In-situ pellet growth and quality monitoring diagnostics for the ITER DMS Support Laboratory

Abstract

The ITER disruption mitigation system is based on shattered pellet injection. The ITER DMS requires the use of large, 19 × 38 and 28.5 × 57 mm (diameter x length) cryogenic pellets, made of hydrogen, neon, or a mixture of those. The production of such large pellets, especially protium, is not straightforward, therefore the freezing process is monitored in real-time. A barrel view camera diagnostic was developed at the Centre of Energy Research for this purpose: the pellet is illuminated from one side of the barrel via a special vacuum adaptor featuring a high-pressure rated window, while observing it from the other side using a digital camera. It was found that the growth of the pellet can be followed easily, and issues such as bad heat contact between the cold head and the barrel can clearly be observed as well. Once the pellet is launched, its integrity and other parameters can be determined in an optically assessable vacuum chamber, using a shadowgraphy diagnostic. The pellet size can routinely be determined to sub-mm precision, and voids (or other non-solid material regions) can be identified as dark areas on the captured shadowgraphs.