One of the main plasma fuelling methods in a tokamak is the injection of deuterium ice pellets at high velocity. This technique allows direct fuelling of the plasma core as well as the possibility of mitigating plasma instabilities such as Edge Localised Modes (ELM). The control of the ELM activity is an important issue since the energy ejected during each ELM could lead to the erosion of the divertor plates, reducing their lifetime.

The system shall be able to produce and accelerate at 50- 500 m/s unlimited stream of both small pellets (1 – 2 mm3) at high frequency (up to 60 Hz) for ELM control, and large pellets (35 – 70 mm3) at lower frequency (up to 15 Hz) for deep fuelling, with a high level of reliability.

The new High Frequency (50-60 Hz) deuterium ice Pellet Injector (HFPI) will allow the mitigation of ELMs and deep fuelling experiments as part of the development of the JET operating scenarios with direct relevance to ITER. The new injector will be designed on the basis of the injector recently installed on Tore Supra at CEA Cadarache (France) for deep fuelling experiments (shown in the figure).

Recent ASDEX Upgrade experiments showed that pellet injection can be a method to mitigate the ELMs. It has been demonstrated that their frequency can be imposed by a pellet injection frequency corresponding to 1.5 to 3 times the natural ELM frequency (“Pace Making”) and that the energy ejected during each ELM is proportional to the ELM period, as for intrinsic ELMs. In JET the natural type-I ELM frequency is typically 10 Hz but can reach 20 to 25 Hz for high plasma densities. Thus the pellet injection frequency needed for an efficient ELM mitigation in JET is about 50-60 Hz, which is not achievable with the present JET pellet injection system.

The injector will be delivered as a turn-key system and will be installed in the Torus Hall close to the existing centrifuge pellet injector. It will be connected to the vacuum vessel through an appropriate pumping line followed by a selector and by the existing pellet guide tubes. The selector system will allow the selection of either the new injector or the existing one and also the track to be used to convey the pellets to the plasma. This will allow the use of the three tracks already in place, giving the new system maximum flexibility.

Deep pellet fuelling, which matches the main fuelling scheme on ITER, will allow a range of important issues to be addressed on JET, including core density control, density limit physics and the effect of core vs. edge fuelling on confinement. Additionally, in Advanced Scenarios, deep fuelling could be explored as a tool to increase the bootstrap current fraction and to study particle transport inside Internal Transport Barriers. Simultaneous operation of the HFPI with the current Pellet Centrifuge will also enhance the flexibility to investigate profile effects.