• Thematic program: Turbulence and Fusion (2015/2016)
• Event: 8th Plasma Kinetics Working Group Meeting (2015)

• Thematic program: Turbulence and Fusion (2015/2016)
• Event: 8th Plasma Kinetics Working Group Meeting (2015)

• Thematic program: Turbulence and Fusion (2015/2016)
• Event: 8th Plasma Kinetics Working Group Meeting (2015)

A real-time capable core turbulence tokamak transport model is developed. This model is constructed from the regularized nonlinear regression of quasilinear gyrokinetic transport code output. The regression is performed with a multilayer perceptron neural network. The transport code input for the neural network training set consists of five dimensions, and is limited to adiabatic electrons. The neural network model successfully reproduces transport fluxes predicted by the original quasilinear model, while gaining five orders of magnitude in computation time. The model is implemented in a real-time capable tokamak simulator, and simulates a 300s ITER discharge in 10s. This proof-of-principle for regression based transport models anticipates a significant widening of input space dimensionality and physics realism for future training sets. This aims to provide unprecedented computational speed coupled with first-principle based physics for real-time control and integrated modelling applications.

• Thematic program: Turbulence and Fusion (2015/2016)
• Event: 8th Plasma Kinetics Working Group Meeting (2015)

A Lie- Poisson bracket is presented for a four-field gyrofluid model with compressible ions and magnetic field curvature, thereby showing the model to be Hamiltonian. In particular, we find that in addition to commonly adopted magnetic curvature terms present in the continuity equations, analogous terms must be retained also in the momentum equations, in order to have a Lie-Poisson structure. The corresponding Casimir invariants are presented, and shown to be associated to four Lagrangian invariants, that get advected by appropriate ''velocity'' fields during the dynamics. This differs from a cold ion limit, in which the Lie-Poisson bracket transforms into the sum of direct and semidirect products, leading to only three Lagrangian invariants.

• Thematic program: Turbulence and Fusion (2015/2016)
• Event: 8th Plasma Kinetics Working Group Meeting (2015)

The impact of electromagnetic stabilization and flow shear stabilization on ITG turbulence is investigated. Analysis of a low-β JET L-mode discharge illustrates the relation between ITG stabilization and proximity to the electromagnetic instability threshold. This threshold is reduced by suprathermal pressure gradients, highlighting the effectiveness of fast ions in ITG stabilization. Extensive linear and nonlinear gyrokinetic simulations are then carried out for the high-β JET hybrid discharge 75225, at two separate locations at inner and outer radii. It is found that at the inner radius, nonlinear electromagnetic stabilization is dominant and is critical for achieving simulated heat fluxes in agreement with the experiment. The enhancement of this effect by suprathermal pressure also remains significant. It is also found that flow shear stabilization is not effective at the inner radii. However, at outer radii the situation is reversed. Electromagnetic stabilization is negligible while the flow shear stabilization is significant. These results constitute the high-β generalization of comparable observations found at low-β at JET. This is encouraging for the extrapolation of electromagnetic ITG stabilization to future devices. An estimation of the impact of this effect on the ITER hybrid scenario leads to a 20% fusion power improvement.

• Thematic program: Turbulence and Fusion (2015/2016)
• Event: 8th Plasma Kinetics Working Group Meeting (2015)

• Thematic program: Turbulence and Fusion (2015/2016)
• Event: 8th Plasma Kinetics Working Group Meeting (2015)

To rigorously model fast ions in fusion plasmas, a non-Maxwellian equilibrium distribution must be used. In this work, the response of high-energy alpha particles to electrostatic turbulence has been analyzed for several different tokamak parameters. Our results are consistent with known scalings and experimental evidence that alpha particles are generally well confined: on the order of several seconds. It is also confirmed that the effect of alphas on the turbulence is negligible at realistically low concentrations, consistent with linear theory. It is demonstrated that the usual practice of using a high-temperature Maxwellian, while previously shown to give an adequate order-of-magnitude estimate of the diffusion coefficient, gives incorrect estimates for the radial alpha particle flux, and a method of correcting it in general is provided. Furthermore, we see that the timescales associated with collisions and transport compete at moderate energies, calling into question the assumption that alpha particles remain confined to a flux surface that is used in the derivation of the slowing-down distribution.

• Thematic program: Turbulence and Fusion (2015/2016)
• Event: 8th Plasma Kinetics Working Group Meeting (2015)

• Thematic program: Turbulence and Fusion (2015/2016)
• Event: 8th Plasma Kinetics Working Group Meeting (2015)

A rotating and magnetized three-dimensional axisymmetric equilibrium for hot plasma confined by a gravitational field is found. The plasma density and current can exhibit strong equatorial plane localization, resulting in disk equilibria with open magnetic field lines. The associated equatorial plane pinching results in magnetic field flaring, implying a strong gravitational squeezing of the plasma carrying ambient magnetic field lines toward the gravitational source. At high plasma pressure, the magnetic field becomes strongly radial outside the disk. The model predicts the rotation frequency bound, the condition for a plasma disk, and the requirement for strong magnetic field flaring.

• Thematic program: Turbulence and Fusion (2015/2016)
• Event: 8th Plasma Kinetics Working Group Meeting (2015)

• Thematic program: Turbulence and Fusion (2015/2016)
• Event: 8th Plasma Kinetics Working Group Meeting (2015)

• Thematic program: Turbulence and Fusion (2015/2016)
• Event: 8th Plasma Kinetics Working Group Meeting (2015)

• Thematic program: Turbulence and Fusion (2015/2016)
• Event: 8th Plasma Kinetics Working Group Meeting (2015)

• Thematic program: Turbulence and Fusion (2015/2016)
• Event: 8th Plasma Kinetics Working Group Meeting (2015)

Hybrid-kinetic numerical simulations of firehose and mirror instabilities in a collisionless plasma are performed in which pressure anisotropy is driven as the magnetic field is changed by a persistent linear shear S . For a decreasing field, it is found that mostly oblique firehose fluctuations grow at ion Larmor scales and saturate with energies ∝S 1/2 ; the pressure anisotropy is pinned at the stability threshold by particle scattering off microscale fluctuations. In contrast, nonlinear mirror fluctuations are large compared to the ion Larmor scale and grow secularly in time; marginality is maintained by an increasing population of resonant particles trapped in magnetic mirrors. After one shear time, saturated order-unity magnetic mirrors are formed and particles scatter off their sharp edges. Both instabilities drive sub-ion-Larmor–scale fluctuations, which appear to be kinetic-Alfvén-wave turbulence. Our results impact theories of momentum and heat transport in astrophysical and space plasmas, in which the stretching of a magnetic field by shear is a generic process.

• Thematic program: Turbulence and Fusion (2015/2016)
• Event: 8th Plasma Kinetics Working Group Meeting (2015)

Many naturally occurring and man-made plasmas are collisionless and turbulent. It is not yet well understood how the energy in fields and fluid motions is transferred into the thermal degrees of freedom of constituent particles in such systems. The debate at present primarily concerns proton heating. Multiple possible heating mechanisms have been proposed over the past few decades, including cyclotron damping, Landau damping, heating at intermittent structures and stochastic heating. Recently, a community-driven effort was proposed (Parashar & Salem, 2013, arXiv:1303.0204) to bring the community together and understand the relative contributions of these processes under given conditions. In this paper, we propose the first step of this challenge: a set of problems and diagnostics for benchmarking and comparing different types of 2.5D simulations. These comparisons will provide insights into the strengths and limitations of different types of numerical simulations and will help guide subsequent stages of the challenge.

• Thematic program: Turbulence and Fusion (2015/2016)
• Event: 8th Plasma Kinetics Working Group Meeting (2015)