Design and operational characteristics of the cryopump high-vacuum system for the 25000-l target chamber of the Helios Laser Facility

A vacuum system utilizing two 25 000 l/s (H2O) two-stage (20 and 70 K) helium refrigerator (cold gas) cryopumps and a 1500 l/s turbomolecular pump has been provided for a 25 000-l-volume stainless steel chamber as a part of the Helios Laser Facility. Design goals for the performance of the vacuum system are to produce a chamber pressure of less than 1.33×10−4 Pa after two hours pumping from atmospheric pressure. Details of the vacuum system configuration, and typical operating conditions are described. Observations made during operation indicate the cryopumps utilized initially produce expected pumping performance followed by variable performance (pumping speed and ultimate pressure) depending upon pumping history. The turbomolecular pump utilized has demonstrated reproducible pump speed and ultimate pressure. Both pumps produce some vibration which can cause interference with critical components in the target chamber; vibration damping/attenuation has been successfully employed for the turbomolecular pump with similar techniques being studied for use with the cryopumps. Reliability of the machines utilized has been excellent with only routine maintenance required in the first year of operation. Considerable instability in ionization gauges (Bayard-Alpert type) has been experienced where the expected gauge sensitivity of 10 per Torr has been observed to vary from 7.5 to ≳11.