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The effect of neon glow discharge cleaning (GDC) on the reduction of water outgassing and the production of ultrahigh vacuum was studied using a 304 stainless steel chamber. Two pumping systems having different effective pumping speeds of 10 l s-1 and 182 l s-1 were used. By switching these pumping systems after GDC, the effect of GDC on the reduction of outgassing was investigated. The ion dose on the wall surface in a 2 h GDC trial at a neon gas pressure of 2 Pa was 1.25×1018 ions cm-2. During pump-down, where the pressure decrease is inversely proportional to time, the degassing effect of GDC was not so large compared to that in pump-down of the chamber at a higher pumping speed without the assistance of GDC. However, GDC in which the pressure reaches a limit after a long pump-down was very effective in reducing the pressure limit. When a cryopump that has an effective pumping speed of 274 l s-1 was used, a very low pressure of the order of 5×10-8 Pa was attained by GDC. Experimental results are discussed from the point of view of coverage reduction of adsorbed molecules on the wall surface and also using a theoretical model. |
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