Recent Upgrades of the DIII-D Impurity Granule Injector

An impurity granule injector on the DIII-D tokamak (IGI) injects granules into the plasma to trigger Edge Localized Modes (ELMs). Impurities, such as lithium, carbon, and boron, are used. The IGI drops granules (0.3–1.0 mm diameter) from a four chamber segmented storage hopper into a down-tube. The downtube guides the granules into a spinning impeller, rotating at a maximum frequency of 170 hz. The granules’ collisions with the impeller propel the granules (maximum velocity 120 m/s) through a drift tube, through an open torus interface valve shield, and into the plasma. This device underwent substantial upgrades to improve its functionality, to minimize the device footprint, and to automate post injection analysis. Upgrades include: (1) a drop-tube positioner to account for impeller/granule collision trajectories; (2) a granule drop monitor using an LED and a photodetector in the drop-tube; (3) a photodiode based granule ablation monitor; (4) DC isolation from the DIII-D vacuum vessel; and (5) an electric motor impeller drive with an integrated rotational speed sensor. These modifications improved the operability and efficiency of the IGI, leading to the successful triggering of ELMs using gasless impurity injection. These recent upgrades are discussed in detail.