Interaction of SPI pellets with plasma on JET and associated disruptions

Abstract

The presented data refer to the Shattered Pellet Injector (SPI) experiments carried out at JET in 2019-2020. This paper is a full journal version of the data originally presented as posters at TMPDM_2020 and EPS_2021. This paper presents various aspects of the interaction of pellets with plasma and associated disruptions. The experiment was performed with Ip = (1.1 - 3.1) MA plasmas and mainly with Ne + D2 pellet composition, but also with Ar pellets. The Current Quench (CQ) time, τ80-20, is the key characteristic of mitigation effectiveness. A pellet with a high content of Ne or Ar can reduce the CQ duration below the upper required JET threshold. Plasmas with high (thermal + internal poloidal magnetic) pre-disruptive plasma energy require a high content of Ne pellets to obtain a short CQ duration. Pellets with a small amount of Ne (and accordingly large amount of D), instead of causing a mitigated CQ, create the conditions for a “cold” Vertical Displacement Events (VDE).
The SPI was applied to plasma with different status: mainly to normal (“healthy”) plasma, i.e. not prone to disruption, post-disruptive and VDE plasma. This study shows that SPI effectiveness in terms of CQ duration and, accordingly, EM loads does not depend on the state of the plasma, whether it is “healthy” or post-disruptive plasma. SPI has been shown to reduce axisymmetric vertical vessel reaction forces by about (30-40)% compared to unmitigated disruptions.
On JET, the VDE, whether “hot” or “cold”, always creates the conditions for a toroidal asymmetry in the plasma, so the VDE on the JET is referred to as Asymmetric VDE (AVDE). The interrupting of VDE and prevention of AVDE with SPI has been demonstrated. Thus, the effectiveness of disruption mitigation using SPI has been confirmed.