AVS2011 Session VT-TuM: Accelerator and Large Vacuum System Design, Outgassing and Pumping

The Jefferson Lab DC, high voltage polarized electron source utilizes deep-UHV pressures to limit photocathode damage due to ionization and acceleration of residual gasses into the photocathode, as well as preserve the surface chemistry necessary for electron emission. Continued efforts toward improving and quantifying pressures below 5x10^-12 Torr for electron source vacuum chambers will be presented, including operational experience with the newly available Watanabe Bent-Belt Beam (BBB) gauge, and the incorporation of a bakable cryopump into the pumping configuration.

National Synchrotron Light Source II is a new medium energy, ultra low emittance, high flux and high brightness synchrotron radiation facility. NSLS-II consists of a 200-MeV linac, a 3-GeV booster synchrotron, and a 3-GeV 500-mA storage ring with a circumference of 792 meter and over 60 beam lines. The construction of NSLS-II started in 2009 and will be ready for users in 2014. The storage ring vacuum chambers are made of extruded aluminium, with ante-chamber for photon extraction and for distributed NEG pumping. The precision machined extrusions are welded to aluminium-to-stainless bi-metallic flanges using robotic welding machines. Due to the high heat loads, all the photon absorbers are made of GlidCop. The details of the storage ring vacuum system will be presented and compared with those of similar facilities. The challenges encountered in the fabrication of vacuum chambers, photon absorbers and RF shielded bellows will be described. The status of the project will also be summarized.

The ITER Neutral Beam Injection system (NBI) is one of the heating systems to achieve the required plasma temperatures to start the fusion process. Thereby, the NBI system is basing on the principle of provision and acceleration of deuterium and protium ions and the re-neutralization of the high energy ions to be injected into the plasma through the confining magnetic fields. Each heating NBI is designed to insert 16 MW of heating power to the plasma and presents major technical and physical challenges.

In order to solve these and to demonstrate the achievement of the required parameters, a robust R&D program is under way. A central milestone for this development is the establishment of a full scale test facility, which will be built on site of Consorzio RFX, Padova. Part of this test facility is MITICA (Megavolt ITER Injector and Concept Advancement), the test bed for the entire neutral beam injection system. Karlsruhe Institute of Technology (KIT), which is the lead party in design and R&D of the ITER cryopumps since more than a decade, is supporting this project with the development of a customized cryopump design which ensures that the requested density profiles for optimum beam performance can be produced.

The main operational task which has to be provided by the cryogenic pump at a speed of ~5000 m³/s is to handle very high gas loads of protium and deuterium. As basic pumping concept, cryosorption was chosen and the cryopump is operated with supercritical helium at 4.5 K for the adsorbing and gaseous helium at 80 K for the shielding circuit. As demonstrated in other NBI applications, cryosorption provides a wide and robust operational window at acceptable cryogenic loads to the cryoplant. The design was driven by two competing requirements: The high thermal heat loads ask for a closed pump, whereas the need for a high pumping speed asks for an open structure. To reconcile both objectives in an optimized geometry, modeling simulations were performed using the Test Particle Monte Carlo code MOVAK3D. To properly describe the density distribution in the NBI vessel with large thermal and pressure gradients, the time-of-flight cell code ProVac3D was developed.

Additional to the design activities for an optimized cryopump, a considerable effort has been spent to investigate the thermal hydraulic properties of the cryopump during the different operational and failure modes.

Reports of hydrogen outgassing usually involve large vacuum chambers made of stainless steel. Typically, the chambers were baked at temperatures up to 400 °C, but the outgassing was measured only at room temperature. In contrast, the chamber used for the present measurements had a volume of only 29 cm³, with a correspondingly large surface-to-volume ratio, and the outgassing rate was measured at temperatures as high as 250 ° C.

The present outgassing measurements were compared with a numerical model that included (1) diffusion of hydrogen atoms in the steel, (2) recombination at the surface into hydrogen molecules (plus the reverse process), and (3) release of hydrogen from traps. A trap is a site, such as a dislocation or a grain boundary, where the hydrogen is bound more strongly than in the surrounding metal. Traps allow stainless steel to hold much more hydrogen than implied by the small solubility of hydrogen in pure iron. The larger binding energy means that increasing the temperature does more than speed up diffusion; it also increases the amount of mobile hydrogen in the steel.

A cryopump is probably the most versatile and flexible vacuum pump. In large R&D applications where cryoplants are anyway available and high pumping speeds at high throughputs are requested, it is often advantageous to exploit a directly cryogen-supplied cryopump. The nuclear fusion project ITER is a perfect example for such a project, which triggered the development of customized cryopumps. To name just two advantages of this approach, a cryopump can be designed to perfectly fit the available space, and can be installed in-situ without any conductance losses, if regeneration frequency allows for that.

Karlsruhe Institute for Technology (KIT) is developing tailor-made cryosorption pumps for fusion applications over the last 20 years. This has been associated with an extensive design supporting R&D programme which has provided a broad parametric database and stimulated the development of modeling and design tools.

This paper will delineate the essential steps one has to consider when designing a cryosorption pump. The design process of a customized cryopump starts with the proper identification of the set of requirements, which defines the requested integral pumping speed at the given pump location and space. The tools needed for individual cryopump design are described and typical examples are given. This includes the calculation of capture coefficients and distributed pumping on the cold surfaces by means of Test Particle Monte Carlo methods.

Cryopump examples are taken from the area of large cryopumps for ITER, such as the torus cryopumps (~ 80 m³/s) and the cryopumps for high energy neutral beam injection development (~ 5000 m³/s). Although being cryogenic pumps, these applications are characterised by relatively moderate vacua due to the high gas throughputs during pumping. This also leads to the fact that transitional flow conditions prevail inside the pump, which results in additional challenges with regard to modelling and operation. Both pump types are currently in the stage of build-to-print design finalization and prototypes will be manufactured to validate this design in dedicated testbeds at KIT and Padova, Italy.