2D simulations of transport dynamics during tokamak fuelling by supersonic molecular beam injection

Abstract

Time-dependent transport of both plasma and neutrals is simulated during supersonic molecular beam injection (SMBI) yielding the evolution of edge plasma and neutral profiles. The SMBI model is included as a module, called trans-neut, within the original BOUT++ boundary plasma turbulence code. Results of calculations are reported for the realistic divertor geometry of the HL-2A tokamak. The model can also be used to study the effect of gas puffing. A seven-field fluid model couples plasma density, heat, and momentum transport equations together with neutral density and momentum transport equations for both molecules and atoms. Collisional interactions between molecules, atoms, and plasma include dissociation, ionization, recombination and charge-exchange effects. Sheath boundary conditions and particle recycling are applied at both the wall and the divertor plates. A localized boundary condition of constant molecular flux (product of density times speed) is applied at the outermost flux surface to model the SMBI. Steady state profiles with and without particle recycling are achieved before SMBI. During SMBI, the simulation shows that neutrals can penetrate several centimetres inside the last closed (magnetic) flux surface (LCFS). Along the SMBI path, plasma density increases while plasma temperature decreases. The molecule penetration depth depends on both the SMBI flux and the initial plasma density and temperature along its path. As the local plasma density increases substantially, molecule and atom penetration depths decrease due to their higher dissociation and ionization rates if the electron temperature does not drop too low. Dynamic poloidal spreading of the enhanced plasma density region is observed due to rapid ion flow along the magnetic field (parallel) driven by a parallel pressure asymmetry during SMBI. Profile relaxation in the radial and poloidal directions is simulated after SMBI termination, showing that the plasma returns to pre-SMBI conditions on a time scale of 60 ms.

Summary of the fundamental plasma physics session in the first AAPPS-DPP conference

“…fast/slow components (Fig. 30), and blocking effect during SMBI are consistent with the experimental measurements 53 . Simulation of the neutral gas jet injection reveals that the fueling efficiency and penetration depth can be improved by adjusting injection fluxes and distances between plasmas and injector.…”

Numerical Study of Supersonic Molecular Beam Injection System in Thailand Tokamak I

“…[9]. It is also experimentally and computationally demonstrated to be more efficient than the GP method since the fuels can be delivered deeper into the plasma [10]. In addition, the SMBI yields comparable penetration depth of the deposited fuel as fueling scheme using the injection of pellets.…”

Influence of injection parameters on fueling efficiency of supersonic molecular beam injection into turbulent fusion plasmas

“…However, it should be emphasized that the critical density defined in the simulations with the trans-neut module of the BOUTþþ code is lower, than the experimental values, but at the same time the plasma density in the simulations increased in several orders of magnitude. 13 This observation indicates the importance of the correct implementation of the density source in the model and its spreading, especially in the parallel direction.…”

“…A wide range of questions regarding SMBI has been investigated with the trans-neut module of the BOUTþþ code. [13][14][15][36][37][38] This model couples plasma density, heat, and momentum transport equations together with neutral density and momentum transport equations for both molecules and atoms. Plasma neutral interactions include dissociation, ionization, recombination and charge-exchange reactions.…”

“…Numerical simulations are a powerful tool for the investigation of fueling properties of SMBI and for determining the optimal injection parameters. Numerical simulations performed with a MHD code for supersonic gas jets 12 as well as simulations performed with the trans-neut module 13 pointed out that the molecular beam intensity is one of the main parameters determining the penetration depth of neutrals. As demonstrated in Ref.…”