Formation and sustainability of H-mode regime in tokamak plasma via sources perturbations based on two-field bifurcation concept

A set of coupled particle and thermal transport equations is used to study a formation and sustainability of an edge transport barrier (ETB) in tokamak plasmas based on two-field bifurcation. The two transport equations are numerically solved for spatio-temporal profiles of plasma pressure and density. The plasma core transport includes both neoclassical and turbulent effects, where the latter can be suppressed by flow shear mechanism. The flow shear, approximated from the force balance equation, is proportional to the product of pressure and density gradients, resulting in non-linearity behaviors in this calculation. The main thermal and particle sources are assumed to be localized near plasma center and edge, respectively. It is found that the fluxes versus gradients regime illustrates bifurcation nature of the plasma. This picture of the plasma implies hysteresis properties in fluxes versus gradients space. Hence, near marginal point, the perturbation in thermal or particle sources can trigger an L-H transition. Due to hysteresis, the triggered H-mode can be sustained and the central plasma pressure and density can be enhanced

ELM triggering conditions for the integrated modeling of H-mode plasmas

Recent advances in the integrated modeling of ELMy H-mode plasmas are presented. A model for the H-mode pedestal and for the triggering of ELMs predicts the height, width, and shape of the H-mode pedestal and the frequency and width of ELMs. Formation of the pedestal and the L-H transition is the direct result of ExB flow shear suppression of anomalous transport. The periodic ELM crashes are triggered by either the ballooning or peeling MHD instabilities. The BALOO, DCON, and ELITE ideal MHD stability codes are used to derive a new parametric expression for the peeling-ballooning threshold. The new dependence for the peeling-ballooning threshold is implemented in the ASTRA transport code. Results of integrated modeling of DIII-D like discharges are presented and compared with experimental observations. The results from the ideal MHD stability codes are compared with results from the resistive MHD stability code NIMROD.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004, Nice (France

Burning Plasma Projections Using Drift Wave Transport Models and Scalings for the H-Mode Pedestal

OAK-B135 The GLF23 and Multi-Mode (MM95) transport models are used along with a model for the H-mode pedestal to predict the fusion performance for the ITER, FIRE, and IGNITOR tokamak designs. The drift-wave predictive transport models reproduce the core profiles in a wide variety of tokamak discharges, yet they differ significantly in their response to temperature gradient (stiffness). Recent gyro-kinetic simulations of ITG/TEM and ETG modes motivate the renormalization of the GLF23 model. The normalizing coefficients for the ITG/TEM modes are reduced by a factor of 3.7 while the ETG mode coefficient is increased by a factor of 4.8 in comparison with the original model. A pedestal temperature model is developed for type I ELMy H-mode plasmas based on ballooning mode stability and a theory-motivated scaling for the pedestal width. In this pedestal model, the pedestal density is proportional to the line-averaged density and the pedestal temperature is inversely related to the pedestal density

Integrated predictive modelling of JET H-mode plasma with type-I ELMs

It is well known that edge plasma parameters influence performance in many different ways (profile stiffness is probably one of the best known examples). In ELMy H-mode a thin region with improved transport characteristics (Edge Transport Barrier) c

Simulations of ITER in the presence of ITB using the NTV intrinsic toroidal rotation model

Abstract Simulations of a standard H-mode International Thermonuclear Experimental Reactor (ITER) scenario in the presence of internal transport barrier (ITB) are carried out using the 1.5D BALDUR integrated predictive modelling code. The intrinsic offset toroidal rotation, which can play an essential role in turbulent transport suppression that results in the ITB formation, is theoretically calculated using a model based on the neoclassical toroidal viscosity (NTV) concept. The core transport in this simulation is a combination of a mixed Bohm/gyro-Bohm anomalous transport model and an NCLASS neoclassical transport model. The boundary condition of the simulations is taken to be at the top of the pedestal where the pedestal value is calculated using the pedestal model based on a combination of pedestal width scaling determined by magnetic/flow shear stabilization and an infinite-n ballooning pressure gradient model. It is found that the predicted intrinsic rotation can result in the formation of ITB, locating mostly between r/a = 0.6 and 0.8 and having a strong impact on the plasma performance in ITER. It is also found that the variations of plasma density and heating power result in a minimal change in toroidal rotation; whereas the increase in plasma effective charge can considerably reduce the toroidal velocity peaking.</jats:p

Simulations of ITER with combined effects of internal and edge transport barriers

Predictive simulations of ITER with the presence of both an edge transport barrier (ETB) and an internal transport barrier (ITB) are carried out using the BALDUR integrated predictive modelling code. In these simulations, the boundary is taken at the top of the pedestal, where the pedestal values are described using theory-based pedestal models. These pedestal temperature models are based on three different pedestal width scalings: magnetic and flow shear stabilization (Δ ∝ ρi s 2), flow shear stabilization ( ) and normalized poloidal pressure ( ). The pedestal width scalings are combined with a pedestal pressure gradient scaling based on the ballooning mode limit to predict the pedestal temperature. A version of the semi-empirical Mixed Bohm/gyroBohm (Mixed B/gB) core transport model that includes ITB effects is used to compute the evolution of plasma profiles. In this model, the anomalous transport in the core is stabilized by the influence of E r × B flow shear and magnetic shear, which results in the formation of ITB. The combination of the Mixed B/gB core transport model with ITB effects, together with the pedestal model, is used to simulate the time evolution of plasma current, temperature, and density profiles for ITER standard type I ELMy H-mode discharges. It is found that ITER fusion performance using the BALDUR code with Mixed B/gB transport model without the presence of ITB is quite pessimistic (Fusion Q ∼ 3). The presence of ITB is crucial and can result in a significant improvement, which is needed for achieving a target Fusion Q of 10. The improvement due to the presence of ITB is almost the same for all simulations with those three pedestal temperature models. This is caused by the predicted pedestal temperature from each pedestal temperature model varying just slightly. The presence of ITB has a strong impact on both temperature profiles, especially near the centre of the plasma, but has a small impact on electron, deuterium, tritium and carbon density profiles, except the helium density profile. The formation of ITB does not impact on the pedestal. It is also found that during a sawtooth crash, the temperature profiles drop significantly, but there is a small change in the density profiles. However, the sawtooth oscillation has no impact on the pedestal. When the auxiliary heating power is turned off, it is found that significant fusion power is sustained.</jats:p