HIGH-SPEED REPEATING HYDROGEN PELLET INJECTOR FOR LONG-PULSE MAGNETICCONFINEMENT FUSION EXPERIMENTS

The projected fueling requirements of future magnetic confinement fusi on devices [e.g., the International Thermonuclear Experimental Reactor (ITER)] indicate the need for a flexible plasma fueling capability, i ncluding both gas puffing and low- and high-speed pellet injection. Co nventional injectors, based on single-stage pneumatic guns or centrifu ges, can reliably provide frozen pellets (1- to 6-mm-diam sizes) at sp eeds up to 1.3 km/s and at suitable repetition rates (1 to 10 Hz or gr eater). Injectors based on two-stage pneumatic guns and ''in situ'' co ndensation of hydrogen pellets can reliably achieve velocities over 3 km/s; however, they are not suitable for long-pulse repetitive operati ons. An experiment in collaboration between Oak Ridge National Laborat ory (ORNL) and ENEA Frascati is under way to demonstrate the feasibili ty of a high-speed (>2 km/s) repeating (similar to 1 Hz) pneumatic pel let injector for long-pulse operation. A test facility has been assemb led at ORNL, combining a Frascati repeating two-stage light-gas gun an d an ORNL deuterium extruder, equipped with a pellet chambering mechan ism/gun barrel assembly. The main issues to be investigated were the s trength of extruded deuterium ice as opposed to that produced by in si tu condensation in pipe guns (hence the highest acceleration which can be given to the pellet without fracturing it), and the maximum repeti tion rate at which the system can operate without degradation in perfo rmance. Pellet velocities of up to 2.55 km/s have been achieved in joi nt experiments at ORNL. A new pressure tailoring valve was developed b y the Frascati group for this application and proved to be a crucial c omponent for good performance. Tests carried out in repeating mode, at frequencies of 0.2-0.5 Hz and speeds up to 2.2 km/s, indicate no sign ificant degradation in performance with increasing repetition rate. So me preliminary tests using 3.7 mm pellets gave very encouraging result s. The equipment and the experimental results are described in this ar ticle. (C) 1996 American Institute of Physics.