stuart.aston@LIGO.ORG - posted 14:34, Friday 03 April 2015 - last comment - 17:38, Friday 03 April 2015(17662)
ETMX charging update
Measurements were carried out on the ETMX effective charge voltage over a period of 3.5 hours this morning. Firstly, some housekeeping, I svn'd up the charge scripts directory to obtain updates from LLO /ligo/svncommon/SusSVN/sus/trunk/QUAD/Common/Scripts/ and persevered local changes. ETMX was selected since it does not currently have low-pass filters installed on each quadrant. It was not necessary to re-align the ETMX optic, since the OpLev was already well centred. Linearization was bypassed (turned off) for the duration of these measurements. The ESD_UL_LL_UR_LR_charge_07-H1.py script was run to drive the ESD at 4 Hz with an amplitude of 130k counts. The script attempts to write a bias to H1:SUS-ETMX_L3_LOCK_BIAS_OFFSET, however, this is nominally turned OFF at LHO. Therefore, it will need to be engaged, and the bias in DAC counts residing in H1:SUS-ETMX_L3_LOCK_INBIAS set to zero. Also, the ramp time should be reduced from 10s to 5s for the duration of the charge measurements. Data was processed using the ESD_UL_LL_UR_LR_analysis_07_H1.m script, charging and charge deviations from today’s measurement can be seen below (all times in UTC). A significant negative effective voltage is observed on the lower quadrants, which are quite variable. Upper quadrants exhibit less charge and are more stable (similar to what was seen for LLO ETMY, before it was discharged) n.b. there is also a large discrepancy between the effective charge reported by OpLev Pitch and Yaw. Previous automated charge measurements carried-out on ETMX by Brett after the most recent vent did not show as larger effective charge (see LHO aLOG entry 16057). Follow-up measurements, may help determine how the charge is evolving. The infrastructure is in place to help determine if any future discharging is successful. Processed results, and analysis scripts have been committed to the sus svn, raw data files have not.
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At Jeff K's request, I've included v2 of my charge measurements notes, so as to help ensure the measurement can be repeated.
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If the ion pump is open to the chamber you will have significant charge fluctuations over 5 to 10 hours.
Rai -- agreed. This was more to establish that (a) our measurement suite at LHO was functional and blessed by Stuart, (b) to confirm that, "yup! we've still got plenty of charge," and (c) there are locals on site who know how to run the measurement suite. We know that the charge will continue to swing around because, as you say, experience has shown when ion pumps are valved in, and there is some charge on the mass, it varies greatly. As such, we're not going to bother to continue tracking it on a day-to-day basis, or really at all because we know there's nothing we can do about it until we vent and remove the charge as LLO has done.