Aspect ratio effects on limited scrape-off layer plasma turbulence

Abstract

The drift-reduced Braginskii model describing turbulence in the tokamak scrape-off layer is written for a general magnetic configuration with a limiter. The equilibrium is then specified for a circular concentric magnetic geometry retaining aspect ratio effects. Simulations are then carried out with the help of the global, flux-driven fluid three-dimensional code GBS [Ricci et al., Plasma Phys. Controlled Fusion 54, 124047 (2012)]. Linearly, both simulations and simplified analytical models reveal a stabilization of ballooning modes. Nonlinearly, flux-driven nonlinear simulations give a pressure characteristic length whose trends are correctly captured by the gradient removal theory [Ricci and Rogers, Phys. Plasmas 20, 010702 (2013)], that assumes the profile flattening from the linear modes as the saturation mechanism. More specifically, the linear stabilization of ballooning modes is reflected by a 15% increase in the steady-state pressure gradient obtained from GBS nonlinear simulations when going from an infinite to a realistic aspect ratio. V C 2014 AIP Publishing LLC. [http://dx

Comparison of 3D flux-driven scrape-off layer turbulence simulations with gas-puff imaging of Alcator C-Mod inner-wall limited discharges

“…Finally, we stress that this initial validation exercise will be expanded, on one hand, by using a more advanced model including ion temperature dynamics and plasma shaping [20,31], and on the other hand, by comparing GBS against new experimental data from C-Mod's 2014 campaign. The new experimental series will include other advanced diagnostics, such as the mirror Langmuir probe and CXRS-GPI measurements of T i .…”

Theory of the scrape-off layer width in inner-wall limited tokamak plasmas

“…the local non-linear flattening of the pressure profile caused by the turbulent structures. The gradient-removal model has been used to explain experimental observations of L p in a number of tokamaks [11], the transition between different unstable modes depending on the plasma parameters [19], the effects of finite aspect ratio [20], and the transition between the electrostatic and the ideal ballooning unstable regimes [10]. A short summary of the model follows below.…”

Plasma shaping effects on tokamak scrape-off layer turbulence

“…First, the Z component is discretized using a fourth order finite difference scheme. Second, Dirichlet boundary conditions are imposed to δn, δφ, and δT e , while no boundary conditions are applied to the ion and electron parallel velocities (the impact of the boundary conditions on the growth rate of the instability is negligible when considering RBMs and RDWs and a limitier at the inner midplane [18]). Finally, the discretized system of equations is integrated implicitly in time starting from random noise.…”

“…The main instabilities driving the SOL turbulence were characterized in the simplest SOL configuration (i.e. limited and caracterized by a circular magnetic equilibrium) [17][18][19], and a predictive first-principle model of the SOL width was derived and compared with experimental observations [20,21]. Despite the fact that the effects of plasma shaping on core turbulence have been extensively studied, both experimentally and numerically (see, e.g., Refs.…”

“…Consequently, it provides quasi-steady state plasma profiles which are selfconsistently obtained from the balance between plasma sources, perpendicular transport, and parallel losses at the vessel. GBS has been extensively used to study the turbulent processes occurring in the SOL [17][18][19][33][34][35], and its results were recently rigorously verified and validated against experimental measurements [36][37][38][39].…”