ITER disruption mitigation system development and port plug integration

Abstract

ITER is designed to withstand a certain number of full scale plasma disruptions, which are the abrupt termination of the plasma within a time frame of a few 10's of millisecond. Each disruption event can induce excessive thermal loads, electromagnetic loads (EM), and runaway electrons (REs) onto the vacuum vessel and in-vessel components; the consequences of unmitigated events are extremely serious in terms of reduced component lifetime. ITER vacuum vessel and in-vessel components are designed mechanically to withstand the EM loads from the expected three thousand “typical” 15 MA disruptions and four hundred “typical” vertical displacement events. However, local thermal loads during unmitigated plasma disruptions significantly exceed power handling capabilities (by an order of magnitude) of divertor targets and first wall panels. RE currents higher than 2 MA cannot be tolerated. In response to current physics requirements a disruption mitigation system (DMS) is needed to inject a certain amount of particles into the disrupting plasma, at once within a very short time period or repetitively with a few millisecond intervals. As part of the design process, analysis and testing is to be provided to ensure the success of the DMS operation and an appropriate level of redundancy supplied to meet the high reliability needed. Massive gas injection (MGI) and shattered pellet injection (SPI) are considered as the most promising candidates for the disruption mitigation system in ITER. Both concepts need to be tested in the laboratory as well as on existing machines to demonstrate their feasibility. One of the main design challenges is to make the system compatible with the harsh environment in ITER port plugs and port cells. This paper describes the design concepts, their integration into the ITER machine, and on-going developments of the DMS. The two different system concepts installation into the upper and equatorial port plugs are presented. The DMS has recently passed the Conceptual Design Review, and moved to the Preliminary Design stage where the installation and integration of the system will be enhanced, such as the implementation of cryogenic cooling for shattered pellets and detailed design of fast operating gas valves for massive gas injection.