Terminated the data taking at 8:35Am this morning. The termperature traces of the cryo chamber show a couple of discontinuities in the gradient. I don't know what the cause is,
Initial running of analysis code puts the max QE at ~62 + /- 1% around 130-150K. I want to explore this temperature regime manually and see if we're saturating the PD or not.
3:30PM - Chamber is still under vacuum. Cryocooler turned back on.
We turned off the heater and the cryocooler this morning to around 11:30AM when the temperature of the diode was around 123K and are letting it gradually warm up.
Through the next 30K, we experimented with different distances between the fiber output and the collimating lens. Bias voltage always set to 1000mV. Laser diode current was set manually on the controller (the input from the DAC was unplugged as this is a little noisy).
It looked like the optimum translation stage setting was 9.75mm - however, i discovered something very interesting ...
If you run the maximize power script at 100mA to the laser diode, then drop the laser current to 30mA and rerun the script, you find that there is a different optimum alignment. This means that the output beam shape/pointing is power dependent. In other words, the output of the fiber is not properly mode-cleaned by the 2m patch cord we have.
Switching to 25mA, I optimized the alignment and continued exploring the optimum translation stage position. Dropping the stage position to 8.0mm maximized the response (at 25mA). Note that the code maximizes an EPICS channel called C4:TST-PD_RESPONSE which is (JPL_PD - DarkV)/REFPD. The reference PD filter bank has an offset applied so OUT16 has a mean value of zero when the laser is off. DarkV for the JPL PD is the PD voltage when the laser is off and this was manually updated every 5-10 minutes or so. C4:TST_PD_RESPONSE is not stored but the JPL_PD and REFPD channels are stored in frames.
On 27-Sept, I measured the ratio of power incident on the JPL PD to the voltage output from the REF PD: dP/dV = 9.3E-4 W/V. The JPL PD DC photocurrent sees a transimpedance gain of 1000Ohm. Therefore, QE is calculated using the following formula:
QE = (RESPONSE)*(1E-3/9.3E-4)* h*c/(e*lambda) = RESPONSE*0.667
Using this calculation and a peak response value of 1.334 at about 145K, the peak QE was estimated to be about 89%. An error analysis is needed on this. And we need to figure out how to get a better output beam shape from the optical fiber (use a really long fiber?)
Note that the translation stage reading of 8.0mm corresponds to a fiber holder to lens mount distance of 30.9mm
its worth looking into how fiber optic mode cleaning actually works:
In order to get a lot of cleaning you have to have a clean beam to begin with. There's a way to pre-clean by putting the laser output into a pinhole before coupling int othe single-mode fiber. Also, use a ~40-50m fiber to make sure the mod-mistanatched beam actually goes into the cladding rather than recombine into the Gaussian beam.
Definitely. I think the lack of beam profiling/imaging equipment is something we want to address too. We will waste a lot of time in Mariner if we can't profile our beams.
Some of the data recorded during the current/micrometer scanning yesterday.
Highlighted change to 25mA and also highest QE.
agreed. You should put into Voyager chat and cryo/ET slack questins about 2 um beam profiling. We'll want it for anything 1.8-2.1 um.
Somewhere around the labs there should be a DataRay Beam'R2 scanning slit profiler, with an extended InGaAs detector that works out to 2.5 µm.
Rebooted the workstations and FB4.
Restarted the model on the FB4:
The cryocooler was switched off last Thursday to do testing on the JPL_PD. I turned the heater back on during this testing and neglected to turn it off when I finished at the end of the day. As a result, the workpiece reached ~400K over the weekend.
We are now allowing it to slowly cool down.
The CTC100 has a feature to specify an upper limit on temperature and then shut off the heater if that temperature is exceeded. We should engage this going forward.
We're at 300K as of 7AM this morning.
Instructions on how to enable the alarm and heater shut off for the CTC100.
Status: This reports the status of the alarm. If LATCH is enabled, this must be manually set to OFF once it has been enabled.
Mode: Set to "Level"
Latch: Optional to set to "YES" if desired.
Output: Set to "Heater"
Max: Set to desired maximum temperature.
The attached photos show:
I've attached a timeline of our inspection this afternoon along with today's pumping timeline,
Here is a brief summary of observations from previous pumping timelines. Today's pump down is consistent with previously observed timelines.
2:17PM – Assessing the impact of the 408K event in the MegaStat
Coldhead reached 363K (90C)
Innershield reached 403K (130C)
2:24PM – Aquadag E service temperature (149C)
Maximum service temperature in air* : 300°F (149°C)
*Service temperature under vacuum conditions is significantly higher. Contact Acheson for specifics.
2:25PM – Bringing Megastat back up to air for initial inspection
2:37PM – chamber is at air
2:41PM – removing bolts
3:13PM – initial inspection looks normal. No elevated amount of black particulates found on surfaces – consistent with or less than the amount seen last time we opened.
Stephen detected faint smell different from last time (“campfire”?)
3:14PM - Stephen reattaching the RTD wiring that had delaminated. I wiped up visible particulates with isoproponal soaked wipe.
3:21PM – putting lid back on
3:30PM – lid on. Screws in finger tight
3:35PM – screws tight – ready to pump
3:40PM – pumping station on
(Cryo-cooler normally turned on around this time)
Not in this instance though
Here is the analyzed data from Test 3 of the A1 JPL PD.
Note about the QE measurement:
Dark current is the output from the Keithley scan - the vertical scale is correct in Amps [ignore question mark]
Dark noise spectra are included for different bias levels and at different temperatures. Stll need to add ADC noise floor for these plots.
Notes from Test 3
6-Oct-2021: done with cryocooler off and temperature increasing
PREAMP GAIN = 1E3
SR560 gain = 500
LD temp set point = 20.2kOhm
IT would also be good if you could plot the RMS noise around 10 Hz and 100 Hz as a function of the bias and temperature, so we can see what the trends are. And how about post the data and scripts to the elog so we can munge data later?
This post describes upgrade efforts from 10 - 16 November, with the following goals:
- introducing a solid copper bar thermal linkage
- shifting the setup away from PD testing
- preparing for the next test (radiative cooling of Si)
Here are some highlights of the effort:
The rest of the installation effort is captured in the next log post QIL/2695, to partition the items relevant to the radiative cooling of the silicon mass.
The photos here (and others) are posted to the QIL Cryo Vacuum Chamber photo album.
In this phase, we are working toward improving our setup with a rigid copper bar, and obtaining a new data point for our radiative cooling thermal models for a suspended silicon mass. Since the past cooling runs of a silicon test mass did not yet incorporate aquadag-painted shields, we wanted to obtain a new data point in the model (in other words, we painted the shields in QIL/2645, but the next test was a PD measurement, so this is the first silicon test mass measurment after shields were painted). The improvement to the thermal linkage, now using a rigid copper bar with higher conductivity (ref. QIL/2666), is a second variable being changed simultaneously in the spirit of improving the cooldown time.
Refer to the prior post (QIL/2694) for the bulk of the blow-by-blow of configuring the chamber to use the rigid copper bar linkage. This post will describe the mounting of the Si mass, and the pump down and cool down.
Closeout photos will be posted to the QIL Cryo Vacuum Chamber photo album.
I came and looked around for a Y1S HR coated 2" optic. I found two ATFilms labeled optics with information of only their substrate. The coating info is encoded in the run# but I could not find a place for what it means. So I'm gonna take these and measure the transmission in 40m.
11/16 was the first Megastat cooldown after exchanging the copper braid linkage for a copper bar. Attachment 1 compares the cooldown trends for the test mass, inner and outer shields, and cold head. The solid curves are the new cooldown trends (copper bar), and the faded dashed curves are the previous cooldown trends (copper braid).
- The coldhead has a reduced heat load, and interestingly a second time constant governs cooldown from ~2-35 hours.
- The inner shield time constant is reduced significantly, but the inner shield experiences a slightly greater heat load at steady state.
- The test mass cooling is improved as expected, given inner shield cooldown.
- The coupling between the outer shield and inner shield has increased, resulting in greater cooling of the outer shield. This could explain the added heat load to the inner shield.
This week I have been working towards a Markov-chain Monte Carlo (MCMC) approach for fitting Megastat cooldown data and obtaining estimates on various emissivity values. I started with a simple model, only simulating the radiative cooling from the inner shield to the test mass. I supply the test mass and inner shield temperature data, and the emissivity of Aquadag (coating both the TM and inner surface of IS) as the only fit parameter. I am using Stan for the modeling, and Attachment 1 is a copy of my stan model.
The results of the simple model are in Attachment 2. The emissivity of Aquadag is estimated as a gaussian centered around 0.7. I am still determining whether this result is "real", or if the simulation is simply returning my prior. I will look into this more before adding complexity to the model.
thats looking good
You should try to use either the corner or getdist packages to plot the 'corner' plots commonly used when showing correlated posteriors (cf. https://emcee.readthedocs.io/en/v2.2.1/user/line/) so that we can see what's up with the other uncertainties
Started a new run this afternoon, with the following goals:
1) confirm that the first run (QIL/2695) went smoothly, by performing a visual inspection in the chamber while setting up for the first run.
- kapton tape affixing inner shield RTD lead junctions to inner shield had fallen. These junctions were simply hanging - not ideal, but apparently not too harmful. Not likely to impact temperatures, in my opinion, but could have led to shorts or glitches in data.
- all RTDs appeared to be fixed and well-contacted to surfaces
- Everything seemed to be in good shape with the copper bar, no apparent issues
2) obtain a second run with similar configuration, now that the rigid copper bar linkage has been implemented.
3) vary a single important parameter relative to the first run, namely the inner surface emissivity of the inner shield, so that the impact of that parameter may be observed.
- Added Aluminum foil (matte side visible) to the inner shield inner surfaces (lid and cylinder, both). Anywhere there was previously black Aquadag, there is now matte aluminum foil.
- Kept the same apertures for viewport access and for electrical and thermal connection passthrough, basically attempted to achieve identical shield coverage.
- There is one small sliver of black aquadag visible at the location of the electrical leads, but I didn't worry about patching that small area.
- Pumps on at ~3:40 pm
- Cooling started at 4:13 pm (pressure ~6 mTorr, rapidly falling with turbo pump spinning up from ~70% to ~85% over a 1 minute interval). Coldhead RTD is responsive.
- All photos will be posted to the QIL Cryo Vacuum Chamber photo album.
- Note from check in on Monday afternoon, ~ 69 hours after start: everything looks good, and the workpiece temperature (~127 K) seems to reflect the emissivity change.
This post will host plots and trends from this radiative cooling run. At a glance, the tuned CTC100 PI control was able to control the workpiece steady state temperature in this radiative cooling test within .005 K.
Run description: At 4 pm Wednesday, the workpiece temperature was at steady state from the QIL/2701 cooldown, a little less than 120 K. From 4pm Wednesday thru 5pm Thursday (25 hours) the CTC100 controller was actuating on the workpiece RTD temperature (cryovarnished to the suspended Si mass) using the resistive heater (dog clamped to baseplate with indium foil gasket). The conductive heating of the cold plate, and therefore the inner shield, led to radiative heating capacity (via ΔT) that actuated on the temperature of the suspended test mass. As found in QIL/2643, the suspended Si mass is well isolated from conduction to the cold plate.
Before the run, the CTC100 PID controller was allowed to autotune using a long lag (600 s) and a moderate acutation step (10 W). After autotuning, the D term was still 0, which seemed fine.
Data: Attachment 1 plots cooldown curves for all RTDs during this run. Attachment 2 compares this run's test mass and inner shield temperature curves to those from the previous run (Aquadag on inner surface of inner shield). The expected result of this change (coating inner surface of inner shield with Al foil) is a weakened radiative coupling between the inner shield and test mass, leading to less effective cooling of the test mass.
Initial observations from data:
1) The cold head temperature curve again suggests 2 time constants, and cooldown is identical.
2) The inner shield's cooldown is roughly unchanged.
3) The outer shield's temperature drops significantly more, indicating a stronger coupling to the inner shield. We will check for a conductive short the next time we open up.
4) The test mass's cooldown matches expectations (weaker radiative coupling).
[WIP - The data will be fitted and discussed]. More detailed analysis from fit to come, including from heater runs.
This ELOG serves as a compilation of known/measured geometric parameters of Megastat. This is informative for thermal modeling of the system, so I wanted to create a centralized reference. A reference to these dimensions has been added to the Wiki page.
Outer Radius = 0.3048 m (12")
Wall thickness = .00477 m (.188")
Height = 0.3048 m (12")
Flange thickness = .0254 m (1")
Outer shield specs
Outer Radius = 0.2794 m (11")--> CAD .265 m (10.433")
Height = 0.2286 m (9") --> CAD .206 m (8.110")
Thickness = 2.90 mm (0.114") (CAD nominally 3 mm, but 9 gauge aluminum is standard)
Inner shield specs:
Outer Radius = 0.244983 m (9.645") --> CAD .225 m (8.858")
Height = 0.205994 m (8.11") --> CAD .192 m (7.559")
Thickness = 2.90 mm (0.114")
Cold plate specs:
Radius = CAD .254 m (10")
Thickness = CAD .01498 m (.5897")
Test mass specs: (confirmed)
Radius = 0.0508 m (2")
Length = 0.1016 m (4")
Copper bar specs:
Length = 0.508 m (20") --> note that center to center length is .440 m in CAD
Width1 = 0.03175 m (1.25") (bulk cross section, could be approximated accross full length)
Width2 = 0.049784 m (1.96") (cross section at cold head bolting interface)
Thickness = 0.011684 m (0.46")
Radius = 0.03175 m (1.25")
Thickness = .0516 m
CAD (.EASM) is located at https://caltech.app.box.com/folder/131056505764 (File path: Voyager > Mariner > CryoEngineering). Screenshot of current state is added as Attachment 1.
CAD (source file, .sldasm - SolidWorks 2021) may be accessed via the PDM Vault (File path: llpdmpro > voyager > rnd qil cryostat)
I opened the QIL cryostat today for a health check and visual inspection before the next run. Because I saw a couple of interesting issues, I decided to redo the same run with more attention to detail on the closeout. I'm worried the outer shield may have been linked to the inner shield and baseplate enough to affect comparability with the prior run.
issues with this run, requiring redo:
And since I didn't get to implement any of the intended next runs, here are some notes on other future runs of interest:
This post will host plots and trends from this radiative cooling run (QIL/2704).
Preliminarily, it looks like the reconfiguration to remove a hardware mistake or two led to a healthier run. The comparison below clarifies the two runs:
Run ended with cryocooler shutdown at 12:27 pm (actual duration just under 92 hours). System will warm up with pumps on for the rest of the break, unless I am inspired to come in and run one of the next intended runs discussed in QIL/2704. I did not run any heat input test for this data set, as I am not planning to come in frequently enough to monitor the heating safely.
Attachment 1 compares QIL/2704 (solid) to QIL/2702 (dashed). As expected, the outer shield temperature from the latter run stays warm since the conductive short was resolved. Due to the reduction of the inner shield's thermal load, the inner shield is able to cool faster and plateau at a colder temperature. As Stephen pointed out, however, the test mass is not cooled as efficiently compared to when the outer shield was conductively cooled.
Attachment 2 is a current model diagram of the various components being considered, and their thermal couplings. Attachment 3 plots the fitted model (dashed) over the temperature data (solid). The fit parameters were the following emissivities: aluminum foil, rough aluminum, and aquadag. Notes from the fit:
1. With the conductive shorting of the outer shield resolved, the model (which considers only radiative cooling of the OS) is well fit to the OS temperature data.
2. The inner shield model is missing some key term(s) affecting its time constant and steady state temperature.
3. The above error propagates to the test mass model (I believe).
Given these caveats, the fit results are as follows: aquadag e = 0.92, Al foil e = 0.04, rough Al e = 0.19. These all initially seem reasonable, and I'm happy to see that the aquadag emissivity is higher than previously estimated.
1. Separate the cold plate from the inner shield, and model their conductive and radiative link. Also model the radiative link between the cold plate and the test mass.
2. Cover the test mass in foil (to best of our ability) to refine the radiative link between the test mass and inner shield. Doing so will mean both elements have the same emissivity, so there is only one unknown parameter.
As discussed during the 07 Jan 2022 meeting, the next cryostat run will seek the fastest radiative cooling through the following configuration choices:
Actions completed 14 Jan 2022
This afternoon, we entered the QIL to look for a 2 in optic from the old GYRO experiment that might work as the PR2 in the BHD upgrade in 40m. We found some in the right most cabinet near where MEGASTAT is. We ended up taking two pieces (see Attachment #1 for evidence) one labeled "COATED 2"dia F.S. SUB S1: HR R>99.99% @ 1064NM S2: AR R<0.15% S/N:" and the other one from ATFilms, labeled "2"dia x 0.375" thk FS HR/AR @ 1064nm Run#s': V6-704 & V6-705" and a datasheet which specifies T = 0.02 % or 200 ppm.
I ended the first cooldown of the run at 12:49 today, approximately 92 hours after it had started. Workpiece temperature was 100 K - more data and model fitting will reveal more insights on the results.
I started a 5 W injection through the heater (dog-clamped to baseplate with indium gasket), which I intend to leave running until steady state temperature is reached, at least overnight. This power level will not present a risk to the system, so I feel comfortable with this static power input even though it is not interlocked.
Update after 25 hours [ATTACHMENT 1]
The 5 W heating of the shields has not quite leveled off yet, and the workpiece temperature resumed falling, showing the temperature had not yet reached a steady state. I will leave 5 W on until I see a steady state, then I will plan to turn off 5 W and allow the workpiece trend to level off fully. Summary:
See attachment 1 for data viewer screenshot which reflects the above summary.
Update after 145 hours [ATTACHMENT 2]
Continued injecting 5 W for the whole weekend. Over the last 24 hours, the workpiece temperature seems to have leveled off into a new cooling rate, while both shields alternated between heating and cooling. I think this can be described as a steady-enough state that I'm ending the cooldown.
Now, I'll let the chamber come up to temperature with the help of the 5 W load.
I took a deep dive into Ekin 2.6 to understand heat tranfer between various joint types, specifically pressure joints and grease joints. This was motivated by the fact that modeling of the cold plate and inner shield temperatures seemed to be missing some key physics.
Heat conductance through grease joints is dependent on the contact area of the surfaces:
, where = the heat conductance. The heat conductance of a grease contact with area 1 cm2 can be found in Attachment 1, indicated by the region between the blue and red boxes (our temperature range).
Heat conductance through pressure joints is dependent on the force holding the surfaces in contact:
, where the heat conductance of a pressure contact with force of 50kgf is found in Attachment 1, indicated by the orange box. (Megastat pressure contacts are between the cold plate and shields, so the Al-Al contact was referenced.)
[Added] Heat conductance through varnish joints are similar to grease joints, in that the conductance scales with contact area of the joint. (It follows the same formula as above for grease joints.) The heat conductance of a varnished contact with area 1 cm2 can be found in Attachment 1, and note that it is higher than that of greased contacts.
My efforts this week went into incorporating these heat links into the model. This required splitting up components into various parts (verging on a finite-element approach), since every joint is considered in addition to the elements themselves. A few notes:
1. For now, I assume the force between the inner shield and cold plate is 50 kg, for simplicity. Therefore, the heat conductance being used for this pressure joint is 4.5 W/K (from chart), and I am using this constant value across temperatures until a better estimation can be found.
2. For the grease joints, I estimate the heat conductance at 1 cm2 to be ~ 1 W/K, for the 50-300K range.
3. The contact between the outer shield and cold plate is not actually uniformly touching, as noted in 2706. I am not sure how to estimate the actual force between the surfaces, so I will add this as a fit parameter in the model.
The new model with this incorporation still needs some adequate debugging, but I felt these were vital steps to ensure we get realistic answers regarding cooling power of the copper bar vs. LN2. Once I feel the model can be trusted, I will follow up with analysis of the new optimized cooldown.
Is there a units issue? 50 kg is a mass, but not a force.
For one surface resting on another, the force of the contact is the grativational force of the top surface. There's an implicit factor of that cancels out from the ratio, so it becomes a ratio of masses. The heat conductance listed for a 50 kg Al-Al pressure joint is interpreted as the heat conductance for a force of .
I think its least confusing to just replace 50 kg g with 500 N. Writing 50 g can be misleading, it seems like 50 grams.
There's also a convention to write "50 kgf" to designate "kilograms of force" (implying the same conversion Rana describes, multiplying by g). I see kgf enough in the mechanical engineering world that I wouldn't have been confounded, so I wanted to pass that along.
As discussed during the 21 Jan 2022 meeting, the next cryostat run will seek the fastest radiative cooling (again, see QIL/2706) through the following configuration choices:
Actions completed 27, 28, and 31 Jan 2022
Model updates required to reflect new configuration:
Attached are best fits for cooldown runs on 01/14 and 01/31. The setup for both cooldowns can be found in the previous ELOGs. We noticed that the outer shield did not cool as significantly on 01/31 than on 01/14, hinting that there might have been more thermal contact between the outer shield and cold plate / copper bar.
The model considers the resistances of the following conductive elements (and uses these resistances as fit parameters):
These additions helped the model more closely resemble our recorded data, with a few exceptions:
- At early cooldown times, the model seems to be underestimating the heat load on the inner shield and outer shield.
- The best fit was performed on the inner shield and outer shield data (to fine tune elements of the cold linkage), so the test mass fit is not optimized. (This will be performed next to refine emissivity predictions.)
To identify bottlenecks in the cold linkage, I used the 01/31 model and tweaked the resistances to see which would provide the largest gains in cooldown. The results from such tweaks are below:
From these observations, it seems like the greased joints are thermally efficient, and the bulk area of the copper bar appears to be the largest bottleneck.
The double copper bar configuration pictured in Attachment 1 has been implemented. We completed the updates within work sessions on Thursday and Friday. Here's a pseudo-log:
All images are (or soon to be) posted to the QIL Photo Dump.
The heater was turned on at 3:13pm on Friday 2/4.
We specified a set temperature of 123K. However, the CTC100 PI control included a 1 W lower limit on the input to the heater, so there was a steady load of 1 W applied to the Silicon Workpiece over the weekend.
At 16:01 the cryocooler was turned off to start the warmup.
The CTC100 PI control was configured with a setpoint of 250 K on the Workpiece RTD, to aid in the warmup, and an allowable power range from 0 W to 22 W.
In the plots comparing data and models, can you use the legend to indicate which is which? e.g. use dots for data and solid lines for models, and then label them as that in the legend. Also nice to include error bars on the temperautre measurements; I think there's a python way to plot this as a shaded region as well.
*Note: The RTD spring-clamped to the cold head gave spazzy readings for this cooldown, so the last cooldown's cold head temperature data was used instead for reference.
Looking at the data, there are some initial noteworthy observations:
It could be that somehow the resistances of re-bolted joints increased significantly to compensate the lowered resistance of the bar, but this doesn't seem too likely. The more likely answer is the model overestimated the original resistance of the bulk of the copper bar relative to other components/joints in the chain. This means more work needs to be done, and hopefully a more realistic model will also resolve the discrepancy in early cooldown of the inner shield data.
Attachment 2 shows the best fit for the new cooldown.
I've been assuming that the inner shield can be treated as a point mass, but perhaps the thinness make a significant delay between the temperature of the cold plate and the inner shield during the initial cooldown.
Could you model the cold shield to estimate what the temperature gradient would look like during the rapid cooldown? Not full 3D, but something approximate that takes into account the conductivity, thinness, and heat capacity.
The heater was turned on on Wed, 2/16 at 11:30am, with control setpoint 123K. The lower power limit was verified to be 0W.
The cryocooler was turned off on Thu, 2/17 at 12pm. The heater control setpoint was changed to 295K for warmup. The plan is to address the wacky cold head RTD on Monday.
On Friday, we came down to QIL to poke around the WOPO setup. The first thing we noticed is that the setup on the wiki page is obsolete and in reality, the 532nm light is coming directly from the Diablo module.
There were no laser goggles for 532nm so we turned on the 1064nm (Mephisto) only. The pump diode current was ramped to 1A. We put a power meter in front of Mephisto and opened the shutter. Rotating the HWP we got 39mW. We dialed it back so that 5mW is coming out of the polarizer.
The beam block was removed. We disconnected the LO fiber end from the fiber BS - there is light coming out! we connected a power meter to the fiber end using an FC/PC Fiber Adapter Plate. The power read 0.7mW. By aligning the beam into the LO fiber we got up to 3.3mW.
We connected the BHD PDs to the scope on the table to observe the subtraction signal. Channel 2 was negative so we looked at the sum channel.
Time ran out. We ramped down the diode current and turned off Mephisto.
Next time we should figure out the dark current of the BHD and work toward observing the shot noise of the LO.
The goals for this cooldown are:
On Tuesday 2/22, we opened up the bottom conflat of the T to check on the RTD spring-clamped to the cold head. I re-inserted the RTD and tightened the nut further than last run, and it seemed much more secure [Attachment 1]. I re-inserted the mylar "cap" covering the cold head [Attachment 2].
In the chamber body, we carefully passed the RTD leads through the inner shield and outer shield apertures to remove the outer shield. We did this without having to unclamp/remove the inner shield or any components inside, to preserve consistency with the last cooldown. A few pins were damaged in this process (from inner shield).
Once the outer shield was removed, we used kapton tape to secure strips of peek sheets to its bottom rim [Attachments 3,4]. The strips were taped at 2 points along the rim associated with the most wobble, with hopes of stabilizing the shield as much as possible.
On 2/23 I repaired the pins previously damaged. I also added kapton tape labels to the socket leads, corresponding to the shapes found on the RTD leads (semi-circle example in Attachment 5). This way it will be much easier to match the right pins and sockets in the future.
I then bolted up the chamber (close-out pictures can be found on the QIL Google photo dump). The vacuum pump was turned on at 5:45pm, and the cryocooler was turned on at 7:08pm.
Yehonathan brought over 532nm/1064nm laser goggles from the 40m.
Our next step would be to measure the LO shot noise.
The heater was turned on on Tue, 3/1 at 4pm, with control setpoint 123K.
*UPDATE: After checking a few hours later, I noticed the test mass temperature hadn't risen, and the heater power was reading nan. When I initially turned the heater on, I watched the power ramp up to 22W (max power limit) and the test mass temperature start to rise. I wonder if somehow the lead pins shorted after it was turned on. For now I have turned the heater output off and will check on this after warmup.
The cryocooler was turned off at 5:45pm.
We made some a list of some random questions and plans for the future. We then went down and found answers to some of those:
1. Why is there no Faraday isolator in the 1064nm beam path? (edit: turns out there is, but inside the laser, see pictures in this elog).
2. Do the fibers joined by butt-coupling have similar mode field diameter? If not it can explain many loss issues.
a. In the green path we find that according to the SPDC datasheet the 532nm fiber (coastalcon PM480) is 4um while the input thorlabs fiber (P3-488PM-FC2) coupled to it has an MFD of 3.3um. This mismatch gives maximum coupling efficiency of 96%. Ok not a big issue.
b. At the 1064nm output the SPDC fiber is PM980 with MFD of 6.6um while the BS fiber is 6.2um which is good.
3. What is the green fiber laser damage threshold? According to Thorlabs it is theoretically 1MW/cm^2 practically 250kW/cm^2 for glass air interface. With 3.3um MFD the theoretical damage threshold is ~ 80mW and practically ~ 20mW. It doesn't sounds like a lot. More so given that we could only get 50% coupling efficiency. How much is needed for observable squeezing? There is the possibility to splice the fiber to an end cap to increase power handling capabilities if needed.
4. Is stimulated Brillouin back scattering relevant in our experiment? According to this rp photonics article not really.
5. How much green light is left after the dichroic mirrors? Is it below the shot noise level? Should check later.
In addition, we found that the green fiber input and the 1064nm fiber output from the SPDC were very dirty! We cleaned them with a Thorlabs universal fiber connector cleaner.
Since we had left the lasers ON with the shutters closed we wanted to see if the powers measured after opening the shutter would be similar to what it was when we left. We realized that opening and closing the green shutter destabilizes the doubling cavity (the FI is after the shutter and the shutter does not seem to be a good dump), which in turn changes the SHG crystal temperature (possibly because of the power fluctuation within the crystal). Re-opening the shutter requires some tuning of the temperature and offset to recover similar output power. Finally, after some tuning, we were able to see 156 mW of green light.