This special issue was contrived by education committee. In this specia issu, We present maintenances for vacuum equipment and components.

The helium leak test method that is the highest sensitivity among various types of leak tests is reviewed on the application for a vacuum equipment maintenance. The notes and important things on each test condition are explained from the application example.

Maintenance procedure for instrument used under ultra-high vacuum is described particularly focused on soft X-ray beamlines in synchrotron radiation facilities. The main concern is how to maintain UHV condition during and after the maintenance process.

An experimental system was developed for the physisorption study of hydrogen at substrate temperatures between 1.8 K and 10 K and at pressures down to 10^－10 Pa. The vaccum system was separated into two rooms by a bulkhead ; the upper chamber contained a cryostat equipped with a 4 K mechanical refrigerator and the lower one a substrate kept in an extremely high vacuum. Helium was liquefied at the refrigerator in the upper chamber and was stored in a pot in the lower one. The substrate was directly attached to the helium pot. The substrate temperature was controlled with an accuracy of 0.1 K by a heater located in the helium pot. The adsorption density of hydrogen physisorbed on the copper surface was measured with use of the electron stimulated desorption method. The mean residence time of hydrogen was directly determined by applying this technique to the transient state of adsorption. The adsorption isotherms were obtained by monitoring the hydrogen ion yield as a function of the pressure in the adsorption equilibrium state.

In application of one dimensional diffusion theory to gas molecular flow in the steady state through long ducts of uniform cross section, diffusion coefficient 𝒟 is generally represented as

here λ is a kind of mean free path and v is avarage velocity of molecules. Up to now rigorous expressions of λ＝λ_S where λ_S is Smoluchowski's expression for λ which have been given by Smoluchowski himself and other author for rectangular, ellipsoidal and annular cross section. As for triangular cross section, the rigorous expression has been obtained only on equilatera-triangle cross section. In this report, the concrete expression of λ_S and the correction factor α defined by the ratio of λ_S/λ_K where λ_K is Knudsen's expression for λ, are deduced for long ducts with isosceles-triangle cross sections. The values for λ_S and α, which are given by these deduced formula, are compared with those obtained from the Monte-Carlo simulation, which uses the mean free path in the duct ℓ and the mean square of free path in the axial direction of the duct .