JET quasistationary internal-transport-barrier operation with active control of the pressure profile

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.

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.

Radio-frequency current drive for thermonuclear fusion reactors

Principal research on energy from thermonuclear fusion uses Deuterium-Tritium plasmas magnetically trapped in toroidal devices. As major scientific problem for an economic (i.e., really feasible) reactor, we must understand how to lead strongly heated plasmas to sustain a high fusion gain while large fraction of current is self-produced via the presence of strong pressure gradient. To suppress turbulent eddies that impair thermal insulation and pressure tight of the plasma, current drive (CD) is necessary. However, tools envisaged so far in ITER (International Thermonuclear Experiment Rector) are unable accomplishing this task that requires efficiently and flexibly matching the natural current profiles of plasma. Consequently, viability of a thermonuclear reactor should be problematic. Multi-megawatt radio-frequency (RF) power coupled to plasma would produce the necessary CD, but modelling results based on previous understanding found difficult the extrapolation of this CD concept to reactor conditions of high temperature plasma, and greater flexibility of method would also be required. Here we present new model results based on standard quasilinear (QL) theory that allow establish conditions to drive efficiently and flexibly the RF-driven current at large radii of the plasma column, as necessary for the goal of a reactor.

Modeling of a lower-hybrid current drive by including spectral broadening induced by parametric instability in tokamak plasmas

By incorporating parametric instabilities of lower hybrid (LH) waves into a ray-tracing Fokker-Planck code, accurate simulations of the LH deposition profiles are provided, which are useful for interpreting the long-lasting internal transport barriers (ITBs) sustained by lower hybrid current drive (LHCD) on JET (Joint European Torus). Utilizing the new model, the simulation of the q-profile evolution results in agreement with that provided by the motional Stark effect reconstructed equilibria. Low magnetic shear (s approximately equal to 0) is produced by LHCD in a layer close to the ITB radial foot.

Modification of the current profile in high-performance plasmas using off-axis electron-cyclotron-current drive in DIII-D

Recent DIII-D experiments using off-axis electron cyclotron current drive (ECCD) have demonstrated the ability to modify the current profile in a plasma with toroidal beta near 3%. The resulting plasma simultaneously sustains the key elements required for Advanced Tokamak operation: high bootstrap current fraction, high beta, and good confinement. More than 85% of the plasma current is driven by noninductive means. ECCD is observed to produce strong negative central magnetic shear, which in turn acts to trigger confinement improvements in all transport channels in the plasma core.