Session CO4: DIII-D Tokamak

Overview of Recent DIII-D Experimental Results

DIII-D experiments added to the ITER physics basis and to physics understanding for extrapolation to future devices. Experiments with a mock-up of the ITER Test Blanket Modules showed mainly a reduction in toroidal rotation but only a small decrease in confinement. Baking the DIII-D vessel in 2~Torr O$_2$ at 350$^{\circ}$C showed removal of carbon co-deposits in agreement with predictions and high performance hybrid operation was then recovered within 15 plasma discharges. Improved control of post-disruption runaway electrons suggested new approaches to mitigation in ITER. ELM pacing was demonstrated with both LFS pellets and oscillating RMP fields. New diagnostics proved Alfv\'en eigenmodes cause fast ion losses. Locked NTMs were stabilized by aligning the island O-point to the ECCD absorption zone. Advances in physics understanding included: 1)~fully noninductive AT scenarios, 2)~plasma response and turbulence changes due to resonant and non-resonant 3D fields vs $\beta$, 3)~validation of core turbulence and thermal transport models, 4)~new intrinsic rotation physics and 5)~SOL/secondary divertor heat flux studies.   

Confinement in Advanced Inductive Plasmas - Gyroradius and Rotation

Advanced inductive (AI) plasmas promise long pulse operation and high fusion yield. We address issues of extrapolation to ITER and beyond. First, joint JET \& DIII-D work studies the scaling of transport with size ($\rho^*$). Preliminary 0-D analysis indicates that global scaling is close to Bohm-like: $B\tau_E\propto\rho^{*-\alpha}$ with $\alpha\approx 2.16$. For matched discharges $H_{98y2}$ depends weakly if at all on $\rho^*$. Second, the dependence of confinement on rotation and on the presence of an NTM island was studied in DIII-D. Rotation was varied by a factor of $\sim$4.6 in plasmas with similar $n_e$ and $\beta$, with $3.1\leq q_{95}\leq 4.9$. $H_{89}$ increased from $\sim$2.0 to $\sim$2.5, with weak $q_{95}$ dependence. Increasing $E\times B$ flow shear is dominant, accompanied by a decrease in turbulence. Decreasing NTM island width is less important, but not negligible. Third, with added ECH, $T_e/T_i$ increases but energy and momentum transport increase as well. Matching NBI and ECH heated plasmas shows that the reduction in density with ECH is a consequence of reduced rotation rather than changing $T_e/T_i$.   

Impact of Current Profile on Transport and Stability in High Noninductive Fraction DIII-D Discharges

Experiments addressing the issue of $J_{BS}$ and $J_{EC}$ alignment and the optimum $q$ profile for stable noninductive operation show the $J_{NI}$ and $J$ profiles are best aligned at $q_{min}\sim 1.5$, $q_{95}\sim 6.8$. The kinetic profiles vary systematically with $q_{min}$ and $q_{95}$. Transport analysis shows that electrons dominate losses at low $q_{min}$, while at high $q_{min}$ ions dominate. Drift wave stability analysis with the TGLF model shows trends in the linear growth rates that contradict these observations. Systematic scans of EC deposition indicate that a broad ECCD profile at $\rho\sim 0.3-0.55$ yields a $J$ profile that is more stable to the tearing modes that limit the duration of the discharges. Optimal alignment of $J_{EC}$ for tearing stability coincides with the region where additional NI current is needed for $f_{NI}=1$.   

High-Power Fast Wave Coupling Experiments in Advanced Regimes in DIII-D

Up to 1.5~MW of fast wave (FW) power has been successfully coupled to the core of ELMing H-mode discharges with $\beta_N\sim 2.5$ in conjunction with $\sim$7~MW of neutral beam injection and 1.5~MW of electron cyclotron (EC) heating. FW core heating efficiency is observed to be comparable to that of an equal amount of incremental EC power, as expected with first-pass FW absorption on core electrons of 76\%. Antenna loading is in quantitative agreement with modeling when edge density profiles measured with a reflectometer adjacent to the antenna are used in the model. Local deuterium puffing is used in some cases to increase the loading and hence the coupled power. Comparison of gas puffing and other techniques to increase the antenna loading are evaluated with respect to core plasma performance. Extension of the coupled FW power levels towards 3.5~MW is projected.   

Finite Orbit Monte Carlo Simulation with Full Wave Fields for ICRF Wave Heating Experiments in DIII-D, NSTX, KSTAR and ITER

Fast-ion spatial profile measured by fast ion $D_\alpha$ (FIDA) diagnostic in DIII-D and NSTX high harmonic ICRF heating experiments indicates outward radial shifts of fast ions from the magnetic axis. Finite orbit theory suggests that the fast-ion radial excursion may be due to finite drift orbit width effects, which can directly affect the ICRF wave propagation and absorption. Recent ORBIT-RF coupled self-consistently with AORSA simulations predicted outward radial shift qualitatively consistent with FIDA. A noted discrepancy is that simulations compute further outward shift than FIDA. To investigate this discrepancy, we improve physics such as Coulomb collision and quasi-linear heating model, and also develop a synthetic diagnostic technique to explore improved means to best model diagnostic measurements. Effects of this improvement on previous results will be discussed including details of finite orbit effects of fast ions on KSTAR and ITER   

New Measurements of Fast-ion Transport

Many new fast-ion diagnostics were commissioned during the 2010 campaign, including a scintillator-based fast-ion loss detector, high bandwidth neutral-particle analyzers and fast-ion $D_\alpha$ (FIDA) detectors, spectroscopic FIDA views that are sensitive to co-passing ions, and improved FIDA imaging capabilities. Fluctuations at mode frequencies are detected during Alfv\'en eigenmodes, neoclassical tearing modes, energetic-particle driven geodesic acoustic modes, and $q=2$ fishbones. The transport of passing and trapped ions differ at the sawtooth crash. Drift-wave transport is more evident in lower-energy channels than in higher-energy channels. High time resolution toroidal rotation measurements detect local sub-millisecond changes associated with non-ambipolar fast-ion transport.   

Fuel Retention and Removal from the Carbon First-Wall in DIII-D

Experiments to determine the short-term retention of deuterium fuel (D) in a graphite first wall are done using a global particle balance. The global particle balance is calculated continuously through a discharge and shows a majority of the D wall retention occurs during the initial ohmic and L-mode phases. Typical wall uptake rates in these phases are $\sim$30$\pm$3.5~Torr-L/s which is $\sim$2\% of the measured divertor ion flux. The continuous global particle balance is compared with a shot-integrated balance, and they agree to $\sim$5\%. During the H-mode phase of typical ELM-y discharges, the uptake is near zero ($\sim \pm$5~Torr-L/s), which is $\leq$0.2\% of divertor ion flux. This fluctuating retention rate leads to a wall inventory reduction during the H-mode phase that can be as much as 100\% though typically $\sim$20-30\%. A vacuum bake of first wall at 350$^{\circ}$C after plasma operations recovered $\sim$80\% of the retained D, and left $\sim$9\% of the total fuel injected during the run-day in the first wall.   

Investigation of Divertor Heat Flux Width in \hbox{DIII-D} for 2010 Joint Research Target

The 2010 Joint Research Target for NSTX, C-Mod, and DIII-D aims to improve prediction of divertor heat flux profile width for future divertors. In DIII-D we varied input power, toroidal field, plasma current $I_p$, and density. Divertor heat flux was obtained using IR thermography. We find that $w_{q,div}$ is most sensitive to $I_p$. Mapped to the outer midplane $w_{q,div}$ scaled like $w_{q,mid}$~(mm)~= 5.38/$I_p^{1.24}$~(MA). Scrape-off layer and pedestal density and temperature fluctuations were measured using midplane and x-point plunging Langmuir probes at the lower powers. From midplane fluctuation data, we obtained energy transport measurements, which we compare with transport coefficients obtained from onion-skin modeling using density profiles, and with experimentally determined heat flux widths in the divertor.   

Variation of Turbulence and Transport with the $T_e/T_i$ Ratio in H-mode Plasmas

Confinement and transport vary strongly with $T_e/T_i$. Recent experiments on DIII-D have sought to examine the physical mechanisms behind this dependence by systematically varying $T_e/T_i$ in L- and H-mode plasmas, while $T_i$, rotation, density and gradient scale lengths are held roughly constant. $T_e/T_i$ is increased by 25\% (achieving $T_e/T_i\leq 1$) in non-sawtoothing, long-pulse hybrid H-mode plasmas using 3.3~MW of ECH power, reducing $\tau_E$ by 30\%. The magnitude of low-k density fluctuations, measured with BES, is found to correspondingly increase by 30\%-50\% over the radial range $0.35 < r/a < 0.8$. This turbulence behavior contrasts with that observed in L-mode experiments, in which confinement is reduced at higher $T_e/T_i$, but little change is observed in the magnitude of low-k density turbulence. $S(k_r,k_\theta)$ spectra and related turbulence properties for L- and H-mode plasmas will be compared. Calculations of growth rates and predicted turbulence levels will be performed.   

Thermal Ion Orbit Loss and Intrinsic Toroidal Velocity Near the Last Closed Flux Surface

Recent Mach probe measurements in DIII-D have revealed a relatively universal co-$I_p$ directed, localized toroidal velocity of the main $D^+$ ion in the edge of DIII-D discharges, centered near the outboard last closed flux surface. The ion orbit loss model [1] formerly applied to the region near the top of the pedestal in H-mode discharges has been extended to the edge, and into the scrape-off layer. This model gives relatively good agreement with the width of this intrinsic velocity peak, and with the magnitude given uncertainties in the probe velocity measurements due to uncertainties in $T_e$ and $T_i$. The extensions of the former model include limiting surfaces and a radial electric field.\par \vskip6pt \noindent [1] J.S.\ deGrassie et al., Nucl.\ Fusion {\bf 49}, 085020 (2009).   

Dependence of ELM Size on Rotation at High-Triangularity and High Beta-Poloidal in the DIII-D Tokamak

The effect of rotation on edge localized mode (ELM) size and frequency was studied in the DIII-D tokamak. The regime chosen was one where JT60-U, ASDEX-U and JET reported evidence of rotational dependence to ELM stored energy loss ($\Delta W_{\rm ELM}$) and frequency ($f_{\rm ELM}$), changing from low-frequency, high heat-flux Type-I at high-rotation to the high-frequency, low-heat flux ``grassy" ELMs at low rotation. In DIII-D the experiments were performed at triangularity ($\delta$) $> 0.7$, $\beta_p > 1.6$, $q_{95} > 6$ and with a broad edge rotation scan (+90 to -70~km/s). In the DIII-D experiments, the ELM frequency was found to be weakly dependent on rotation and the ELMs appeared to be always Type-I ELMs. Comparison of $\Delta W_{\rm ELM}$ and $f_{\rm ELM}$ to discharge parameters ($P_{\rm aux}$, $\delta$, $\beta_p$, $q_{95}$) and with other grassy ELM experiments, will be presented. The stability to peeling-ballooning modes and the pedestal height for the ELMs in the rotation scan will be calculated by the ELITE and EPED1 codes. Future grassy ELM experiments in DIII-D based on this analysis will be outlined.   

Impact of Resonant Magnetic Perturbations (RMPs) on Turbulence Drive, Damping, and Transport

It has been previously reported that broadband density fluctuations increase in RMP ELM-suppressed discharges in DIII-D, suggesting that electrostatic turbulence plays a role in RMP ELM suppression similar to the Edge Harmonic Oscillation in QH-modes: increasing particle transport to stabilize ELMs. Recent results show that the RMP-induced changes for ion-scale turbulence vary with radius. In the core, ion-scale fluctuations ($k_\theta\rho_i\approx 0.2$) increase, while in the H-mode pedestal, they decrease. These changes correlate with $E\times B$ shearing rate changes. However, the $E\times B$ shearing rate doesn't scale with increasing RMP-coil current as the density pump-out does, suggesting that turbulence drive (ion pressure gradient) or intermediate-scale modes ($k_\theta\rho_i\approx 1$) are important or that the turbulence and $E\times B$ shear changes are linked to remnant islands as seen in previous devices with stochastic fields.   

Pellet ELM Pacing Results from DIII-D

Small deuterium pellets have been injected into DIII-D H-mode plasmas from the low field side to trigger ELMs at a higher frequency than natural occurring 5 Hz ELMs. The resulting 25 Hz ELM frequency leads to smaller stored energy loss per ELM by a factor of 4 and reduced peak heat flux to the divertor by a factor of 2.5. The pellets result in no increase in plasma density with a modest $\sim$10\% decrease in energy confinement compared to an identical non-pellet discharge. Fast camera images of the pellet injection event show that a single filament of plasma becomes visible at the front edge of the pellet perturbation as the pellet reaches the separatrix. The filament is rapidly ejected during the ELM release and hits the vessel wall locally near the pellet injection location within 200~microseconds. Implications for controlled ELM triggering on ITER will be discussed.   

Improving Stability and Confinement of Slowly Rotating Tokamak Plasmas Using Static Nonaxisymmetric Magnetic Fields

A high-confinement regime without edge localized mode instabilities (QH-mode) has been demonstrated for the first time in tokamak plasmas with near-zero rotation and zero-net neutral beam injected torque. Edge rotation shear required for QH-mode operation is generated by the counter-$I_p$ torque driven, through neoclassical toroidal viscosity, by externally applied static, nonaxisymmetric, nonresonant magnetic fields. In this regime, the nonresonant magnetic fields also provide improved resilience to locked modes. Furthermore, the energy confinement improves with higher plasma pressure and slower rotation. The reduction in energy transport is correlated with a reduction in turbulent fluctuations.   

A New Resistive Response to 3-D Fields in Low Rotation H-modes

A new resistive response to 3-D fields is identified in low rotation H-modes that are far from ideal MHD $\beta$ limits. The response increases as natural ($\Delta^\prime$) tearing stability limits are approached, either by lowering plasma rotation or by raising $\beta$. This leads to 2/1 tearing modes that degrade performance, with threshold fields to trigger modes falling to zero as the natural tearing limit is approached. These applied static 3-D field appear to act through rotation braking to decrease intrinsic tearing stability, thereby leading to formation of rotating modes at modest $\beta_N\sim 1-2$ in low torque plasmas. A formalism has been developed based on the observed physics mechanisms to account for the 3-D field threshold scaling. Further scans have been executed in toroidal field and density to determine coefficients of this scaling in torque-free H modes. These yield a more adverse toroidal field scaling for future devices than previous Ohmic studies, though the main increase in field sensitivity comes about because of proximity to the natural tearing $\beta$ limit at $\beta_N\sim 2$.