Content Shortcut menu Shortcut



Disruption mitigation by symmetric multiple injection of shattered pellets in KSTAR


Date of registration 2020-09-24


Figure 1. Toroidally symmetric dual shattered pellet injectors,

which are 180 degree apart from each other, installed in KSTAR.

They share the ports with ECE imaging and ECH antenna, respectively.

ITER adopts a strategy to evenly distribute the radiated power during disruption mitigation and reduces the time to prepare pellets using multiple shattered pellet injections (SPI) at the same time [1]. However, since there were no existing devices with fully symmetrical SPIs, as planned in ITER [2], sufficient studies on the effects of simultaneous multiple injections have not been conducted. To confirm the feasibility of ITER's disruption mitigation strategy, KSTAR installed two SPIs of exactly the same design in opposite locations as shown in figure 1 [3]. Each SPI can use three barrels of different diameters to selectively control the number of particles injected. The species used can vary deuterium, neon, argon, or mixtures thereof, depending on mitigation purposes, such as mitigating heat loads or suppressing runaway electrons.

   Last year, we investigated differences in disruption mitigation, primarily by deliberately changing the arrival times of the two SPIs to assess the possible jitter effects between multiple SPIs. As can be seen in figure 2a), the current quenching rate changes proportionally as the time difference varies from a few percent to tens of percent over the thermal quenching (TQ) period (1 to 2 ms). This has been experimentally proven that more energy can be released when multiple SPIs are injected simultaneously, as planned by ITER. The results resolved the ambiguity for simultaneous multiple injections observed in previous experiments performed with two SPIs 120 degrees apart [4]. In addition, the sensitivity to jitter identified by KSTAR experiments provided guidance for the design of the ITER Disruption Mitigation System (DMS). 

On the other hand, in the disruption mitigation process, it is also important to form a high plasma density to prevent the transfer of magnetic energy towards the runaway electrons. For this study, a dispersion interferometer with a short wavelength of 1064 nm was developed and installed to measure the density during the disruption process where the conventional interferometer with a long wavelength typically suffers the cutoff and refraction. The dispersion interferometer can measure 1 or 2 orders of magnitude higher than that of a conventional interferometer. For the dual SPI, we measured a peak density of 1.2x1021 m-3 near the TQ end, which is almost twice the single SPI value.


Figure 2. a) Current quench rates depending on the difference of arrival time between two SPIs,

b) density rise during TQ in single SPI case (KSTAR #23456), and c) density rise during TQ

in well-synchronized dual SPIs case (KSTAR #23464). Red vertical dash-lines.

Excessive particle injection of SPI and subsequent radiation creates a strong MHD instability in the plasma. Conversely, this MHD mode significantly affects the behaviour of the injected particles. As can be seen in figure 3, the well-synchronized dual SPI showed a much milder MHD instability than the asynchronous SPI.


Figure 3. n=1 MHD mode amplitudes during TQ depending on the synchronization of SPIs.

Mitigation of disruption with SPI is a complex phenomenon depending on plasma and SPI parameters. Studies of interactions with existing MHD modes, such as the cause of disruption, are also important in establishing realistic mitigation strategies. Among the various topics of DMS, we first plan to focus on multiple injections at different toroidal locations by changing the parameters mentioned above, as well as multiple barrel injections at the same poloidal/toroidal location according to ITER DMS's plan. To do this, the largest sized barrel (8.5 mm) is changed to medium size barrel (7.0 mm), simulating an ITER SPI with all the same sized barrels. It is expected to provide the data underlying the ITER DMS design.

Authorities concerned