J. P. Graves

Saturated ideal modes in advanced tokamak regimes in MAST

MAST plasmas with a safety factor above unity and a profile with either weakly reversed shear or broad low-shear regions, regularly exhibit long-lived saturated ideal magnetohydrodynamic (MHD) instabilities. The toroidal rotation is flattened in the presence of such perturbations and the fast ion losses are enhanced. These ideal modes, distinguished as such by the notable lack of islands or signs of reconnection, are driven unstable as the safety factor approaches unity. This could be of significance for advanced scenarios, or hybrid scenarios which aim to keep the safety factor just above rational surfaces associated with deleterious resistive MHD instabilities, especially in spherical tokamaks which are more susceptible to such ideal internal modes. The role of rotation, fast ions and ion diamagnetic effects in determining the marginal mode stability is discussed, as well as the role of instabilities with higher toroidal mode numbers as the safety factor evolves to lower values.

Mechanism for blob generation in the TORPEX toroidal plasma

Reference CRPP-CONF-2009-011Afficher la publication dans Web of Science Notice creee le 2009-01-23, modifiee le 2017-05-12

Control of magnetohydrodynamic stability by phase space engineering of energetic ions in tokamak plasmas

Virtually collisionless magnetic mirror-trapped energetic ion populations often partially stabilize internally driven magnetohydrodynamic disturbances in the magnetosphere and in toroidal laboratory plasma devices such as the tokamak. This results in less frequent but dangerously enlarged plasma reorganization. Unique to the toroidal magnetic configuration are confined circulating energetic particles that are not mirror trapped. Here we show that a newly discovered effect from hybrid kinetic-magnetohydrodynamic theory has been exploited in sophisticated phase space engineering techniques for controlling stability in the tokamak. These theoretical predictions have been confirmed, and the technique successfully applied in the Joint European Torus. Manipulation of auxiliary ion heating systems can create an asymmetry in the distribution of energetic circulating ions in the velocity orientated along magnetic field lines. We show the first experiments in which large sawtooth collapses have been controlled by this technique, and neoclassical tearing modes avoided, in high-performance reactor-relevant plasmas.

Anisotropic pressure bi-Maxwellian distribution function model for three-dimensional equilibria

A formulation of the anisotropic pressure magnetohydrodynamic equilibrium problem for three-dimensional plasmas with imposed nested magnetic surfaces is developed based on a bi-Maxwellian model of the distribution function for the energetic particle species. The hot particle distribution function satisfies the constraint . Large parallel and perpendicular anisotropy factors can be explored within the model through the choice of the hot particle perpendicular to parallel temperature ratio T⊥/T||. A fixed boundary version of the VMEC code has been adapted to numerically compute three-dimensional anisotropic pressure equilibria. Applications to a 10-field period Heliotron device and a 2-field quasiaxisymmetric stellarator demonstrate that the pressures do not vary significantly around the magnetic surfaces when the total parallel pressure p|| is larger than its perpendicular counterpart p⊥. For off-axis hot particle deposition with p⊥ > p||, p⊥ concentrates in the region where the energetic particles are generated. On the other hand, p|| is distributed roughly uniformly around the flux surfaces in the Heliotron but is localized on the low field side in the quasiaxisymmetric machine. The hot particle density structure correlates more closely with the corresponding perpendicular rather than with parallel pressure. The specific form for the definition of β that best correlates with the Shafranov shift is identified.

The effect of plasma triangularity on turbulent transport: modeling TCV experiments by linear and non-linear gyrokinetic simulations

The effect of plasma shape on confinement has been experimentally explored in the TCV tokamak revealing that the core electron heat transport is significantly reduced by a negative triangularity configuration, which could indicate a (partial) stabilization of the microinstabilities at play in a conventional positive triangularity configuration.This work is a theoretical investigation of the effect exerted by triangularity on plasma turbulence. In particular, it compares the TCV experimental results with non-linear local gyrokinetic simulations performed on the basis of actual MHD equilibrium reconstructions.In both the linear and non-linear phases, negative triangularity is found to have a stabilizing influence on ion-scale instabilities, specifically on the so-called trapped electron mode (TEM) which is the dominant instability in the conditions of the TCV experiments considered; more specifically, the variation of the heat flux with triangularity calculated by the non-linear simulations is in fair agreement with the experimental results.The resulting stabilization is a result of a rather complex modification of the toroidal precessional drift of trapped particles exerted by negative triangularity.

Helical ITER hybrid scenario equilibria

Numerical computations of ITER equilibria in the hybrid scenario using a three-dimensional (3D) magnetohydrodynamic equilibrium code with nested magnetic flux surfaces demonstrate the formation of internal 3D helical cores similar to saturated ideal internal kink modes under reversed magnetic shear conditions when the minimum value of the inverse rotational transform is in the neighbourhood of unity.

Neoclassical equilibria as starting point for global gyrokinetic microturbulence simulations

The implementation of linearized operators describing inter- and like-species collisions in the global gyrokinetic particle-in-cell code ORB5 [ S. Jolliet, Comput. Phys. Commun. 177, 409 (2007) ] is presented. A neoclassical axisymmetric equilibrium with self-consistent electric field can be obtained with no assumption made on the radial width of the particle trajectories. The formulation thus makes it possible to study collisional transport in regions where the neoclassical approximation breaks down such as near the magnetic axis. The numerical model is validated against both analytical results as well as other simulation codes. The effects of the poloidally asymmetric Fourier modes of the electric field are discussed, and the contribution of collisional kinetic electrons is studied. In view of subsequent gyrokinetic simulations of turbulence started from a neoclassical equilibrium, the problem of numerical noise inherent to the particle-in-cell approach is addressed. A novel algorithm for collisional gyrokinetic simulation switching between a local and a canonical Maxwellian background for, respectively, carrying out the collisional and collisionless dynamics is proposed, and its beneficial effects together with a coarse graining procedure [ Y. Chen and S. E. Parker, Phys. Plasmas 14, 082301 (2007) ] on noise and weight spreading reduction are discussed.

Magnetohydrodynamic properties of nominally axisymmetric systems with 3D helical core

Magnetohydrodynamic equilibrium states with a three-dimensional helical core are computed to model the MAST spherical tokamak and the RFX-mod reversed field pinch. The boundary is fixed as axisymmetric. The MAST equilibrium state has the appearance of an internal kink mode and is obtained under conditions of weak reversed central shear. The RFX-mod equilibrium state has seven-fold periodicity. An ideal magnetohydrodynamic stability analysis reveals that the reversal of the core magnetic shear can stabilize a periodicity-breaking mode that is dominantly m/n = 1/8 strongly coupled to a m/n = 2/15 component, as long as the central rotational transform does not exceed the value of 8.

JET Snake Magnetohydrodynamic Equilibria

Magnetohydrodynamic (MHD) equilibrium states with a three-dimensional helical core that display the characteristics of a saturated ideal internal kink mode are computed to model snake structures that have been observed in the JET tokamak (Weller et al 1987 Phys. Rev. Lett. 59 2303). The equilibrium states are calculated with a peaked pressure profile and a weak to moderate reversed core magnetic shear with a minimum safety factor qmin near unity in the neighbourhood of the mid-radius of the plasma. Snake equilibrium states are computed in the range 0.94 <q min < 1.03. This range aligns with linearly unstable ideal MHD internal kink solutions of the purely axisymmetric branch of the equilibrium states. The energy difference between the bifurcated axisymmetric and helical snake equilibrium solutions is minimal. One very important novelty is that the helical structures are computed with an equilibrium code developed for three-dimensional (3D) stellarator applications in a tokamak context and cannot be obtained with standard Grad–Shafranov equation solvers. (Some figures in this article are in colour only in the electronic version)

Ion cyclotron resonance heating with consistent finite orbit widths and anisotropic equilibria

Minority ion cyclotron resonance heating is studied using the self-consistent numerical model SCENIC. This model includes 3D geometries with full shaping and anisotropic pressure effects, warm contributions to the dielectric tensor and full orbit effects. It evolves the equilibrium, wave field and hot particle distribution function iteratively until a self-consistent solution is found. We will show applications to JET-like two-dimensional equilibria with minority heating scenarios. The effects due to different heating locations on the hot particle distribution function, the hot dielectric tensor and the equilibrium will be studied for symmetric wave injection. Finally, the RF-induced particle pinch is investigated using asymmetric wave injection.