Abstract. This paper presents particle-in-cell simulations of the plasma behaviour in the vicinity of gaps in castellated plasma-facing components (PFCs). The point of interest was the test limiter of the TEXTOR tokamak, a PFC designed for studies of plasma-wall interactions, in particular, related to impurity transport and fuel retention. Simulations were performed for various plasma conditions in the vicinity of the castellated surface, where the gaps can be either shaped or unshaped. It was observed that depending on plasma parameters the transport of plasma particles inside the gap can be either in potential-or geometry-dominated regimes. The mechanisms responsible for the formation of a potential peak inside the poloidal gap and its consequences on plasma deposition profiles are discussed. Study of gap shaping was performed in order to validate its effectiveness.Submitted to: Plasma Phys. Control. Fusion PIC simulations of plasma interaction with shaped and unshaped gaps in TEXTOR 2

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This paper presents the first three-dimensional (3D) particle-in-cell (PIC) simulations of castellated plasma-facing components (PFCs) in tokamaks. Special focus is given to crossings between poloidal and toroidal gaps where elevated heat loads are expected to occur. Moreover, the crossings may affect the plasma penetration into the gaps between tiles. Both of these problems are of high importance for ITER when estimating the lifetime of its PFCs. Localized heat loads can potentially lead to damage of the tiles, while the plasma penetration is related to fuel retention in the gaps due to redeposition of eroded wall material. This problem has previously been targeted by 2D PIC simulations using our in-house code SPICE2, where toroidal and poloidal gaps (PGs) had to be simulated separately. This paper presents the results of a full 3D3V code SPICE3, which allows us to simulate a more realistic geometry of the tiles including the gap crossings and includes the complete E × B drift, which could not be simulated in 2D. The results of self-consistent simulations show that the crossing acts as a transport channel for electrons, allowing them to enter the plasma shadowed region in PGs. As a consequence, the potential near the gap entrance is modified allowing more ions to flow deep inside the gap. The combination of the plasma flow and an E × B drift in the crossing directs ions onto one tile corner, which receives elevated heat load.

Ion sensitive probes serve to measure the ion temperature in magnetized plasma. Such a probe typically consists of a collector submerged inside a hollow tube, which is oriented perpendicularly to the magnetic field. The principle of the probe is based on geometrical shielding of the ion collector from plasma electrons. According to the basic theory, when the collector is retracted in the tube, electrons with their small Larmor radii should not be able to reach it and the collector becomes sensitive to ions. However, experimental results show that the electron shielding is in general inefficient, it only works in the case when the potential of the collector is the same as the potential of the inside surface of the tube.This problem is investigated using a full 3D particle-in-cell Cartesian code with a fast multigrid Poisson solver. We simulate the plasma behaviour in the vicinity of a model of the ion sensitive probe. A positive potential structure is formed at the entrance of the tube due to the space charge of ions that gyrate inside. This structure produces E × B drifts, which push electrons into the shielded space. A stream of electrons hitting the collector is observed for various potentials of the collector. Simulations revealed that electrons can penetrate inside the geometrical shadow in all studied cases; however, they do not reach the collector when the potential of the collector is equal to the potential of the tube.

In this paper, we present results of Particle-In-Cell simulations of a simplified model of the Ball-pen probe (BPP). The BPP is a probe designed for direct measurements of plasma potential in the scrape-off layer (SOL) of tokamaks. The probe is based on a standard Langmuir theory, which states that when the ratio of ion and electron saturation currents flowing to the probe collector approaches zero, the floating potential of the collector is close to the plasma potential. However, it is unclear whether the Langmuir theory is still valid in such a complex geometry, where the probe tunnel can work as a filter in velocity phase-space and as such violates the Maxwellian distributions of particle velocities. To verify the validity of the theory a simplified model of BPP has been studied using the SPICE2 code.