Study of pressure variation in very long UHV chamber with change in pump location

In a very large vessel, maintain ultrahigh vacuum (UHV) is a challenge. The LIGO Laser Interferometer Gravitational Wave Observatory vacuum envelope is one of the large vacuum chambers in the world. LIGO vacuum system consists of two arms of 4 km long beam tubes of 1.2 meter diameter. Ultrahigh Vacuum (UHV) environment is required to reduce residual gas phase noise to enhance accuracy of measurement to detect such low fractional changes by this detector. The total volume of the vacuum system is about 10,000 m3 and surface area is about 32,000 m2. To achieve UHV in such very long tube, outgassing from the surface must be much less. The outgassing can be reduced by proper material selection, surface treatments, fabrication procedures, various testing procedure etc. After assembly, initially, it will be evacuated to rough and high vacuum with dry vacuum pump and turbo molecular pump. After achieving ~10-3 pa, it must be baked up to allowable temperature, ~150 C for sufficiently long time to achieve UHV, ~10-7 pa. It will be pumped for longer time using turbo molecular pump, Cryopump and Ion pumps connected to beam tubes. To avoid any vibration to the beam tube, both the pumps have to be switched off. To maintain achieve UHV, ~10-7 pa in the beam tube, silent pump, viz. ion pump will be kept on. The ion pumps will be installed at the ports at both the ends of the tube. This study described the pressure variation from one end to the other end of a 2 km long tube for different locations of the ion pumps. This type of study may help to locate pumping ports and decide the required number of pumps to maintain UHV condition in very long tube for years.