Session UI2: Tokamak Scenarios, Transport Barriers, and Stability

Research on DEMO Physics Issues at High Density on ASDEX Upgrade

Conceptual design studies of DEMO, the step that should bridge the gap between ITER and an FPP, heavily rely on the physics assumptions for its operational scenario. Usual DEMO designs exceed the parameters of the ITER Q$=$10 baseline scenario in a number of points, such as $\beta_{N}$, $n/n_{GW} $and $f_{rad,core}=P_{rad,core}/P_{tot}$. Research on present day devices cannot address these issues simultaneously at the high density and low collisionality that will occur in ITER or DEMO. In the last years, work on the ASDEX Upgrade tokamak has therefore mainly focused on the high density regime, consistent with the operational range set by the unique all-W wall of ASDEX Upgrade. In this contribution, we will report in particular on the following results: \begin{itemize} \item ELM mitigation with magnetic perturbation coils at high densities: ASDEX Upgrade has demonstrated reliable ELM mitigation using $n=$\textit{1}, $n=$\textit{2} and $n=$\textit{4} coil configurations at high density with no loss in confinement, in contrast to RMP ELM suppression at low density in DIII-D. We will discuss differences and commonalities. \item H-Mode operation at line averaged density well above the empirical Greenwald limit: small ELM regimes, lead to good pellet fuelling efficiency and have allowed achieving stationary H-modes at $n/n_{GW} =$ 1.5 with peaked density, the pedestal top density staying below $n_{GW}$. These findings may open a route to operation of DEMO beyond the empirical Greenwald limit. \item Upper density limit for H-mode operation: recent studies reveal the coupling of an energy loss and the saturation of the density increase, which lead to the degeneration of the H-mode at high edge densities. Hence, also this limit can be viewed as an edge density limit. \item Exhaust at high $P_{sep}/R$ or high $f_{rad,core}$: both ITER and DEMO will have to operate with (semi)detached divertor at $P_{sep}/R \ge $\textit{15} MW/m to stay in H-mode. We show stationary operation at 7 MW/m with average divertor heat flux below 5 MW/m$^{2}$ and $T_{e,div}$ $\sim$\textit{5} eV by simultaneous feedback control of two seed impurities. In DEMO, this regime calls for $f_{rad,core}$ \textgreater\ 70{\%}; we have also shown this, albeit necessarily at lower $P_{sep}/R$ values. \end{itemize} While these studies target DEMO operation parameters, they have a high relevance for the exhaust problem in ITER as well, which faces the same challenges as DEMO in terms of $P_{sep}/R$ and the need to mitigate ELMs.   

Projecting High Beta Steady-State Scenarios from DIII-D Advanced Tokamk Discharges

Fusion power plant studies based on steady-state tokamak operation suggest that normalized beta in the range of 4-6 is needed for economic viability. DIII-D is exploring a range of candidate high beta scenarios guided by FASTRAN modeling in a repeated cycle of experiment and modeling validation. FASTRAN is a new iterative numerical procedure coupled to the Integrated Plasma Simulator (IPS) that integrates models of core transport, heating and current drive, equilibrium and stability self-consistently to find steady state ($d/dt=0$) solutions, and reproduces most features of DIII-D high beta discharges with a stationary current profile. Separately, modeling components such as core transport (TGLF) and off-axis neutral beam current drive (NUBEAM) show reasonable agreement with experiment. Projecting forward to scenarios possible on DIII-D with future upgrades, two self-consistent noninductive scenarios at $\beta_N>4$ are found: high $q_{min}$ and high internal inductance $l_i$. Both have bootstrap current fraction $f_{BS}>0.5$ and rely on the planned addition of a second off-axis neutral beamline and increased electron cyclotron heating. The high $q_{min}>2$ scenario achieves stable operation at $\beta_N$ as high as 5 by a very broad current density profile to improve the ideal-wall stabilization of low-n instabilities along with confinement enhancement from low magnetic shear. The $l_i$ near 1 scenario does not depend on ideal-wall stabilization. Improved confinement from strong magnetic shear makes up for the lower pedestal needed to maintain $l_i$ high. The tradeoff between increasing $l_i$ and reduced edge pedestal determines the achievable $\beta_N$ (near 4) and $f_{BS}$ (near 0.5). This modeling identifies the necessary upgrades to achieve target scenarios and clarifies the pros and cons of particular scenarios to better inform the development of steady-state fusion.   

ELM Suppression and Pedestal Structure in I-Mode Plasmas

The I-mode regime is characterized by the formation of a temperature pedestal and enhanced energy confinement ($H_{98}$ up to 1.2), without an accompanying density pedestal or drop in particle transport. Unlike ELMy H-modes, I-mode operation appears to have naturally-occurring suppression of large ELMs in addition to its highly favorable scalings of pedestal structure (and therefore overall performance). Instead, continuous Weakly Coherent Modes help to regulate density. Extensive study of the ELMy H-mode has led to the development of the EPED model, which utilizes calculations of coupled peeling-ballooning MHD modes and kinetic-ballooning mode (KBM) stability limits to predict the pedestal structure preceding an ELM crash. We apply similar tools to the structure and ELM stability of I-mode pedestals. Peeling-ballooning MHD calculations are completed using the ELITE code, showing I-mode pedestals to be generally MHD-stable. Under certain conditions, intermittent ELMs are observed in I-mode at reduced field, typically triggered by sawtooth crashes; modification of the temperature pedestal (and therefore the pressure profile stability) by sawtooth heat pulses is being examined in ELITE. Modeled stability to KBM turbulence in I-mode and ELMy H-mode suggests that typical I-modes are stable against KBM turbulence. Measured I-mode pedestals are significantly wider (more stable) than the width scaling with the square root of poloidal beta characteristic of the KBM-limited pedestals in ELMy H-mode. Finally, we explore scalings of pedestal structure with engineering parameters compared to ELMy H-modes on C-Mod. In particular, we focus on scalings of the pressure pedestal with heating power (and its relation to the favorable scaling of confinement with power in I-mode) and on relationships between heat flux and pedestal temperature gradients.   

Resolving the Mystery of Transport Within Internal Transport Barriers

The Trapped Gyro-Landau Fluid (TGLF) quasilinear model, which is calibrated to approximate non-linear gyro-kinetic 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 in excellent agreement with data from the DIII-D tokamak. This is a strong validation of gyro-kinetic theory of ITBs, requiring multiple instabilities responsible for transport in different channels at different scales. Inside the ITB, the ion energy transport is observed to be reduced to the neoclassical level which is consistent with the theory of turbulence suppression by $E\times B$ velocity shear acting on low wavenumber turbulence. The electron energy transport is observed to be far above the neoclassical level which is consistent with electron energy transport due to high wavenumber electron temperature gradient (ETG) modes. Since the ETG modes do not produce particle and ion momentum transport, and low wavenumber modes are suppressed, these channels are expected to be reduced to the neoclassical level in striking disagreement with experimental measurements. A possible resolution of this conundrum was found in 2005 when gyro-kinetic turbulence simulations showed that the parallel velocity shear driven Kelvin-Helmholtz (KH) mode can arrest the suppression of transport by the shear in the $E\times B$ velocity Doppler shift at high toroidal flow 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 $E\times B$ shear and to recent improvements to TGLF that allow the KH mode to be faithfully modeled. The resolution of this long-standing mystery of the missing particle and momentum transport in an ITB is the result of the steady advances in gyro-kinetic simulations and quasilinear modeling.   

Recent Advances in Long Pulse High Confinement Plasma Operations in EAST

Significant progress has been made in the EAST superconducting tokamak toward steady-state operations, achieving a new H-mode regime with a record duration over 30 s, using predominantly Lower Hybrid Current Drive (LHCD) and Lithium (Li) wall conditioning [1]. A key feature of the long pulse H-modes relies on the achievement of tiny ELMs with a dramatic reduction in ELM energy, compared to Type I ELMs. The small ELMs are rather benign with a frequency of 0.5 $-$ 1 kHz and peak heat fluxes largely below 2 MW/m2. This new small ELMy H-mode regime exhibits a confinement quality modestly lower than Type I, but higher than Type III ELMy H-modes, with an confinement enhancement factor, H$_{98(y,2)}$ $\sim$ 0.9, similar to Type II ELMy H-modes. What is truly remarkable is that LHCD induces a three dimensional distortion of the edge magnetic topology by driving helical current filaments at the edge [2], thus mitigating ELMs, similar to RMP (Resonant Magnetic Perturbations). Another important facet is that the small ELMs are accompanied by a quasi-coherent MHD mode at 30 $-$ 50 kHz throughout the H-mode phase, which can provide continuous particle and heat exhaust, hence facilitating long pulse operations. This new, small ELM regime, enabled by Li wall conditioning and LHCD, exhibits a dramatic reduction in ELM transient power loads and good global confinement without significant impurity accumulation, thus potentially opening a new avenue in long pulse H-mode operations.\\[4pt] [1] J. Li, H. Y. Guo*, B. N. Wan et al., ``Achievement of a new long pulse high confinement plasma in EAST superconducting tokamak,'' submitted to Nature Physics (under review).\\[0pt] [2] Y. Liang, X. Z. Gong, K. F. Gan et al., Phys. Rev. Lett. \textbf{110}, 235002 (2013).   

Enhanced understanding of the MHD dynamics and ELM control experiments in KSTAR

In KSTAR, H-mode discharges have been achieved reliably at toroidal fields from 1.4 to 3.5 T with a heating power of $\sim$ 5 MW. Using real-time plasma shape control [1] the flattop time in H-mode has been extended to over $\sim$ 16 s at 600 kA in the 2012 campaign and the extended plasma operation boundary has surpassed the $n=$\textit{1} no-wall limit with $\beta_{N}/l_{i}$ up to 4.1. In order to achieve a high beta steady state operation in KSTAR, establishment of predictive MHD simulation and first-principle-based control of the harmful MHD are the first steps. Visualization of MHD dynamics via a 2-D Electron Cyclotron Emission Imaging (ECEI) [2] has significantly enhanced the level of understanding of the MHD dynamics. Following the first 2-D ELM measurements in H-mode plasmas [3] in KSTAR the measured 2-D ELM images were compared with synthetic images [4] from the BOUT$++$ code. The physics of ELMs is characterized based on a wide range of measured mode numbers ($n$, $m)$ local magnetic shear and pressure gradients. The observed ELM dynamics during~control experiments have been enlightening and consistent with the stability models. Near the \textit{q $\sim$ 2} surface, the island width and $\Delta '$ of the $m=$\textit{2} tearing mode have been verified~through the modified Rutherford model based on the 2-D images. With the aid of a second (toroidally separated) ECEI system installed in the 2012 KSTAR campaign, a 3-D reconstruction of the MHD instabilities has allowed further validation of the computed magnetic field pitch angles, rotation speeds, and toroidal asymmetries of the MHDs Work supported by NRF of Korea under contract No. 20120005920 and the U.S. DoE under contract No. DE-FG-02-99ER54531\\[4pt] [1] Kwak, J.G. . 2012 24th IAEA conference, SanDiego, CA\\[0pt] [2] Park, H. 2004 Rev. Sci. Instrum. Vol. 74, 3787\\[0pt] [3] Yun, G.S. 2011 Phys. Rev. Lett. 107, 035001\\[0pt] [4] Park, H. 2012 24th IAEA conference, SanDiego, CA