I am trying again to measure a temperature step response on the reference cavity on the PSL table.
I have been working to relock the NPRO to the cavity. I unblocked the laser beam, reassembled the setup described in my previous elog entry: 5202. I then did the following:
1) Monitored error signal (from LB1005 PDH servo), transmitted signal, and control signal sent to drive PZT on oscilloscope.
2) With loop open, swept through 0,0-mode resonance, saw a peak in the transmission, saw an accompanying error signal similar to the signal shown in 5202.
3) Tried to lock. Swept the gain on the LB1005 and could not find a gain that would make it lock. Tried changing the PI-corner freq. from 10 kHz to 30 kHz and back and still could not lock.
4) Noticed that the open loop error signal displayed on the scope was DC-offset from zero. Changed the offset to zero the error signal.
5) Tried to lock again and succeeded.
6) Noticed that upon closing the loop, the error signal became offset from zero again. Turning on the integrator on the LB1005 increased DC-offset.
7) Reduced the gain on the SR560 being used as a low pass filter from 5 to 1. Readjusted the open loop error signal offset on the LB1005.
8) Closed the loop and locked. Closing the loop then caused a much smaller DC change in the signal than I had seen with the larger gain (now around 3mV). RMS fluctuations in error signal are now 1 mV (well within the linear region of the error signal).
9) Noticed transmission has a strange distorted harmonic oscillation in it a 1MHz. (Modulation freq is 230kHz, so it's not that). Checked reflected signal and also saw a strange oscillation--in a sawtooth-like pattern.
I intend to
1) Post oscilloscope traces here showing transmitted and reflected signal when locked.
2) Look upstream to see if the sawtooth-like oscillation is in the laser beam before it enters the cavity:
3) At some point, try to close the slow digital loop, perhaps readjusting the gain.
4) Try to measure a temperature step response.
I decided to go forward and try to close the digital loop in spite of the unexplained oscillations in the transmission.
1) Plugged the 20dB attenuator into the slow port on the laser drive. This pushed the laser out of lock and, for some reason, made the laser temperature stop responding to sweeping the set point manually with the knob.
2) Plugged the output from the digital system into the slow port (with the attenuator still in place).
3) Displayed the beam seen by the camera on a monitor in the control room
4) Stepped the laser temperature using MEDM until finding the 0,1 mode. Locked to that mode.
5) Closed the digital loop (input to slow laser drive attenuated 20dB attenuator). Gain 0.010
6) Loop appeared stable for 30 minutes, then temperature began shooting off. I opened the loop, cleared history, reduced the gain to 0.008, and started it again. Loop appears stable after 15 minutes of watching. I'm going to leave it for a few hours, then come back to check on it and, if it's stable, step the can temperature.
Using the equation for thermal resistance
Rthermal = L/(k*A)
where k is the thermal conductivity of a material, L is the length, and A is the surface area through which the heat passes, I could find the thermal resistance of the copper and stainless steel on the reference cavity. To reduce temperature gradients across the vacuum chamber, the thermal resistance of the copper must be the same or less than that of the stainless steel. Since the copper is directly on top of the stainless steel, the length and width will be the same for both, just the thickness will be different (for ease of calculation, I assumed flat, rectangular strips of the metal). Assuming we wish to have a thermal resistance of the copper n times less than that of the stainless steel, we have
RCu = RSS/n
L/(kCu*w*tCu) = L/(kSS*w*tSS*n)
tCu/tSS = n*kSS/kCu
We know that kSS = 401 W/m*K and KCu = 16 W/m*K, so
tCu/tSS = 0.0399*n
By using the drawings for the short reference cavity vacuum chamber (the only one I could find drawings for online) I found a thickness of the walls of 0.12 in or 0.3048 cm. So for the same thermal resistance in both metals, the copper must be 0.0122 cm thick and for a thermal resistance 10 times less, it must be 0.122 cm thick. So we will have to keep wrapping the copper on the vacuum chamber!
From the trend, it seems that the Reference Cavity's temperature servo is working fine with the new copper foil. I was unable to find the insulating foam anywhere, but that's OK. We'll just get Frank to make us a new insulation with his special yellow stuff.
The copper foil that Steve got is just the right thickness for making it easy to form around the vacuum can, but we just have to have the patience to wrap ~5-10 more layers on there. We also have to get a new heater jacket; this one barely fits around the outside of the copper wrap. The one we have now seems to have a good heating wire pattern, but I don't know where we can buy these.
I also turned the HEPA's Variac back down to the nominal value of 20. Please remember to turn it back up to 100 before working on the PSL.
Rana and I
1) took the temperature sensors off the reference cavity;
2) wrapped copper foil around the cavity (during which I learned it is REALLY easy to cut hands with the foil);
3) wrapped electrical tape around the power terminals of the temperature sensors (color-coded, too! Red for the out of loop sensor, Blue for the first one, Brown for the second, Gray for the third, and Violet for the fourth. Yes, we went with an alphabetical coding system, excluding the out of loop sensor);
4) re-wrapped the thermal blanket heater;
5) covered the ends of the cavities with copper, ensuring that the beam can enter and exit;
6) took pretty pictures for your enjoyment!
We will see if this helps the temperature stabilization of the reference cavity.
The end of the reference cavity, with a lovely square around the beam.
The entire, well-wrapped reference cavity!
I was aware of a problem on those units since I acquired the data. Then it wasn't totally clear to me which were the units of the data as downloaded from the Agilent 4395A, and, in part, still isn't.
It's clear that the data was in units of spectrum, an not spectral density: in between the two there is a division by the bandwidth (100KHz, in this case). Correcting for that, one gets the following plot for the FSS PD:
Although the reason why I was hesitating to elog this other plot is that it looks like there's still a discrepancy of about 0.5dBm between what one reads on the display of the spectrum analyzer and the data values downloaded from it.
However I well know that, I should have just posted it, including my reserves about that possible offset (as I'm doing now).
This evening we measured the noise spectrum of the reference cavity PD used in the FSS loop. From that we estimated the transimpedance and found that the PD is shot-noise limited. We also found a big AM oscillation in correspondence of the FSS modulation sideband which we later attenuated at least in part.
Some more words about the RFAM: I noticed that there was an excess RFAM by unlocking the RC and just looking at the RF out with the 50 Ohm input of the scope. It was ~100 mVp-p! In the end our method to minimize the AM was not so sensible - we aligned the waveplate before the EOM so as to minimize the p-pol light transmitted by the PBS cube just ahead of the AOM. At first, this did not minimize the RFAM. But after I got angry at the bad plastic mounting of the EOM and re-aligned it, the AM seemed to be small with the polarization aligned to the cube. It was too small to measure on the scope and on the spectrum analyzer, the peak was hopping around by ~10-20 dB on a few second timescale. Further reduction would require some kind of active temperature stabilization of the EOM housing (maybe a good SURF project!).
For the EOM mount we (meaning Steve) should replace the lame 2-post system that's in there with one of the mounts of the type that is used in the Mach-Zucker EOMs. I think we have spare in the cabinet next to one of the arms.
After the RFAM monkeying, I aligned the beam to the RC using the standard, 2-mirror, beam-walking approach. You can see from the attached plot that the transmission went up by ~20% ! And the reflection went down by ~30%. I doubt that I have developed any new alignment technique beyond what Yoichi and I already did last time. Most likely there was some beam shape corruption in the EOM, or the RFAM was causing us to lock far off the fringe. Now the reflected beam from the reference cavity is a nice donut shape and we could even make it better by doing some mode matching! This finally solves the eternal mystery of the bad REFL beam (or at least sweeps it under the rug).
At the end, I also fixed the alignment of the RFPD. It should be set so the incident angle of the beam is ~20-40 deg, but it was instead set to be near normal incidence ?! Its also on flimsy plastic legs. Steve, can you please replace this with the new brass ones?
Teflon feet removed and heavy brass-delrin pd base installed. Ref-cavity reflected light remains to be beautiful doughnut shape on camera.
I made the changes to the psl.db to handle the new Temperature box hardware. The calibrations (EGUF/EGUL) are just copied directly from the LHO .db file (I have rsync'd their entire target area to here).
allegra:c1psl>diff psl.db~ psl.db
< field(DESC,"TIDALOUT- drive to the reference cavity heater")
< field(SCAN,".5 second")
< field(INP,"#C0 S28 @")
< field(DESC,"TIDALINPUT- tidal actuator input")
< field(SCAN,".5 second")
< field(INP,"#C0 S3 @")
> field(DESC,"TIDALINPUT- tidal actuator input")
> field(SCAN,".5 second")
> field(INP,"#C0 S3 @")
> field(DESC,"TIDALOUT- drive to the reference cavity heater")
> field(SCAN,".5 second")
> field(INP,"#C0 S28 @")
Summary: This afternoon we managed to get the temperature control of the reference cavity working again.
We bypassed the MINCO PID by connecting the temperature box error signal directly into EPICS.
We couldn't configure the PID so that it worked with the modified temperature box so we decided to just avoid using it.
Now the temperature control is done by a software servo by using the channel C1:PSL-FSS_MINCOMEAS as error signal and driving C1:PSL-FSS_TIDALSET (which we have clip-doodle wired directly to the heater input).
We 'successfully' used ezcaservo to stabilize the temperature:
ezcaservo -r C1:PSL-FSS_MINCOMEAS -s 26.6 -g -0.00003 C1:PSL-FSS_TIDALSET
We also recalibrated the channels:
with Peter King on the phone by using ezcawrite (EGUF and EGUL) but we didn't change the database yet. So please do not reboot the PSL computer until we update the database.
More details will follow.
Basically, in addition to the replacement of the resistors with metal film ones, Peter replaced the chip that provides a voltage reference.
The old one provided about 2.5 V, whereas the new one gets to about 7V. Such reference voltage somehow depends on the room temperature and it is used to generate an error signal for the temperature of the reference cavity.
Peter said that the new higher reference should work better.
It turned out that half an hour was too long. In less than that the reference cavity temperature passed the critical point when the temperature controller (located just below the ref cav power supply in the same rack) disables the input power to the reference cavity power supply.
The controller's display in the front shows two numbers. The first goes with the temperature of the reference cavity; the second is a threshold set for the first number. The power supply gets enabled only when the first number comes under the threshold value.
Now the cavity is cooling down and it will take about another hour for its temperature to be low enough and for the heater power supply to be powered.
The cavity temp cooled below SP2 set point 0.1 The Minco SP1 (present temp in Volts) now reading -0.037 so DC power supply was turned on and set to 12V 1A
The 40m Lab reference cavity temperature box S/N BDL3002 was modified as per DCN D010238-00-C.
R1, R2, R5, R6 was 10k now are 25.5k metal film
R11, R14 was 10k now are 24.9k metal film
R10, R15 was 10k now are 127k thick film - no metal film resistors available
R22 was 2.00k now is 2.21k
R27 was 10k now is 33.2k
U5, the LM-336/2.5 was removed
An LT1021-7, 7 V voltage reference was added. Pin 2 to +15V, pin 4 to ground, pin 6 to U6 pin 3.
Added an 8.87k metal film resistor between U6 pin 1 and U4 pin 6.
Added an 8.87k metal film resistor between U6 pin 1 and U4 pin 15.
The 10k resistor between J8 pin 1 and ground was already added in a previous modification.
In addition R3, R4, R7, R8, R12 and R13 were swapped out for metal film resistors of the same value
The jumper connection to the VME setpoint was removed, as per Rana's verbal instructions.
This disables the ability to set the reference cavity vacuum chamber temperature by computer.
There's no elog entry about what work has gone on today, but it looks like Peter took apart the reference cavity temperature control around 2PM.
I touched the reference cavity by putting my finger up underneath its sweater and it was nearly too hot to keep my finger in there. I looked at the heater power supply front panel and it seems that it was railed at 30 V and 3 A. The nominal value according to the sticker on the front is 11.5 V and 1 A.
So I turned down the current on the front panel and then switched it off. Otherwise, it would take it a couple of days to cool down once we get the temperature box back in. So for tonight there will definitely be no locking. The original settings are in the attached photo. We should turn this back on with its 1A setting in the morning before Peter starts so that the RC is at a stable temp by the evening. Its important NOT to turn it back on and let it just rail. Use the current limit to set it to 1 A. After the temperature box is back in the current limit can be turned back up to 2A or so. We never need the range for 3A, don't know why anyone set it so high.
While Peter King is still working on the reference cavity temperature box, I turned the power supply for the reference cavity's heater back on. Rana turned it off last night since the ref cav temperature box had been removed.
I just switched it on and turned the current knob in the front panel until current and voltage got back to their values as in Rana's picture.
I plan to leave it like that for half an hour so that the the cavity starts warming up. After that, I'll turn the current back to the nominal value as indicated in the front panel.
Good settings for acquisition:
MC INPUT GAIN = 6 dB
FAST polarity MINUS
VCO Gain -3 dB
MC LIMITER Disable
FSS TEST1 TEST
FSS CG -3 dB
FSS FG 13 dB
[HV pulse] ----+ +-->-- [PD2]
->--[HWP]->-- [EOM] -->-- [PBS] --<->-- [QWP] --<->-- [Reference Cavity] -->-- [PD1]