(Aidan, Stephen, Koji)
Building off of QIL/2597 after more thorough discussion in Mariner meeting today. Aidan and I will confirm details today and make moves toward installation of PDs next week.
Necessary capabilities (note: no need to scavange anything from the IRLabs dewar):
- Cold temperatures = ready (via conductive mounting we have seen workpiece ~ 80 K, workpiece heating and CTC100 temp control is demonstrated)
- Optical interface = ready (existing 1700 nm AR-coated window will be used at 2 micron, input power will just be calibrated with a power meter)
- Electrical interface = almost ready, pending cabling (the cryo vacuum chamber has RTDs instrumented, so we just need the leads for the PDs, and we can create Cu twisted pair leads with in-vac crimp/solder sockets a la Koji's OSEM cabling) (also need in-air cabling with DB9 plug - trivial)
Improvements in advance of PD testing:
- RTD cabling and Heater cabling (in vacuum) should be split with in-vac pins and insulated with PTFE tube - Koji has the magic material recommendations
Desired improvements (not necessarily in advance of PD testing):
- Cu solid linkage (being fabricated).
- Inner radiation shield should be clamped well to cold plate (consistent with most recent trial, but do we want better/more clamps?).
- RTD mounting option on shields without cryo varnish (new threaded hole, new clamps).
- RTDs consistent with planned Mariner RTDs (ref. QIL/2590).
- Heater mounting should be directly to the workpiece via an improved clamp on the 2" x 2" grid (rather than on unused Si cantilever workpiece holder).
We were able to successfully integrate the Megastat CTC100 temp data into EPICS and Frame builder. Picking up from my last entry, we completed the following steps:
1. Restarted the EPICS st.cmd service - this "connected the wires" from the CTC100 to EPICS and allowed us to be able to read out live-time channel values. Koji created a script file to restart st.cmd:
Note that if these channels need to be edited, this is the script one should call to restart EPICS. At this point, we could get the values by caget commands like
P.S. Chris sent us a permanent solution for the service. The process was killed and the service was started by the following command
systemctl restart CTC100
2. Appended new channel names to an existing .ini file (/opt/rtcds/caltech/c4/chans/daq/C0EDCU.ini). This allowed us to record these channels in Frame builder. I was originally going to create a new .ini file with these channels and add the name of the file to master, but we could not find the master file. Plus, instructions of fb4 told us to append to C0EDCU.ini specifically.
3. Restarted firstname.lastname@example.org on fb4:
sudo systemctl restart email@example.com
We used DataViewer to verify that the channels were being recorded. Success!
During the process, we corrected the channel labeling for RTD #3/#4. So for a few first data points, the numbers for the workpiece and the outer shield were swapped.
The test mass temperature indicates 121K@100hr but there seemed a few sensor glitches for the test mass (𝛥=-4.2K) and the inner shield (𝛥=-0.43K).
So the actual test mass temperature could be 125K.
The temp was read to be 119K@114hr (Attachment 1)
There was very little cooling capability left for the test mass (Attachment 2)
The OSEM reading is now stable @12.3V (Attachment 3)
The raw temp data and the minimal plotting code are attached (Attachment 4)
With Test Mass RTD at 119K, and with all of Koji's trials completed, I started the warmup this afternoon.
- Cryocooler off at 12:41
- Heater on at 13:05 (forgot to complete this task durming my initial visit).
Anticipated warmup duration of ~ 1 day, as improvements to cooldown (higher emissivity test mass, better conduction to inner shield) should also improve the efficiency of our active warming, which took a bit more than 30 hours last time (ref QIL/2615).
I added an EPICS channel for the heater power controlled by the Megastat CTC100.
1. Added a function to the protocol file to query the heater power: /opt/rtcds/caltech/c4/target/qil-nfs/CTC100/iocBoot/iocCTC100/protocols/ctc100.proto
2. Created a new channel C4:CTC-MS_HEATER_POWER_VAL in the database file: /opt/rtcds/caltech/c4/target/qil-nfs/CTC100/db/ctc100.db
3. Restarted st.cmd:
At this point I confirmed I could access the channel via
4. Restarted firstname.lastname@example.org
The legit way to restart st.cmd is
- Confirmed the heating stopped in the evening -> The heater was deactivated @~23:00
- Made some measurements and checks - the oplev spot was approximately on the center of the QPD before warming up. Now it is ~4mm above the center (note that the QPD size is 0.5" in dia) (Attachment 2). This corresponds to ~2mrad misalignment.
- Dismantled the OSEM electronics and power supply from the table. The electronics were salvaged into the OMC lab -> to be returned to the 40m.
- A 2" Al mirror package was brought to the OPLEV periscope so that the gold mirror (too thin) can be replaced. (Attachment 1)
This afternoon I met with Aidan and Stephen about near-term plans for the megastat. Aidan and I got familiar with the various viewports, and identified that if we want to set up another window for laser access we would need to move the existing window to the viewport 60 deg clockwise from its current location (at the SE corner of the chamber). We also noted that there is a single instrument RTD currently available (plus three others monitoring each heat shield and the cold head), so setting up two separate measurements each requiring temperature readout simultaneously would require opening up the electrical feedthrough and wiring up another RTD.
This week, I will be installing a clamped cantilever into the existing op lev setup in the cryostat. We will then pumpdown the chamber on Friday, and cool down over the weekend. At the beginning of next week, I will make some Q measurements of the clamped cantilever around 123K, then warm back up so Aidan can set up PD testing at the end of next week or early the following.
Today, I pulled a moderately smooth cantilever from cryo, and brough it over to QIL to clamp it. I hadn't used this clamp before, and found that the cantilevers Zach was producing in 2018 are larger than the slot in the clamp that was in the megastat. Since the clamping screws are not centered around the cantilever itself, one end of the clamp is touching steel on steel and the other is slightly open. I found in the elog that Zach was using a spare, broken off cantilever piece to balance the clamp. Later, he updated the clamp design and used an alignment jig to set the cantilever in the clamp. I will bring these items from cryo lab and install them in the megastat tomorrow.
Additional photos are on the ligo.wbridge google drive, under Google Photos -> CantileverQ.
I've sketched out a way to change the beam size on the in-vacuum PD. I think the beam diameter coming from the collimator is 1.2mm, but I need to check this. If we add a telescope outside the vacuum system and put the second lens of this on a translation stage, then this provides us with wide control of the beam size on the PD inside the vacuum system (about 400mm away).
I've used the following model for cooling of the coldplate and testmass in Megastat:
where , and e_cp and e_bp are the emissivities of the coldplate and baseplate, respectively. The first term is conductive cooling of the cold plate via copper braid, and the second term is radiative heating of the coldplate from the baseplate (roughly room temp). In the model, the coefficient c is the fit parameter.
where and e_is and e_tm are the emissivities of the inner shield and test mass, respectively. This equation considers radiative cooling of the test mass from the surrounding inner shield. Here, the fit parameter is e_tm.
Attachment 1 (top plot) shows the results of the fitting. For conductive cooling of the coldplate, the best fit parameter is c=0.62. This means that 62% of the calculated conductive cooling power is actually being delivered to cool the coldplate, according to this model. Another way to look at it is that the constant factors (A, l of copper braid) that are used in the model need a correction of 0.62. Regardless, the model predicts a plateau temperature a few degrees cooler than the data shows. This means there must be a heat source we are not considering that delivers extra heating power at lower temperatures.
The testmass cooldown best fit parameter is e_tm = 1. I supplied bounds on e_tm from 0 to 1, since it is an emissivity value; the fit hits the upper limit. This is consistent with Koji's result that the calculated test mass emissivity is over 1. It is not clear why/how the test mass is cooled so quickly, since the black paint realistically has an emissivity between 0.5-1. Just like for the coldplate, the current model predicts a plateau temperature lower than what the data shows.
The bottom plot of Attachment 1 shows the difference between the fits and the data. The coldplate model does fairly well at high temperatures, but starts to break down around 100K. Then, other effects must be kicking in that we are failing to consider.
Next I plan to simplify further and model cooling power as a polynomial of T, and fit for its coefficients. Hopefully this can give insight into the temperature dependence of cooldown curve.
Here's a more detailed layout of the in-air design for the PD testing. Things to note:
Actions on my to-do list, before we are able to pump down for Aaron and Shruti’s Si Cantilever Q measurement:
0. Confirm Aaron has completed cantilever mounting and is happy with shield alignment.
- if needed, might have to rotate shields and/or clamp down workpiece holder.
[Done 2021.08.07] 1. Solder on new RTD, then mount RTD and heater to workpiece holder.
2. Verify electrical continuity of RTDs and heater.
3. Close up (shield lids, chamber lids).
4. (anytime, optional) complete RTD and Heater disconnection junctions with in-vac crimped pins.
[Koji indicated these were likely scavanged from DB connector kits, Stephen ordered more] - crimped plug pins were in 40m lab parts tower, but didn’t see any receptacle pins in the vicinity.
[update - Aaron indicated interest in deferring 1 week to establish more permanent setup. Likely to reattach outer shield RTD to cantilever clamp, allowing two different workpiece sensors] Planning to complete items on this list Friday, by Mariner meeting timeslot at the latest.We are in good shape to pump down Friday early afternoon, and for Aaron to collect data via controlled warmup on Monday/Tuesday (could run through Wednesday, if needed).
Actions on my to-do list, once we are warm and up to air, before Aidan and I are able to run the JPL PD test. This list complements the optical setup tasks and data acquisition setup tasks that are also mentioned by Aidan in this thread.
Update 23 August - using this list to [comment on details] and highlight items which are still outstanding instead of duplicating in a daily progress log entry :)
0. Remove Aaron and Shruti’s Si cantilever clamp, and return to them for safekeeping [I set the clamp aside, need to coordinate return of cantilever to with Aaron!]
1. Solder a connector equivalent to the testpiece (ref. QIL/2465) but with the cryo wire, In-Ag solder on the connector end, and crimped plug pins on the free end. The crimped plug pins interface the with receptacle pins of the existing leads, which are connected to in-vac side of DB9 feedthrough).
- need to locate some In-Ag solder! [done, and checked connectivity - used standard lead-tin solder following Rana's recommendation (ref. QIL/2418) born out by Koji's testing (ref. QIL/2462 and QIL/2465)]
2. Confirm which PD holder will be most useful for this effort (Koji’s newly machined holders might be useful?) and mount to cold baseplate. If using the tombstone from the IRLabs dewar, which is likely shorter than the beam height, it sounds like we would need to mount it on a pedestal or post. [Aidan used Koji's taller PD mount from the same purchase (ref. QIL/2459). The beam height required no modification, nice!]
3. Mount RTD to PD holder, likely with cryovarnish (unless there is a lucky extra hole for a screw/clamping post). [Radhika and Aidan's G2100807 demo shows the problem with the prior lug, not super stable mounting for the RTD as the same hole is being used to host two screws. Instead, I retrofitted the upper screw from the mount's retaining ring to host one of the trusty spring clamps, see Attachment 1. I checked for clipping or connector interference throughout, and found none.]
4. Mount heater to ___ (TBD, ideally on PD holder but possibly on cold baseplate nearby). [dog-clamped the heater to the baseplate, directly adjacent to the mount - see overview images in Attachment 2 and Attachment 3. We may go through some PID growing pains with this configuration, and we also need to learn whether the 22 W heater power locally applied can overcome the cryocooler's ~50 W cooling power at our operating temperatures (ref. Radhika's QIL/2585). Might be necessary to intermittently power cycle the cryocooler.]
5. Confirm alignment of shields and PD. [aligned both shields, clamped inner shield, but could reposition if there is an issue.]
6. Verify electrical continuity of PD cable, RTDs, and heater. [note need to add indium and finalize clamping of PD holder, also note routing of pins to be connected to PD connector per Koji's QIL/2605 as described in Attachment 3]
7. Close up (shield lids, chamber lid). [note that in particular, the covering up (with foil sheets) of unused shield apertures is still WIP but wasn't originally mentioned!]
8. (anytime, optional) complete RTD and Heater disconnection junctions with in-vac crimped pins. [done, with mounting isolation achieved by kapton tape as ptfe tubing has not yet arrived. Attachment 4 shows one example, from the inner shield.]
- crimped plug pins were in 40m lab parts tower, but didn’t see any receptacle pins in the vicinity. [ordered a few hundred new socket pins, I should share some with the 40m parts tower]
Pending Aaron and Shruti’s measurements, it is likely that heater-assisted warmup will occur starting Tuesday, in time for Wednesday/Thursday access. Friday 13th August could be the start of the cool down if everything goes to plan. [Nothing goes well on Friday the 13th. Aaron and Shruti do not need in-vacuum measurements anytime soon. The current plan is for cooldown to begin Tuesday and, if everything goes to plan, we will collect data through the week, then likely swap out the PDs on Monday for another run next week. The next experiment slated for the QIL Vacuum Cryostat is another Si mass radiative cooling run w/ black paint on inner surfaces of inner shield.]
Also borrowed Mini-Circuits amplifier ZFL-500LN+ from same setup.
Yesterday I came in the QIL and performed an express kidnapping of the 2um in fiber AOM (Brimrose) and the 5 W RF amplifier that was hooked to the RF in port (though it seems it saturates at ~ 600 mW from past elogs). I will test it with the 1419 nm ECDL fiber pickoff port to see that it works and if it doesn't I will reinstall it in the 2um testing facility.
I've made simplifications to the testmass cooling model. Assuming 2 possible cooling mechanisms, radiative and conductive, the ODE must be a function of only and . If conductive cooling/heating of the testmass is treated as negligible, as we previously assumed, then:
, where a is the fit parameter. I include in the equation because it would appear as a constant in any radiative transfer model. I use the measured coldplate/inner shield temperature data for . Note that , and here and throughout I use a temperature-dependent .
The best fit parameter is a = 0.014, and the result of this fit can be seen in Attachment 1. The disagreement between the best-fit model and data suggest that cooling is not only dependent on , i.e. it cannot be radiative alone. I added back a conductive heating/cooling component:
, where a and b are the parameters of the fit.
The result of the fit can be seen in Attachment 2. The best fit parameters [a, b] = [0.022, -0.0042]. This model matches the data much better than the purely radiative model, but the negative sign on b is non-physical (I think) since should always be < 0, so the sign of the conductive term should also be < 0. I'm not sure how to interpret this result, but it almost seems like there is conductive heating to the test mass even though physically this shouldn't be the case.
How about incorporating radiative and conductive terms from the object at 300K?
Per Koji's suggestion, I added two terms to the expression for test mass cooling power for radiative/conductive cooling from 295K (see 2641):
I added 0 as a lower bound for fitting a, b, c, and d. The best fit for d came out to be neglible (on the order of 10-17): conductive heating from room temperature can be ignored. The best-fit parameters for [a, b, c] came out to: [0.025, 0.003, 0.001]. The result can be seen in Attachment 1.
This fit would imply that there is some radiative heating of the test mass from room temperature, in addition to expected radiative/conductive cooling from the inner shield / coldplate. It is worth investigating what might be causing this, and if nothing can be determined then the model needs to be revisited.
Wow, nice fit!
How is this c compared to the 300K exposure from the open aperture for the oplev beam?
We tried to cover all the ports but one. A naive assumption is that we have this opening with this aperture size, but the heat leakage could be more than that.
e.g. the aperture allows angled infrared to come into the chamber and the inner shield reflects the hot radiation inside (and a part of it reaches the test mass).
We applied Aquadag painting on the inner side of the inner shield.
Stephen and I spent about an hour today tidying up the North Table in preparation for 2um photodiode testing in the Megastat.
I ordered two more wire racks (24" deep, 36" wide, 27" high with two shelves each) to go under the table and serve as instrumentation racks.
2643], I realized that the conductive cooling term proportional to was also negligible. I added back in physical parameters for the 2 radiative terms (one from the cold plate, one from 295K) and used the emissivities of the test mass and inner shield as fit parameters:
I used Koji’s input to model radiative heating from 295K. I approximated the radius of the aperture (from which room temperature could be exposed) to be 3cm, and assumed radiative heat is emitted from this circle.
The result of this fit can be seen in Attachment 1: [e_testmass, e_innershield] = [0.59736291, 0.20177643]. This would imply that Aquadag has an emissivity of about 0.6, and that the emissivity of rough aluminum is much higher than expected at 0.2.
I then used these parameters to model the cooldown, given: 1. both the test mass and inner shield surfaces are painted in Aquadag; 2. only the inner shield surface is painted in Aquadag. These models are shown in Attachment 2.
Painting the inner shield in addition to the test mass would yield marginal improvement, as expected. However, painting the inner shield while removing Aquadag from the test mass would, according to this model, weaken the coupling further compared to the reverse case. This makes sense, since in Fe the effect of the inner shield’s emissivity is scaled by the ratio , which is quite small. Increasing the emissivity of the test mass therefore makes more of a difference in the coupling.
I think the asymptotic temperature in the model is missing the data. i.e. the steady state temperature should match up, but the recorded data terminates too soon. Probably should figure out what the missing term is.
Can you post the covariance matrix of your fit so that we can see what the fractional errors are on the physical parameters? (i.e. construct the fit function so that physical parameters which are unknown are the fit parameters.)
The fit parameters are 1. the emissivity of the test mass and 2. the emissivity of the inner shield. Turning on absolute_sigma=True, the covariance matrix is:
Interpreting this, the standard deviation of the parameters is:
e_testmass (painted with Aquadag): 0.00066398.
e_innershield (rough Al): 0.00038737.
As discussed in today's meeting, these values much lower than expected. I'll look more into how scipy.curve_fit calculates these values, and will use the fitting script discussed to better quantify the error.
Worked toward aligning and characterizing beam on PD. Will complete next session.
Some difficulty aligning to the 2um beam, which is sensed by a thermal card. Aidan intends to upgrade with a fiber coupled visible laser, which could then be swapped interchangably for alignment.
The 1" mirror at the top of the periscope doesn't make sense, given larger apertures in shields and viewport. We looked for a nearby 2" replacement but did not have luck. Ended up swapping back in the gold-coated 2" mirror, even though it is thin enough to be a pain to mount.
Instrumented connector pins to DB9 pins using the following translation (ref Aidan's drawing for connector / PD pinout, ref drawing from QIL/2639)
DB9 - 1 6 2 7 3 8
Connector - 2 3 4 5 6 7
We're getting close to running this (2um beam is focussed onto the PD and we have piezo mirror steering and beam size tuning). However, all DAC channels are currently non-responsive. I'm going to rebooting the front-end.
Incident power calibration was performed with a S148C power meter head placed directly in front of the PD. For varying current levels, I recorded the pick-off PD voltage and the power meter reading.
POW [mW] = 0.862[mW/V]*REF_PD [V] -0.067[mW]
I bought two racks to house all our electronics which is currently taking up space on the North Table. The legs were exactly the same length as the clearance between the floor and the bottom fo the table - but not after I got the hacksaw involved. Thankfully the legs have plugs on the top whic cover up the sharp bits that I cut.
Next step is to start transferring the equipment below the table.
Got the DAC working by reactivating entries in the C4TST_cdsMuxMatrix.
No problems with channels 12-14. However, channel 15 doesn't output anything at the AI chassis.
Using channel 14 on the AI chassis with FM15 input into it.
After Aidan validated that model inputs were creating physical parameter changes, we proceeded with some last few checks before closing up. Notes:
- Aidan set up a helpful script turning laser power on and off, and strip tool to follow PD and monitor PD signals. A strip tool chart was used to confirm that there was no loss of functionality or alignment during pump down.
- Checked Heater function and Workpiece RTD response - all good.
- Confirmed alignment by steering at periscope output mirror and watching PD voltage.
At this point Aidan gave a green light for pumpdown prep, and to start pumpdown and start cooldown. Notes:
- Stephen disconnected the PD connector accidentally while trying to add strain relief (the irony is palpable). Reattachment of connector didn't seem to affect any signal levels.
- During installation of radiation shield lids, shields became misaligned and PD signal fell (presumably due to clipping). Recovered previous signal levels by realligning outer shield.
- Double checked that everything seemed good to go, no issues!
- Timeline of pumpdown and cooldown:
17:32 - Pumpdown started.
17:47 - Turbo spin up started (pump station delay parameter).
17:55 - Pressure dropped below 1 mTorr, so I started cryocooler.
18:05 - Healthy so far - Pressure had come down half a decade more, and Coldhead RTD was reading 235 K.
Note on the script - it's running in a TMUX session on WS1: On for 15s, Off for 15s. A much better way to do this would've been to turn on an EXC in the laser current channel (I only remembered the AWG once I'd left). I'll pop in this morning to check on things and see if I can change the modulation.
The purpuose of the laser modulation is to continually monitor the PD dark current level as well as the change in the nominal QE as a function of temperature. I might change it to a sawtooth wave - if the PD is saturating from too much intensity, we'll see a decrease in the QE each time the laser power increases. FYI - we're getting about 0.8mW onto the PD and it should be close to the beam waist (numbers to follow on that);
The DAC outputs are really wandering around a lot. We should diagnose what's going on there as this makes the laser current set point very noisy.
10:00AM Saturday update: Temperature of the workpiece (photodiode) is around 192K. I terminated the script and started an excitation RAMP on the laser diode current. The response of the diode has increased dramatically and I think I see signs of the QE rolling off as the incident power increases.
Sunday morning update:
11:17AM - Temperature of photodiode is ~86K
Also shown is the photodiode current time series. The laser is being ramped from minimum to maximum every 50s. The span of the signal increases as temperature is decreased. The response is almost nil at room temperature. Then it peaks around 140K and is clearly reduced by the time we get below 100K. (Caveat: the beam was initially aligned onto the PD at room temperature but there is currently no auto-alignment. The diode is quite large, 1.5mm sq, but we can;t rule out that the beam is moving off the PD as temperature decreases - so we can only put an lower limit on QE vs temperature from this data).
Attached are time series of the A1 photodiode temperature (and megastat temperatures) along with the PD response and reference PD measurement of delivered power. The laser is being modulated by a ramp wave (sawtooth) so the response shows the maximum and minimum. The difference between the maximum and minimum trend lines is an indication of the PD response (although the PD is saturating as well at higher intensities so further data analysis is required).
I was working in the QIL on Friday and I heard a clicking sound coming from the rack where the DAQ is installed. It turned out to be the DC power supply for the AI/AA chassis. One of the voltage was floating around from ~14.2V to ~14.8V and the unit was clicking as it did this. Since the AA/AI chassis expect +/-18V which is regulated down to +/-15V, this was, to use the scientific term, bad.
I set the low voltage channel back to 18V. We have noticed previous drifts DAC channels - it's possible this was the cause.
I borrowed (retrieved?) the TED200C temperature controller from the north table in QIL to use in the cryo lab.
We should not have a bench power supply installed permanently. Can you install a Sorensen in that rack or use one of the nearby ones?
I turned off the cryocooler and the A1 PD is slowly coming up to room temperature from ~53K (it's currently at 78K).
There is an automated script (autorun2021.sh) running to acquire data from the PD during this process (it is attached):
Example output is attached.
Note - I was originally running the code with a manual realignment each time. I switched to maximize_output_power.py around the 2:30PM mark (~78K) and this yielded a 30% increase in photocurrent. So QE results below this are going to be low.
CALC channel (~/JPL_PD/Ioc/QIL/db)
field(SCAN, ".1 second")
11:47AM – Script is ready to record data
12:15PM – start run with chiller on
12:26PM- chiller off
12:36PM – third loop
12:48PM – 1315165720-1004: 60.98K
12:59PM – 1315166396-1005: 63.2K
1:10PM – 1315167064-1006: 65.2K
1:24PM – 1315167908-1007: 67.6K
1:35PM – 1315168569-1008 :69.4K
1:47PM – 1315169251-1009: 71.3K
1:57PM – 1315169873-1010: 72.9K
2:07PM – 1315170489-1011 :74.45K
2:26PM – running the max power script. Getting 30-40% more power!!
2:32PM – 1315171982-1001: 78.145K
NOW WITH AUTOALIGNMENT
2:34PM – 1315172074-1001: 78.3K
2:45PM – restarted code with longer pause (60s) between end of loop and maximize output power
3:18PM – restarted code with 120s pause between end of loop and Maximize Output
3:43PM – ADDED OFFSET OF 141 to FM29 (REF PD) which sets the zero power level to about zero. (in the middle of 1315176124-1003 measurement at 87.5K)
caput C4:TST-FM15_OFFSET 0
while :; do
# dark current
echo ----- TOP OF LOOP -----
# script to maximize the output power of the piezo
import os, sys, subprocess
import numpy as np
PD_gain_DC FM30 C4:TST-FM30_GAIN 2
We're at 170K as of 9AM this morning. At the current rate, we should reach 273K tomorrow morning.
9AM: At 232K this morning at 9AM. Turned on the heater to 1W around 9:07AM to speed up the return to room temperature.
10AM - set heater to 7W
11:50AM - At GPS = 1315334850 - Set heater to 20W. Also noticed that SR560 output was railing so set gain to 500
I terminated the data taking around 5PM when the photodiode was at about 4C (277K).
Monday I completed the vent that Aidan had started by turning off the cryocooler. During the afternoon I turned off the pumps, unbolted the chamber lid, and removed the radiation shield lids.
Next, Aidan was going to run some characterization measurements and determine whether to swap the diode or repeat with A1.
Koji asked me to calculate the thermal resistance between the cold head and cold plate from Megastat cooldown data, to compare to the theoretical thermal resistance of the copper braid. This way we can determine if cold plate cooling is limited by the braid itself or by the contact(s) between the braid and cold head / cold plate.
After folding the copper braid in half, its cross-sectional area is 1.34e-4 m2 (source here) and I estimated its length to be 30 cm. I used a room-temperature value for the thermal conductivity of copper, for simplicity (~400 W/m*K). The "theoretical" thermal resistance of the copper braid should therefore be 5.57 K/W.
I used existing cooldown data from the cold head and cold plate to fit the thermal resistance between the two. I ignored effects from room temperature and simply modeled conductive cooling from the cold head to the cold plate. The result of the fit was a thermal resistance of 5.75 K/W, obtained from data. This value is pretty consistent with the calculation above, implying that the cold plate cooling is hitting the physical limits of the copper braid.
If the copper strap were instead a solid bar with the same nominal diameter (0.483"), the thermal resistance would drop to 3.15 K/W (a factor of 0.57 in cooldown time).
I was setting up for some characterization measurements of the JPL PD and I noticed that there are flecks of black paint all through the chamber. There were a couple of visible bare sections on the wall of the inner shield where paint had been removed.
The attached files are the scripts used to take data during the PD temperature cycling/testing and to retrieve and analyze data after the fact.
# analysis od the A1 JPL PD diode
# Aidan Brooks - 10-Sept-2021
import numpy as np
import matplotlib.pyplot as plt
import os, glob
you can put these in the GIT repo for the QIL Cryo tests that Radhika set up. Otherwise, they'll get lost. And we should probably change autorun to a .py script and document these in the README on the repo.
I performed some occlusion measurements of the 2um laser going into the cryo chamber. For different values of dz on the collimating lens translation stage, I moved the power meter into the beam using it's translation stage by an amount dx.
One the beam was on the power meter (aperture = 5mm diameter) the power stayed constant for several MM before dropping again (indicating all the laser beam was on the power meter).
There was a big inrcease in incident power as dz was increased. This, and the constant power across the PD aperture, indicates that the beam is clipping or sees an aperture somewhere like the focussing lens (f=75mm) or further upstrean. I will review the expected beam size as a function of position, assuming the given NA fof the fiber.
Length from Focussing lens to POW METER = 80mm
2.73mm 3.73mm dz 4.73mm 5.73mm 6.73mm 7.73mm 8.73mm 9.73mm 10.73mm 11.73mm 12.73mm 13.73mm
dx (0.01mm) PM(uW) dx PM(uW) dx PM(uW) PM(uW) PM(uW) PM(uW) PM(uW) PM(uW) PM(uW) PM(uW) PM(uW) PM(uW)
60 191 40 3 200 241.9 272 305 348 395 454 523 605 697 798
50 189 50 7.25 175 241.7 270 307 346 394 452 519 595 685 777
35 181.7 65 23.6 150 234.5 262 296 333 377 432 492 563 645 727
20 169 75 85.5 130 218.8 243.6 274 308 356 395 445 507 579 657
Precise distances required between:
accounting for thickness of optic mounts, sunken fiber launcher plane, back focal length of lenses, dispersive variation in focal lengths of lenses from nominal and distance between PD surface and base of PD mount. Also shown are the distances between the steering mirrors (PZT steering mirror, lower periscope mirror and upper periscope mirror).
Beam propagation through this system is shown in the attached PDF. The upper plot shows a paraxial beam propagation as the collimating lens is displaced from the nominal position. The purpose is to indicate the beam size (radius) all the way through the system. We would like this to be less than about 6mm radius (12mm diameter) on all of our 1 diameter optics. The second plot shows the waist size at the PD as the collimating lens is moved by +/- 2mm. The purpose is to allow us to tune the beam size on the PD without clipping the beam on intervening optics.
Keeping the collimating lens Delta Z to a range of +/- 2mm is safe for beam propagation in terms of clipping on apertures or on the 1.5mm diameter PD.
We borrowed the fiber alignment pen from the QIL for quickly coupling the AOM first-order beam in the ECDL experiment in DOPO lab.
Pump down sequence executed tonight; Aidan plans to automate data collection during cooldown and warmup both, and the script will be activated early in the coming week.
- Carbon paint flakes (mentioned by Aidan in QIL/2667) were either picked up or scrubbed by IPA wipe, except the biggest, which were nudged near the closest 1/4-20 hole and picked up with tweezers for removal. There are still some small flakes of paint as there was less benefit to cleaning flakes closer to the PD or lens, and I opted not to risk any bumps. Aidan was correct, some areas of inner shield ID wall have flaked, but it seems the main location of delaminated paint is on the wrinkly foil excess covering the rim of the shield, an accidental paint location.
- While adding the lids, I monitored the PD outputs using Aidan's strip tool kindly left running. I never noticed any clipping in the trends - should I have been more skeptical?
- While adding the lids, I forgot to monitor the RTD outputs to the CTC100 controller, and the outer shield ended up shorting and ceasing to read any temperature. Didn't notice until I had turned on the roughing pump! Had to reopen and fix the short. Good reminder.
- Turbo came on at 12:07 am on 03 October at a pressure of 30 Torr - the setting is actually a timer and not a gauge reading.