The use of curved guide tubes for transporting frozen hydrogen pellets offers great flexibility for pellet injection into plasma devices. While this technique has been previously employed, an increased interest in its applicability has been generated with the recent ASDEX Upgrade experimental data for magnetic high-field side (HFS) pellet injection. In these innovative experiments, the pellet penetration appeared to be significantly deeper than for the standard low-side injection scheme, along with corresponding greater fueling efficiencies. Thus, several of the major experimental fusion devices are planning experiments with HFS pellet injection. Due to the complex geometries of experimental fusion devices, installations with multiple curved guide tubes will be required for HFS pellet injection. To more thoroughly understand and document the capability of curved guide tubes, an experimental study is under way at the Oak Ridge National Laboratory (ORNL).

In the first phase of the study, a repeating pneumatic injector (RPI) at ORNL was fitted with components to test curved guide tubes; the RPI gas gun provides deuterium pellets of 2.7-mm nominal diam. Tests have been carried out with guide tubes of different curvature (5 to 80 cm nominal radii and 90 arc). Previously, B|chl and Sandman (Fusion Technology 1982, p. 1507) presented some experimental data for curved guide tubes and a simple model for predicting their performance; the model assumes that the centrifugal forces produce stresses in the pellet and that fracturing occurs whenever the magnitude of the stress exceeds a limiting value as determined by the strength of the deuterium ice. Thus, the model predicts a maximum speed that pellets can survive for any given radius of curvature. The ORNL experimental data and the model were in reasonable agreement for only some test parameters; for instance, with a curved guide tube of 20-cm radius, the highest speed of an intact pellet was 480 m/s and in good agreement with the model.

For phase II, a pipe-gun facility for producing and propelling 10-mm-diam (nominal size) pellets is under construction. This pellet size is more applicable for the International Thermonuclear Experimental Reactor (ITER) application. The model discussed above suggests that the maximum speed that the large pellets propelled through curved guide tubes can survive will be more limited. In this paper, the experimental results from phases I and II will be presented and compared to the simple model. Since many parameters and factors are significant in determining curved guide tube performance, it is advisable to evaluate and document the pellet speed limits experimentally for any given installation.

* Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp. for the U.S. Department of Energy under contract number DE-AC05-96OR22464.