Abstract
The performance has been studied of a 12 in. diameter titanium getter-ion pump with an evaporated-getter area of 3400 cm2. The titanium was evaporated from an electron-bombarded molten bead suspended via a frozen zone from a cooled anode; a triode electrode system was used for ionization pumping. The sorption rates of a number of gases were measured in the pressure range 10-4-10-8 mm of mercury. The pumping speed for nitrogen was enhanced by operating the ionization source during evaporation. The h.t. voltage used with the vapour source influenced the form of the titanium bead, and at low voltages (approximately 800 V) and high power inputs (approximately 465 W) large metal droplets were obtained giving the highest evaporation and sorption rates. Sorption rates were greatly increased if a diffusion pump was in operation during gettering, because impurity gases slowly sorbed by the getter-ion pump were more effectively removed. Pumping speeds in l./s at 10-6 mm of mercury for the getter-ion pump with a diffusion pump (10 l./s) in operation and a titanium evaporation rate of 40 mg/min were as follows: oxygen2 - 2450; nitrogen2 - 4500; hydrogen2 - 1600; air - 600; argon - 2.5; Calor gas - 90. The low pumping speed of air was due to the low sorption rate of the argon component. Hydrocarbon gases were slowly sorbed and when mixed with active gases greatly reduced their pumping speed. The pump-down characteristics of the getter-ion pump were examined and it was found that the ultimate pressure was only slowly regained after exhausting oxygen. The sorption efficiency can be found from the equation: Sm = α S0, where Sm is the measured sorption rate per unit getter area (l./s cm2), S0 is the ideal sorption rate and α the sorption coefficient. Values of α measured at 10-6 mm of mercury were as follows: oxygen2 - 0.068; nitrogen2 - 0.12; hydrogen2 - 0.012; possible reasons for the low value of α for hydrogen are considered.