in ELMing discharges with 40 Hz ELMs. For selected discharges with impurity injection, the impurity particle confinement time compared to the energy confinement time is in the range of τp/τe≈2−3. In QH-mode operation, the impurity confinement time is shown to be smaller for intense, coherent magnetic, and density fluctuations of the edge harmonic oscillation than weaker fluctuations. Transport coefficients are derived from the time evolution of the impurity density profile and compared to neoclassical and turbulent transport models NEO and TGLF. Neoclassical transport of fluorine is found to be small compared to the experimental values. In the ELMing and RMP ELM-suppress...

Experiments were conducted with Peltier-actuated microvalves to determine the maximum stand-off pressures possible with such valves, and to investigate the effect of supercooling on valve operation. Stand-off pressures were found to exceed the limits of measurability at 10 MPa. The experimental data, supported by thermal modeling, indicated that supercooling in linear-junction microvalves was typically about ?15 C, while supercooling in tube-type Peltier valves ranged from ?8 to ?20 C.

Experimental tests of ion poloidal rotation theories have been performed on DIII-D using a novel impurity poloidal rotation diagnostic. These tests show significant disagreements with theoretical predictions in various conditions, including L-mode plasmas with internal transport barriers (ITB), H-mode plasmas, and QH-mode plasmas. The theories tested include standard neoclassical theory, turbulence driven Reynolds stress, and fast-ion friction on the thermal ions. Poloidal rotation is observed to spin up at the formation of an ITB and makes a significant contribution to the measurement of the E→×B→ shear that forms the ITB. In ITB cases, neoclassical theory agrees quantitatively with the experimental measurements only in the steep gradient region. Significant quantitative disagreement with neoclassical predictions is seen in the cores of ITB, QH-, and H-mode plasmas, demonstrating that neoclassical theory is an incomplete description of poloidal rotation. The addition of turbulence driven Reynolds stress ...

The Trapped Gyro-Landau Fluid (TGLF) quasi-linear model [G. M. Staebler, et al., Phys. Plasmas 12, 102508 (2005)], which is calibrated to nonlinear gyrokinetic turbulence simulations, is now able to predict the electron density, electron and ion temperatures, and ion toroidal rotation simultaneously for internal transport barrier (ITB) discharges. This is a strong validation of gyrokinetic theory of ITBs, requiring multiple instabilities responsible for transport in different channels at different scales. The mystery of transport inside the ITB is that momentum and particle transport is far above the predicted neoclassical levels in apparent contradiction with the expectation from the theory of suppression of turbulence by E×B velocity shear. The success of TGLF in predicting ITB transport is due to the inclusion of ion gyro-radius scale modes that become dominant at high E×B velocity shear and to improvements to TGLF that allow momentum transport from gyrokinetic turbulence to be faithfully modeled.

We present direct evidence of low frequency, radially sheared, turbulence-driven flows (zonal flows (ZFs)) triggering edge transport barrier formation preceding the L- to H-mode transition via periodic turbulence suppression in limit-cycle oscillations (LCOs), consistent with predator-prey dynamics. The final transition to edge-localized mode-free H-mode occurs after the equilibrium E × B flow shear increases due to ion pressure profile evolution. ZFs are also observed to initiate formation of an electron internal transport barrier (ITB) at the q = 2 rational surface via local suppression of electron-scale turbulence. Multi-channel Doppler backscattering (DBS) has revealed the radial structure of the ZF-induced shear layer and the E × B shearing rate, ωE × B, in both barrier types. During edge barrier formation, the shearing rate lags the turbulence envelope during the LCO by 90°, transitioning to anti-correlation (180°) when the equilibrium shear dominates the turbulence-driven flow shear due to the increasing edge pressure gradient. The time-dependent flow shear and the turbulence envelope are anti-correlated (180° out of phase) in the electron ITB. LCOs with time-reversed evolution dynamics (transitioning from an equilibrium-flow dominated to a ZF-dominated state) have also been observed during the H-L back-transition and are potentially of interest for controlled ramp-down of the plasma stored energy and pressure (normalized to the poloidal magnetic field) in ITER. © 2014 IAEA, Vienna.

Charge-exchange spectroscopy is a powerful diagnostic tool for determining ion temperatures, densities and rotational velocities in tokamak plasmas. This technique depends on detailed understanding of the atomic physics processes that affect the measured apparent velocities with respect to the true ion rotational velocities. These atomic effects are mainly due to energy dependence of the charge-exchange cross-sections, and in the case of poloidal velocities, due to gyro-motion of the ion during the finite lifetime of the excited states. Accurate lifetimes are necessary for correct interpretation of measured poloidal velocities, specially for high density plasma regimes on machines such as ITER, where l-mixing effects must be taken into account. In this work, a full nl-resolved atomic collisional radiative model coupled with a full kinetic calculation that includes the effects of electric and magnetic fields on the ion gyro-motion is presented for the first time. The model directly calculates from atomic physics first principles the excited state lifetimes that are necessary to evaluate the gyro-orbit effects. It is shown that even for low density plasmas where l-mixing effects are unimportant and coronal conditions can be assumed, the nl-resolved model is necessary for an accurate description of the gyro-motion effects to determine poloidal velocities. This solution shows good agreement when compared to three QH-mode shots on DIII-D, which contain a wide range of toroidal velocities and high ion temperatures where greater atomic corrections are needed. The velocities obtained from the model are compared to experimental velocities determined from co- and counter-injection of neutral beams on DIII-D.

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.

We describe a simple and inexpensive method, which corrects the astigmatism of a Czerny-Turner spectrometer. Initial characterization of the astigmatism for a particular Czerny-Turner spectrometer was performed and the design of the corrective optic is described. The optic is a thin piece of glass, which is used as a one-dimensional waveguide between the light source and the spectrometer such that the sagittal and tangential focal planes are brought to the same position. This method is demonstrated to work well between 360 and 900 nm for an f/4.7 spectrometer. With appropriate materials, corrections for longer and shorter wavelengths should also be possible. When using an inexpensive glass plate, light intensity lost with this method is approximately 12%. Improved surface finish should reduce this loss.

A dimensionless parameter dependence study of intrinsic torque has been performed on a database of H- and I-mode plasmas from the Alcator C-Mod tokamak. The torque was determined by comparing intrinsic angular momentum density profiles just before and just after L–H and L–I transitions. The intrinsic torque has been found to scale as β N 1.5 ρ * − 1.0 ν * 0.1 , with the parameter ranges 0.3 ⩽ β N ⩽ 1.5, 0.004 ⩽ ρ * ⩽ 0.011 and 0.04 ⩽ ν * ⩽ 0.9. Comparison with results from JET and DIII-D suggests that the intrinsic torque should be normalized by some measure of the device size. Depending upon this normalization, the estimated total intrinsic torques for ITER, SPARC and ARC are ∼20, ∼4 and ∼8 Nm, respectively.