High density operation in H mode discharges by inboard launch pellet refuelling

Abstract

Operating a tokamak at plasma densities near the empirical Greenwald limit n̄eGw in H mode could yield significant advantages for a fusion reactor. Trying to avoid the strong confinement degradation observed with gas puff refuelling, pellet injection from the magnetic high field side was applied. Sufficient pellet particle flux was supplied to achieve persistent density rampup and to enable density control in H mode at a level beyond n̄eGw for the first time. The pellet induced density increase decays in a fast phase with τ = 10 ms until about half of the latest pellet inventory remains, and decays thereafter to the base density on the particle confinement timescale with τ = 120 ms. The fast decay is the result of strong ELM events following each injected pellet, accompanied by a loss of energy, causing a transient reduction of the plasma energy content by convective heat flux. Recovery of the plasma energy after the ELM sequence takes place with τ = 25 ms, enabling transient operation at appropriately high densities without significant confinement degradation. To reach this scenario, however, confinement degradation caused by other factors must be inhibited. Other factors causing confinement degradation were found to be the increase of neutral gas pressure by pellet born gas puff at insufficient pumping speed or the occurrence of neoclassical tearing modes triggered by pellets when the temperatures close to rational surfaces were reduced too strongly.

High-density H-mode operation by pellet injection and ELM mitigation with the new active in-vessel saddle coils in ASDEX Upgrade

“…As known from earlier pellet fuelling experiments, confinement is best preserved at high density when pellet induced convective losses are minimised. The preferred approach to density enhancement with a pellet particle flux as low as possible consequently aims at maximising pellet particle sustainment time in the plasma [16]. This can be achieved by the deepest possible pellet penetration and deposition, i.e.…”

Role of shielding in modelling cryogenic deuterium pellet ablation

“…Of course for the simple NGS we use (Q 4e and E 4e ) and the original pellet radius. The ablation rate in the LLP code is considered to be the heat flux (Q 2e = Q 1e ) divided by the sublimation energy (see Equation (2)). The time evolution of the four ablation rates NGS, NGSE, LLP and Hybrid (HYB on the plots and formulas) are plotted in Figure 2c.…”

Two-dimensional simulation of pellet ablation with atomic processes

“…Especially, since the first term on the right-hand side of Eq. ͑18͒ becomes (p nϩ1 Ϫ p*)/͓␥p*(⌬t) 2 ͔ in an ideal fluid, Eq. ͑18͒ is reduced to temporal evolution for pressure in the nonadvection phase: p nϩ1 Ϫ p*ϭϪ␥p*"…”

“…1 Pellet penetration well inside the separatrix has enabled the plasma to surpass the empirical density limit, 2 which is clearly of importance for next step devices such as the International Tokamak Experimental Reactor ͑ITER͒. 3 Additionally, peaking the density profile with strong central fueling by pellet injection significantly improves the energy confinement time 4 -6 and increases the fusion power density at fixed ͗␤͘ and B t .…”