Study of electromagnetic disruption forces for plasma detachment measurements in DEMO

Abstract

Global plasma instabilities such as plasma disruptions and vertical displacement events (VDEs) can generate huge transient electrical currents in the conductive parts of the divertor. The interaction of the currents with the magnetic field induces Lorentz force loads. Particularly, the force induced by a VDE can impose a critical impact on the divertor water-cooling pipes due to high current density. A recently proposed isolated-target design solution for the DEMO divertor introduces a shunt element between the target plate and the cassette body to divert electrical current towards more massive cassette body. Such a design solution is also considered as a diagnostic tool for measuring thermo currents (voltages) through a shunt resistor to control plasma detachment.In this study, a finite-element analysis is performed to provide a conservative upper value for the shunt resistance by investigating the impact of the electromagnetic disruption forces on the integrity of water-cooling pipes in the isolated-target design solution of the DEMO divertor. In the analysis, a maximum allowed pipe current is estimated assuming realistic material properties and magnetic fields, and true geometries of the divertor cassette and water-cooling pipes fixed to the cassette. The results show that local pipe yielding (representing damage initiation) develops for electrical currents larger than ∼3.5 kA, which sets a conservative (assuming extreme VDE halo currents) upper value for the shunt resistance to ∼40 µΩ.

Shunt analysis in the isolated-target divertor model for plasma detachment measurement in DEMO

“…The FE modelling and simulations are performed with Abaqus [12] using an updated isolated-target divertor model DEMO baseline 2017. Contrary to the previous modelling strategy [6], the cassette body and water-cooling pipes are modelled jointly within the same FE model. Electrical conduction between the two parts is modelled at several (4) locations by introducing electrical surface-surface interaction to model shunt elements with adjustable resistance (Fig.…”

“…• To account for plasma attachment scenario, a thermo-current of 150 A is applied uniformly over the inner and outer target plate regions of the cooling pipes [6].…”

“…Thermo-current is driven by a temperature difference between the inboard and outboard PFC target plates which causes a current flow to compensate the difference in the sheath potentials [3,4,5]. In DEMO this current is estimated to have a steady state value of approximately 150 A per divertor when plasma attachment begins [6]. When the plasma is fully detached, however, no current should run on average through the divertor.…”

“…One of the few feasibility studies [6,9] on thermo-current measurement in isolated-target divertor model in DEMO was presented in our previous work [6]. In that study a finite-element (FE) electro-mechanical analysis was conducted to calculate the conservative upper value of shunt resistance.…”

“…The objective of this study is to reduce the conservatism used in our first analysis [6] in order to enhance the thermo-current shunt signal while still retaining the integrity of the water-cooling pipes. This is achieved through several mechanical, thermal and electrical FE analyses by (i) using a recently upgraded isolated-target divertor model [10,11] with optimized pipe fixation system, where water-cooling pipes are firmly embedded (and thus not allowed to bend) in specially designed cassette channels (see Fig.…”