Cryopump design development for the ITER Neutral Beam Injectors

Abstract

Forschungszentrum Karlsruhe (FZK) is developing the cryopumps for the ITER Neutral Beam Injectors (NBI). The system is characterized by high gas flows against which the cryopumping system must maintain the required pressures down to 10 −3 Pa in the beam line vessel. During detailed investigations of the gas dynamics in the neutral beam system it showed up that in the close geometry of the beam line design a cryopump with a gas capture probability of 30% is needed to cover the low pressures. This cannot be achieved by a classical cryopump design using a chevron baffle because it is limited to gas capture probabilities below 24%. Therefore, a novel cryopump has been developed which is characterised by a gas capture probability of 33%. The engineering design of the pump takes into account that it will be operated under the conditions of the ITER cryosupply. Under these conditions the heat load to the low temperature circuits of the new cryopump design is only 20% higher than for the classical cryopump which is in regard of the 50% higher pumping speed a very good improvement of the entire efficiency. The new cryopump design is now the reference design for the ITER Heating NBI. Two cryopumps will be integrated in the beam line vessel, each of them about 8 m long and 2.75 m high resulting in an overall pumping speed for hydrogen (H 2) of 5000 m 3 s −1. This paper describes the vacuum conditions of the ITER Heating NBI and summarizes the pumping properties of the cryosorption pump as well as to the investigations on the heat loads to the cryogenic circuits.