Gas permeation through a high density polyethylene microwave window

Journal Article · Thu Jul 01 04:00:00 EDT 1993 · J. Vac. Sci. Technol. A 11, 1584 (1993)

DOI:https://doi.org/10.1116/1.578508· OSTI ID:791703

Nguyen-Tuong, Viet

Due to its low dielectric constant and low loss tangent, high density polyethylene (HDPE) has been selected for use as a high power microwave vacuum window in the Continuous Electron Beam Accelerator Facility cryounit. This window isolates the cryounit waveguide vacuum from the dry air in the external waveguide system. Gas permeation through the window will lead to cryopumping of the gas onto the cold waveguide walls and the cold ceramic window of the superconducting cavity. The gas load from permeation and outgassing of the window have to be minimized, due to the possibility of arcing when high power is applied through the waveguide. The outgassing and permeation of air through the 3.2 mm thick HDPE window were measured using the throughput method. A typical outgassing rate of 5.0 x 10{sup -1} Torr l/s/cm{sup 2} for samples baked out at 70 C was observed 20 h after pump down and bakeout. The gas load due to permeation through 34 cm{sup 2} of the window was 1.6 x 10 {sup -7} Torr l/s. The gas permeation through the 3.2 mm thick HDPE coated with a 300 nm barrier layer of SiO{sub x} was also investigated. No improvement was observed. It was presumably due to the presence of defects in the deposited SiO{sub x} layer.

• Research Organization: Thomas Jefferson National Accelerator Facility, Newport News, VA (US)
• Sponsoring Organization: USDOE Office of Energy Research (ER) (US)
• DOE Contract Number: AC05-84ER40150
• OSTI ID: 791703
• Report Number(s): CEBAF-PR-93-055; DOE/ER/40150-2003
• Journal Information: J. Vac. Sci. Technol. A 11, 1584 (1993), Journal Name: J. Vac. Sci. Technol. A 11, 1584 (1993)
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
43 PARTICLE ACCELERATORS
CEBAF ACCELERATOR
DEFECTS
DEGASSING
GASES
PERMEABILITY
POLYETHYLENES
SILICON OXIDES
VACUUM SYSTEMS
WAVEGUIDES
WINDOWS