Description
The ITER Disruption Mitigation System (DMS) [1] utilizes Shattered Pellet Injection (SPI) technology [2], for which it is critical that the pellet arrives at the shattering head intact, with minimal propellant gas and debris preceeding it, in order to maintain disruption mitigation efficiency. An SPI system was developed in the ITER DMS Support Laboratory at the HUN-REN Centre for Energy Research [3] to study pellet formation, launch and shattering.
In schlieren imaging [4], density gradients are visualized by placing a camera and a point light source at twice the focal length of a spherical mirror. We developed and installed a schlieren diagnostic on our SPI test bench, which is now routinely operated and is capable of detecting the gas flow around the pellet mid-flight. Previously, we obtained direct evidence of propellant gas flow ahead of the pellet by observing shock waves with the schlieren diagnostic in the expansion chamber of the SPI [5].
A calibration was performed for quantitative analysis of schlieren measurements using a calibration lens to determine the relationship between density gradients and light-intensity variations. Using this calibration, the density profiles of the shock waves observed behind the pellet and in the flight tube were quantified.
According to ITER requirements, a gas suppressor chamber was installed in place of the expansion chamber to retain propellant gas. With the schlieren diagnostics installed on the second half of the suppressor, no shock wave was detected during the passage of the pellet. Several milliseconds after the pellet passed, a shock wave generated by the propellant gas flow was observed, and its density profile was reconstructed. The maximum density jump in this case was only a fraction of that measured for the shock waves in the expansion chamber, implying that an even smaller density perturbation occurs during the passage of the pellet.
These results demonstrate that the suppressor effectively reduces propellant gas flow during pellet launch, providing critical information for the ITER DMS and highlighting the capabilities of Schlieren diagnostics.
[1] T. Luce et al 2020 IAEA Fusion Energy Conference, Nice, TECH/1-4Ra
[2] L.R. Baylor et al Nucl. Fusion 59 (2019) 066008
[3] S. Zoletnik et al, Fusion Engineering and Design 190 (2023) 113701
[4] G.S. Settles, Schlieren and Shadowgraph techniques, Springer (2001)
[5] M. Vavrik et al., EPS 2024 pp. 437-440., P4.077