Enhanced confinement and stability in DIII-D discharges with reversed magnetic shear

Self-Organized Evolution of the Internal Transport Barrier in Ion-Temperature-Gradient Driven Gyrokinetic Turbulence

Understanding the self-organization of the most promising internal transport barrier in fusion plasmas needs a long-time nonlinear gyrokinetic global simulation. The neighboring equilibrium update method is proposed, which solves the secularity problem in a perturbative simulation and speeds up the numerical computation by more than 10 times. It is found that the internal transport barrier emerges at the magnetic axis due to inward propagated turbulence avalanche, and its outward expansion is the catastrophe of self-organized structure induced by outward propagated avalanche.

Nontwist field line mapping in a tokamak with ergodic magnetic limiter

For tokamaks with uniform magnetic shear, Martin and Taylor have proposed a symplectic map which has been used to describe the magnetic field lines at the plasma edge perturbed by an ergodic magnetic limiter. We propose an analytical magnetic field line map, based on the Martin-Taylor map, for a tokamak with arbitrary safety factor profile. With the inclusion of a nonmonotonic profile, we obtain a nontwist map which presents the characteristic properties of degenerate systems, such as the twin islands scenario, shearless curve, and separatrix reconnection. We estimate the width of the islands and describe their changes of shape for large values of the limiter current. From our numerical simulations about the shearless curve, we show that its position and aspect depend on the control parameters.

A sustained high-temperature fusion plasma regime facilitated by fast ions

Nuclear fusion is one of the most attractive alternatives to carbon-dependent energy sources¹. Harnessing energy from nuclear fusion in a large reactor scale, however, still presents many scientific challenges despite the many years of research and steady advances in magnetic confinement approaches. State-of-the-art magnetic fusion devices cannot yet achieve a sustainable fusion performance, which requires a high temperature above 100 million kelvin and sufficient control of instabilities to ensure steady-state operation on the order of tens of seconds^2,3. Here we report experiments at the Korea Superconducting Tokamak Advanced Research⁴ device producing a plasma fusion regime that satisfies most of the above requirements: thanks to abundant fast ions stabilizing the core plasma turbulence, we generate plasmas at a temperature of 100 million kelvin lasting up to 20 seconds without plasma edge instabilities or impurity accumulation. A low plasma density combined with a moderate input power for operation is key to establishing this regime by preserving a high fraction of fast ions. This regime is rarely subject to disruption and can be sustained reliably even without a sophisticated control, and thus represents a promising path towards commercial fusion reactors.

Threshold Heat-Flux Reduction by Near-Resonant Energy Transfer

Near-resonant energy transfer to large-scale stable modes is shown to reduce transport above the linear critical gradient, contributing to the onset of transport at higher gradients. This is demonstrated for a threshold fluid theory of ion temperature gradient turbulence based on zonal-flow-catalyzed transfer. The heat flux is suppressed above the critical gradient by resonance in the triplet correlation time, a condition enforced by the wave numbers of the interaction of the unstable mode, zonal flow, and stable mode.

Review of plasma turbulence experiments

Understandings of turbulent plasma have been developed along with nuclear fusion research for more than a half century. Long international research has produced discoveries concerning turbulent plasma that allow us to notice the hidden nature and physics questions that could contribute to other scientific fields and the development of technologies. Guiding concepts have been established up to now that stimulate investigations on turbulent plasma. Research based on concepts concerning symmetry breaking and global linkage requires observing the entire field of plasma turbulence for an ultimate understanding of plasma. This article reviews the achievements as well as contemporary problems regarding turbulence experiments associated with strongly magnetized plasmas in the last and present century, and introduces forthcoming experimental issues, including new diagnostics and physics-oriented devices related to plasma turbulence.

Suppression of electron temperature gradient turbulence via negative magnetic shear in NSTX

Negative magnetic shear is found to suppress electron turbulence and improve electron thermal transport for plasmas in the National Spherical Torus Experiment (NSTX). Sufficiently negative magnetic shear results in a transition out of a stiff profile regime. Density fluctuation measurements from high-k microwave scattering are verified to be the electron temperature gradient (ETG) mode by matching measured rest frequency and linear growth rate to gyrokinetic calculations. Fluctuation suppression under negligible E×B shear conditions confirm that negative magnetic shear alone is sufficient for ETG suppression. Measured electron temperature gradients can significantly exceed ETG critical gradients with ETG mode activity reduced to intermittent bursts, while electron thermal diffusivity improves to below 0.1 electron gyro-Bohms.

Localized turbulence suppression and increased flow shear near the q=2 surface during internal-transport-barrier formation

Broadband turbulent fluctuations in the plasma density are transiently suppressed when low-order rational q surfaces first appear in negative central magnetic shear plasmas on the DIII-D tokamak, which can lead to the formation of internal transport barriers. Increased localized flow shear is simultaneously observed. It transiently exceeds the measured turbulence decorrelation rate, providing a mechanism to trigger the formation of the transport barrier. This increased flow shear and turbulence suppression propagates radially outward, following the q=2 surface.

Observations of reduced electron Gyroscale fluctuations in national spherical torus experiment H-mode plasmas with large ExB flow shear

Electron gyroscale fluctuation measurements in National Spherical Torus Experiment H-mode plasmas with large toroidal rotation reveal fluctuations consistent with electron temperature gradient (ETG) turbulence. Large toroidal rotation in National Spherical Torus Experiment plasmas with neutral beam injection generates ExB flow shear rates comparable to ETG linear growth rates. Enhanced fluctuations occur when the electron temperature gradient is marginally stable with respect to the ETG linear critical gradient. Fluctuation amplitudes decrease when the ExB flow shear rate exceeds ETG linear growth rates. The observations indicate that ExB flow shear can be an effective suppression mechanism for ETG turbulence.

The motional Stark effect diagnostic on NSTX

This work describes the implementation and recent results from the motional Stark effect (MSE) collisionally induced fluorescence diagnostic on NSTX. Due to the low magnetic field on NSTX the MSE diagnostic requires a new approach for the viewing optics and spectral filter. This has been accomplished with a novel optical design that reduces the geometric Doppler broadening, and a high throughput, high resolution spectral filter to optimize signal-to-noise ratio. With these improvements the polarization fraction is approximately 30%-40% and, combined with the large throughput, a time resolution of approximately 5 ms. The MSE diagnostic presently has 16 sight lines operating, providing measurements of the magnetic field line pitch from the plasma center to near the outboard edge of the plasma.

Transition between internal transport barriers with different temperature-profile curvatures in JT-60U Tokamak plasmas

A spontaneous transition phenomena between two states of a plasma with an internal transport barrier (ITB) is observed in the steady-state phase of the magnetic shear in the negative magnetic shear plasma in the JT-60U tokamak. These two ITB states are characterized by different profiles of the second radial derivative of the ion temperature inside the ITB region (one has a weak concave shape and the other has a strong convex shape) and by different degrees of sharpness of the interfaces between the L mode and the ITB region, which is determined by the turbulence penetration into the ITB region.

Observation of stable superdense core plasmas in the large helical device

In reduced recycling discharges in the Large Helical Device, a super dense core plasma develops when a series of pellets are injected. A core region with density as high as 4.5 x 10(20) m(-3) and temperature of 0.85 keV is maintained by an internal diffusion barrier with very high-density gradient. These results may extrapolate to a scenario for fusion ignition at very high density and relatively low temperature in helical devices.

Multitude of core-localized shear Alfvén waves in a high-temperature fusion plasma

Evidence is presented for a multitude of discrete frequency Alfvén waves in the core of magnetically confined high-temperature fusion plasmas. Multiple diagnostic instruments confirm wave excitation over a wide spatial range from the device size at the longest wavelengths down to the thermal ion Larmor radius. At the shortest scales, the poloidal wavelengths are comparable to the scale length of electrostatic drift wave turbulence. Theoretical analysis confirms a dominant interaction of the modes with particles in the thermal ion distribution traveling well below the Alfvén velocity.

Rapid and localized electron internal-transport-barrier formation during shear inversion in fully noninductive TCV discharges

Clear evidence is reported for the first time of a rapid localized reduction of core electron energy diffusivity during the formation of an electron internal-transport barrier. The transition occurs rapidly (approximately = 3 ms), during a slow (approximately = 200 ms) self-inductive evolution of the magnetic shear. This crucial observation, and the correlation of the transition with the time and location of the magnetic shear reversal, lend support to models attributing the reduced transport to the local properties of a zero-shear region, in contrast to models predicting a gradual reduction due to a weak or negative shear.

Evolution of lower-hybrid-driven current during the formation of an internal transport barrier

Evolution of the lower-hybrid(LH)-driven current profile was measured during the formation of an internal transport barrier (ITB) in a reversed magnetic shear discharge. As the ITB developed, the initially centrally peaked LH-driven current profile gradually turned hollow and was sometimes accompanied by an off-axis peak in the electron temperature profile. These observations indicate the concentration of LH power deposition to the ITB for this case as a result of nonlinear coupling between the LH waves and the target plasma.

MHD stability of advanced tokamak scenarios with reversed central current: an explanation of the "current hole"

A region of zero current density in the plasma center has been observed in the advanced tokamak scenarios with off-axis lower-hybrid current drive in the JET and JT-60U tokamak experiments. Significantly, the central current density does not become negative, although this is expected based on conventional current diffusion. In this paper, it is shown that the zero central current density and the absence of negative central current can be explained by the influence of a resistive kink magnetohydrodynamic instability.

Observation of zero current density in the core of jet discharges with lower hybrid heating and current drive

Simultaneous current ramping and application of lower hybrid heating and current drive (LHCD) have produced a region with zero current density within measurement errors in the core ( r/a< or =0.2) of JET tokamak optimized shear discharges. The reduction of core current density is consistent with a simple physical explanation and numerical simulations of radial current diffusion including the effects of LHCD. However, the core current density is clamped at zero, indicating the existence of a physical mechanism which prevents it from becoming negative.

Reaching high poloidal beta at Greenwald density with internal transport barrier close to full noninductive current drive

In the ASDEX Upgrade tokamak, high poloidal beta up to beta(pol) = 3 at the Greenwald density with H-mode confinement has been reached. Because of the high beta, the plasma current is driven almost fully noninductively, consisting of 51% bootstrap and 43% neutral beam driven current. To reach these conditions the discharge is operated at low plasma current ( I(P) = 400 kA) and high neutral beam heating power ( P(NBI) = 10 MW). The discharge combines an edge (H mode) and internal transport barrier at high densities without confinement-limiting MHD activities. The extrapolation to higher plasma currents may offer a promising way for an advanced scenario based fusion reactor.

Neoclassical radial electric field and transport with finite orbits

Neoclassical transport in a toroidal plasma with finite ion orbits is studied, including for the first time the self-consistent radial electric field. Using a low-noise deltaf particle simulation, we demonstrate that a deep electric-field well develops in a region with a steep density gradient, because of the self-collision-driven ion flux. We find that the electric field agrees with the standard neoclassical expression, when the toroidal rotation is zero, even for a steep density gradient. Ion thermal transport is modified by the electric-field well in a way which is consistent with the orbit squeezing effect, but smoothed by the finite orbits.

Maximum-J capability in a quasiaxisymmetric stellarator

Maximum-J (J is the second adiabatic invariant) capability, i.e., the radial derivative of J has the same sign as that of pressure, is investigated in a quasiaxisymmetric (QA) stellarator to investigate improved confinement. Due to the existence of nonaxisymmetry of the magnetic field strength, a local maximum of J is created to cause the drift reversal. External controllability of the maximum-J condition is also demonstrated, by which the impact of magnetic configuration on turbulent transport can be studied.

Internal transport barrier triggered by neoclassical transport in W7-AS

The three-dimensional magnetic configuration of a stellarator offers two specific mechanisms for a transition to improved particle and energy confinement. One route goes through the so-called electron-root confinement regime, which leads to a reduction of neoclassical transport via strong radial electric fields. In this Letter evidence for a second route is presented. It opens due to the layer of a strongly varying radial electric field which is present in the transitional region from neoclassical electron to ion-root confinement. This type of improvement acts on turbulent transport.

Dynamic modeling of stepwise internal transport barrier formation in DIII-D negative-central-shear discharges

The GLF23 transport model is used to dynamically follow bifurcations in the energy and toroidal momentum confinement in DIII-D discharges with an internal transport barrier. The temperatures and toroidal velocity profiles are evolved while self-consistently computing the effects of E x B shear stabilization during the formation and expansion of internal transport barriers. The barrier is predicted to form in a stepwise fashion through a series of sudden jumps in the core-electron and ion temperatures and toroidal rotation velocity. These results are consistent with experimental observations. In the simulations, the step transitions are a direct result of local E x B driven transport bifurcations.