Link do seminarium: https://bit.ly/3IwbWDA

Abstract:

Due to the predicted high opacity of the ITER SOL to neutrals, in ITER the plasma density will have to be controlled through injection of cryogenic pellets. This is different to the situation in present day machines, for which gas dosing has been traditionally used. In addition, ELM pacemaking with pellets is one of the techniques foreseen in ITER to control ELMs. Due to technical limitations pellet velocity is limited to 300 m/s. In combination with the high pedestal temperatures expected in ITER, this implies a pellet ablation depth which is comparable to the width of the edge transport barrier (ψpel ∼ 0.95). Due to its size, the JET tokamak is capable of reaching ITER-like pellet ablation depths. In addition, JET can reach smaller ratios of pellet to target plasma particle content compared to medium sized tokamaks. This together with the presence of Be/W-based first wall material mix (ITER-like Wall, or ILW), renders JET ideally suited for the study of pellet fuelling and pellet ELM pacing physics under ITER-relevant conditions.

Dedicated experiments have been carried out on JET to study systematically the effect of cryogenic pellet fuelling and pellet ELM-pacemaking on type-I ELMy H-modes in the presence of an ITER-like Wall (ILW) and with ITER-like pellet ablation depths. The main aim here is to document and understand how shifting the radial particle source localisation from the plasma edge (gas dosing) to inside the H-mode transport barrier (pellets) impacts the boundary plasma as well as the structure and height of the pedestal under ITER-relevant conditions. Additionally, the impact of pellet ELM pacing (fpel > fELM) and the role of gas dosing location (main chamber vs divertor) have also been assessed.

The work is presented in two parts. In the last IPPLM Seminar (part-I, Feb-2022), we looked into the changes in SOL and divertor conditions with pellets. In summary, we found that in the divertor, the substitution of (main chamber) gas dosing by pellets at fixed fuelling rate (Γtot) yields no significant differences in terms of neutral pressures, divertor radiation, inner and outer target densities or outer target temperature. This is a reflection of the fact that the conditions in the divertor are mainly being set through recycling processes. In the upstream SOL, the measured pressure of neutrals is not found to be sensitive to the fuelling scheme but lower electron densities (shorter radial fall-off lengths) are measured with pellets than with gas, also when normalising against the separatrix density (n_e /n_e,sep ). n_e,sep itself remains essentially unaffected when main chamber gas is replaced by pellets at constant Γtot . Moving gas dosing location from the main chamber to divertor at fixed ELM frequency does however result in colder and denser divertor plasmas and higher n_e,sep.

In the coming IPPLM Seminar (part-II), we will look into the changes to the pre-ELM pedestal height and structure, and how the differences are linked to the previous SOL findings.