Effect of Terminal Straight Sections on Cryogenic Pellet Dispersion in Curved Guide Tubes

Injection of solid cryogenic pellets comprising hydrogenic isotopes is the primary method of fueling a burning fusion plasma. Pellets can also be utilized for plasma edge stability control through edge localized mode pacing. The pellet trajectory is defined by a set of guide tubes that curve due to navigating infrastructure and to guide pellets to their injection location. These guide tube systems also contain gaps created by isolation valves, diagnostics, and tube connections. In designing these guide tube systems, the dispersion angle of the pellets leaving the tube is an important consideration to ensure that pellets are captured by the downstream tube across a gap en route to the plasma. Current dispersion angle experiments have been conducted using straight guide tubes, which provide a design allowance for dispersion of 1 to 2 deg. However, verification of how a curve will impact the dispersion angle has not been conducted. In this experiment, we detail the dispersion of hydrogen (H2) pellets through a 0.5'' OD, 0.405'' ID stainless steel guide tube geometry based on an ITER guide tube section at the injection cask exit. The tube has a 0.6-m radius bend through an angle of 15 deg. A length of straight guide tube terminating this bend was varied from 0.15 m to 0.61 m to determine its collimating effect. Pellets were created using a continuous single screw extruder developed by Oak Ridge National Laboratory at a frequency of 1 Hz with 3 pellets per firing cycle. By controlling the opening duration of the helium propellant valve, pellet speed was varied within the range of 139 m/s to 1089 m/s. The broad speed range of the data enhances their applicability to a wide range of fusion experiments. We found that the pellet dispersion indicated that the curve resulted in a spiraling trajectory that collimated as we increased the terminating straight guide tube and that a minimum length of 0.30 m for the terminating guide tube was required for the dispersion angle to remain within 3 deg. The experimental results provide design criteria to improve reliability in the pellet delivery system to fusion plasmas.