Session GO3: DIII-D Tokamak

Overview of Recent DIII-D Experimental Results

The 2007 DIII-D experimental campaign focuses on resolving issues of importance to the ongoing ITER Design Review. Recent experiments have established the importance of island overlap for ELM suppression and confirmed the previous observation of a low rotation threshold for RWM stabilization in AT plasmas with $\beta _{N}$~$\approx $~4, providing critical information for the design of internal coil systems for RWM and ELM control in ITER. Experiments simulating the ITER startup scenario with an outer wall limited plasma exhibit high internal inductance during the current ramp, potentially presenting challenges for vertical stability in ITER. Studies performed in the high performance hybrid scenario establish that pedestal pressure increases with triangularity and plasma $\beta $, but with little dependence on plasma rotation. Increasing T$_{e}$/T$_{i}$ with ECH results in increased low to intermediate $k$ fluctuations, but little change in confinement. In other studies, radial profiles of electron temperature and density fluctuations have been simultaneously measured, and are observed to behave similarly.   

Effects of Resonant Magnetic Perturbations on Edge Turbulence and Profiles in DIII-D

Resonant magnetic perturbations (RMPs) applied to the plasma edge can cause changes in average density and in the turbulence measured by various diagnostics at the edge and scrape-off layer (SOL). The change in turbulence can modify the edge profiles, which can affect the RMP ELM suppression, which is important for ITER. Two main regimes have been explored: 1) low power and collisionality discharges, where it is seen that the RMPs affect the edge profiles across the SOL and into the core; and 2) high power, varying (low, medium and high) collisionality discharges where the average density can increase or decrease but the turbulence in the SOL always increases. In these discharges, the pedestal fluctuations can increase or decrease in narrowly localized radial regions near the pedestal top. When the RMP are rotated toroidally, the fluctuations change amplitude and/or location, indicating that the RMP-induced changes are toroidally localized.   

Resistive Wall Mode and Plasma Stability at High $\beta $ and Slow Rotation

DIII-D experiments extended the observation of resistive wall mode (RWM) stabilization by slow plasma rotation to various scenarios, including high-$\beta $ advanced tokamak scenarios, and confirmed that magnetic feedback increases stability against equilibrium disturbances, such as large ELMs. At high $\beta $, magnetic disturbances that resonant with marginally stable RWM can lead, depending on torque input and momentum confinement, to loss of torque balance followed by plasma locking, perturbation growth, and confinement loss. Reconnection may take place once plasma is locked. Magnetic feedback can maintain or quickly restore axisymmetry and avoid locking. With very low torque input, however, error field threshold for locking may be below feedback sensitivity. Residual uncorrected error fields may explain why minimum sustainable rotation profiles are generally higher than those predicted by ideal-plasma RWM stability theory.   

Plasma Initiation and Startup in DIII-D Simulating the ITER Scenario

DIII-D similarity experiments have investigated the ITER baseline startup scenario, specifically outer wall low field side (LFS) limited discharges with an I$_{p }$ ramp at constant safety factor, q$_{95}$. Optimizing startup may be necessary for ITER advanced tokamak (AT) discharges and to minimize limiter heating. Although I$_{p}$ initiation in DIII-D occurred near the inner wall (in the region of highest E$_{\phi }$L, where E$_{\phi }$ is the inductive electric field and L is the wall connection length), it moved outward in $<$5 ms (scaling to $\sim $0.25 s in ITER) and then limited on the LFS limiters. In this ITER-like shape, I$_{p }$was ramped to $\le $1.2 MA with q$_{95}$ held constant during the limited phase by a simultaneous I$_{p}$ and $\kappa $ ramp. In addition to presenting LFS startup results, we will discuss other startup issues for ITER, i.e. limiter heat flux, compatibility with AT scenarios, sawteeth, and vertical stability. Startup scenarios other than constant q$_{95}$ will also be presented.   

High Performance Operation on DIII-D With Reduced Frequency of Wall Conditioning

Recent DIII-D experiments have demonstrated the capability to obtain high performance plasmas, ($\beta _{N}H_{ITER89}$/$q_{95}^2 \sim $ 0.38), in both hybrid and steady-state scenarios over an extended operations period (6000 plasma seconds) with no intervening boronization or bakes. Over the same period, impurity influx monitored with daily reference shots remains at low levels. With adequate divertor pumping, good hybrid performance can also be maintained in several sequential discharges with no between-shot helium glow. These findings on DIII-D, which has $>$95{\%} graphite plasma facing wall, are in sharp contrast to recent studies on tokamaks with high-Z metallic walls, where frequent boronizations are found necessary to prevent radiative collapse of high-confinement, high-beta discharges [1,2]. \newline [1] B. Lipschultz, \textit{et al., }Phys. Plasma \textbf{13}, 056117 (2006). \newline [2] R. Neu, \textit{et al.,} J. Nucl. Mater. \textbf{363-365}, 52 (2007).   

Observation of Carbon Dust in the DIII-D Divertor and SOL

Dust accumulation is a serious safety concern for ITER. In DIII-D carbon dust is observed in divertor and scrape-off layer (SOL) by optical imaging. After an extended entry vent, thousands of dust particles are observed in the first 2-3 plasma discharges. Individual particles moving at velocities up to $\sim $500 m/s, and breakup of larger particles into pieces are observed. After $\sim $70 discharges, dust levels are reduced to a few observed events per discharge except in discharges with disruptions that produce significant amounts of dust. Using the divertor materials evaluation system (DiMES), micron-sized carbon dust is injected into DIII-D ELMing H-mode discharges. When the outer divertor strikepoint is swept onto DiMES, $\sim $2{\%} of the dust carbon content penetrates the core, raising the core carbon density by a factor of $\sim $4. Dust particles from the injection are observed in the outboard SOL. The observed dust trajectories and velocities are in qualitative agreement with the modeling of the 3D DustT code.   

Extension of DIII-D Hybrid Plasmas Towards Operation with T$_{e}$ $\sim$ T$_{i}$ and Low Rotation

DIII-D hybrid plasmas typically operate in a hot ion mode (T$_{i}$~$>$~T$_{e})$ with high plasma rotation, which tend to reduce turbulent transport. Recent DIII-D experiments extend hybrid operation to more reactor relevant conditions, with low plasma rotation and T$_{e}\approx $T$_{i}$. Using electron cyclotron (EC) heating to replace part of the neutral beam heating, T$_{e}$/T$_{i}$ has been increased to $\sim $0.8 in hybrid plasmas with $\beta _{N}$=2.6, with minimal effect on confinement time and a modest reduction in plasma rotation. The plasma turbulence level increased significantly in the EC heated hybrid plasmas, at both low and intermediate wavenumbers, as measured by BES and FIR scattering systems. Central Mach number in DIII-D hybrid plasmas has been scanned across a wide range, from 0.07 to 0.6. At low rotation, the confinement factor H$_{89}$ degrades, typically by 10{\%}-30{\%}. Transport modeling using the GLF23 code indicates that the change in transport with rotation can be accounted for by changes in the ExB shearing rate.   

Towards Demonstration of Steady-State High-Performance Scenarios in DIII-D

Experiments on advanced scenarios in DIII-D are focused on extension to the resistive time scale, optimization, and exploration for higher performance. Optimization studies use ECCD and counter-NBI to modify the $q$ profile shape, looking at the effect on MHD stability and bootstrap current. Feedback control of the current formation is also a key element of optimization. Closed-loop experiments and modeling of open-loop tests have been carried out. Experiments seeking $\beta _{N}$=5 used two approaches -- high $q_{min}$ with rotational stabilization and high magnetic shear. High shear experiments achieved $\beta _{N}$~$>$~4.5 transiently. Attempts to use the longer-pulse (5 s) ECCD system to extend the duration of noninductive high-performance discharges to resistive equilibrium will be presented.   

Integrated Scenario Modeling for Advanced Scenario Development in DIII-D

Integrated predictive simulations are carried out to guide design of Advanced Tokamak (AT) experiments with upgraded DIII-D hardware. Recent advances in the theory-based modeling include improved transport models for particle and momentum and integration with realistic feedback control algorithms as used in the DIII-D experiments. The modeling tools are validated successfully against recent AT experiments: (i)~sustained ($\sim $2 s) operation with $\beta _{N}$~$\approx $~4 (50{\%} above the no-wall stability limit) with internal transport barrier using toroidal field ramp, and (ii) fully noninductive operation [in-principle steady state (SS)] with $\beta _{N}$~$\approx $~3.5. Present simulation efforts focus on optimizing SS conditions at higher $\beta $ by utilizing the increased electron cyclotron and fast wave power in a pumped double-null configuration, indicating that SS can be achieved with $\beta _{N}$~$\ge $~4 for longer than a current relaxation time using the upgraded hardware planned for DIII-D.   

Locked Neoclassical Tearing Mode Control on DIII-D by Electron Cyclotron Current Drive and Magnetic Perturbations

Magnetic perturbations were used at DIII-D to unlock, reposition or spin locked tearing modes and so assist their electron cyclotron durrent drive (ECCD) stabilization. While the island was slowly (0.66 Hz) dragged in the toroidal direction and illuminated by 1.3~MW ECCD, current was alternatively driven in its O-point and X-point. Correspondingly, a modulation of the mode amplitude by up to a factor 2 was observed, consistent with the stabilizing/destabilizing effect of ECCD in the O/X point. Faster sustained rotation, at up to 180~Hz, was also demonstrated. This brings the locked mode case into the well-studied rotating neoclassical tearing mode (NTM) case. It also opens up the possibility to synchronize and phase-lock the ECCD modulation to the entrained mode rotation, which is simpler than adapting the ECCD to the natural mode frequency and phase.   

Local Turbulence Suppression and Flow Shear Dynamics During \textit{q}$_{min}$-Triggered Internal Transport Barriers

Turbulence is observed to transiently decrease locally during the formation of internal transport barriers (ITBs) following the appearance of low-order rational $q_{min}$ surfaces in negative central shear discharges on DIII-D. Simultaneously, increased poloidal flow shear is observed. To further study this phenomenon, localized 2D density fluctuation measurements of turbulence and turbulence flow were obtained over 0.3 $<$ r/a $<$ 0.7 via the high-sensitivity beam emission spectroscopy diagnostic. Both the reduction in fluctuations and the poloidal velocity shear are found to propagate radially outward at about 1 m/s. Initial observations suggest that these effects follow the $q$=2 surface. Related GYRO simulations suggest transient zonal flows form near the $q$=2 surface to trigger these ITBs. High-frequency poloidal velocity measurements will be used to examine this mechanism.   

Role of Pedestal in Hybrid Discharges in DIII-D

Studies of hybrid discharges in DIII-D show that the best performance in the hybrid regime is due to a combination of pedestal and core effects. The role of the H-mode pedestal in this regime has been studied with scans of $\beta _{N}$ (heating power) in discharges with 2 different values of average triangularity $\delta $ = 0.25 and 0.50. For both $\delta $s, the pedestal beta, as determined from measurements of pedestal temperatures and densities, increased as the global $\beta $ was increased. For a given value of $\beta _{N}$, discharges with the higher delta had significantly higher values of pedestal $\beta $ and also of H98(y,2) than discharges with the lower $\delta $, a result which is consistent with improved MHD stability of the pedestal expected at higher $\delta $. However, at the highest values of $\beta _{N}$ in the high $\delta $ discharges, some of the increase in core energy with increased power cannot be attributed directly to changes in the pedestal. Thus, both core and pedestal physics are important in determining the global confinement achieved in hybrid operation.   

Effect of Reverse Shear Alfv\'{e}n Eigenmodes on Delivered Neutral Beam Torque

Modifications to the torque deposition profile from neutral beam injection is investigated under varying levels of reverse shear Alfv\'{e}n eigenmode (RSAE) activity. The rotation profile is found to be altered in the presence of strong RSAE activity, but the total mechanical angular momentum appears relatively independent of the RSAE intensity. This indicates that the neutral beam ions are being redistributed in the presence of RSAEs, rather than lost, consistent with present understanding of how RSAEs affect fast ions. Analysis of the momentum transport channel provides a technique for determining how the fast ions are redistributed. In particular, by matching the local momentum diffusivity and global momentum confinement time for plasmas with strong RSAEs with plasmas without RSAEs, one can precisely infer the modification to the neutral beam torque profile from the classical prediction, and estimate the anomalous fast ion diffusion caused by the RSAEs.   

DIII-D Quiescent H-mode Experiments With Co Plus Counter Neutral Beam Injection

QH-mode discharges have all the confinement advantages of H-mode with none of the detrimental effects caused by ELMs. Edge density control is instead provided by an edge MHD mode, the edge harmonic oscillation (EHO). The edge parameter operating regime in QH-mode is consistent with peeling-ballooning mode stability calculations. Using these as a guide, we developed a double-null plasma shape that has a much broader range of edge stability compared to the upper single null plasmas used previously. Altering the edge rotation in these plasmas using the co plus counter NBI capability on DIII-D, we could control the edge density and pressure by a factor of $\sim $2 while retaining the ELM-free state. The change in rotation alters the EHO and, apparently, the edge particle transport. Pedestal densities up to $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ the Greenwald density have been achieved in these plasmas, which is the same as the ELMing H-mode value. Accordingly, we now have a technique for actively controlling edge pressure in QH-mode plasmas.   

Prediction of Sawtooth Periods in Fast-Wave Heated DIII-D Experiments Using Extensions of the Porcelli Model

Validation of a predictive sawtooth model is important for burning plasma experiments such as ITER. The Porcelli model using simplified expressions for the key contributions has been found to predict average sawtooth periods reasonably well in existing tokamaks. We evaluate this model using realistic models for the ideal MHD contribution from GATO, and a nonisotropic fast ion contribution using ORBIT-RF and TORIC for the rf-modified fast-ion pressure. Application to the first giant sawtooth cycle in a DIII-D discharge where beam ions accelerated by fast waves modify the sawteeth shows the model can predict the crash time consistent with the experimental crash. The stabilizing contributions depend strongly on uncertainties in the magnetic shear at q=1 and the fast ion poloidal beta. The model will be also applied to other sawteeth in the same discharge and compared to predictions from the more complete NOVA-K stability code with full anistropy.