We can limit the EPICS values giving some parameters to the channels. cf https://epics.anl.gov/tech-talk/2012/msg00147.php

But this does not solve the MC1 issue. Only we can do right now is to make the output resister half, for example.

I will be at the 40m, in the Clean and bake lab today from ~9am to ~3pm.

Summary

For the initial phase of BHD testing, we recently discussed whether the mode-matching telescopes could be built with 100% stock optics. This would allow the optical system to be assembled more quickly and cheaply at a stage when having ultra-low loss and scattering is less important. I've looked into this possibility and conclude that, yes, we do have a good stock optics option. It in fact achieves comprable performance to our optimized custom-curvature design [ELOG 15357]. I think it is certainly sufficient for the initial phase of BHD testing.

Vendor

It turns out our usual suppliers (e.g., CVI, Edmunds) do not have enough stock options to meet our requirements. This is for two reasons:

• For sufficient LO1-LO2 (AS1-AS4) Gouy phase separation, we require a very particular ROC range for LO1 (AS1) of 5-6 m (2-3 m).
• We also require a 2" diameter for the suspended optics, which is a larger size than most vendors stock for curved reflectors (for example, CVI has no stock 2" options).

However I found that Lambda Research Optics carries 1" and 2" super-polished mirror blanks in an impressive variety of stock curvatures. Even more, they're polished to comprable tolerances as I had specificied for the custom low-scatter optics [DCC E2000296]: irregularity < λ/10 PV, 10-5 scratch-dig, ROC tolerance ±0.5%. They can be coated in-house for 1064 nm to our specifications.

From modeling Lambda's stock curvature options, I find it still possible to achieve mode-matching of 99.9% for the AS beam and 98.6% for the LO beam, if the optics are allowed to move ±1" from their current positions. The sensitivity to the optic positions is slightly increased compared to the custom-curvature design (but by < 1.5x). I have not run the stock designs through Hang's full MC corner-plot analysis which also perturbs the ROCs [ELOG 15339]. However for the early BHD testing, the sensitivity is secondary to the goal of having a quick, cheap implementation.

Stock-Part Telescope Designs

The following tables show the best telescope designs using stock curvature options. It assumes the optics are free to move ±1" from their current positions. For comparison, the values from the custom-curvature design are also given in parentheses.

AS Path

The AS relay path is shown in Attachment 1:

• AS1-AS4 Gouy phase separation: 71°
• Mode-matching to OMC: 99.9%

LO Path

The LO relay path is shown in Attachment 2:

• LO1-LO2 Gouy phase separation: 67°
• Mode-matching to OMC: 98.6%

Ordering Information

I've created a new tab in the BHD procurement spreadsheet ("Stock MM Optics Option") listing the part numbers for the above telescope designs, as well as their fabrication tolerances. The total cost is $2.8k + the cost of the coatings (I'm awaiting a quote from Lambda for the coatings). The good news is that all the curved substrates will receive the same HR/AR coatings, so I believe they can all be done in a single coating run.

We consider the astigmatism effects of the stock options. The conclusions are:

1. For the AS path, the stock should work fine for the phase-one of BHD, if we could tolerate a few percent MM loss. The window for length adjustment to achieve >99% MM for both s and t is only 1 mm for 1% RoC error (compared to ~ 1 cm in the customized case). 

2. The LO path seemed tricky. As LO3 & LO4 are both significantly curved (RoC<=0.5 m), the non-zero angle of incidence makes the astigmatism quite sever. For the t-plane the nominal MM can be 0.98, yet for the s-plane, the nominal MM is only 0.72. We could move things around to achieve a MM ~ 0.85, which is probably fine for the phase-one implementation but not long term. 

Details:

Attachments 1-3 are for the AS path; 4-6 are for the LO path. 

1 & 4. Marginalized MM distribution for the AS/LO paths. Here we assumed 5 mm positional error and 1% fractional RoC error. Due to the astigmatism, the nominal s-plane MM is only 0.72 for the LO path. 

2 & 5. Scattering plots for the AS/LO paths. We color coded the points as the following: pink: MM>0.99; olive: 0.98<MM<=0.99; grey: MM<=0.98. For the AS path, MM is mostly sensitive to the AS1 RoC and can be adjusted by changing AS1-AS3 distance. For the LO path, the LO3 RoC and LO3-LO4 distance are most critical for the MM. 

3 & 6. Assuming +- 1% AS1 (LO3) fractional RoC error, how much can we compensate for it using AS1-AS3 (LO3-LO4) distance. For the AS path, there exists a ~ 1 mm window where the MM for s and t can simultaneously > 99%. For the LO path, the best we can do is to make s and t both ~ 85%. 

Can you describe the mode matching  in terms of the total MM? Is MM_total = sqrt(MM_vert * MM_horiz)?

MM_total = (MM_vert + MM_horiz) / 2. 

The large astigmatic MM loss in the LO case is mainly due to the strong LO4 curvature (R=0.15m) with a 10 deg AOI. I looked again at whether LO1 could be increased from R=5m to the next higher stock value of 7.5m, as this would allow weaker curvatures on LO3 and LO4. However, no, that is not possible---it reduces the LO1-LO2 Gouy phase separation to only 18 deg.

There is, however, a good stock-curvature option if we want to reconsider actuating LO4 instead of LO2 (attachment 1). It achieves 99.2% MM with the OMCs, allowing positions to vary +/-1" from the current design. The LO1-LO4 Gouy phase separation is 72 deg.

Alternatively, we could look at reducing the AOI on LO3 and LO4 (keeping LO1-LO2 actuation).

Summary:

Around 5pm local time, the three vertex FEs crashed. AFAIK, no one was in the lab or working on anything CDS related, so this is worrying.

Details:

• Reboot script was used to bring all FEs back - only soft reboots were required.
• The IMC and arms can now be locked.
• I think combination of burt + SDF would have reverted all the settings as they should be, but if something appears off, it could be that some EPICS value didn't get reset correctly.

Hmm? T1300364 suggests MM_total = Sqrt(MM_Vert * MM_Horiz)

I will be in the Clean and Bake lab today from 9:30am to 4pm.

I don't think we ever discussed why the angular RMS of the ETMs is so much higher than the ITMs. Maybe that's a separate matter because, even assuming the worst case, the actuation range requirement is

(0.82 μrad RMS) x (15 μrad/μrad) x (10 safety factor) = 0.12 mrad

which is still only order 1% of the pitch/yaw pointing range of the Small Optic Suspensions, according to P1600178 (sec. IV. A). Can we check this requirement off the list?

Using the updated AOI's for the LO path: (4.8, 47.9, 2.9, 4.5) deg for (LO1, LO2, LO3, LO4), we obtain the following results. 

First two plots are scattering plots for the t and s planes, respectively. Note that here we have changed to 0.5% fractional RoC error and 3 mm positional error. We have also changed the meaning of the colors: pink:MM>0.98; olive 0.95<MM<=0.98, and grey MM<=0.95. It seems that both planes would benefit statistically if we make the LO3-LO4 distance longer by a few mm. 

We also consider how much we could compensate for the MM error in the last plot. We have a few mm window to make both planes better than 0.95. 

I will be in the Clean and Bake lab from 10am to 4pm today. I will also replace an empty N2 cylinder.

After further astigmatism/tolerance analysis [ELOG 15380, 15387] our conclusion is that the stock-optic telescope designs [ELOG 15379] are sufficient for the first round of BHD testing. However, for the final BHD hardware we should still plan to procure the custom-curvature optics [DCC E2000296]. The optimized custom-curvature designs are much more error-tolerant and have high probability of achieving < 2% mode-matching loss. The stock-curvature designs can only guarantee about 95% mode-matching.

Below are the final distances between optics in the relay paths. The base set of distances is taken from the 2020-05-21 layout. To minimize the changes required to the CAD model, I was able to achieve near-maximum mode-matching by moving only one optic in each relay path. In the AS path, AS3 moves inwards (towards the BHDBS) by 1.06 cm. In the LO path, LO4 moves backwards (away from the BHDBS) by 3.90 cm.

AS Path

LO Path

I will be in the Clean and Bake lab today from 11am to 4pm.

There appears to have been some sort of vacuum failure.

ldas-pcdev1 was down, so the summary pages weren't being generated. I have now switched over to ldas-pcdev6. I suspect some forepump failure, will check up later today unless someone else wants to take care of this.

There was no interlock action, and I don't check the vacuum status every half hour, so there was a period of time last night there was high circulating power in the arm cavities when the main volume pressure was higher than nominal. I have now closed the PSL shutter until the issue is resolved.

Summary:

It looks like the main vacuum interlock was tripped due to a serial communication error from the TP2 controller. With Rana/Koji's permission, I will open V1 and expose the main volume to TP1 again (#2 in last section).

Details:

• The vacuum interlock log file at /opt/target/vac.log on c1vac suggests that the interlock was tripped because "TP2 is too warm".
• Looking back at the diagnostics channels, it looks like the TP2 temperature channel registered a rise in temperature of >30 C in <0.2 seconds, see Attachment #1 - seems highly unlikely, probably some kind of glitch in the serial communication? This particular pump is relatively new from Agilent (<2 years installed I think)
• The PSL shutter was automatically closed at ~1150 am today, see Attachment #2. There is some EPICS logic on c1psl (Acromag server) that checks if C1:Vac-P1a_pressure is greater than 3 mTorr (or greater than 500 Torr for in-air locking of the IMC), in which case it closes the shutter, so this seems consistent with expectations.

Recommended course of action:

1. Code in some averaging in the interlock code, so that the interlock isn't triggered on some unphysical glitch like this. As shown in Attachment #3, this has been happening for the past 24 hours (though not before, because the interlock wasn't tripped). Probably need the derivative of the temperature as well, and the derivative should be less than 5 C/s or something physical (in addition to the temperature being high) for the interlock to trip.
2. Re-open V1 to pump down the main volume to nominal pressure so that the interferometer locking activity can resume.
   • One option in the interim is to bypass the TP2 temperature interlock condition.
   • The pressure-based interlocks are probably sufficient to protect the main volume / pumps during the nominal operations - the temperature interlocks are mainly useful during the pumpdown where the TPs have a large load, and so we want to avoid over-stressing them.

Summary:

The pumpspool UPS has its "Replace Battery" indicator light on. Might be a good chance to change the UPS, but at the very least, we should put in fresh batteries (last replacement was in Aug 2017).

I'll say this again - the pumpspool area is noisier than I remember it being, I think one / both of the roughing pumps backing TP2 / TP3 need tip-seal replacements.

BTW, EX is 5C hotter than EY, by virtue of the tarnac outside? In fact, judging by Steve's thermometers, EX reports a 12C swing in 24 hours between 30 C and 18 C (so almost no temperature control) while EY reports a 5C swing between 20 and 25 C. This is borne out by the ETM Oplev data I think...

1. I agree that it's likely that it was the temp signal glitch.
Recom #2: I approve to reopen the valves to pump down the main volume. As long as there is no frequent glitch, we can just bring the vacuum back to normal with the current software setup.

2. Recom #1 is also reasonable. You can use simple logic like if we register 10 consecutive samples that exceed the threshold, we can activate the interlock. I feel we should still keep the temp interlock. Switching between pumping mode and the normal operation may cause unexpected omission of the interlocks when it is necessary.

3. We should purchase the UPS battery / replacement rotary TIP seal. Once they are in hand, we can stop the vacuum and execute the replacement. Can one person (who?) accomplish everything with some remote help?

4. The lab temp: you mean, 12degC swing with the AC on!?

 

I will be in the Clean and Bake lab today from 12pm to 4pm.

Jon and Koji remotely supported Jordan's resetting the TP2 controller.

From the operator's console in front of the vac rack:

1. Open a terminal window (click the LXTerminal icon on the desktop)
2. Type "control" + enter to open the vac controls screen
3. Toggle all the open valves closed (edit by KA: and manually close RV2 by rotating the gate valve handle )
4. Turn OFF TP2 by clicking the "Off' button. Make sure the status changes and the rotation speed falls to zero (you'll also hear the pump spinning down) 
5. The other pumps (TP1, TP3) can be left running
6. Once TP2 has stopped spinning, go to the back of the rack and locate the ethernet cable running from the back of the TP2 controller to the IOLAN server (near the top of the rack). Disconnect and reconnect the cable at each end, verifying it is firmly locked in place.
7. From the front of the rack, power down the TP2 controller (I don't quite remember for the Agilent, but you might have to move the slider on the front from "Remote" to "Local" first)
8. Wait about 30 seconds, then power it back on. If you had to move the slider to shut it down, revert it back to the "Remote" position.
9. Go back to the controls screen on the console. If the pump came back up and is communicating serially again, its status will say something other than "NO COMM"
10. Turn TP2 back on. Verify that it spins up to its nominal speed (66 kRPM)
11. At this point you can reopen any valves you initially closed (any that were already closed before, leave closed)

TP2 was stopped and at this moment the glitches were gone. Jordan powercycled the TP2 controller and we brought up the TP2 back at the full speed.
However, the glitches came back as before. Obviously we can't go on from here, and we've decided to stop the recovery process here today.

- We left TP1/2/3 running while the valves including RV2 were closed.

- When Jordan is back in the lab next week, we'll try to use TP3 as the backing of TP1 so that we can resume the main volume pumping.

- Currently, TP3 does not have interlocking and that is a risk. Jon is going to implement it.

- Meanwhile, we will try to replace the controller of TP2. We are supposed to have this in the lab. Ask Chub about the location.

- Once we confirm the stability of the diagnostic signals for TP2, we will come back to the nominal pumping scheme.

I still don't understand why restoring the vacuum is contingent on this functionality working. All the TPs have their own internal logic to shutdown the pump if some damage threshold is exceeded. Plus, we have the pressure-sensor based interlocks to protect the main volume as well as pumps. While the extra redundancy from the readbacks from the controller is useful, clearly it isn't the first line of defense?

The main volume pressure is currently ~10mTorr. If we pump down before this reaches 500mTorr, the procedure is pretty straightforward. Otherwise, we have to do the dance with the manual throttling valve (judging by current rate of increase, unlikely to exceed this over the weekend, but I lose IFO time).

Obviously I don't want to rush this and have some permanent damage, so I'll stay out of this unless otherwise instructed.

The vacuum safety policy and design are not clear to me, and I don't know what the first and second defense is. Since we had limited time and bandwidth during the remotely-supported recovery work today, we wanted to work step by step.

The pressure rising rate is 20mtorr/day, and turning on TP3 early next week will resume the main-volume pumping without too much hustle. If you need the IFO time now, contact with Jon and use backing with TP3.

Didn't mean to sound whiny. I will wait until the vacuum team tells me it is okay.

I will be at the 40m today at 10am to deliver optics to Downs and to replace the TP2 controller.

ITMU01 / ITMU02 as well as the five E1800089 mirrors came back to the 40m. Instead, the two ETM spares (ETMU06 / ETMU08) were delivered to GariLynn.
Jordan worked on transportation.

Note that the E1800089 mirrors are together with the ITM container in the precious optics cabinet.

[Jon, Jordan, Koji]

Today Jordan reconfigured the vac system to allow pumping of the main volume resume, with Jon and Koji remotely advising. All clear to resume normal IFO activities. However, the vac system is operating in a temporary configuration that will have to be reverted as we locate replacement components. Details below.

Procedure

Since serial readback of the TP2 controller seems to be failing, we reconfigured the system with TP3 now backing for TP1. TP2 was valved off (at V4) and shut down until we can replace its controller.

TP3 has its own problems, however. It was valved off in January after its temperature readback began glitching and spuriously triggering the interlocks [ELOG 15140]. However the problem appears to be limited only one readback (rotation speed, current, voltage are fine) and there is enough redundancy in the pump-dependent interlock conditions to safely connect it to the main volume.

We also discovered that sometime since January, the TP3 dry pump has failed. The foreline pressure had risen to 165 torr. Since the TP2 and TP3 dry pumps are not interchangeable (Agilent vs. Varian), we instead valved in the auxiliary dry pump and disconnected the failed dry pump using a KF blank. This is a temporary arrangement until the permanent dry pump can be repaired. Jordan removed it to replace the tip seals and will test it in the bake lab before reinstalling.

With this configuration in place, we proceeded to pump down the main volume without issue (attachment 1). We monitored the pumpdown for about 45 min., until the pressure had reached ~1E-5 torr and TP3 had been transitioned to standby (low-speed) mode.

Summary of topology changes:

• TP2 valved off and shut down until controller can be replaced
• TP3 temporarily backing for TP1
• Auxiliary dry pump temporarily backing for TP3
• TP3 dry pump has been removed for repairs

I replaced an empty N2 cylinder, there are now two empty tanks in the outside rack.

I missed the vacuum discussion on the call today, but I have some questions/comments:

• Isn’t it true that we didn’t digitally monitor any of the TP diagnostic channels before 2018 December? I don’t have the full history but certainly there wasn’t any failure of the vacuum system connected to pump current/temp/speed from Sep 2015-Dec2018, whereas we have had 2 interruptions in 6 months because of flaky serial communications.
• According to the manuals, the turbo-pumps have their own internal logic to shut off the pump when either bearing temperature exceeds 60C or current exceeds 1.5A. I agree its good to have some redundancy, but do we really expect that our outer interlock loops will function if the internal ones fail?
• In what scenario do we expect that all our pressure gauge readbacks fail, but not the TP readbacks? If so, won’t the differential pressure conditions protect the vacuum envelope, and the TPs internal shutoffs will protect the pumps? Except during the pump down phase perhaps, when we want to give a little more headroom to the small TPs to stress them less?

At the very least, I think we should consider making the interlock code have levels (like interrupts on a micro controller). So if the pressure gauges are communicating and are reporting acceptable pressure readings, we should be able to reject unphysical readbacks from the TP controllers.

I still don’t understand why TP2 can’t back TP1, but we just disable all the software interlock conditions contingent on TP2 readbacks. This pump is far newer than TP3, and unless I’ve misunderstood something major about the vacuum infrastructure, I don’t really see why we should trust this flaky serial readbacks for any actionable interlocks, at least without some AND logic (since temperature, current and speed aren’t really independent variables).

I also think we should finally implement the email alert in the event the vacuum interlock is tripped. I can implement this if no one else volunteers.

This might also be a good reminder to get the documentation in order about the new vacuum system.

I will be at the 40m today from 9:30am to 4pm.

Looking at images of the old vac screens, the TP2/3 rotation speed and status string were digitally monitored. However I don't know if there were software interlocks predicated on those.

The temperature and current interlocks are implemented precisely because the pumps can shut themselves off. The concern is not about damaging the pumps (their internal logic protects against that); it's that a pump could automatically shut down and back-vent the IFO to atmosphere. Another interlock (e.g., the pressure differentials) might catch it, but it would depend on the back-vent rate and the scenario has never been tested. The temperature and current interlocks are set to trip just before the pump reaches its internal shut-down threshold.

One way we might be able to reduce our reliance on the flaky serial readbacks is to implement rotation-speed hardware interlocks. The old vac documentation alludes to these, but as far as Chub and I could determine in 2018, they never actually existed. The older turbo controllers, at least, had an analog output proportional to speed which could be used to control a relay to interrupt the V4/5 control signals. I'll look into this for the new controllers. If it could be done, we could likely eliminate the layer of serial-readback interlocks altogether.

That would be awesome if you're willing to volunteer. I agree this would be great to have.

I agree there were MEDM fields, but I can't find any record of these channels being recorded till 2018 December, so I don't agree that they were being digitally monitored. You can also look back in the elog (e.g. here and here) and see that the display fields are just blank. I would then assume that no interlocks were dependent on these channels, because otherwise the vacuum interlocks would be perpetually tripped.

Sorry but I'm having trouble imagining a scenario how the pressure gauges wouldn't register this before the IFO volume is compromised. Is there some back of the envelope calculations I can do to understand this? Since both the pressure gauges and the TP diagnostic channels are being monitored via EPICS, the refresh rate is similar, so I don't see how we can have a pump temperature / speed / current threshold tripped but NOT have this be registered on all the pressure gauges, seems like a bit of a contrived scenario to me. Our thresholds currently seem to be arbitrary numbers anyway, or are they based on some expected backstreaming rate? Isn't this scenario degenerate with a leak elsewhere in the vacuum envelope that would be caught by the differential pressure interlocks?

For the email alert, can you expose a soft channel that is a flag - if this flag is not 1, then the service will send out an email.

Right, I doubt they were ever recorded or used for interlocks. But the readbacks did work at one point in the past. There's a photo of the old vac monitor screen on p. 19 of E1500239 (last updated 2017) which shows the fields once alive.

I don't disagree that the pressure gauges would register the change. What I'm not sure about is whether the change would violate any of the existing interlock conditions, triggering a shutdown. Looking at what we have now, the only non-pump-related conditions I see that might catch it are the diffpres conditions:

• abs(P2 - PTP2) > 1 torr (for a TP2 failure)

• abs(P3 - PTP3) > 1 torr (for a TP3 failure)

• abs(P1a - P2) > 1 torr (for either a TP2 or TP3 failure)

For the P1a-P2 differential, the threshold of 1 torr is the smallest value that in practice still allows us to pump down the IFO without having to disable the interlocks (P1a-P2 is the TP1 intake/exhaust differential). The purpose of the P2-PTP2/P3-PTP3 differentials is to prevent V4/5 from opening and suddenly exposing the spinning turbo to high pressure. I'm not aware of a real damage threshold calculation that any one has done; I think < 1 torr is lore passed down by Steve.

If a turbo pump fails, the rate it would backstream is unknown (to me, at least) and likely depends on the failure mode. The scenario I'm concerned about is if the backstream rate is slower than the conduction time through the pumspool and into the main volume. In that case, the pressure gauges will rise more or less together all the way up to atmosphere, likely never crossing the 1 torr differential pressure thresholds.

There's already a channel C1:Vac-error_status, where if the value is anything other than an empty string, there is an interlock tripped. Does that work?

I removed the backing pumps for TP2 and TP3 today to test ultimate pressure and determine if they need a tip seal replacement. This was done with Jon backing me on Zoom. We closed off TP3 and powered down TP3 and the auxilliary pump, in order to remove the forepumps from the exhaust line.

1. Close V1
2. Close V5
3. Turn off TP3
4. Turn off aux dry pump (manually)
5. Once the PTP3 foreline pressure has come up to atmosphere, you can disconnect the TP3 dry pump and cap the exhaust line with a KF blank.
6. Restore the vac configuration in reverse order: dry pump ON, TP3 ON, open V5, open V1

Once pumps were removed I connected a Pirani gauge to the pump directly and pumped down, results as follows:

TP2 Forepump (Agilent IDP 7):

• Ultimate Pressure: 123 mtorr
• Hours: 10903

TP3 Forepump (Varian SH 110):

• Ultimate pressure: ~70 torr
• Hours: 60300

TP3 forepump defintely needs a new tip seal, and while the pressure on TP2 Forepump was good there was a significant amount of particulate that came out of the exhaust line, so a new tip seal might not be needed but it is recommeded.

So why not just have a special mode for the interlock code during pumpdown and venting, and during normal operation we expect the main volume pressure to be <100uTorr so the interlock trips if this condition is violated? These can just be EPICS buttons on the Vac control MEDM screen. Both of these procedures are not "business as usual", and even if we script them in the future, it's likely to have some operator supervising, so I don't think it's unreasonable to have to switch between these modes. I just think the pressure gauges have demonstrated themselves to be much more reliable than these TP serial readbacks (as you say, they worked once upon a time, but that is already evidence of its flakiness?). The Pirani gauges are not ultra-reliable, they have failed in the past, but at least less frequently than this serial comm glitching. In fact, if these readbacks are so flaky, it's not impossible that they don't signal a TP shutdown? I just think the real power of having these multi-channel diagnostics is lost without some AND logic - a turbopump failure is likely to result in an increase in pump current and temperature increase and pump speed decrease, so it's not the individual channel values that should be determining if an interlock is tripped.

I definitely think that protecting the vacuum envelope is a priority - but I don't think it should be at the expense of commissioning time. But if you think these extra interlocks are essential to the safety of the vacuum system, I withdraw my request.

It would be better to have a flag channel, might be useful for the summary pages too. I will make it if it is too much trouble.

I agree with your assessment, Jordan.  If I'm not mistaken the scroll pump for TP2 is new; we had a very early failure with the last new scroll pump (the forepump for TP3) tip seals at just over 5000 hours.  Glad to see my replacement seals held up for over 60K hours. If this is the trend with these pumps, we can simply run them to  around 60000 hours and replace the seals at that time, rather than waiting for failure! - Chub

I think we should discuss interlock possibilities at a 40m meeting. I'm reluctant to make the system more complicated, but perhaps we can find ways to reduce the reliance on the turbo pump readbacks. I agree they've proven to be the least reliable.

While we may be able to improve the tolerance to certain kinds of hardware malfunctions (and if so, we should), I don't see interlocks triggering on abnormal behavior of critical equipment as the root problem. As I see it, our bigger problem is with all the malfunctioning, mostly end-of-lifetime pieces of vacuum equipment still in use. If we can address the hardware problems, as I'm trying to do with replacements [ELOG 15412], I think that in itself will make the interlocking much less of an issue.

Ok, this can be added pretty easily. Its value will just be toggled between 1 and 0 every time the interlock server raises/clears the existing string channel. Adding the channel will require restarting the whole vac IOC, so I'll do it at a time when Jordan is on hand in case something fails to come back up.

I will be at the 40m today from 9am to 3pm.

For this particular email service, ideally the email should be sent out as soon as the interlock is tripped, so this would require a line of code to be added to the main interlock code. Which I guess would require a restart of the interlock service. So let me know when you guys plan to do the dry-pump tip seal replacement operation (when I presume valves will be closed anyways) so that we can do this in a minimally invasive way.

The four 4x25DSUB and single 8x25DSUB feedthrough flanges have arrived and will be picked up from the dock and brought to the 40M lab.

Tip Seals were replaced on the forepumps for TP2 and TP3, and both are ready to be installed back onto the forelines.

TP2 Forepump Ultimate Pressure: 180 mtorr

TP3 Forepump Ultimate Pressure: 120 mtorr

Summary:

In ELOG 15368, I had claimed that the POP QPD based feedback servo actuating on the PRM stabilized the lock. I now believe this scheme of sensing using the POP QPD and feeding back to the PRM is not a good topology for stabilizing the PRC angular motion.

Details:

• I was never able to get a measurement of the OLTF of this loop that made sense 
  • the loop was initally commissioned with the PRMI locked on the carrier, and the settings hence inferred to give a ~5 Hz UGF loop were used in the PRFPMI lock.
  • In the PRFPMI configuration, however, the loop gain seemed way too low when I measured using the usual IN1/IN2 method.
  • So it is critical for the lock stability that the angular feedforward works well, which it kind of does now (not that I have changed anything, but the glitches in the seismometer have not resurfaced recently).
  • Hopefully, this becomes less of an issue once we replace the TTs with SOS and OSEM based damping.
• To get some more insight, I did some finesse modeling
  • Attachment #1 shows the sensing response at the QPDs we have available currently (POP and TR). 
  • I included the telescopes (propagation distances, in-air lenses) to these QPDs as best as I could.
  • A simplified model (3 mirror coupled cavity) is used, so there isn't really a common/differential mode in this picture, but we still get some insight I think.
  • Specifically, once the full lock is realized, the PRC optic motion isn't sensed well with our QPDs, and so it was some fluke that turning on these PRC angular feedback loops worked. 
  • Attachment #2 shows the same info as Attachment #1, but with the pendulum transfer functions (and radiation pressure effects) included. The SOS suspensions are modelled as f0=0.7/0.8 Hz (for P/Y), Q=5, while the tip-tilts have f0~5 Hz, Q~10. The high frequency phase is 0 degrees and not 180 as expected because of the pendulum complex pole pair because of the way the quantity is computed in Finesse.
• The current scheme I use is:
  • DC couple the ITM oplevs, using their individual Oplev QPDs.
  • Use the TR QPDs, mixed to actuate on the ETMs in a common/differential way.
  • I think the system is under-determined with the sensors we currently have - we wan't to sense the 10 angular modes - PIT and YAW for the PRC, Csoft, Chard, Dsoft and Dhard (using the terminology from Kate's thesis), but we only have 6 sensors of the same field (POP, TRX and TRY QPDs, PIT and YAW from each).
  • So we need more sensors?
• One thing that can easily be improved I think is to make the ASS system work at high power. 
  • I think this should be as simple as scaling the gain for the loops to work for the high power.
  • Then we can counteract the input pointing drift at least.
  • But the ITM Oplev DC coupling would need to be turned OFF and then ON again, I'm not sure if this will introduce some transient that will destroy the lock...

I would also like to bring up the topic of implementing some WFS for the interferometer fields again, there doesn't seem to be any mention of this in the procurement/planning for the BHD. It is not obvious to me yet that we need WFS and not just DC QPDs from a noise point of view, but at least we should discuss this.

Summary:

I want some input about what the short-term (next two weeks) commissioning goals should be.

Details:

Before the vacuum fracas, the locking was pretty robust. With some human servoing of the input beam, I could maintain locks for ~1 hour. My primary goals were:

1. Transition the vertex length DoF control from 3f signals to 1f signals.
2. Turn on some MICH-->DARM feedforward cancellation, because the noise between ~100 Hz and ~1kHz is dominated by this cross-coupling.

I didn't succeed in either so far.

1. I find that there is poor separation of the length DoFs in the 1f sensors, which makes this transition hopeless.
   • Why should this be? I can't get any sensing matrix in Finesse to line up with what I measure in-lock.
   • One hypothesis I came up with (but haven't yet tested) is that the offsets from the 3f photodiodes are changing from time to time, which somehow changes the projection of the various DoFs onto the photodiode quadratures. 
   • The attached GIF shows the variation in the measured sensing matrix on two days - while the sensing of MICH/PRCL in the 3f photodiodes have hardly changed, they are significantly different in the 1f photodiodes. Note that the I and Q have changed for REFL11 and REFL55 between the two days because I changed the demod phase.
   • I also thought that maybe the CARM suppression isn't sufficient for REFL11 to be used as a PRCL sensor - but even after engaging a CM board SuperBoost, I was unable to realize the PRCL 3f-->1f transition, even though the CARM-->REFL11 coupling did get smaller in the measured sensing matrix (red line in the GIF). I don't think we can juice up the CARM gain much more without modifying the CM board boosts, see Attachment #1.
2. I was able to measure the MICH CTRL --> DARM ERR transfer function with somewhat high coherence (~0.98).
   • I then used the infrastructure available in the LSC model to try and implement some cancellation, but didn't really see any effect.
   • Perhaps the TF needs to be measured with higher coherence.
   • It may also be that if I am able to successfully execute the 3f-->1f transition, the coupling gets smaller because the 1f sensing noise is lower?

I guess apart from this, we want to run the ALS scan to try and infer something about the absorption-induced thermal lens. I guess at this point, the costs outweigh the benefits in trying to bring in the SRC as well, since we will be changing the SRC config?

The PSL shutter was closed from the vacuum interlock trip. Today, I did the following:

• Re-aligned input beam to PMC to recover high transmission / low reflection.
• Re-set the LSC offsets.
• ETMX watchdog was tripped. Reset it.
• Opened the PSL shutter, IMC autolocker was able to lock the cavity almost immediately.
• Tested POX/POY locking, ran the ASS to maximize single arm transmission.

All looks good for now. I will probably get back to PRFPMI locking Monday.

I will be at the 40m today from 11am to 4pm.

The machine needed a hard reboot as it was un-ssh-able. 

The exact time that the machine went down is unknown because the blinkys were not DQ-ed. I've now added these to the EDCU to make these channels actually useful, and we may look back on the reliability (or otherwise) of the Acromag system. To my memory, this is the ~5th time one of the new Acromag servers has needed a hard reboot. While this may be less frequent (?) than the VME machines, perhaps there is some other reason for these dropouts. Maybe something to do with the martian network?

Anyway the machine is back up and running now.

I will be in the Clean and Bake lab today from 10am to 4pm.

Per the discussion at the meeting today, the plan of action is:

1. Lock the PRMI on carrier and measure the sensing matrix, see if the MICH and PRCL signals look sensible in 1f and 3f photodiodes.
2. Try locking CARM on POP55 (since there is currently no POP55 photodiode, can we use POX/POY as an intermediary?).
3. For the ASC, can we hijack one of the IMC WFS heads to study what the AS port WFS signals would look like, and maybe close a feedback loop on the ETMs?
   • My guess is no, because currently, the L2A is so poorly tuned on MC2 that the CARM length control messes with the alignment of the IMC significantly.
   • So we need the IMC WFS loops to maintain the pointing.
   • Of course, the MC2 L2A can be tuned to mitigate this problem. 
   • I also believe there is something funky going on with the WFS heads. More to follow on that in a later elog.
   • Apart from these issues, for this scheme to be tested, some mods to the c1ioo model will have to be made so that we can route the servo output to the ETMs (as opposed to the IMC mirrors as is the usual case).

If I missed something, please add here.

This earthquake tripped all suspensions and ITMX got stuck. The watchdogs were restored and the stuck optic was released. The IFO was re-aligned, POX/POY and PRMI on carrier locking all work okay.

I will be in the Clean and Bake lab from 11pm to 4pm