Thanks Chris! Did you run startc4iop in /opt/rtcds/caltech/c4/scripts/?

CW: Yes, that's right. Perhaps after this cooldown we can find some time to upgrade the system to the new RCG that starts things automatically on reboot (like at the 40m).

I think that's a good idea - I'll get started on it.

The framebuilder is now running again. It relies on the c4iop front end model for timing, and that was not running either. (Maybe there was a power outage and the system wasn't brought back up afterward.)

The cryo lab has a gitlab CI script that complains to us on mattermost when its DAQ stops working. The QIL can probably use something like that, too.

[UPDATE]

I realized on Sunday that the frame builder was not storing channel data. I turned off the cryocooler so that the cooldown could be restarted once the past temperature data could be extracted. 

In the meantime, I wrote a python script to query the EPICS channels associated with the temperatures of the workpiece, inner shield, outer shield, cold head, and the heater power. The script writes out the data to a .txt file. Once Megastat reaches room temperature, I'll have someone switch on the cryocooler to start the cooldown. Until then I'm working to debug the cds issue, but we can at least use the new script to record the data if it comes down to it.

The pipe is currently being replaced. Should be finished up by the end of the day.

I re-attempted to bond a 120um alumina strip to a Si wafer. I applied a thicker layer of varnish to the wafer and spread it with the dull side of a razor blade. Attachment 1 shows the contact right after placing the alumina on top. I pressed down to try to spread the varnish and cracked the alumina in a few places - although the damage was minimal and the majority of the surface was intact. Attachment 2 shows the bonding after placing a weight on the sample for ~2 hours. There were still a few air bubbles remaining, but as a whole the contact area was well bonded.

I de-bonded the RTD element from the last undoped Si cooldown and varnished it to the bottom of the new sample - directly on the Si wafer [Attachment 3]. Next I soldered a new PT-1000 RTD element (label J - calibration) for the coldhead [Attachment 4], and updated the CTC100 settings with the new resistance-temperature calibration.

I placed the new sample on top of the ceramic ball bearings and connected the RTD pins and sockets (replaced a few in the process). Then I closed up.

Vacuum pump started at 9:35pm, cryocooler turned on at 10:05pm.

Pasting this link here for easy access: https://www.thermalspray.com/application-spotlight-emissivity/. 

With Radhika's OK, I removed the Edwards WRG-S vacuum gauge from the megastat to borrow on the PSOMA cryostat until we receive our replacement gauge. The WRG-S uses a Pirani gauge at high pressure and inverted magnetron at UHV.

I garbed up in gloves, face mask, hood, and lab coat. I removed the gauge and replaced it with a DN40CF rotatable blank that was recently sonicated and baked at the 40m. I used a copper gasket from cryo lab, but used bolts from the QIL lab to close the flange. I couldn't find 12-point screws, so used hex screws, See attachment 1.

On removing the gauge, I found the same black, flaky material that I've seen around failed cold cathode (inverted magnetron) gauges. I wiped down the inside of the megastat flange with a lint free cloth before closing up. Remaining photos are of the black particulate.

Today I decided to go ahead and bond a 120um strip of Al2O3 to an undoped Si wafer. I cleaned both samples and then applied varnish to the surface of the wafer [Attachment 1]. I pressed the alumina strip on top and covered it with a weight (with a dry cloth in between) to squeeze the contact and eliminate any bubbles [Attachment 2]. However, I noticed bubbles were not smoothing out, so I tried to undo the work and remove the alumina strip. In the process, the strip split in half and led to more shatters as I tried to remove the pieces [Attachment 3]. Sadly, we are down 1 alumina strip of 120um thickess (4 remaining). The Si wafer can be cleaned and reused as needed.

Looking back, the varnish application was definitely questionable and I can find a way to smooth out the layer before placing on the strip. I can also apply more varnish towards the edges (I assumed varnish from the middle would spread out). From modeling it looks like we can get away with applying a bit of a thicker layer without sacrificing too much on thermal conductance across the joint.

My goal is to perform a cooldown with this bonded wafer-strip sample in our existing Megastat setup, while we are waiting for the new aluminum plate. Our last measurement was of a bare undoped Si wafer, so seeing increased radiative coupling from the high-emissivity alumina would be informative. This measurement can be compared to the cooldown models in [2812].

In Attachments 1 and 2 I've modeled the cooldowns of an undoped Si wafer (no coating - assumed e=0.06), and of an alumina-coated Si wafer (bonded with existing cryo varnish). I've considered the 120um Al2O3 strip and used e=0.65 from the DielectricThinFilm model [Attachment 3].

[*Note that while the undoped Si wafer model shows that its temperature should not dip below 200K, in our last cooldown it reached ~75K. We believe this was due to some conductive shorting, which is still being investigated.*]

For the Al2O3 strip model, the cryo varnish has thermal conductivity of 0.22-0.44 W/(m*K) from 77-300K. Even with this value, there is overall high thermal conductance across the varnish joint (, assuming 1 mm varnish thickness - in practice, probably thinner). As a result, the modeled temperature difference between the Si wafer and the alumina strip does not exceed 0.5K at any point during the cooldown - the wafer follows the Al2O3 strip temperature almost indistinguishably. Given this I don't think we need purchase an epoxy with higher thermal conductivity, at least not until we try bonding with the varnish to confirm this experimentally. 

Otherwise the EP30TC looks good, but it's stated as serviceable above 200K. The benefit of EP21TCHT-1 is it's explicitly serviceable above 4K and is stated to widthstand thermal cycling.

On their web page (https://www.masterbond.com/properties/thermally-conductive-epoxy-adhesives), they have one with Aluminum Nitride that's twice as conductive. Is that useful to get?

In order to get the strong cooling from the test mass, do we need a more conductive epoxy? (I found this related paper on low temp epoxies).

I exchanged emails with someone at MasterBond about the thermal conductivity of this epoxy. They said they do not expect the thermal conductivity to drop significantly from its room temperature value (1.44 W/m*K) until about 30K. They were confident the value would stay above 1 W/m*K in the 77-123K range. However, they didn't provide any data or qualitative measure of this.

On Monday Nov 7, Koji found a puddle of water in the beside the lab’s toolbox. I contacted facilities to check this out and find the source of the issue. 

Plumbing from facilities came today at 8:00 am. After moving the toolbox aside and checking the pipe, there is a very evident crack running down the side. This pipe is running from the first floor level. Steve from plumbing began to poke around to see if he could where the pipe was coming from and found that the source is a kitchen sink on the first floor. The sink has been labelled “Out of Order”, so the leaking should stop soon. 

Plumbing will contact the carpenters today to see if try to get this done next week. Radhika and I will cover electronics with tarps and use other tarps as curtains the block off the lab area while plumbing is fixing this issue. I will provide updates with the dates soon. 

Today Koji noticed a water puddle along the south wall of QIL (265B). Photos attached. I lifted the cables from the main rack off the ground, and JC contacted Facilities.

[Stephen, JC, Radhika] [WIP]

We met today to discuss the design of the aluminum insert for the cold plate. [Diagram to come]

For a setup with an elevated base plate (sample mount), Stephen suggested having an inner aluminum disk on standoffs and an outer ring that would extend to the wall of the inner shield. The raised outer ring would eliminate the need to align tapped holes with the coldplate, and it wouldn't interfere with any components currently bolted down to the cold plate (dog clamps, cold finger flange). The outer ring would also have a slit to fit around the copper bar/flexible strap. The inner disk would first be bolted down according to the base plate's position, and the outer ring's slit would be aligned with the copper bar/strap. Having 2 elements provides an extra degree of freedom for alignment. 

We took the following measurements inside the chamber:

*We forgot to take a measurement of the smallest inner shield diameter, to inform max diameter of outer ring. I'll add this soon.*

Here is a promising epoxy for bonding an alumina strip to a Si wafer (full data sheet in Attachment 1): 

Ekin recommends silver-based epoxies for high thermal conductivity and high electrical conductivity [Attachment 2].

[Stephen, Radhika]

We met today to regroup and plan out next steps for Megastat:

1. Design and machine/order aluminum sheet to cover cold plate. Thick enough to not bulge, thin enough to be somewhat maneuverable around cold strap.

2. Thoroughly clean Megastat.

3. Paint aluminum sheet with aquadag, install onto cold plate.

4. Replace cold head RTD with newly-ordered (calibrated) PT100.

5. Cool down with alumina strip, on existing mount w/ ceramic ball bearings.

I'm documenting new 75x75mm alumina strip samples brought to the QIL by Chris. We have 5 strips 120um thick [Attachments 1, 2] and another 5 strips 40um thick [Attachments 3,4].

I've modeled cooldowns in Megastat's current configuration for the two strip thicknesses [Attachments 1, 2]. For both, the strip is modeled sitting atop 3 ceramic ball bearings, as in the Si wafer setup.

An important note is that the emissivities of these strips is much lower (effectively transparent) than typically quoted for alumina ceramic, due to how thin they are (emissivity proportional to bulk absorption, dependant on bulk thickness). This means the 120um and 40um strips also likely have different effective emissivities. Bulk transmission decays exponentially as exp(-c*z) for some c, so the effective emissivity of the 40um strip is likely much much smaller than that of the 120um strip. The model doesn't consider this dependence analytically - I've made a guess for emissivity of 0.02 for the 120um strip, and 0.005 for the 40um strip. Despite these low values, the plots still show significant cooling of the samples because their mass is very tiny. 

This analysis could be improved by deriving an analytic model of alumina emissivity as a function of sample thickness.

I torqued down the toothed washer and bolt holding the ground wire to the Maglite body and locally flattened the aluminum, securing the electrical connection. With the Maglite wired up, I connected a Tenma power supply to a mini breadboard and attached the Maglite leads [Attachment 1]. At 3V, 0.38A flowed through the incandescent bulb [Attachment 2].

Next steps:

- I will wire a current-limiting circuit to protect the bulb.

- Normally the Maglite batteries press up against an internal component to complete the flashlight circuit. Since we’ve cut off the body and of course have no batteries, I plan to use vacuum/cryo-safe epoxy to hold it in place (or use some spring mechanism to apply pressure to that component).

- The head of bolt used to connect the ground wire to the aluminum body sticks out and interferes with screwing on the flashlight's reflector. I will need to cut away some of the metal to allow the piece to screw in all the way.

Once these steps are complete, we need to address the feedthrough for the maglite leads into Megastat. Then it will be ready to be mounted inside the chamber and used for wafer heat actuation.

I acquired internal tooth lock washers to bolt down the ground wire to the aluminum maglite body, but I soon recognized that the curvature of the aluminum does not allow for flush contact of a washer or nut. Attachment 1 shows a 4-40 screw and nut relative to the maglite body. 

I am continuing to search for the best way to get electrical contact, given these constraints and the issues with soldering in the last ELOG. 

I recall at one point we had one of these NF1811 with a broken power suply pin.  It was from a limited production run with the smaller micro 3-pin power connectors. Maybe check yours is not that one.  

Long story short it still responded with only the positive rail but DC will gave a bad photovoltaic mode response and the AC had a large unstable oscillation that was only viewable on a high speed scope (if I recall right higher than the 125 MHz nominal bandwidth).  I would check the power-in pins aren't bent/broken and also check the AC out on a higher speed scope (i.e. >=500 MHz).

[Clare, Radhika]

We performed a calibration procedure on the newly ordered RTDs for Megastat, in LN2 and ice water. We noted that these are 1kohm RTDs at 0C, as opposed to the 100ohm RTDs previously calibrated that are inside the chamber. We chose to calibrate 6 RTDs and labeled them alphabetically starting with G.

The first calibration point was taken at the boiling point of LN2. We dispensed the LN2 into a dewar and dipped each RTD just below the surface of the liquid. Since the temperature right below the surface is 77K with relatively high accuracy, we only took 1 measurement of each RTD resistance. The results are tabulated below:

The second calibration point was the melting point of ice. We created an ice bath by filling a beaker with crushed ice and adding deionized water until 1/2" below the surface of the ice. We let this equilibriate for a few minutes and then proceeded to dip each RTD and record its resistance. When we started this process, we realized we had not let the system equilibriate for long enough, since the temperature of the water was dropping with every measurement. The water surrounding the ice had not yet been cooled sufficiently. We scrapped the first set of resistances, but the 2 rounds of valid measurements are tabulated below. The multimeter was overloaded when set to display the resistance in ohms, so we had to display kohms and thus lost a significant figure. 

RTD G was dropped and lost after calibration, but we calibrated more than we needed in anticipation of an accident like this. 

- RTD calibration with Clare scheduled for Wed 8/3 morning.

- A heater will be bolted down to the cold plate for the next cooldown (to aid in chamber warmup).

- For the maglite, I took the wire intended for ground and wrapped it through the drilled hole and around [Attachment 1]. When I applied solder, it bonded to the wire but made no contact with the Maglite aluminum surface. The near-instant oxidation of the aluminum still prevents the solder from bonding to it. Previously when we tried scratching the surface and immediately using mineral oil (vaseline) to prevent oxidation, the solder joint still did not form due to the layer of oil. So I am not convinced that applying vaseline will help this joint with the drilled hole - I am in favor of considering an alternative connection. In addition, the wrapped wire does not allow the maglite shaft to slide completely into the "head", and thus the bulb does not make it through the opening of the reflector. 

Update to QIL/2795:

• RTDs (Digi-key P/N 615-1123-ND) brought to lab (see pic).
• Heaters (Digi-key P/N A102128-ND) brought to lab (see pic).
• Maglite hole drilled; remains to be seen whether than improves solderability, but we could also look at a lug / ring connector as an alternative connection.

{Yehonathan, Paco}

We comfirmed that the DC ouput of one of the 1811s is bad. We set out to measure the AC response of the PDs.

For this, we decided to use the current modulation on the Diabolo laser which is rated to have 0.1 A/V and a bandwidth of 5kHz. We calibrated the current to optical power by swiping the current and measuring the power at the homodyne PDs using a power meter. The laser power before the 1064nm PZT mirror was measured to be 5mW.

Attachment shows the measurement and a linear fit with slope=0.97 mW/A.

We drove the current modulator using a sine wave from a function generator with 1kHz 0.5Vpp. When we looked on the PD AC signal port in the Oscilloscope we saw 2 Vpp 12Mhz signal. We passed the signal with a low pass filter but again we saw mostly noise.

We took the PDs to the 40m PD test stand but we accidently fried Jenne's laser.

Next, we should just use the Moku network analyzer instead of the scope to measure the response in the QIL using the Diabolo current modulator.

[Stephen, Clare, Hiya, Radhika]

We started a cooldown on Thursday with an undoped Si wafer in Megastat [2794]. The chamber does not have the Maglite flashlight installed, due to struggles with soldering the ground wire to the aluminum surface of the flashlight. We decided to pivot to drilling a hole in the aluminum, wrapping the ground wire through and around, and then applying solder to the joint. This will be completed by next week.

Tasks:

• Order new RTDs [done]
• Order new heater to aid in chamber warmup (not for actuating on wafer) [Stephen]
• Calibrate new RTDs in an ice bath and LN2 bath [Hiya, Clare]
• Finalize aluminum sheet design [Stephen]
• Finalize baseplate design [Stephen]
• Drill hole in Maglite to solder ground wire [Stephen, if time]
• Continue optimal excitation analysis [Hiya]
• Finalize final report abstract [Hiya]
• Continue MCMC model development [Clare]

These tasks will ideally lead up to a Si wafer cooldown next week with optimal heat excitation and with an RTD re-installed on the cold head, and potentially with the finalized baseplate and aquadag-coated Al sheet on the cold plate. It is with this cooldown that we can start to estimate the sample’s emissivity, first with least-squares fitting and eventually with MCMC analysis.

[Stephen, Clare, Hiya, Radhika]

On Thursday 7/14 we started a cooldown of an undoped Si wafer in Megastat. Our goals were to swap out the glass wafer from the previous cooldown for the new undoped Si wafer, to verify that there is sufficient radiative coupling between thin Si and the inner shield. This way we ensure that our cooldown data can be fit for the wafer's emissivity. 

When we opened up the chamber, we noticed some of the aquadag-coated foil was touching the bottom face of the glass wafer [bottom edge of wafer in Attachment 1]. This is a potential conductive short, and analyzing the cooldown data will reveal how significant the conduction was. We also noticed several aquadag flakes on the ceramic ball bearings/wafer [Attachment 1]. Stephen pointed out ways to improve the clamping of one end of the base plate to the cold plate, given that the hole spacings of the two did not line up (explained in 2792). We mapped out a new bolting/clamping approach for the baseplate using metal spacers instead of folded Al foil [Attachment 2]. We applied grease to the bottom surface of the baseplate and then bolted/clamped it down to the cold plate [Attachment 3].

We removed the glass wafer and debonded the varnish from the RTD. We then re-varnished this RTD to the undoped Si wafer [Attachment 4] and placed it on top of the ceramic ball bearings on the bolted/clamped baseplate [Attachment 5].

Lastly, we trimmed off excess aquadag-coated Al foil to ensure there would be no future shorting to the wafer. We then verified all RTDs were responsive and closed up the chamber.

The vacuum pump was turned on at 5:35pm, and the cryocooler was turned on at 6pm. 

The cryocooler was switched off on Tue 7/5 at 11:45am.

[Radhika, Clare]

On Thursday 6/30 we worked towards a cooldown with a glass wafer rested on ceramic ball bearing supports. We used a glass wafer for this cooldown because it has a higher emissivity than a Si wafer, and therefore could be more easily validate our cryostat setup. 

The procedure went as follows:

1. RTD preparation [Attachments 1, 2]

  - Re-soldered the leads of the 3 RTDs we previously calibrated (2791)

  - Bonded RTDs to the inner shield, outer shield, and glass wafer with varnish

  *Note that we did not have any RTDs left to clamp to the coldhead

2. Wafer support structure [Attachments 3, 4]

  - Applied grease to the bottom of the baseplate and bolted it down to the cold plate using 2 screws and a dog clamp. The hole spacings on the cold plate and base plate did not align align along the longer axis of the baseplate, so we needed the dog clamp to hold down one end. To get the dog clamp elevated to the right height, we folded a piece of aluminum foil until it was tall enough to rest the dog clamp flush on top of the base plate.

  - Rested 3 ceramic ball bearings and wafer on top (*The ball bearings created enough clearance between the dog clamp and the wafer)

We closed up the chamber and started the vacuum pump [Attachments 5, 6], but soon after I realized the wafer RTD was glitching/shorting. I turned off the pump and re-opened the chamber, and upon inspection noticed that a few of the RTD lead pins were fraying. I replaced these pins and left for the day.

On Friday I verified that all RTDs were operating properly and closed up the chamber. The vacuum pump was started at 3:10pm and the cryocooler was switched on at 4:10pm on 7/1.

[Radhika, Hiya, and Clare]

We debonded the RTDs from the chamber using isopropanol and acetone, then soaked the RTDs in isopropanol for about 15 minutes to remove residue. We then took resistance measurements in ice water detailed below:

Ice Water Calibration Measurements (completed 28-06-2022):

1) Prepared ice water bath by filling beaker with crushed ice and then water to one half inch below the ice surface. Let the ice bath sit for about 1 minute. 

2) All six RTDs were measured (labeled A-F). For each RTD, the resistance in the ice water bath was measured by swirling the RTD held by the DMM leads in the water until the DMM readout stabilized. 

3) These measurements were repeated in the reverse direction (Started with RTD F).

*Note: in the process of ice bath calibration, RTD D broke. 

RTD	Trial 1 ()	Trial 2 ()
A	100.2	100.15
B	100.15	100.3
C	100.1	100.1
D	X	X
E	100.1	100.1
F	100.1	100.1

Liquid Nitrogen Calibration Measurements (completed 30-06-2022):

Procedure: We clipped the DMM leads to the RTDs and taped the two clips together. This is very secure - good method for the future. We dipped the taped clips with the RTD into the LN2, and swirled under the liquid surface until the DMM readout stabilized. We only took one set of measurements because we are very confident of the boiling point of N2. 

*Note: RTD A broke. This is because we spread the RTD wires out to put into the aligator clips, making them very prone to snapping when we move them. This means that we do not have an RTD on the cold head.

Results (ohms):

A: X

B: 19.8

C:18.2

E:18.5

F:19.8

Here I analyze potential gains from using LN2 to kick off a Megastat cooldown, and transition to a cryocooler to cool under 77K. I've removed the heat loads in the system by modeling an empty chamber (no wafer) and no exposed apertures in the inner shield. I've also modeled the flexible strap is a solid copper element, for an ideal cooldown scenario.

Attachment 1 is a reminder of the cooldown of the cold head of the cryocooler. The cooling power of the cryocooler is a function of the temperature at the cold head, and this is tabulated in its manual. The cold head reaches 77K at ~3.8 hours from past data. This provides an upper limit on gains from using LN2 - if the cold head is cooled to 77K instantly, its cooldown curve will be shifted to the left by 3.8 hours, and thus the system will reach steady state 3.8 hours faster. In reality, there would be a time constant for the cold head to reach 77K due to the LN2-copper interaction, and the mass/heat capacity of the cold head. 

Attachment 2 is a plot from Ekin which shows the heat transfer rate (Qdot) per m^2, for the boundary between LN2 and a solid. The different curves correspond for different  between the LN2 and cold head, and for a majority of the cooling from RT, we will use the curves for  ~ 100K. These have lower heat transfer because film boiling is occurring, and a layer of film "insulates" the surface of the solid from the 77K bath. These curves vary in the diameter of the contact area, and we use the bottom curve corresponding to D >= 1.0 cm. Once  reaches 10K, the heat transfer rate jumps up to the nucleate boiling curves. We use the one corresponding to 1atm of pressure. 

Here I analyze potential gains from using LN2 to kick off a Megastat cooldown, and transition to a cryocooler to cool under 77K. 

[JC, Paco]

I took the Moku and iPad from QIL over to 40m for PLL Measurement at the 40m. (SURF)

Here I summarize the analysis of thermal conduction through 3 ceramic ball bearings supporting a test wafer in Megastat. I considered a baseplate or housing for the ceramic ball bearings at the temperature of the cold plate, the 3 ceramic ball bearings themselves, and the wafer seated on top. This pathway can be broken down into the following components:

1. Thermal resistance across joint from Al baseplate to ball bearing

2. Thermal resistance of bulk of ball bearing

3. Thermal resistance across joint from ball bearing to wafer

These resistances in series can be summed, and there exist 3 of these pathways in parallel (1 for each bearing) and the net resistance can then be found using the corresponding formula.

Attachment 1 is a plot of the thermal conductance of alumina ceramic (Al2O3), and it was taken from from this paper. The peak value of 99% Al2O3 agrees with what is listed in the ceramic ball bearing spec sheet: 28 W/mK. I've used these tabulated values to obtain component 2 above. 

Attachment 2 is a plot of the thermal conductances across various joints, taken from Ekin. I could not find data for the conductance across Al--Al2O3 pressure joints or Al2O3--Si pressure joints, but I have used the value corresponding to a stainless steel--stainless steel pressure joint as a conservative upper bound. (*When I was calculating non-negligible conduction last Friday, I had forgotten to update the joint conductance value from an Al--Al pressure joint (orange box in Attachment 2). This greatly overestimated the joint conduction, and the ceramic joints should be a few orders of magnitude below this.) 

The results of this modeling show that the maximum cooling power delivered via conductive contact is below 7e-5 W. Compared to the 6e-3 W of radiative cooling power (~1%), this is negligible. Attachment 3 shows the nominal difference in wafer cooldown - the solid and dashed orange curves are barely distinguishable (T difference of ~0.2K at steady state - RTD uncertainty is +- 0.15K). We should proceed with this design plan for emissivity testing in Megastat. 

RTD Calibration Plan Developed on 16-06-2022:

1. Prepare an ice bath and liquid nitrogen bath to calibrate the RTD.

   1. Ice bath: fill cup (?) with ice. Fill with water until 2 inches below the top of the ice. Let sit for 2 minutes before calibration. 

2. We will use a DMM to measure the resistance across the leads. Right now, we are thinking that we will connect the leads to the resistor using alligator clips or solder them together.

3. Use linear fit to calibrate the RTD value as a function of resistance using these two reference values. 

4. Repeat for all three RTDs

-----------------------------------

1. Q: What is the tolerance of the resistor?

   1. 100 ohm +/- .06%. Means that our calculation if done correctly will be between 99.4 → 100.6 ohms in the ice bath. Our RTD is class A, which has tolerance +/- .15 degrees. Pt-100 SHOULD be 100 ohms at 0C – the temp changes linearly.

From the plot, it seems like the slop is ~50 Ohms / 120 K. So a difference of 5 K corresponds to ~2 Ohms.

It could be that your measurement of the resistance is off slightly due to the lead resistances, etc. Are you using a 3-wire or 4-wire probe?

You can get a higher accuracy relative calibration by dunking all the RTDs together in ice water and then in boiling water.

https://us.flukecal.com/literature/articles-and-education/temperature-calibration/application-notes/how-calibrate-rtd-or-pla

I looked closer at the RTDs, and they are all PT100s with the same resistance-temperature curve [Attachment 1]. So we might need to re-calibrate them.

The chamber is currently warming up, and I plan to cool down again on Monday to a) obtain data for the entire cooldown and b) see if this inner shield behavior is repeated.

I encountered several puzzles in the data from this cooldown (Attachment 1):

1. Due to residual challenges with getting the Megastat SLOW channels recorded to frames (after flood), we've been using a USB to extract data form the CTC100. However, only ~48 hours of cooldown were recorded on the drive - we lost data from the rest of the cooldown. I'm hoping this is due to storage limits on the USB, and if so I will decrease the sampling rate of the CTC100 and/or source a USB with larger storage. Of course this is a temporary solution, and efforts would be better spent on getting data recorded in frames. For now, since every component had reached steady state (except the test mass), most parameters can theoretically still be inferred from the data we obtained. (Note that data logging started a bit after the cryocooler was switched on.)

2. The inner shield appears to have reached a steady-state temperature lower than that of the cold head. This is bizzare, and non-physical - the cold head should be the component supplying cooling power to the rest of the system. Our current Megastat model would not predict such behavior under any choice of parameters. I replaced the inner shield RTD before this cooldown, so I am hoping the new RTD has a different calibration curve than what the CTC100 was expecting. If this is the case, I should be able to use the data to undo the existing calibration and obtain correctly-calibrated temperature for the inner shield. It is worth noting that the new RTD accurately measured room temperature, so if this theory is correct then the calibration error only exists at low temperatures.

[Aidan, Radhika]

On Tuesday 6/7 we met to discuss next steps for getting PD testing up and running in CAML (cryo auxiliary mariner lab). Here is a rough 2-month plan:

Step 0 [1 week]: Get CAML workstation running and confirm connection to EPICS/CDS. 

    - Organize BNC cables coming in from QIL and connect to the PD testing table. 

Step 1 [1 week]: Outline desired measurements and sketch diagram of electronics/components.

    - Decide how we want these component organized; i.e. how many boxes, number of inputs/outputs per box. 

Step 2 [2 weeks]: Implement desired setup.

Step 3 [2 weeks]: Replicate QE measurements on JPL PDs

    - Tweak setup as necessary

Step 4 [2 weeks]: Test MCT detectors

The cryocooler was turned off 6/8 at 11am.

I've modeled the theoretical "best" achievable configuration in Megastat, replacing the flexible strap with a solid copper element, and leaving all but one aperture open in the inner shield [Attachment 1]. In this configuration, the wafer can reach 123K in ~20 hours.

  - The difference in time constant between the copper bar end and cold head (purple and blue curves) is due to the thermal resistance across the bar and 2 joints, plus realistic radiative losses from bar.

  - The difference in time constant between the cold plate and copper bar end (brown/green and purple) is the thermal resistance across the additional solid copper finger and 2 joints.

  - The stead-state temperature difference between the inner shield and the wafer (orange and green) is maintained by heat leaking from 1 aperture in the shield. (We've closed up 4, but best case there will need to be 1 open for the copper bar and leads.

Attachment 2 shows power budgeting of heating and cooling power delivered to various components: 1. copper bar, 2. cold plate, 3. inner shield, 4. Si wafer. The points of intersection of the heating and cooling power curves correspond to the steady-state temperature for each component, which can be verified by Attachment 1. The total heat load on the copper bar and cold plate is just over 70W. 

The ideal model shows that there are two areas of potential gains:

Analyzing the most recent cooldown data will hopefully validate the efforts of closing up apertures and painting the entire wafer enclosure with aquadag. 

[Aidan]

I bought a new photodiode for the West Bridge Labs. It will be housed in the QIL (WB265B) in the central cupboards. There is a QR code on it linking to this page. 

[Stephen, Radhika, Hiya]

This entry describes efforts towards the most recent Megastat cooldown with: 

1.) the inner shield repainted with aquadag and pieces of foil covering the apertures,

2.) aquadag-painted aluminum foil lining the cold plate,

3.) the original test mass and suspension used in previous cooldowns.

Because of the lead time for ordering/fabricating components for wafer supports, we proceeded to use the previous test mass. This way, the improvements made to the chamber can be quantified by comparing the test mass cooldown to past cooldown data. 

On Tuesday 5/31, Hiya and I coated the outside surface of the inner shield with Al foil (previously removed for hole tapping / aquadag repainting -  see 2775). We inserted the inner shield into the chamber and cut a circular piece of aquadag-coated Al foil to cover the cold plate [Attachments 1, 2]. A slit was cut in the piece of foil in order to pass it over the copper bar and flexible strap. Then, the foil was lightly rested over the bulk created by the flexible strap and its flange [Attachment 3]. We poked holes in the foil in order to bolt down dog clamps for securing the inner shield; then we tightened these dog clamps all around [Attachment 4]. 

After completing this, we realized we hadn't applied grease to the bottom lip of the inner shield, for maximizing thermal conductance across the cold plate <--> inner shield joint. We also noticed that the inner shield RTD, previously removed for hole tapping / aquadag repainting, was glitching and read an unphysical temperature value. We wrapped up for the day and made note of these items to address next time.

On Wednesday 6/1, Stephen and I picked up with removing the inner shield and aquadag foil. We applied grease to the bottom lip of the inner shield, in the same manner as previous setups. Stephen noticed that the solder joints of the inner shield RTD (that was glitching) had debonded, and so we re-soldered those joints. We re-inserted the inner shield and aquadag foil and bolted the components down. We placed the suspension frame with the test mass back inside the chamber and connected all RTDs [Attachemnt 5]. Since the heater from previous runs had been damaged, we wrapped the exposed leads with kapton tape and proceeded without a heater. Lastly, a piece of foil was placed under the copper bar opening to cover the cut-out area of the inner shield [Attachment 6]. 

I closed up the chamber and pumped down on Thursday 6/2 at 2:20pm. The cryocooler was switched on at 2:55pm. 

[Shruti, Yehonathan]

We took newport 1811 PDs, one from CTN lab (suspicious) and one from (I forgot) for their high gain and low dark noise.

The detector diameter is small 0.3mm, but our focusing is sufficient:

The mode field diameter of the PM980 fiber is ~ 6.6um. The beam is collimated by a Thorlabs F240APC-1064 with f = 8.07mm and focused with an f = 30mm lens. It means that the diameter at the focus should be roughly 6.6um*30/8.07 = 0.024mm which is well within the PD active area.

We place the PDs at the focal point of the lens at the BHD readout. The impinging optical power was set to be ~ 0.6mW at each port. In one of the PDs, we measure the DC response with a scope to be ~ 5.5 V/0.6 mW ~ 9e3 V/W. According to the specs, the DC monitor as a response of 1e4 V/A while the responsivity of the PD itself is ~ 0.8 A/W at 1064nm so the overall responsivity is ~ 8e3 V/W.

However, the second PD's DC response was bonkers: we measured it to be ten times less. The AC response might still be OK since it is a different port but we haven't measured it yet.

that's a good start, but we want the cooldown to be fast, so what we would need from the model is for you to change some parameters and find out what kind of possible physical changes we can make to make the cooldown fast. i.e. change some parameters and see what configurations would give a fast cooldown, and then we can discuss which of those is the easiest.

Does the true cold-head temperature follow the model? Or is it less good than the model?

[Stephen, Radhika]

Today we further discussed ideas for the enclosure, suspension/support, and heat actuation for wafers in Megastat. 

Enclosure: We re-painted the inner surface of the inner shield with aquadag, including the top side of the bottom lip [Attachments 1,2]. The folded sheets of aluminum foil covering the apertures were unbolted and also painted with aquadag [Attachment 3]. 

Wafer support and heat actuation: Attachment 4 shows sketches and a plan forward (drawn by Stephen). The first round of upgrades includes the components underlined in blue: a baseplate to house the points of contact to the wafer, the ceramic ball bearings which will serve as the points of contact, an aquadag-painted sheet metal insert that will bolt down to the coldplate, and a mount for a Maglite mini flashlight. 

We will obtain more wire-wound resistors to serve as a back-up to flashlight heating, if the need arises.

I modeled the cooldown of a 2" Si wafer in Megastat with cooling from a) the existing cryocooler and b) LN2. The only difference between the two models is the temperature that the cold end of the copper bar "sees" - in the cryo-cooler case, I've used cold head temperature data from a previous cooldown; in the LN2 case, I've used 77K. In Attachment 1, the cryo-cooler models are solid lines and the LN2 models are dashed. 

It is clear from the plot that the cold head gets ~30K colder than LN2 at 77K. This explains the discrepancy between the inner shield models for the two cases at steady state. While the initial temperature rates of change are greater in the LN2 case, the cold head crosses the 77K line in roughly 5 hours. 

I think this flashlight would work

This post will serve as a running list of modifications for Megastat for emissivity estimation (brain dump). It is divided into categories:

Category 1: Inner shield / cold plate

We want to cover the cold plate with Aquadag, either directly or by placing aquadag-coated tiles or Al foil on top of the cold plate. It seems the latter approach is preferable, to avoid directly removing and coating the cold plate. Stephen has prepared aquadag-coated aluminum foil which we will soon assess for this purpose. If for some reason it doesn't seem to be sufficient, we will need to identify/design tiles or chunks of aluminum that we can paint with aquadag and lay on top of the cold plate. While we're coating things with aquadag, there are some spots on the inner shield that could use a touch-up.

Category 2: Suspension of sample

There are several options for suspending a 2" Si wafer or 1" optic.

1. RTD lead suspension

This method would involve wrapping the RTD wires around the sample and somehow hanging or dangling the sample from another surface. The wire would be strain-relieved around another component, like a post, before being varnished to the sample. I will have to play around and try out various configurations to determine if this is feasible / would not strain the RTD contact. This approach would not require additional components in the chamber. 

2. Insulating posts 

In the case where we cannot achieve RTD lead suspension, we will need to rest the sample on support posts while minimizing conductive heat transfer through said posts. Stephen suggested using ceramic ball bearings bolted down to the cold plate. Further modeling is needed to calculate conduction through these supports. Using a bunch of tiny "pins" was also suggested, but these would need to be similarly modeled, and eventually procured. 

Category 3: Heat actuation on sample

There are several options for applying heating power to the suspended sample, each with its own drawbacks.

1. Wire-wound resistor

Binding this directly to the sample will be challenging, especially if the sample is wire-suspended. The resistor will occupy a non-negligible amount of area on the sample, which is not ideal for maximum thermal coupling between the sample and the inner shield. Furthermore, since the emissivity an uncoated Si wafer is quite low (low bulk absorption given thin wafer), the emissivity of the heater body (or the varnish/epoxy) could dominate the coupling and lead to an inaccurate fit for sample emissivity. 

2. Lamp source + parabolic reflector

The lamp and reflector would be placed and mounted inside the chamber and directed towards the sample. I found parabolic reflectors in the TCS lab and would need to purchase a suitable light source. Here is an example from my search: https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=7541. Its emission is broadband and could probably be modeled as a blackbody, but I would need to look into this further. I don't know how efficient the heat transfer to the sample would be, i.e. how much heating power would actually hit the sample. This would also make the control of heating power much more difficult.

3. Laser heating source

A laser could be set up outside Megastat and send its beam through one of the viewports to hit the sample. There would be some reflection from the viewport glass, but the transmission would directly heat the suspended sample without crowding the inside of the chamber. The heating power reaching the sample could more easily be determined and controlled in this method, although a laser would need to be sourced for this purpose. This would also require uncovering an additional viewport, which could contribute more heat leakage into the enclosure. 

The results from tapping are in the previous entry, and it seemed to work well. As for the emissivity of the inner surface, it would definitely be better to have it black but I wasn't sure how feasible it would be to coat the coldplate. If this is something we can consider, I won't remove the aquadag from the shield.

I'd recommend drilling a through hole and using a nut, rather than tapping as I suggested earlier. The shield is not thick enough to have many 4-40 threads.

Is it better to have the inner shield inner surface low or high emissivity? \Depending on the effect on the MCMC uncertainties, we may want to make everything black, including the cold plate. i.e. we could mount something like black aquadag tiles on the cold plate.