High Frequency Pellet Injector Project

A key component of the JET enhancements in support of ITER is the new High Frequency Pellet Injector, which will be capable of shooting 50-60 deuterium ice pellets per second into JET plasmas (see Fig. 1). This project has two main experimental objectives: deep plasma fuelling (i.e. shifting the deuterium supply significantly towards the hot plasma core) and mitigation of the edge instabilities called ELMs (Edge Localised Modes). At the moment it appears to be mandatory for ITER to establish a reliable ELM mitigation or avoidance technique. The fact that every pellet injected into a high confinement mode plasma with ELMs is triggering an ELM makes pellet injection a very promising option. It has been demonstrated in the ASDEX Upgrade tokamak in Garching, Germany, that the ELM frequency can be controlled by the pellet injection frequency, leading to a significant reduction of the energy ejected during each ELM.

The first sub system in the injection line (see Fig. 3, below) is the High Frequency Pellet Injector designed on the basis of a injector (see Fig. 2) installed on Tore Supra at CEA Cadarache, France. The injector is based on the screw extruder technology developed by the Russian company PELIN, which allows the injection of an unlimited number of pellets at a very high level of reliability. The new High Frequency Pellet Injector will be capable of uninterrupted operation during an entire JET pulse. Cylindrical pellets are cut from the different deuterium ice ribbons by the sharp edge of thin wall tubes of fixed diameter moved axially by three solenoid actuators. Then a short pulse of propellant gas (Helium up to 20bar), driven by three electromagnetic valves, accelerates each pellet in three gun barrels (1.25mm, 1.25mm and 4mm). For the ELM control, the new injector will produce small pellets at high frequency (pellet volume 1-2 mm³, up to 60 pellets per second, with pellet velocity 50-200 metres per second), while for the deep plasma fuelling it will be able to produce large pellets at lower frequency (pellet volume 35-70 mm³, up to 15 pellets per second, with pellet velocity 100-500 metres per second).

Fig. 3: The injection line of the JET HFPI system

A CCD camera coupled to a set of light barriers, for pellet velocity measurement, is installed at a diagnostic chamber located at the injector exit. Special software has been developed by PELIN to re-construct the pellet volume from CCD pictures, recording a combination of two perpendicular views of the pellets.

After passing the diagnostic chamber the pellet flies through an injection line that has been designed to connect the injector to the three existing flight tubes already in place on the JET machine allowing pellet injection at three different poloidal locations. The functions of the injection line are:

• pumping of the propellant gas
• measurement of the pellet mass in the dual microwave cavity
• selection of the flight tube to be used
• pumping of a part of the gas produced from the pellet erosion inside the flight tubes in the last section of the injection line.

The HFPI system is expected to become fully operational in early 2008.