S. Dwyer, J. Kissel, B. Weaver, R. Weiss

Sheila, Betsy, Rai, and myself measured the electric field inside BSC10 (ETMY's) chamber yesterday using an electric field meter developed by Rai (see T1700569). The experimental setup is show in the collection of pictures attached below.

We had three goals with the measurement:
(1) Characterize the broadband field.
(2) Confirm that the BSC-ISI's capacitive position sensors (CPS) don't create an excessive field inside the chamber (as was the case at MIT)
(3) Use the data to decide whether we need to vibrationally isolate the future in-chamber field sensor (see D1700365).

I attach several sets of plots comparing different measurement scenarios:
- In the lab, on the EE bench, immediately after calibration
- In chamber, with the sensor sensitive to the longitudinal direction (i.e. +/- Y global), exposed to the entire chamber that is sealed with a foil door.
- In chamber, in the longitudinal direction, shielded with a crude foil Faraday cage about 4 inches larger than the sensor in all directions.
- In chamber, in the transverse direction (i.e. +/- X global), exposed to the full chamber, sealed with foil
- In chamber, in the transverse direction, shielded with a foil smaller faraday cage, but the data is junk because I forget to reset the input range of the SR785.
The above data sets are shown in the set of plots at various levels of zoom in frequency. I also attach the raw ASCII data for your perusal if need be.

EDIT: The previous posting of this aLOG made all statements about and plots of electric field strength in V_{rms}.Hz^{-1/2}, but that was merely taking the raw voltage output of the SR785. The SI units for electric field strength is Volts.meter^{-1}. As Rai comments in LHO aLOG 40597, the calibration of this voltage into field strength is Volts / (Volt/m) = 2.1e-2. I've now calibrated the data both in the posted plots and text files and corrected all statements. Apologies for the confusion.

The data is collected from 1 Hz to 50 kHz, which completes goal (1). However, is not as satisfying as one could hope -- namely the crude shielding did not cleanly isolate / identify which spectral features were of the chamber or that of the sensor. But we think it's enough to accomplish goals (2) and (3).

Yet -- We can confirm that the BSC-ISI's CPS clock frequency at 24.82 kHz has an amplitude of only (at worst) 6e-5 (V.m^{-1})_{rms}.Hz^{-1/2} [with an 8 Hz bin width], which is not the dominant field strength inside the chamber.

At low frequency, given that we see many broad features (e.g. at ~25, ~55, ~80, ~130, ~191 Hz) that they change with location (lab vs. chamber) and direction (longitudinal vs transverse) we assume are acoustic. Rai suggests this is enough evidence to convince him that the future field meter needs to be vibrationally isolated, i.e. mounted onto Stage 2 of the BSC-ISI.

As Rai also suggests in LHO aLOG 40597, we should continue exploring the chamber with a few extra scenarios, namely attempting to turn off successive electronics, and/or try to mount the sensor on some sort of small isolation table.

A final note on the instrument -- it is sensitive to time-dependent magnetic fields (also present in the chamber) as well as static electric fields. Further the instrument and the cable readout are sensitive to acoustic vibration. During the measurement, all persons were quiet and still, but the end station itself is LOUD due to purge air and clean rooms. Further potential exploratory measurements besides turning things off would be to intentionally drive either a magnetic field or acoustic excitation near the instrument to further shake down the noise budget of the attached spectra. All exercises will be balanced against person power, time, and priority in conversation with all involved.

For now, we've left the sensor in BSC10 with it sealed with the foil door. We (Sheila, Betsy, and I) hope to get back to it on Wed/Thu/Fri.