Non-evaporable getters in a turbomolecular pump backing application
M. Maskell, Old Dominion University; G. Myneni, P. Adderley, Jefferson Lab;
H. Londer, E. Ogris, ALVATEC - Austria; C. Day, Forschungszentrum, Karlsruhe
Non-evaporable getters (NEGs) have certain advantages over traditional mechanical pumps when applied to UHV systems, particularly in the pumping of Hydrogen. Barium NEGs have been shown1 to be capable of maintaining pressures in the 10E-5 to 10E-6 mbar range in sealed systems over extended periods of time. Mechanical pumps used for backing turbomolecular pumps provide a foreline vacuum of 10E-4 mbar at best, and result in open systems where atmospheric Hydrogen can backstream into the foreline and raise the ultimate pressure attainable by the turbo. Additionally, NEGs cost less than one-hundredth the price of a typical mechanical roughing pump. In this work, a Barium NEG is applied as a backing pump for a turbomolecular pump, and the pressures are compared to those obtained using a mechanical scroll pump as the backing.
EXPERIMENTAL SETUP
A stainless steel chamber approximately 2 L in volume was pumped by an Alcatel ATH-31+ turbomolecular pump, alternately backed by a Varian scroll pump and an Alvatec Barium NEG. The structure and operation of the NEG is described in detail in reference 1. Mounted on the chamber was an SRS 100 AMU RGA. The backing system consisted of a small cube with four mini-conflat ports and one standard conflat port. The mini-conflat ports led to the turbo and to three all-metal valves, one leading to the NEG, one to the scroll, and one to another RGA. The standard conflat port led to a convectron gauge. The NEG was mounted to its valve with a swadgelock connection, leaving the NEG tube accessible for direct heating.
Figure 1: Photograph of experimental apparatus. The NEG is attached to the UHV valve closest in the foreground, opposite to it is the valve leading to the scroll pump, and the remaining valve leads to the foreline RGA (not shown).
EXPERIMENTAL PROCEDURE
Figure 2 shows the activation of the NEG as viewed by the foreline RGA. The NEG was activated using a heating tape that heated the NEG to ~300 C for 5 minutes. Water and Nitrogen are desorbed from the tube due to the heating, and a flash of Argon is clearly visible during the activation; this is the Argon that the Barium is stored under behind the Indium sealing. The Argon flash is a good indication that activation has occurred. The heat is removed at the 35 minute mark on the figure. Activation is further indicated by the background Hydrogen partial pressure level falling below the previous baseline once heat is removed. At approximately the 50 minute mark, the Argon flash occurs again, along with a brief spike of Hydrogen; this is likely due to the remaining Indium sealing falling away.
Figure 2: Foreline RGA Pressure vs. Time plot during NEG activation
Figure 3 shows the main chamber pressure vs. time as the turbo is backed solely by the NEG for 54 hours. The Hydrogen partial pressure slowly rises over this time from 6.5e-10 Torr to 8.5e-10 Torr.
Figure 3: Main chamber Pressure vs. Time plot as turbo is backed by NEG for 54 hours
The NEG’s performance can be improved by a second heating to desorb some adsorbed gas. In Figure 4, the NEG is heated to around 150 C for 10 minutes, and then is left to back the turbo for another 24 hours. The main chamber pressure holds at around 5e-10 Torr during this time, an even better performance than backing with the scroll pump. Figure 5 shows the main chamber pressure while backing the turbo with the scroll pump for comparison.
Figure 4: Main chamber pressure after reheating NEG at 150 C for 10 min
Figure 5: Main chamber pressure with turbo backed by scroll pump
Figure 6 shows the foreline spectrum during this reheating. Nitrogen, Hydrogen, and Methane are desorbed from the NEG during heating. The foreline RGA was pumped by its own turbo system, so these gases were removed from the foreline as they were desorbed. Removal of the desorbed gas by some other pump is necessary for NEG regeneration to be successful.
Figure 6: Foreline spectrum of gas desorbed by the NEG during reheating. Hydrogen and Nitrogen are the primary gases desorbed and removed by the foreline RGA turbo. The heat is removed around the 8 minute mark.
RESULTS AND CONCLUSIONS
The NEG has been shown to be capable of producing turbopump performance equal to or better than a traditional mechanical backing pump can provide, and can maintain this performance for days at a time. This time can be extended by regenerating the NEG with heat to desorb collected gas and removing the gas from the system. The implications are significant for projects involving many turbopumps, such as the UHV areas of particle accelerators. Instead of backing each turbopump with its own dedicated, expensive mechanical pump, the turbos can be backed by inexpensive (about 1/100 the cost of a mechanical pump) NEGs, maintaining only a single mechanical pump to facilitate initial roughing of each foreline and periodic removal of gases desorbed during NEG regeneration. Smaller systems can benefit from NEG backing as well; the method provides protection of UHV systems and turbopumps against power failure, and under the right conditions can allow the turbo to produce lower ultimate pressures than a mechanical, open backing system would allow.
REFERENCE:
1. H. Londer, et al. New High Capacity Getter for Vacuum-Insulated Mobile Liquid Hydrogen Storage Systems in Hydrogen in Matter: Second International Symposium on Hydrogen in Matter. Ed. G.R. Myneni and B. Hjorvarsson. American Institute of Physics, 2006, pp 210-220.