Description
Considerable progress has been made in modelling edge turbulence and the L-H transition using fluid and kinetic approaches. However, the gyrokinetic simulation of a complete transition from steady-state
L-mode turbulence to a quasi-stationary H-mode remains a challenge. To simulate the transport properties of the H-mode pedestal, an alternative approach, avoiding the L-H transition is to start the simulation with profiles (density, temperature, current density) approximating the expected H-mode profiles. This approach has, for example, been successfully applied in [1] using the GRILLIX drift-fluid code. In this paper, we apply the gyro-kinetic model from the JOREK code (named JOREK-GK) to obtain quasi steady-state H-mode solutions.
The JOREK-GK code builds on the C1 finite element discretisation and the discrete particle module to implement the well-established electro-static gyrokinetic model including ions and electrons. The conservative collision model from [2] is included, together with a small-angle scattering model for the e-i collisions. In this paper, only ion-ion and small angle electron-ion collisions are retained. A simple model for the divertor sheath is also used. This implementation allows global, full-f, gradient driven, electrostatic simulations in x-point geometry.
Simulations are initialised with H-mode density and pressure profiles in COMPASS-like geometry, as in [3], followed by an axisymmetric phase with collisions, i.e. without turbulence, to establish the neoclassical flows. Adding the toroidal harmonics allows to obtain a quasi-steady H-mode solution including ITG-TEM turbulence, and the stabilisation in the pedestal. The pedestal width is ~1.5cm, in reasonable agreement with experimental observations [3]. The resulting radial electric field is ~36 kV/m. The reduced turbulent transport inside the pedestal is associated with both a decrease in fluctuation amplitudes by a factor of 2–3 and a comparable modification of the cross-phase between pressure and electrostatic potential fluctuations, leading to a local reduction of turbulent transport by about an order of magnitude. In absence of MHD and kinetic ballooning (KBM) modes in the electrostatic model, the pedestal height grows well above the experimental value. A medium-n discrete mode, localised within the pedestal and initially at low amplitude, is found to grow and eventually trigger a rapid collapse of the pedestal radial electric field.
[1] W. Zholobenko et al., Nucl. Fusion 64 (2024) 106066
[2] P Donnel et al., Plasma Phys. Control. Fusion 63 025006 (2021)
[3] R Pánek et al 2016 Plasma Phys. Control. Fusion 58 014015