The injection of frozen deuterium pellets at high speed into tokamak discharges has the potential for more efficient fuelling than the puffing of deuterium gas. The JET programme is investigating the dependence of pellet penetration on three different injection geometries.

The first JET experimental campaign of 2003 benefited from the addition of a third pellet track (Fig. 1), together with improvements in the D2 ice quality and a design of the switch which allows selection between the existing low field side (LFS), high field side (HFS) and new vertical high field side (VHFS) tracks (Fig. 2). The basic function of the new VHFS pellet track is to inject pellets towards the plasma core from the high field side. The previous HFS track is positioned too low to fuel directly the inner core and the LFS track cannot take advantage of  outward drifts which are thought to improve fuelling efficiency. The VHFS guiding track is a 10 mm ID stainless steel tube capable of handling 4 mm and 5 mm (cube size) pellets. Diagnostics include a direct measurement of the pellet mass using a microwave cavity.

The full system offers a doubling of the previous maximum fuelling rate and will allow the penetration and fuelling efficiency of pellets to be studied for a wide range of injection speeds, pellet sizes and injection geometry (which can be changed during a pulse). The pellet sizes currently available are 4×4 mm2 strings, which can be cut to lengths of 2.2-4 mm, at repetition rates range in the range 2 – 10 Hz, respectively.

Initial experiments in 2003 demonstrated reliable operation of the new VHFS pellet track. In Fig. 3, a direct comparison of the density response to pellets injected along different tracks into the same plasma discharge is shown. Preliminary results indicate that the fuelling efficiency is similar for all three tracks at low speed (up to 250 m/s) and is better with the VHFS track at high speed (up to 400 m/s). The HFS track has allowed injection speeds of 310 m/s, much higher than previously achieved.

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