Shielding the Turbomolecular Pump: an update 14 May 2013 Kiril Marinov ASTeC, MaRS, DL 1

Design considerations Compared to the PSU (last week) the shielding requirements of the pump are more stringent. Access is needed to the valve handle, there is a pipe connecting the pump to its chamber and for cabling (?) Implement the pump shield I am about to show would require additional engineering effort needed, e.g. to design support from the floor, safety, etc.

Concept B in max <<<B 0 Air circulation, valve access Air circulation, cabling Top To vacuum chamber 15 mm thick AISI 1010 steel mm thick steel Φ110 Φ60 600

Installation The two halves can be held together by steel bands. The gap can be controlled by a thin gauge during assembly, should not be much bigger than 0.1 mm. Not to scale

Valve access Shield Valve handle

Shielding performance I B External field 40 mT, along Z. The field in the shielded region is well below the target value (5 mT) with 10 mm thick steel and without any “chimneys”

Shielding performance II External field 40 mT, along Z. The field in the shielded region is well below the target value (5 mT) with 10 mm thick steel and without any “chimneys”. B

Shielding performance III (worst case scenario) External field 40 mT, along Z. The peak field in the shielded region is 9 mT with 15 mm thick steel and with the “chimneys”. B

Permeability distribution Saturation in the problematic (colored) regions: μ decreases to Could be fixed by adding additional 5mm of steel there. We know that this works, but I have not done it yet. B

Summary The pump shielding solution is (almost) complete. The shield thickness is, however, 1.5 cm and may need to be increased to 2 cm. Will investigate and report next week. I will look at the effect of the gap between the two halves of the shield but do not expect anything dramatic. As with the PSU this shielding solutions requires additional engineering effort: support from the floor, safety, joining the two halves. The pump generates a fairly small amount of heat and cooling should not be a problem. Documentation is available. Building a prototype and testing it will eliminate uncertainties and reduce the risk. The sooner – the better.