Cryopump design for the ITER heating neutral beam injector

摘要

Forschungszentrum Karlsruhe (FZK) is developing cryopumps for ITER heating neutral beam injectors (NBIs). The system is characterized by high gas flows coming from different sources against which the cryopumps must maintain a pressure between 10~(-2) and 10~(-3) Pa in the beam line (BL) vessel. The design of 2001 had proposed a cylindrical vessel with a cylindrical cryopump inside with a maximized pumping area to cover the vacuum requirements. In this design the only access to all BL components was through the beam source vessel, making it necessary to dismantle the beam source for all maintenance issues on the BL. During the ongoing design investigations on the heating NBI, it was decided in 2006 to use a rectangular vessel for the BL with a top access to improve the maintenance. This necessitated a full redesign of the cryopumps. In the new geometry of the BL vessel the size of the cryopump is limited to a flat rectangular geometry of 8 m length, 2.75 m height and 0.5 m depth. To cover the vacuum requirements two cryopumps of this size must be included for each injector. Gas profile calculations using the detailed BL geometry have been used to detect the needed gas capture probability of the pumping surface to guarantee the beam pulse operation. In this paper we give an outline of the working scope done to come up with a cryopump design covering the needs of the ITER NBI. The design of the novel cryopump and the heat load calculations for the different operation scenarios will be discussed. It is shown that the cryopump for the ITER heating NBI covers all vacuum requirements and it is fully adapted to the ITER cryogenic supply.