Effect of the Dome on the Collisional Neutral Gas Flow in the Demo Divertor

This paper presents the analyses of the dome effect on the pumping efficiency and screening of the neutrals for the European DEMO ITER-like divertor configuration of 2015. The effect of the dome on neutral compression in the private flux region is assessed by using the DIVGAS code based on the direct simulation Monte Carlo method. The numerical analysis includes the calculation of neutral density, temperature, and pressure in the divertor plenum and the overall conductance of the subdivertor structure, which consequently affects the estimation of the effective pumping speed. It is quantified how the presence of the dome impedes the reflux of neutrals toward the plasma. In addition, due to the intermolecular interactions, the dome does not strongly influence the macroscopic quantities near the pumping duct. A strong reflux of neutrals in the domeless case, as it was previously found in the free molecular flow case, is confirmed to be present also in the case of a collisional flow.

I. Introduction

The design of the DEMO divertor cassette requires a new assessment of the role of the divertor dome structure under operational conditions. The primary function of the dome is to achieve a high compression of neutrals in the private flux region (PFR) in order to increase the pumping efficiency and by that make the helium removal from the reactor more effective. In addition, the dome could reduce the neutral reflux to the core plasma through the -point and shield the pump duct from neutrons. On the other side, it shadows that the bottom surface of the divertor cassette from neutrons (which otherwise could be used for the installation of a breeding blanket), is increasing the complexity of the divertor design and, eventually, the machine cost.