I got the LLO test mass charge measuring script running here at LHO. There was an NDS error initially that Dave Barker magically made go away. Otherwise, the script required no modification other than some missing commas in the bias voltage definition. There is a timing error that seems to pop up at pseudo random times. In these cases, the excitation appears shifted from the oplev response by 9 sec. See UR_timing_err.pdf. Of all the ESD quadrants it most often occurs on the UR, though I saw it once on upper left. Not sure if it has something to do with the settings of the script. The way it is now, the timing error has appeared in 3 to 5 out of 20 measurements in each run. I have run the script in the current configuration 7 times.
In any case, I was able to get a 2 hour trend of the ESD charge. See ETMY_Charge_Trend_9Jan2015.pdf. The charge wanders a bit, but has some consistency.
The charge measuring script is:
.../SusSVN/sus/trunk/QUAD/Common/Scripts/ESD_UL_LL_UR_LR_charge_07-H1.py
The analysis script, which I am still updating for LHO is:
...SusSVN/sus/trunk/QUAD/Common/Scripts/ESD_UL_LL_UR_LR_analysis07_BrettTest.m
A lot of work has been done on HAM3-ISI for the past month. I'm trying to summarize here what we know and which path we should follow.
. We see a high Q peak around ~0.65Hz in all the local sensors (GS13+CPS) and all the DOFs, except RZ.
. This peak is present only when the Z sensor correction is ON. It doesn't matter if the Z sensor correction is coming from HEPI or the ISI, the peak is present when one of them is ON (see here and here). It doesn't seem to have any link with the X,Y sensor correction.
. The peak seems non-stationary. First of all, the peak is not the same if the sensor correction is done with the ISI or HEPI (see first attachment). Second of all, the amplitude and frequency of the peak vary with time (see second attachment and here).
. The problem doesn't come from the ground STS. We checked the electronic chain (swapping distribution chassis) and tried another ground instrument (see here). The problem is independant (see here).
. This is not mechanical/rubbing issue. We did a driven transfer function around this frequency with a perfect coherence/no sharp peak.
. The problem doesn't come from HEPI. We turned OFF the HEPI loops with no improvement (see here)
. We don't think it comes from a specific sensor. Everything looks fine wihtout sensor correction, plus the problem shows up in all the sensors (if it was , let's say, a CPS issue, it wouldn't appear in the GS13).
. It might come from the blend: by switching to a 750mHz blend, the peak seems to disappear. However, we know that the 750mHz blend is obsolete, so I wouldn't draw solid conclusions from that...
. It might come from the drive (see here)
Now it seems that the peak appears even when the sensor correction is OFF (see third attachment)! I don't know if it's a good or bad thing, but that's the first time we've seen that. The only thing I did this afternoon is switching blend filters on all DOFs and turning the Z sensor correction ON and OFF... The ground motion doesn't seem any different than usual.
. Keep investigate to see if the sensor correction is the cause of this peak
. Implementation of a slighty different blend in Z to see if it makes a difference.
. PR2 and MC2 have a mode at 0.67Hz. Could it be a weird coupling between the sensor correction and suspension? We'll try to turn the damping loops ON and OFF to see if it makes a difference
. The ISI model has been restarted before, but we haven't tried to restart the actual computer
We'll also take driven transfer functions of MC2 and PR2, to confirm their resonance Q is much lower that this feature. Further, we'll not only try an ON/OFF test of the SUS, but we'll trying *changing* the damping filters (something we want to do eventually anyways).
Since we rolled out the sensor chains from our investigation (I'll do a summary alog about that in a minute), I've looked at the actuation chain.
I took the transfer function between the actual ouput of the actuators (counts) and the related voltage read by the coil driver readback channels. This is the same information in different units, so except some color coming from the electronics, we shouldn't see any sharp peak in the transfer function. I did this exercise on HAM3 (HEPI Z sensor correction ON and OFF) and HAM2 (Sensor correction OFF) for the comparison.
The results are interesting. First I'm not sure I understand why we don't have a perfect coherence (noise?), but I don't think that's linked to our issue. More interesting, we can see a small peak around 0.66Hz on HAM3 when the sensor correction is OFF (which is not the case on HAM2).
This might be an indication that the problem is coming from the actuators.
With Kiwamu's blessing, I modifed the following LSC common files to ground all unused inputs of filter module with control parts
lsc/common/models/lscpsl.mdl
lsc/common/models/lscimc.mdl
These are common parts, so I have not committed the changes to SVN as this may break l1lsc. There appears to be a mismatch between the sites for these files which I'll let the ISC team resolve.
h1lsc0 now compiles with RCG2.9
The BSC-ISI sensor correction screens were updated based on Jeff's request (SEI aLOG #666). A screenshot of the new screens is attached. A link to the old screens is still available on the bottom right of the new ones. Jeff already gave his approval. I will wait for feedback from the other commissioners before commiting to the SVN.
Work was performed under WP #4998 which remains open so the same changes can be applied to the HAM-ISI and HEPI.
I wrote a script which compares a target's autoBurt.req file against its safe.snap file. This is a channel list check, verifying that the safe.snap has no more and no less channels compared to autoBurt.req. I am not checking channel values or channel read-only-status.
I ran the script on all the H1 models (100 of them). For models I manage (IOP and PEM) I fixed any mismatches. For any system which was missing a safe.snap, I created one by snapshotting the system.
The end result is that all safe.snaps are concurrent with the exception of:
certain SUS safe.snaps still retain the GUARD channels which were removed this Tuesday, Jeff K is going to resnap these this weekend
h1pslfss.snap is missing some DINCO channels, I'l work with PSL to resolve this
The psl fss fix was trivial, I have added the missing channel to h1pslfss_safe.snap by hand and committed to svn.
LVEA: Laser Hazard Observation Bit: Commissioning 08:00 Jodi & Rodica – In LVEA looking for IO parts 08:13 Manny – Removing test stand cables from LVEA west bay 08:30 Jodi – Out of LVEA 09:00 Jodi – In LVEA working with Rodica on IO 09:12 Hugh – In LVEA working on 3IFO HAM storage retrofits 09:18 Elli – Working on IOT2R table 09:20 Mitch – Going to LVEA for HAM 3IFO storage work 09:25 Filiberto – Going into LVEA to help Manny with removing cables from the west bay 09:29 Jodi – Out of the LVEA 10:30 Kyle – At End-X to start vent 10:31 Peter & Rick – Going to End-Y to work on PCal camera focus 10:31 Hugh & Mitch – Out of the LVEA 10:34 Travis – Going into the cleaning area to get tools for End-X vent 11:24 Doug – Checking TCS chiller alarm 11:49 Filiberto – Out of LVEA 11:49 Filiberto – Going to End-Y and End-X to check on TCS equipment 12:00 Electrical inspector on site to inspect work at VPW 12:00 Manny – Out of the LVEA 12:05 Doug – Out of the LVEA 12:08 Jodi & Rodica – Out of the LVEA 12:35 Jodi & Rodica – Going to Mid-Y to check on 3IFO IO parts 13:17 Filiberto – Back from end stations 13:20 Jodi & Rodica – Back from Mid-Y 13:36 Sudarshan – Working on PCal cameras at End-Y 13:45 Kyle & Gerardo – At End-X working on BSC9 vent 14:05 Corey – Going to squeezer bay 14:40 Kyle & Gerardo – Back from End-X 14:56 Dale – Taking tour into LVEA 14:57 Bubba & Kyle – Going to End-X to remove BSC9 door 14:57 Travis & Betsy – Gathering tools and heading to End-X for EQ stop work 15:17 Elli – Going to IOTC2R to check on fan noise 15:35 Corey – Out of the squeezer bay 15:43 Betsy – Going to End-X for Quad EQ stop work
Schedule for BSC09 "Abbreviated Vent & Closure"- II: Earth Quake Stop Adjustment of EX PUM Stage, January 9, 2015
Approved work to be done:
Friday, Jan. 9
AM
PM
The last step is at the time and discretion of the Vacuum crew.
Conlog searches unavailable for duration.
Done. Seems to have been successful. I have restarted the crontask for reporting frequently changing channels every hour.
Schedule for BSC09 "Abbreviated Vent & Closure": Earth Quake Stop Adjustment of EX Test Mass, January 7-8, 2015
Approved work to be done:
Wednesday, Jan. 7
Thursday, Jan. 8
The last step is at the time and discretion of the Vacuum crew.
Alexa, Thomas, Evan
Over the past 24 hours, the alignment slider values of MC1, MC2, and MC3 have been changed. (Perhaps the WFS offloading script was run?) However, these new alignments weren't saved.
This morning, we changed the IMC guardian from LOCKED to DOWN to do a scattering test with the Y arm. When we brought the IMC guardian back to LOCKED, we found that the WFSs would cause MC2 trans to ramp continually in a saw-tooth fashion, from about 100 % nominal transmission to less than 50 % transmission.
Eventually Alexa figured out it was because the guardian had restored the old, previously saved alignments for MC1, MC2, and MC3, and this had caused the beam to fall off WFS A and WFS B. Dialing in the new slider values for MC1, MC2, and MC3 (and then saving them on the IFO ALIGN screen) made the WFS loops work again.
The TFs I took this morning show that the rubbing is back on the ETMx main chain now that we are back down to 7 Torr in pressure. The rubbing TF looks exactly like the rubbing TF we had before I opened up the gap of the EQ stops around the test mass. While we knew that the rubbing could have been possibly due to the barrel stops around the test mass OR the PUM mass above the test mass, we assumed it was specifically the stops around the test mass since those were the only ones adjusted during the Dec vent. I did not check the PUM stops (or any other stops further up the chain) yesterday. Darn it, I should have! The temp+buoyancy effect is the same on the PUM as it is on the Test mass, so the gap of the EQ stops should be bigger around the barrel there too.
So, we are re-venting today to now:
- Inspect other EQ stops, look for outliers in gap settings further up the main chain
- Open up the gap around the barrel of the PUM to the 1mm (top stops) and 1.5, (bottom stops)
We'll employ the same stripped down version of the entry/exit checklist as yesterday, namely to inspect and DI Nitrogen blow the HR face of the ETMx on the way out.
The reaction chain TFs continue to look healthy, so no problems there.
Attached here is a trend of ETMx Vertical and Pitch relative to the pressure and temp of the chamber/VEA, with some annotations of know and unknown events.
Note, the vertical seems to stall out as expected after the buoyancy sag, however the pitch trend turns over for some unknown reason at ~1:30pm PT yesterday for a yet uncorrelated reason. (Kyle paused the roughing pump for the evening at ~4pm PT, and no one claims to be doing anything with the ETMx or out at Ex. I've confirmed that the pitch offsets were not changed during any of this trend.)
During today's vent I took a few ETMx TFs at various chamber pressures. At 270 Torr, the suspension still showed rubbing. Now at 550 Torr, the suspension is free of rubbing, however the vertical trend shows that it is only ~10% into it's buoyancy "unsag". It looks like there is a wiggle at ~400 Torr which may indicate when it came fully free from the rubbing stop.
Keita, Elli
Continuing with Evan's and my attempts to lock the auxiliary laser or IOT2 to the PSL with a frequency offset in order to continue the PRC measurements started by Paul Fulda...
Today Keita and I got the auxiliary laser locked for periods of ~20s. Then PLL servo control signal would reach its maximum of 5V and the lock would drop. The beat note between the aux laser and the PSL was continually drifting in one direction- this may have been due to temperature instability on the table (the fan was off to start with and then I turned it on for a while when adjusting the Aux laser temperature.) I am watching
The PLL is controlled by an SR560 servo controller with 10Hz low-pass filter cutoff and 500 gain. A 7dBPm signal is demodulated with the beat signal between the PSL and the auxiliary laser for the error signal.
Today the frequency drift of the auxiliary laser was much slower, I was able to lock it the the PSL and sweep it across 2MHz a few times before the SR560 output railed. I moved the 1611 photodiode back a few cm to where the beam is more tightly focused, increasing the beat note power from -42dBm to -33dBm.
I will switch from using the SR560 to the New Focus LB1005 servo controller. I also plan to add an amplifier with gain 25 output voltage +/- 125 V after the LB1005, to increase the voltage driving the PZTs.
10Hz single pole, combined with a integration of PLL, means that the OLG phase is already -135deg at 10Hz, so the UGF could not possibly be much more than 10Hz.
But it easily acquires lock and stays locked until the output rails. Anyway this is just the first setting that worked while I was changing things non-systematically. Students will improve it to perfection.
As for the 5V output rail of SR560, using something else to go 10V would only be a marginal improvement as the drift is big. The frequency shifted by at least 200MHz while I was watching and it was not slowing down.
According to Evan, offloading to temperature was attempted without much success in the past, but it seems to me that it is still the way to go.
In preparation of allowing a larger offset frequency, I gave Elli a ZFM-2 that is a level 7 mixer for 1MHz to 1GHz.
We should have a New Focus/Newport LB1005 in the lab which is a proper PI controller for laser locking. We also should have +/-120V PZT driver for the JDSU NPROs.
The sensor correction is installed and ON on all the chambers, but with a nominal matching gain of 1.
I made a small script to calculate the correct matching gain for X, Y, Z. The script works only for the HAM-ISIs for now, but it will be pretty easy to adapt. What it does:
- Grab the ground seismometer and GS13 data in X, Y, Z.
- Calculate the ASDs and calibrate them in m/rt(Hz).
- Calculate the GS13 over Ground ratio.
- Take the mean of the ratio for a [0.1Hz 0.4Hz] bandwidth.
Before running this process, the ISI needs to be in High blend mode (750mHz) on all DOFs with sensor correction OFF.
I've done this exercise for HAM4-ISI. The numbers seem a little small (Gain for X: 0.8784, gain for Y: 0.8702, gain for Z: 0.8574) but brings some improvement (see plot attached).
This has been done during the day, we might want to do it overnight for better tuning.
I'll do the other chambers ASAP. The script that I made is for now commited in the HAM4 folder of the svn:
/ligo/svncommon/SeiSVN/seismic/HAM-ISI/H1/HAM4/Misc/gain_matching_calculation.m
After some feedback, I rearranged the script and commited it into:
/ligo/svncommon/SeiSVN/seismic/HAM-ISI/Common/Misc/HAM_gain_matching_calculation.m
What it does now:
HAM_gain_matching_calculation(IFO,Chamber,start_time,duration)
. Grab the ground seismometer and GS13 data in X, Y, Z from start_time to start_time+duration.
. Calculate and plot the calibrated TF between Ground and GS13
. Take the mean of the amplitude for a [0.1Hz and 0.4Hz] bandwidth
Remember. before running this process, the ISI needs to be in High blend mode (750mHz) on all DOFs with sensor correction OFF.
I put HAM4, 5, 6 in this configuration last night and calculated the new matching gains.
HAM4 | HAM5 | HAM6 | |
X | 0.8730 | 0.9280 | 0.96 |
Y | 0.8619 | 0.9161 | 0.9496 |
Z | 0.8487 | 0.9604 | 1.0189 |
I'll put this new numbers in the MEDM screens in a minute.
Sheila, Jeff, Thomas
We changed the H1:SUS-$(OPTIC)_ODC_CHANNEL_BITMASK from 1 to 0 for all the LOCK States for the following optics:
Added the changes to the safe.snap files in userapps, by hand, for the respective optics above. Also committted to SVN.
J. Kissel, R. McCarthy At my request, after seeing that the EY BSC 10 vacuum pressure has dropped below 1e-5 [Torr] (see attached trend), Richard has turned on the H1 SUS ETMY ESD at ~2pm PST. I'm continuing to commission the chain, and will post functionality results shortly. Also -- I've found the ESD linearization force coefficient (H1:SUS-ETMX_L3_ESDOUTF_LIN_FORCE_COEFF) to be -180000 [ct]. I don't understand from where this number came, and I couldn't find any aLOGs explaining it. I've logged into to LLO, their coefficient is -512000 [ct]. There's no aLOG describing their number either, but I know from conversations with Joe Betz in early December 2014 that he installed this number when the LLO linearization was switched from before the EUL2ESD matrix to after. When before the EUL2ESD matrix the coefficient was -128000 = - 512000/4 so we was accounting for the factor of 0.25 in EUL2ESD matrix. I suspect that -128000 [ct] came from the following simple model of longitudinal force, F_{tot} on the optic as a result of the quadrant's signal voltage, V_{S} and the bias voltage V_{B}, (which we know is incomplete now -- see LLO aLOG 14853): F_{tot} = a ( V_{s} - V_{B} )^2 F_{tot} = a ( V_{s}^2 - 2 V_{s} V_{B} + V_{B}^2) F_{lin} = 2 a V_{s} V_{B} where F_{lin} is the linear term in the force model, and a< is the force coefficient that turns whatever units V_{S} and V_{B} are in ([ct^2] or [V_{DAC}^2] or [V_{ESD}^2]) into longitudinal force on the test mass in [N]. I *think* the quantity (2 a V_{B}) was mistakenly treated as simply (V_{B}) which has always been held at 128000 [ct] (or the equivalent of 390 [V] on the ESD bias pattern) and the scale factor (2 a) was ignored. Or something. But I don't know. So I try to make sense of these numbers below. Looking at what was intended (see T1400321) and what was eventually analytically shown (see T1400490), we want the quantity F_{ctrl} ------- 2 k V_{B}^2 to be dimensionless, where F_{ctrl} is the force on the optic caused by the ESD. Note that comparing John / Matt / Den's notation against Brett / Joe / my notation, k = a. As written in T1400321, F_{ctrl} was assumed to have units of [N], and V_{B} to have units of [V_{esd}], such that k has units of [ N / (V_{esd}^2) ], and it's the number we all know from John's thesis, k = a = 4.2e-10 [N/V^2]. We now know the number is smaller than that because of the effects of (we think) charge (see, e.g. LHO aLOG 12220, and again LLO aLOG 14853). In the way that the "force coefficient" has been implemented in the front end code -- as an epics variable that comes into the linearization blockas "Gain_Constant_In," (see attached) -- I think the number magically works out to be ... close. As implemented, the linearized quadrant's signal voltage is as shown in Eq. 13 of T1400490, except that the EPICs record, we'll call it G, is actually multiplied in V_{S} = V_{C} + V_{B}(1 - sqrt{ 2 [ (F_{ctrl} / V_{B}^2) * G + 1 + (V_{C}/V_{B}) + (V_{C}/V_{B})^{2} * 1/4 ] )} Note, that we currently have all of the effective charge voltages set to 0 [ct], so the equation just boils down to the expected V_{S} = V_{B}(1 - sqrt{ 2 [ (F_{ctrl} / V_{B}^2) * G + 1] )} which means that G == 1 / (2 k) or k = 1 / (2 G) and has fundamental dimensions of [V_{esd}^2 / N]. So let's take this "force coefficient," G = -512000 [ct], and turn into fundamental units: G = 512000 [ct] {{LLO}} * (20 / 2^18) [V_{dac} / ct] * 40 [V_{esd} / V_{dac}] * 1 / (V_{B} * a) [(1 / V_{esd}) . (V_{esd}^{2} / N)] G = 9.5391e9 [V_{ESD}^2 / N] ==> k = 5.37e-11 [N/V_{ESD}^2] {{LLO}} where I've used V_{B} = 400 [V_{esd}] and the canonical a = 4.2e-10 [N/V_{esd}^2] originally from pg 7 of G0900956. That makes LLO's coefficient assume the actuation strength is a factor of 8 lower from the canonical number. For the LHO number, G = 180000 [ct] {{LHO}} * (20 / 2^18) [V_{dac} / ct] * 40 [V_{esd} / V_{dac}] * 1 / (V_{B} * a) [(1 / V_{esd}) . (V_{esd}^{2} / N)] G = 3.2697e9 [V_{ESD}^2 / N] ==> k = 1.53e-10 [N/V_{ESD}^2] {{LHO}} Which is within a factor of 3 lower, and if the ESD's as weak as we've measured it to be it may be dead on. So maybe whomever stuck in 180000 is much smarter than I. For now I leave in 180000 [ct], which corresponds to a force coefficient of a = 1.53e-10 [N/V_{ESD}^2].
B. Shapiro, J. Kissel As usual, two heads are better than one when it comes to these nasty dealings with factors of two (go figure). Brett has caught a subtlety in the front-end implementation that further makes it different from the analytical approach used in T1400321 and T1400490. In summary, we now agree that the LLO and LHO EPICs force coefficients that have been installed are closer to the measured values by a factor of 4, i.e. G = 512000 [ct] ==> k = 2.0966e-10 [N/V^2] {{LLO}} and G = 180000 [ct] ==> k = 6.1168e-10 [N/V^2] {{LHO}} which means, though they still differ from the canonical value (from pg 7 of G0900956) k = 4.2e-10 [N/V^2] {{Canonical Model}} and what we've measured (including charge) (see LHO aLOG 12220, and LLO aLOGs 14853 and 15657) k = 2e-10 +/- 1.5e-10** [N/V^2] {{Measured}} they're much closer. **I've quickly guesstimated the uncertainty based on the above mentioned measurement aLOGs. IMHO, we still don't have a systematic estimate of the uncertainty because we've measured it so view times, in so many different ways, infrequently, and with the ion pumps still valved in, and each test mass has a different charge mean, charge location, and charge variance. Here's how the aLOG 15809 math should be corrected: The F_{ctrl} and k = a in the analytic equations is assumed to be for full longitudinal force. However, as implemented in the front end, the longitudinal force F_{ctrl} has already been passed through the EUL2ESD matrix, which splits transforms into quadrant basis force F_{ii}, dividing F_{ctrl} by 4. The EPICs force coefficient, G, should therefore *also* be divided by 4, to preserve the ratio F_{ctrl} F_{ii} ------- = ------------ 2 k V_{B}^2 2 k_{ii} V_{B}^2 inside the analytical linearization algorithm. In other words, as we've physically implemented the ESD, on a quadrant-by-quadrant basis, F_{ctrl} = F_{UL} + F_{LL} + F_{UR} + F_{LR} where F_{ii} = k_{ii} (V_{ii} - V_{B})^2 and k_{ii} = k / 4 = a / 4. As such, the implemented front-end equation V_{ii} = V_{B}(1 - sqrt{ 2 [ (F_{ii} / V_{B}^2) * G + 1] )} means that G == 1 / 2 k_{ii} = 2 / k = 2 / a and still has the fundamental units of [V_{esd}^2 / N]. So nothing changes about the above conversation from G in [ct] to G in [V_{esd}^2 / N], its simply that the conversion from G to the more well-known analytical quantity k was off by a factor of 4, G = 512000 [ct] {{LLO}} * (20 / 2^18) [V_{dac} / ct] * 40 [V_{esd} / V_{dac}] * 1 / (V_{B} * a) [(1 / V_{esd}) . (V_{esd}^{2} / N)] G = 9.5391e9 [V_{ESD}^2 / N] ==> k = 2.0966e-10 [N/V_{ESD}^2] {{LLO}} where I've used V_{B} = 400 [V_{esd}] and the canonical a = 4.2e-10 [N/V_{esd}^2] originally from pg 7 of G0900956. That makes LLO's coefficient assume the actuation strength is a factor of 2 lower from the canonical number, pretty darn close to the measured value and definitely within the uncertainty. For the LHO number, G = 180000 [ct] {{LHO}} * (20 / 2^18) [V_{dac} / ct] * 40 [V_{esd} / V_{dac}] * 1 / (V_{B} * a) [(1 / V_{esd}) . (V_{esd}^{2} / N)] G = 3.2697e9 [V_{ESD}^2 / N] ==> k = 6.1168e-10 [N/V_{ESD}^2] {{LHO}} both of which are closer to the measured value as described above.
N. Smith, (transcribed by J. Kissel) Nic called and fessed up to being the one who installed the -180000 [ct] force coefficient at LHO. Note -- this coefficient only is installed in ETMX, the ETMY coefficient is still the original dummy coefficient of 1.0 [ct]. He informs me that this number was determined *empirically* -- he drove a line at some frequency, and made sure that the requested input amplitude (driven before the linearization algorithm) was the same as the requested output amplitude (the MASTER_OUT channels) at the that frequency, with the linearization both ON and BYPASSED. He recalls measuring this with a DTT session, not just looking at the MEDM screen (good!). Why does this work out to be roughly the right number? Take a look at the front-end equation again: V_{ii} = V_{B}(1 - sqrt{ 2 [ (F_{ii} / V_{B}^2) * G + 1] ) } ) and let's assume Nic was driving V_{ii} at a strength equal and opposite sign to the bias voltage V_{B}. With the linearization OFF / BYPASSED, V_{ii} = - V_{B} Duh. With the linearization in place, V_{ii} = - V_{B} = - V_{B} (1 - sqrt{ 2 [ (F_{ii} / V_{B}^2) * G + 1] ) } ) so we want the quantity (1 - sqrt{ 2 [ (F_{ii} / V_{B}^2) * G + 1] )} = 1 which only happens if (F_{ii} / V_{B}^2) * G = 1. If Nic wants to create a force close to the maximum, it needs to be close to the maximum of F_{ii,max} = 2 k_{ii} V_{B}^2, which makes 2 k_{ii} * G = 1 or G = 1 / (2 k_{ii}) = 2 / k which is the same result as in LHO aLOG 15873. Granted, it's late and I've waved my hands a bit, but this is me trying to justify why it feels like it makes sense, at least within the "factor of two-ish" discrepancy between the canonical value and the accepted measurements of the right number.
I've summarized this exploration of Linearization Science in G1500036.