Particle fueling experiments with a series of pellets in LHD

Abstract

Ice pellet injection is performed in the heliotron LHD (Large Helical Device) [1,2]. The pellets are injected in short series with up to 8 individual pellets. Parameter variations are performed for the pellet ice isotopes, the LHD magnetic configurations, the heating scenario, and some others. These experiments are performed in order to find out, whether deeper fuelling can be achieved with series of pellets compared to single pellets. An increase of the fuelling efficiency is expected since pre-cooling of the plasma by the first pellets within a series could help for deeper penetration of the later pellets in the same series. In addition these experiments have to show up, which boundary conditions must be fulfilled to optimize that technique. The high-field side injection of pellets, as proposed for deep fuelling in a tokamak, will not be feasible with the same efficiency in a stellarator or heliotron, because there the magnetic field gradient is smaller than in a tokamak of comparable size. Hence, too shallow pellet fuelling, in particular in a large device or a fusion reactor, will be an issue that can be overcome only by extremely high pellet velocities, or other techniques that will have to be developed in the future. It turned out that the fuelling efficiency can be enhanced by injection of a series of pellets, however further investigations will be needed in order to optimize this approach to deep particle fuelling.

Overview of first Wendelstein 7-X high-performance operation

“…The motivation for the developing the event synchronized high repetition mode for the W7-X TS was to understand the ablation and deposition physics mechanisms of cryogenic H 2 fueling pellets [31]. The average duration of the pellet ablation is known from the ≈1 ms extent of the H α ablation emission originating from excited neutrals of the ablated pellet material.…”

Pellet Core Fueling in Tokamaks, Stellarators and Reversed Field Pinches

“…However, one of the intrinsic difficulties of density control in helical devices is that, as long as the neoclassical transport dominates as expected in high performance discharges, the density profiles will be hollow in absence of deep core fuel-2102101-6 ing [44]. Using pellet injection for overcoming this difficulty was extensively investigated in LHD [45] and W7-X [46]. In both cases, series of pellets (from a few to a few tens) were injected in the plasma with a high fre-(from a few tens to a few hundreds of Hz).…”

First results from an event synchronized—high repetition Thomson scattering system at Wendelstein 7-X

“…The motivation for developing the event synchronized high repetition mode for the W7-X TS was to understand the ablation and deposition physics mechanisms of cryogenic H 2 fueling pellets [31]. The average duration of the pellet ablation is known from the ≈ 1 ms extent of the H α ablation emission originating from excited neutrals of the ablated pellet material.…”