Speaker
Description
Plasma detachment is a common divertor regime in tokamaks and stellarators. Plasma and power exhaust properties in this regime are described by classical magnetized plasma models including the plasma sheath [1]. However, the classical sheath model can fail with increasing plasma density and collisionality when plasma ions can be demagnetized and no longer follow magnetic field lines. Consequently, in very high density, cold divertor plasmas (density at the sheath entrance, nd > 1021 m-3) a new collisional sheath regime will be formed [2]. In the collisional sheath, plasma transport is dominated by elastic and inelastic plasma and neutral particle collisions. As a result, the sheath properties differ significantly from the classical situation. For example, the plasma flow is subsonic, the normalized divertor heat load and sheath potential drop are reduced and the energy and angular distribution of absorbed particles flatten. Under such conditions, models of particle and power exhaust require revision.
In the present work, we use sophisticated kinetic particle-in-cell simulations with the BIT1 code to explore new properties of the collisional sheath and describe the consequences of their formation in next generation fusion devices such as ITER. We demonstrate that the divertor target plasma profiles will broaden, reducing peak heat loads by a factor ~2, and that a significant portion of these loads will be carried by neutral particles.
In the presence of the collisional sheath the plasma density in front of the target will exceed the value estimated from the classical expressions, affecting plasma-impurity reactions and net sputtering rates of divertor material. For example, in the case of tungsten (W) target material (as in ITER) prompt re-deposition of sputtered W will be negligibly small (below 20%), and contrary to expectations, when typically, more than 90% of sputtered W is promptly redeposited, even a small fraction of energetic ions (e.g. highly ionized impurity ions) can cause non-negligible net erosion. Our model indicates that for detached, high density, cold ITER divertor plasmas, W net erosion resulted by impact of seeded impurity ions (Ne) can reach 1020 atoms/m2s. An additional negative consequence of the collisional sheath will be increased penetration of fuel ions and neutrals into the divertor tile gaps. We finally discuss the impact that the collisional sheath may have on Langmuir probe measurements in ITER.
[1] P.C. Stangeby, The Plasma Boundary of Magnetic Fusion Devices, IOP, 2000.
[2] D. Tskhakaya, invited talk at the 47th EPS conference, Barcelona, 21-25.06.2021.