Gyrokinetic simulations identify microtearing modes (MTM) to be the dominant microinstabilities in the JET-ILW (ITER like wall) pedestal. Nonlinear simulations show that MTM-driven turbulence produces the bulk of the transport in the steep gradient region, demonstrating that MTM may be the principal mechanism limiting JET-ILW pedestal evolution. The combination of MTM, electron temperature gradient (ETG), and neoclassical transport reproduces experimental power balance across most of the pedestal. Kinetic ballooning modes are significant only in the local limit and only at low β, far below the experimental operating point. PACS numbers:The tokamak H-mode [1] is defined by a narrow insulating region-the pedestal-at the plasma edge, where turbulence is suppressed and sharp pressure gradients develop. The pedestal is at the center of the most pressing issues facing fusion energy. ITER, for example, must reach a sufficient temperature at the pedestal's inner boundary in order to achieve its fusion power targets [2]. This work reports the results of, perhaps, the very first, first-principles simulations of the H-mode pedestal dynamics that reproduce experimentally observed transport levels. In addition to providing unprecedented insight into the dynamics of the existing H-mode pedestals, such simulations are likely to advance our capabilities towards predictive modeling of future burning plasma devices.This study targets the JET-ILW (ITER-like wall) pedestal [4][5][6], which approaches ITER conditions in two important ways: 1) as the largest tokamak in operation, it most-closely approximates plasma parameters that are dependent on machine size (like ρ * , the ratio of the gyroradius to minor radius), 2) to approximate ITER conditions even better, JET has recently installed an ITERlike wall (ILW) (composed of a tungsten divertor and beryllium chamber). This modification decreases the global performance of discharges by 20-30%, attributable largely to changes in pedestal structure. In addition to the performance loss, certain observed key properties of the ILW pedestal are inconsistent with predictions of the leading pedestal model (EPED [7,8]).In this work, we elucidate possible mechanisms limiting profile evolution in the JET-ILW discharges. We demonstrate, through simulations using the gyrokinetic code Gene [9,10], that the microtearing mode (MTM) [11][12][13][14] is the dominant instability in the pedestal. Interestingly, the simulations do not find the kinetic ballooning mode (KBM), a basic component of the EPED model, except locally in a narrow region near the separatrix. Most importantly, we determine via nonlinear gyrokinetic simulations that a combination of MTM and electron temperature gradient (ETG) [9,[15][16][17][18] driven turbulence plus the neoclassical flux, produces transport levels closely matching experimental power balance across most of the pedestal, demonstrating the capacity of these mechanisms to limit JET-ILW pedestal evolution.The JET-ILW Pedestal-JET pulse 82585 (described in Ref. [4]) is pa...

The Super-X Divertor ͑SXD͒, a robust axisymmetric redesign of the divertor magnetic geometry that can allow a fivefold increase in the core power density of toroidal fusion devices, is presented. With small changes in poloidal coils and currents for standard divertors, the SXD allows the largest divertor plate radius inside toroidal field coils. This increases the plasma-wetted area by 2-3 times over all flux-expansion-only methods ͑e.g., plate near main X point, plate tilting, X divertor, and snowflake͒, decreases parallel heat flux and hence plasma temperature at plate, and increases connection length by 2-5 times. Examples of high-power-density fusion devices enabled by SXD are discussed; the most promising near-term device is a 100 MW modular compact fusion neutron source "battery" small enough to fit inside a conventional fission blanket.

The limited thermal power handling capacity of the standard divertors (used in current as well as projected tokamaks) is likely to force extremely high (∼90%) radiation fractions frad in tokamak fusion reactors that have heating powers considerably larger than ITER [D. J. Campbell, Phys. Plasmas 8, 2041 (2001)]. Such enormous values of necessary frad could have serious and debilitating consequences on the core confinement, stability, and dependability for a fusion power reactor, especially in reactors with Internal Transport Barriers. A new class of divertors, called X-divertors (XD), which considerably enhance the divertor thermal capacity through a flaring of the field lines only near the divertor plates, may be necessary and sufficient to overcome these problems and lead to a dependable fusion power reactor with acceptable economics. X-divertors will lower the bar on the necessary confinement to bring it in the range of the present experimental results. Its ability to reduce the radiative burden imparts the X-divertor with a key advantage. Lower radiation demands allow sharply peaked density profiles that enhance the bootstrap fraction creating the possibility for a highly increased beta for the same beta normal discharges. The X-divertor emerges as a beta-enhancer capable of raising it by up to roughly a factor of 2.

JET has been unable to recover historical confinement levels when operating with an ITER-like wall (ILW) due largely to the inaccessibility of high pedestal temperatures. Finding a path to overcome this challenge is of utmost importance for both a prospective JET DT campaign and for future ITER operation. Gyrokinetic simulations (using the Gene code) quantitatively capture experimental transport levels for a representative experimental discharge and qualitatively recover the major experimental trends. Microtearing turbulence is a major transport mechanisms for the low-temperature pedestals characteristic of unseeded JET-ILW discharges. At higher temperatures and/or lower ρ * , we identify electrostatic ITG transport of a type that is strongly shear-suppressed on smaller machines. Consistent with observations, this transport mechanism is strongly reduced by the presence of a low-Z impurity (e.g. carbon or nitrogen at the level of Z 2 eff ∼ ), recovering the accessibility of high pedestal temperatures. Notably, simulations based on dimensionless ρ * scans recover historical scaling behavior except in the unique JET-ILW parameter regime where ITG turbulence becomes important. Our simulations also elucidate the observed degradation of confinement caused by gas puffing, emphasizing the important role of the density pedestal structure. This study maps out important regions of parameter space, providing insights that may point to optimal physical regimes that can enable the recovery of high pedestal temperatures on JET.

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International Atomic Energy Agency

The first high fidelity gyrokinetic simulations of the energy losses in the transport barriers of large tokamaks in pursuit of fusion gain are presented. These simulations calculate the turbulent energy losses with an extensive treatment of relevant physical effects-fully kinetic, non-linear, electromagnetic-inclusive of all major plasma species, and in equilibria with relevant shape and local bootstrap current for fusion-relevant cases. We find that large plasmas with a small normalized gyroradius lie in an unexpected regime of enhanced losses that can prevent the projected energy gain. Our simulations are qualitatively consistent with recent experiments on JET with an ITER-like wall. Interestingly and very importantly, the simulations predict parameter regimes of reduced transport that are quite fusion-favorable.

We present the first treatment of the refraction of physical electromagnetic waves in newly developed negative index media (NIM), also known as left-handed media (LHM). The NIM dispersion relation implies that group fronts refract positively even when phase fronts refract negatively. This difference results in rapidly dispersing, very inhomogeneous waves. In fact, causality and finite signal speed always prevent negative wave signal (not phase) refraction. Earlier interpretations of phase refraction as "negative light refraction" and "light focusing by plane slabs" are therefore incorrect, and published NIM experiments can be explained without invoking negative signal refraction.