Assembly, Processing, and Operation of Vacuum Systems
As a general rule, all parts of a vacuum system should be kept as clean as possible through fabrication and until final system assembly. This applies with equal importance if the system is to be baked. Avoid the use of sulfur-containing cutting oil in machining components; sulfur gives excellent adhesion during machining, but is difficult to remove. Rosebury (1965) provides guidance on this point. Traditional cleaning procedures have been reviewed by Sasaki (1991). Many of these use a vapor degreaser to remove organic contamination and detergent scrubbing to remove inorganic contamination. Restrictions on the use of chlorinated solvents such as trichloroethane, and limits on the emission of volatile organics, have resulted in a switch to more environmentally acceptable procedures.Studies at the Argonne National Laboratory led to two cleaning agents which were used in the assembly of the Advanced Photon Source (Li et al., 1995; Rosenburg et al., 1994). These were a mild alkaline detergent (Almeco18) and a citric acid-based material (Citronox). The cleaning
baths were heated to 50º to 65ºC and ultrasonic agitation was used. The facility operates in the low 10¯10 torr range. Note that cleanliness was observed during all stages, so that the cleaning procedures were not required to reverse careless handling. The initial pumpdown of a new vacuum system is always slower than expected. Outgassing will fall off with time by perhaps a factor of 103, and can be accelerated by increasing the temperature of the system. A bakeout to 400ºC will accomplish a reduction in the outgassing rate
by a factor of 107 in 15 hr. But to be effective, the entire system must be heated.
Once the system has been outgassed any pumpdown will be faster than the initial one as long as the system is
not left open to the atmosphere for an extended period. Always vent a system to dry gas, and minimize exposure to atmosphere, even by continuing the purge of dry gas when the system is open. To the extent that the ingress of moist air is prevented, so will the subsequent pumpdown be accelerated.
Currentvacuumpractice forachievingultrahighvacuum conditions frequently involves baking to a more moderate
temperature, on the order of 200ºC, for a period of several days. Thereafter, samples are introduced or removed from the system by use of an intermediate vacuum entrance, the so-called load-lock system. With this technique the pressure in the main vacuum system need never rise by more than a decade or so from the operating level. If the need for ultrahigh vacuum conditions in a project is anticipated at the outset, the construction and operation of the system is not particularly difficult, but does involve a commitment to
use materials and components that can be subjected to bakeout temperature without deterioration.
As a general rule, all parts of a vacuum system should be kept as clean as possible through fabrication and until final system assembly. This applies with equal importance if the system is to be baked. Avoid the use of sulfur-containing cutting oil in machining components; sulfur gives excellent adhesion during machining, but is difficult to remove. Rosebury (1965) provides guidance on this point. Traditional cleaning procedures have been reviewed by Sasaki (1991). Many of these use a vapor degreaser to remove organic contamination and detergent scrubbing to remove inorganic contamination. Restrictions on the use of chlorinated solvents such as trichloroethane, and limits on the emission of volatile organics, have resulted in a switch to more environmentally acceptable procedures.Studies at the Argonne National Laboratory led to two cleaning agents which were used in the assembly of the Advanced Photon Source (Li et al., 1995; Rosenburg et al., 1994). These were a mild alkaline detergent (Almeco18) and a citric acid-based material (Citronox). The cleaning
baths were heated to 50º to 65ºC and ultrasonic agitation was used. The facility operates in the low 10¯10 torr range. Note that cleanliness was observed during all stages, so that the cleaning procedures were not required to reverse careless handling. The initial pumpdown of a new vacuum system is always slower than expected. Outgassing will fall off with time by perhaps a factor of 103, and can be accelerated by increasing the temperature of the system. A bakeout to 400ºC will accomplish a reduction in the outgassing rate
by a factor of 107 in 15 hr. But to be effective, the entire system must be heated.
Once the system has been outgassed any pumpdown will be faster than the initial one as long as the system is
not left open to the atmosphere for an extended period. Always vent a system to dry gas, and minimize exposure to atmosphere, even by continuing the purge of dry gas when the system is open. To the extent that the ingress of moist air is prevented, so will the subsequent pumpdown be accelerated.
Currentvacuumpractice forachievingultrahighvacuum conditions frequently involves baking to a more moderate
temperature, on the order of 200ºC, for a period of several days. Thereafter, samples are introduced or removed from the system by use of an intermediate vacuum entrance, the so-called load-lock system. With this technique the pressure in the main vacuum system need never rise by more than a decade or so from the operating level. If the need for ultrahigh vacuum conditions in a project is anticipated at the outset, the construction and operation of the system is not particularly difficult, but does involve a commitment to
use materials and components that can be subjected to bakeout temperature without deterioration.
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