The compact, four barrel high speed pellet injector for the ignitor experiment

Ignitor is a compact, high field tokamak (R_o = 1.32 m, B _T = 13 T) designed to attain ignition in high density, relatively low temperature plasmas (n,_e0 ≅ n_i0 ≅ 10 ²¹ m^-3, T_e0 ≅ T_i0 ≅ 11 keV), by ohmic heating (or with small amounts of additional ICRF heating). Tailoring of the density profile peaking during the initial plasma current rise is important to optimize ohmic and fusion heating rates. Therefore, a pellet injector has always been included in the Ignitor design. Simulations performed with the NGS ablation model, for the reference ignition plasma parameters in Ignitor, indicate that deuterium pellet of a few mm sizes (≤ 4 mm) injected at 3-4 Km/s from the low field side should achieve sufficient penetration, particularly during the current ramp up. A four barrel, two-stage pneumatic injector for the Ignitor experiment has been built in collaboration between ENEA and Oak Ridge National Laboratory, featuring two innovative concepts: (i) the proper shaping of the propellant pressure pulse to improve pellet acceleration, and (ii) the use of fast closing (∼ 10 ms) valves to drastically reduce the expansion volumes of the propellant-gas removal system. The ENEA subsystem, including four independent two-stage guns and pulse-shaping valves, the gas removal system, and the associated controls and diagnostics, has been extensively tested at CRIOTEC. The ORNL sub-system consists of the cryostat and pellet diagnostics, with related control and data acquisition system. Initial testing with D2 pellets at speeds of ∼1 km/s, using ORNL single-stage propellant valves, are scheduled to be completed by June 2007. The ENEA two-stage drivers will then replace the ORNL propellant valves, and integrated testing at high speeds (>3 km/s) will be finally carried out. The NGS model was also used to assess the maximum ablation depth of D₂ pellets, of the sizes and speeds produced by the Ignitor Pellet Injector, inside JET plasmas. A similar analysis is now extended to the Large Helical Device (LHD), which has recently obtained high density plasma discharges (up to 5×10²⁰ m^-3). Deep pellet penetrations can be achieved over a wide range of plasma parameters in LHD, even at its highest temperature, thanks to the high speed of the IPI pellets.