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Description
Injection of solid non-fuel pellets has been actively used as a tool for pacing and mitigation of edge localized modes (ELMs). In DIII-D, effective ELM pacing has been demonstrated by high frequency injection of Li and C sub-millimeter spheres, using the Impurity Granule Injector (IGI) [1], which injects granules into the plasma at speeds up to 150 m/s, through impact with a rotating impeller. In the IGI, high frequency granule delivery was accomplished through a vibrational granule dropper, in which high time-average rates are obtained at the cost of lack of period control [2].
We present a new in-line granule feeder, capable of delivering granules of size 0.2-2.0 mm with no restriction of material properties, at quasi-periodic rates up to 150 Hz, for 0.7 mm diameter Li granules (600 Hz using 0.3 mm granules). The new dropper mechanism combines two piezo in-line units; one to feed, and one to circulate granules that are filtered out of the feeder path. A remotely adjustable filter eliminates granules that are stacked, oversized, or side-by-side to form a single moving granule injection line. The granules fall off the in-line feeder exit one at a time, hence achieving a quasi-periodic delivery at a rate proportional to the exit speed. At drop rates <60Hz, the granule delivery period has a variation of +/- 25%. At higher rates, the periodicity deteriorates. This behavior was studied using high-speed cameras and electrostatic measurements, and the variation appears caused by to gaps that develop in the last centimeter of the injection line, as granules exit off the moving track.
The linear feeder concept is robust against bridge instabilities and clogging issues, thanks to the simple diverter filter and constant recirculation of granules. Furthermore, the open-top design of the device allows easy access for refilling the device from separate reservoirs, and has easy access for directly monitoring operation and adjustment.
This paper describes the in-line feeder design details, along with several design iterations. The goal is a robust in-vacuum mechanism that can deliver flow ranging from a single particle to a line of particles at 150 per second, using different sizes and materials from the same apparatus.
[1] Bortolon et al. Nucl.Fusion 2016
[2] Nagy Proceedings of SOFE 2015
This work was supported by the U.S. Department of Energy under DE-AC02-09C11466 and DE‑FC02-04ER54698.
| Eligible for student paper award? | No |
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