High-speed multishot pellet injector prototype for the Frascati Tokamak Upgrade

Abstract

The Frascati Tokamak Upgrade (FTU) may require multiple high-speed pellet injection in order to achieve quasi-steady-state conditions. A research and development program was thus being pursued at ENEA Frascati, aimed at developing a multishot two-stage pellet injector (MPI), featuring eight “pipe gun” barrels and eight small two-stage pneumatic guns. According to FTU requirements, the final goal is to simultaneously produce up to eight D2 pellets, and then deliver them during a plasma pulse (1 s) with any time schedule, at speeds in the 1–2.5 km/s range. A prototype was constructed and tested to demonstrate the feasibility of the concept, and optimize pellet formation and firing sequences. This laboratory facility was automatically operated by means of a programmable logic controller (PLC), and had a full eight-shot capability. However, it was equipped as a first approach with only four two-stage guns. In this article we will describe in detail the guidelines of the MPI prototype design, which were strongly influenced by some external constraints. We will also report on the results of the experimental campaign, during which the feasibility of such a two-stage MPI was demonstrated. Sequences of four intact D2 pellets in the 1.2–1.6 mm size 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 aiming dispersion. Some preliminary effort to address the problem of propellant gas handling, based on an innovative approach, gave encouraging results, and work is in progress to carry out an experiment to definitely test the feasibility of this concept.

Addressing the feasibility of inboard direct-line injection of high-speed pellets, for core fueling of DEMO

“…If one takes into account the distance of the target from the gun muzzle, the angular scattering of intact pellets trajectories turns out to be about 0.7° (0.012 rad), which is the same value that had been previously measured for this barrel at speeds of  1 km/s, using the ORNL propellant valve [13]. Moreover, this angular spread is not far from that measured (0.57°) many years ago using the high-speed multiple-barrels pellet injectors for the Frascati Tokamak Upgrade (FTU) [14].…”

Core Fueling of DEMO by Direct Line Injection of High-Speed Pellets From the HFS

“…Our experience is that the angular spread of free-flight trajectories tends to increase, to some extent, when the pellet velocity exceeds 3 km/s, but within this speed limit it remains rather small. A very large number of pellets was indeed launched (thus attesting the statistical relevance of the results), and all pellets were systematically observed to hit the target well within a circle (centered along the barrel axis) of 3 cm in diameter [9], [31]. This indicates that, up to 3 km/s, the scatter cone is expected to have an angle of less than 3 cm / 300 cm = 0.01 rad (0.57°), or about 810 -5 sr. To consolidate these earlier data and, hopefully, extend them to higher pellet velocities, it is necessary to have available a high-speed pellet injector.…”

Steady improved confinement in FTU high field plasmas sustained by deep pellet injection

“…Moderate central temperatures (10-13 keV) and peaked temperature profiles, as foreseen in the high field approach to the burning condition [10,11], together with new injector technology and non-conventional injection schemes, can help in this effort. High speed (up to 4.5 km/s) single barrel injectors are now available [12], and this technology can easily be extended to a multibarrel device [13]. On the other hand, pellets injected from the high field side, taking advantage of the radial particle drift, have proven to be more efficient than those injected from the external equatorial direction [9,14].…”