LIGO Document T1400341-v2

Maximum B-OSEM Current

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T - Technical notes
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Testing is required on the BOSEM assembly in order to establish coil current limits based on thermal considerations. These thermal considerations include:
Maximum operating temperatures of the components and materials comprising the BOSEM assembly, and
Hydrocarbon outgassing rates
The BOSEM report, T050111, contains data on temperature rise vs current, but no outgassing measurements. Moreover this testing was done at relatively high pressure (~50 mTorr) instead of 0.1 mTorr needed to ensure that air conduction/convection is negligible. This memo reports on testing of the outgassing of BOSEMs in a test chamber and the interpretation of this data in terms of the maximum allowed drive current.

Conclusion: A maximum coil temperature of 50C will keep the hydrocarbon outgassing rate low enough (4e-12 torr-L/s). A conservative coil current limit (based on the vacuum test performed) is 105 mA. Some questionable thermal modeling suggests that for the BOSEMs installed in the top stage of a quad suspension, the coil temperature is likely to be well under this temperature limit, even at a coil drive current of 200 mA. However, measurement of the quad installed BOSEM coil temperature (by measuring coil resistance in situ) is recommended before increasing the coil current higher than 100 mA.

Notes and Changes:
Update: A measurement of an in situ BOSEM was made by Stuart Aston on the L1 ITMx quad and reported in the eLog:

Another similar measurement is planned for L1 ITMy. These results are not yet reported in this memo.

Changes for -v2:
1) Corrected temperature axis label for Figure 3, and switched independent variable to the x-axis.
2) Added clarification on the number of assumed BOSEMs per quad suspension that are likely to be at maximum current.
3) Corrected an error in the last sentence.

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