HIGH-SPEED MULTISHOT PELLET INJECTOR PROTOTYPE FOR THE FRASCATI TOKAMAK UPGRADE

The Frascati Tokamak Upgrade (FTU) may require multiple high-speed pel let injection in order to achieve quasi-steady-state conditions. A res earch and development program was thus being pursued at ENEA Frascati, aimed at developing a multishot two-stage pellet injector (MPI), feat uring eight ''pipe gun'' barrels and eight small two-stage pneumatic g uns. According to FTU requirements, the final-goal is to simultaneousl y produce up to eight D-2 pellets, and then deliver them during a plas ma pulse (1 s) with any time schedule, at speeds in the 1-2.5 km/s ran ge. A prototype was constructed and tested to demonstrate the feasibil ity of the concept, and optimize pellet formation and firing sequences . This laboratory facility was automatically operated by means of a pr ogrammable logic controller (PLC), and had a full eight-shot capabilit y. However, it was equipped as a first approach with only four two-sta ge guns. In this article we will describe in detail the guidelines of the MPI prototype design, which were strongly influenced by some exter nal constraints. We will also report on the results of the experimenta l campaign, during which the feasibility of such a two-stage MPI was d emonstrated. Sequences of four intact D2 pellets in the 1.2-1.6 mm siz e range, fired at time intervals of a few tens up to a few hundreds of ms, were routinely delivered in a laboratory experiment at injection speeds above 2.5 km/s, with good reproducibility and satisfactory aimi ng dispersion. Some preliminary effort to address the problem of prope llant gas handling, based on an innovative approach, gave encouraging results, and work is in progress to carry out an experiment to definit ely test the feasibility of this concept. (C) 1998 American Institute of Physics.; [S0034-6748(98)04606-1]