Summary : We tested charging/deionizing of the unsuspended, isolated ITMY (Corning ETM02) with a new electrometer, the Alphalab Ultrastable Surface DC Voltmeter. We first measured the field at the HR surface of the optic interleaving a series of timed deionization runs with the TopGun Deionizer.  Next we applied and ripped FirstContact, and repeated measurements/TopGun nitrogen deionization. The deionization process appeared to take several minutes, having some effect out to nine minutes - however we have to repeat the measurement to assess charge on the barrel and AR optic surface (G. Moreno, N. Robertson, M. Landry, with C. Torrie and R. Abbott).

Details :

We set ETM02 (recently employed in BSC8/1 as the pilot ITMY for single-arm/HIFO experiments) upright on a teflon V-block in a cleanroom of the west bay of the LVEA ("fiber welding cleanroom"). We made measurements of the electric field 1" from the center of HR surface of the optic, and four measurements halfway between the center and the limb of the optic (right, top, left, bottom). Initially we simply applied the TopGun nitrogen in a series of 3 minute applications.  Later we applied FirstContact, let it dry overnight, and the pulled (with concurrent TopGun dionization), and made more field measurements interleaved with de-charging.  The data are below.

All values in negative kiloVolts (-kV) as read from the hand-held meter, unless otherwise signed positive. Field values in kV/m can be obtained by scaling by ~30X.

where

Vc = raw voltage (kV) 1" above center of HR surface
Vr = raw voltage (kV) 1" above center-right (i.e. at a radius of +34cm/2, 0 degrees rotation angle) of HR surface
Vt = raw voltage (kV) 1" above center-top (i.e. at a radius of +34cm/2, 90 degrees rotation angle) of HR surface
Vl = raw voltage (kV) 1" above center-left (i.e. at a radius of +34cm/2, 180 degrees rotation angle) of HR surface
Vb = raw voltage (kV) 1" above center-bottom (i.e. at a radius of +34cm/2, 270 degrees rotation angle) of HR surface

and trial data were taken under the following conditions:

Trial 1 - Initial measure of electric field, HR side of optic, prior to application of FirstContact (FC), or TopGun deionizer (TG)
Trial 2 - same as trial 1, but 24 hours later, to see if there is some discharge in air (there is, ~25% lower than trial 1)
Trial 3 - After 3 min of TG deionization
Trial 4 - After 3 min of additional TG (6 min total)
Trial 5 - After 3 min of additional TG (9 min total)
Trial 6 - No additional TG, but after 3h delay
Trial 7 - FirstContact applied, then removed in a 2 min pull, with concurrent TG (2 min), plus additional 2 min TG thereafter
Trial 8 - After 2 min of additional TG (6 min total)
Trial 9 - After 3 min of additional TG (9 min total)

Individual measurements of the field could take 3-4 minutes to stabilize in the Voltmeter, i.e. values read out on the hand-held unit may have initially read hundreds of volts, falling and stabilizing to a seemingly stable number over the 3-4 minute time frame.

We wil repeat these measurements and include barrel and AR surface tests.  However, in consultation with C. Torrie and R. Abbott, and in order to be conservative in closing BSCs 1, 3 and 9, we opted to extend nominal few-minute TopGun blowing times for test masses and reaction masses to nine minutes total, i.e. 4.5 min for a given test mass HR face, 4.5 min for the AR gap, 4.5 min for the reaction mass upstream face, 4.5 min for the gap. We cycled between surfaces and gap in 1 minute intervals, summing to 9 min per optic.