Plasma Formation in Magnetic Fusion Devices

Magnetic fusion starts with the process of plasma initiation from a very low charged particle number density of 10 m^-3 over the time scale of milliseconds to reach a density of 10¹⁵ m^-3, eventually forming a confined plasma core (P.C. de Vries and Y. Gribov, Nucl. Fusion 59, 096043 (2019)). A key ingredient is the so-called Townsend avalanche that describes the exponential increase of the particle number by collisional ionisation processes of free charged particles colliding with the neutral density background, thus describing the growth rate of the charged particle population.

We use PEPC to simulate the evolution of the charged particle population over time from first principles via a specifically implemented collision and ionisation scattering model using selected particle cross-sections (Junxian Chew et al., Plasma Phys. Control. Fusion 63, 045012 (2021)). This is combined with PEPC's capabilities to calculate the electrostatic potential arising from the spatial distribution of simulated charges around the three-dimensional torus geometry. Another ingredient is to pair PEPC with a background field descriptor which defines the background magnetic and electric field geometries, along with an equation of motion integrator in order to simulate the motion of the magnetically confined charged particles while experiencing acceleration within the vaccum vessel.

This goal is to provide an initial estimate of the expected time required to reach the end of the tokamak breakdown phase with the aforementioned resulting charged particle number density of 10¹⁵ m^-3, which is specific to the defined background electric and magnetic field geometry. Such a simulation can help to gain insights into the time-dependant charged particle concentration in the poloidal plane along the torus geometry and the resulting self-generated magnetic field. Various other physical parameters such as the velocity and energy distribution of charged particles are also accessible from the simulation.

References

• P.C. de Vries and Y. Gribov, ITER breakdown and plasma initiation revisited, Nucl. Fusion 59, 096043 (2019)
• Junxian Chew et al., Three-dimensional first principles simulation of a hydrogen discharge, Plasma Phys. Control. Fusion 63, 045012 (2021)

Cooperation within Forschungzentrum Jülich

The development of fusion reactors, in particular the stability of plasma operation and the intensity of plasma-wall interactions is at the heart of IEK-4 with FZJ. We support our colleagues in developing and evaluating their simulation codes EIRENE and ERO. The former package is used in worldwide fusion research and approxiamtes the tokamak edge plasma along with magnetised charged particles in a fluid dynamics approximation. The latter code is used for 3D simulations of impurity transport in a fusion edge plasma and was developed for the interpretationd and prediction of processes of plasma-wall interactions. The SDL PP helped during upgrade of ERO to ERO2.0 switching to a new, more parallel version of the code (J. Romazanov et al., Phys. Scr. 2017, 014018 (2017)).