On the effects of the equilibrium model in gyrokinetic simulatins: From s-alpha to diverted MHD equilibrium

In order to better identify the role of the magnetic topology on ITG and TEM instabilities, different MHD equilibria with increasing complexity are calculated using the CHEASE code [1]. We start from the geometry of the s-alpha cyclone benchmark case [2], consider the corresponding circular numerical equilibrium, and then successively add a non zero value of alpha consistent with the kinetic profiles, an elongation of 1.68, a triangularity of 0.15, and finally an up-down asymmetry corresponding to a single-null diverted geometry. This gives the opportunity to study separately the effect of each main characteristics of the equilibrium on microinstabilities in core plasmas. Linear local electrostatic gyrokinetic simulations of these different numerical equilibria and of their corresponding analytical descriptions (Miller-type representations [3]) are performed using the codes GS2 [4, 5] and GYRO[6]. It is observed that each modification of the equilibrium has an influence on the results of gyrokinetic simulations. The effect of the alpha parameter can compensate the stabilizing effect of an increase in the elongation. A comparison between the up-down symmetric shaped equilibrium and its corresponding diverted configuration show a non negligible effect on the growth rate of ITG and TEM turbulence. The comparison between the local Miller model and using a full equilibrium shows that it is mainly the indirect change of elongation in the plasma core which influences the results. The global aim is to provide well defined benchmark cases including real geometry and kinetic electrons physics, since this is not analyzed by the cyclone case. In addition, the goal is to define a procedure for testing of local simulations inspired by experimental constraints and results.

Global two-fluid simulations of tokamak SOL turbulence

Federico David Halpern, Annamaria Mosetto, Paolo Ricci

We present non-linear self-consistent global simulations of the SOL plasma dynamics using the Global Braginskii Solver (GBS) code. The code solves on the drift-reduced Braginkii equations, with cold ions. Studied originally for the simulation of the Simple Magnetized Torus (SMT) experiment TORPEX (CRPP, Lausanne), the GBS code has been recently upgraded to describe the SOL turbulence with the introduction of the variable curvature along the magnetic field lines, the magnetic shear, and the electromagnetic effects. The code peculiarity lies in the capability of evolving self-consistently equilibrium and fluctuations as a results of the interplay among the sources, the turbulent transport and the plasma losses at the limiter plates. The non-linear simulations have been interpreted by means of linear analysis of the fluid equations modeling the system. This points out the presence of two main instabilities driving turbulence: the Drift Wave and the Resistive Balloning instabilities. The dependence of the instabilities growth rate and of their properties on the physical parameters of the system, for example the typical length of variation of the plasma density, the safety factor and the magnetic shear have been explained and we identify the regions where each instability dominates.

2011

Global two-fluid simulations of tokamak Scape-Off-Layer turbulence

Federico David Halpern, Sébastien Jolliet, Annamaria Mosetto, Paolo Ricci

We present non-linear self-consistent 3D global fluid simulations of the SOL plasma dynamics using the Global Braginskii Solver (GBS) code. The code solves the drift-reduced Braginkii equations in a collisional plasma, with cold ions. The GBS code, originally developed for an electrostatic, 2D configuration has been recently upgraded to describe the SOL turbulence with the introduction of the variable curvature along the magnetic field lines, the magnetic shear, and the electromagnetic effects. The code peculiarity lies in the capability of evolving self-consistently equilibrium and 3D fluctuations as a results of the interplay among the sources, the turbulent transport and the plasma losses at the limiter plates. The non-linear simulations have been interpreted by means of linear analysis of the fluid equations modeling the system. This points out the presence of two main instabilities driving turbulence: the Drift Wave and the Resistive Balloning instabilities. The dependence of the instabilities growth rate and of their properties on the physical parameters of the system, for example the characteristic length of the plasma density, the magnetic shear and the beta ratio have been explained and the regions where each instability dominates have been identified.

2012

Kinetic simulations and photometry measurements of the E-H transition in cylindrical inductively coupled plasmas

Jacques Lettry, Stefano Mattei, Minh Quang Tran

Inductively coupled plasmas (ICP) are well known to exhibit two modes of operation: a low density capacitive E-mode and a high density inductive H-mode. In this study we investigate the E-H transition in a cylindrical ICP, and show the effect of an external magnetic cusp field on the transition dynamics. The plasma is simulated by an electro-magnetic particlein-cell Monte Carlo collision code in order to take into account spatio-temporal variations of the plasma dynamics as well as kinetic effects. Simulations are compared to photometry measurements on the Linac4 H-ion source plasma chamber. We show that the E-H transition is characterized by strong spatial variations of the plasma parameters, with an axial plasma oscillation in E-mode followed by a centring in the coil region in H-mode. The external magnetic cusp field prevents electrons close to the wall to be accelerated and reduces the inductive power deposition in the plasma. This resulted in a approximate to 50% higher current to achieve E-H transition compared to the configuration without cusp field. The results indicate possible improvements to the magnetic cusp field configuration in order to achieve optimal power transfer.

Institute of Physics2016