Progress on Window thickness measurement

We have so far received two sets of data; one from LBNL and one from Mississippi;

1. The LBNL results

LBNL used a non-contact CMM probe for the measurement. Data was taken at 15 0 interval and at 1mm increment radially up to a radial distance of 84mm from the centre of the window;

It did not cover the full radial distance of the window, but the window gets thicker further away from its centre, and is not that critical to the burst pressure which the window is designed to withstand.

Besides, the window profile beyond this radial distance becomes steeper as has a marked effect on the accuracy of the measurement when using a non-contact probe;

For this reason, the data from LBNL is adequate for what we are trying to achieve.

2. The Mississippi results consist of measurements taken at 3 points only; one at the window centre, two at 25mm from the centre and at 180 0 apart.

The measurement was carried out using a “deep-throat” calibre device designed and made by Mississippi.

It is difficult to judge if this gives us enough data as far as QA is concerned.

3. The initial proposal to have the third set of data coming from FermiLab using Photogrammetry technique was subsequently abandoned because of the lack of resource.

The lack of independent measurement data (so far only the LBNL data is good enough for QA assessment) had prompted a need to do more measurements elsewhere.

Oxford had attempted to do this using its own CMM machine. But the probe touching force (7 gram) has proved too much for the central crown area of the window. It deflects noticeably when a point load of 7 gram was exerted at the window centre.

Consequently we sought assistance from RAL whose has a CMM machine with a much lighter contact force – only 2 gram. Although it still causes the thinnest section of the window to deflect, the magnitude of the deflections on the concave and convex surface is such that they balance each other out, resulting in a negligible nett deflection. The stress so caused is low enough to have any effect on the window structural integrity.

Although RAL is also equipped with a non-contact CMM machine, we did not ask that to be used because it would be repeating what LBNL had done, albeit with a different, but same type of machine.

This work is currently underway and we were advised that the results should be ready before the CM16.