436
F. Bordry et al.
(experimental areas and ceramic chambers), decoupling of baked vacuum parts,
which are at room temperature, from non-baked parts at cryogenic temperature,
radiation issues, etc.
For UHV beam vacuum systems, all-metal gate valves are preferred in order
to allow for bake-out at temperature above 250 ◦ C. VITON-sealed valves are less
expensive but limited in bake-out temperature. A special treatment of the VITON
seals is recommended prior to their use nearby NEG coatings or pumps since minor
outgassing of Fluor will degrade the pump characteristics.
Interlocking the sector valves is not an obvious task. Indeed, increasing the
number of sensors will provide more pressure indications but might results in a
degradation of the overall reliability. Protection at closure (pressure rise, leaks) is
treated differently from the protection while recovering from a technical stop. In the
latter case, parts of the accelerator beampipe vented or being pumped down must
not be put in communication with upstream or downstream sectors which remained
under vacuum.
8.5.4.4 Corrosion Issues
In vacuum systems, feed-throughs and bellows are particularly exposed to corrosion.
Feed-throughs, particularly those of the ion pumps where high voltage is permanently present, are critical parts. Despite the design optimization made by many
suppliers, a direct heating of the protective cover to reduce the relative humidity
around the feed-through is the most efficient and commonly used technique to limit
the corrosion.
The bellows are also vulnerable due to their thinness, often between
0.1 . . . 0.15 mm. Aluminum bellows are exposed to corrosion by nitric acid
(HNO 3 ) which is generated by the combination of O 3 and NO x . In presence of
PVC material, radiation can lead to the creation of hydrochloric acid (HCl) which
corrodes stainless steel materials. This corrosion is strongly penetrating, and once
seen at the surface, it is often too late to mitigate the effects.
Relative humidity is the critical parameter and must be kept below 50%.
However, accidental spillage can compromise locally the conditions and therefore,
corrosion-resistant design are strongly recommended.
Corroded components like feed-throughs and bellows normally start to leak
after a mechanical action or a venting to atmosphere. If several components are
affected, this could result in a long sequence of pumping, leak searching, venting,
repumping, etc., leading to a long downtime and often subsequent radiation dose
rates to personnel doing the interventions.
8.5.4.5 Experimental Areas
Their design must include the following additional constraints: integration, reliability, availability, engineering and performances.
F. Bordry et al.
(experimental areas and ceramic chambers), decoupling of baked vacuum parts,
which are at room temperature, from non-baked parts at cryogenic temperature,
radiation issues, etc.
For UHV beam vacuum systems, all-metal gate valves are preferred in order
to allow for bake-out at temperature above 250 ◦ C. VITON-sealed valves are less
expensive but limited in bake-out temperature. A special treatment of the VITON
seals is recommended prior to their use nearby NEG coatings or pumps since minor
outgassing of Fluor will degrade the pump characteristics.
Interlocking the sector valves is not an obvious task. Indeed, increasing the
number of sensors will provide more pressure indications but might results in a
degradation of the overall reliability. Protection at closure (pressure rise, leaks) is
treated differently from the protection while recovering from a technical stop. In the
latter case, parts of the accelerator beampipe vented or being pumped down must
not be put in communication with upstream or downstream sectors which remained
under vacuum.
8.5.4.4 Corrosion Issues
In vacuum systems, feed-throughs and bellows are particularly exposed to corrosion.
Feed-throughs, particularly those of the ion pumps where high voltage is permanently present, are critical parts. Despite the design optimization made by many
suppliers, a direct heating of the protective cover to reduce the relative humidity
around the feed-through is the most efficient and commonly used technique to limit
the corrosion.
The bellows are also vulnerable due to their thinness, often between
0.1 . . . 0.15 mm. Aluminum bellows are exposed to corrosion by nitric acid
(HNO 3 ) which is generated by the combination of O 3 and NO x . In presence of
PVC material, radiation can lead to the creation of hydrochloric acid (HCl) which
corrodes stainless steel materials. This corrosion is strongly penetrating, and once
seen at the surface, it is often too late to mitigate the effects.
Relative humidity is the critical parameter and must be kept below 50%.
However, accidental spillage can compromise locally the conditions and therefore,
corrosion-resistant design are strongly recommended.
Corroded components like feed-throughs and bellows normally start to leak
after a mechanical action or a venting to atmosphere. If several components are
affected, this could result in a long sequence of pumping, leak searching, venting,
repumping, etc., leading to a long downtime and often subsequent radiation dose
rates to personnel doing the interventions.
8.5.4.5 Experimental Areas
Their design must include the following additional constraints: integration, reliability, availability, engineering and performances.