Just to confirm that my noise estimates make sense, here's a plot of the not-normalized QPD signal that gives the X motion (sum and difference of all four quadrants):

This is the signal after compensating for the whitening filter. If I remove this compensation, the following plot gives the noises in terms of the voltage directly in input to the ADC (or in output of the analog board):

So the total "dark" electronic noise is about 13 uV/rHz.

I did a roughly estimate of the sources of electronic noise:

• QPD dark current noise, from datasheet, at the peak sensitivity is equivalent to 2e-14 W/rHz, or 2 nV/rHz at the output of the TI stage
• First stage: Johnson-Nyquist noise of the TI resistor: 58 nV/rHz
• First stage: output voltage noise of the LT1124: 3 nV/rHz
• First stage: input curent noise of the LT1124, converted to the output: 60 nV/rHz

So the total noise at the outoput of the first stage is about 84 nV/rHz. The second stage adds a gain of 30 at high frequency, and negligible noise. So at the output of the whitening we have 2.5 uV/rHz. The DRV135 adds another gain of 2 and a neglegible output noise. 

So the total electronic noise at the output of each quadrant is 5 uV/rHz. Since we are combining four of them, the total expected electronic noise is 10 uV/rHz, which is not too far from the measured value. 

We are basically dominated equally by the Johnson-Nyquist noise of the TI resistor and by the input current noise of the LT1124. No gain to be obtained by changing the whitening.

So here's the final proof that the glitches I see are digital:

• There is a positive jump (of random size) every second, every time 0.45s after the beginning of the second. There is a negative jump at a less constant time, but between 0.65s and 0.8s after the beginning of each second
• I moved my whole setup to the crackle lab. This included: HeNe laser with power supply, QPD with cable, interface board and power supply. I don't see any glitch there

I swapped the ADC board with a second one in the new cymac, but no change: glitches are still there.

The glitches I saw in the data happens roughly every second, even though not exactly on the second. They are suddend jumps on the signal values over one sample, so of clear digital origin

Yesterday I could clearly see the glitches as jumps in the time domain plot of X and Y signals, and trace them to somewhat harder to see jumps in the quadrant signals. 

• I tried to catch them with an oscilloscope looking at the analog signals, but couldn't see any. However, for a brief period I had a persistent square wave oscillation in all transimpedance stages
• I hooked the single ended output of the whitening filter to an ADC, and I could see the jumps there too. So I can exclude it's a problem of the differential drivers

One suspect was an intermittend oscillation of the transimpendance amplifier, so I looked into the schematics (D1600196) to see what could be the optimal value of the compensating capacitor C7. Following some useful notes online I computed the optimal value of C7 to be close to 2pF (instead of 10pF). I used 30-35 pF as the QPD capacitance, and 10 MHz has the gain-bandwidth product of the opamp. I swapped all 10pF capacitors with 2pF. After this I can still see the glitches in the spectra, but I can't find them anymore in the time domain. So things seem to have improved, although I still have annoying glitches.

Rich suggested to test the stability of the transimpendance stage by driving the output with a square wave and looking for the signal ringing. Here's his note:

I tried this for both the TI stage and the whitening stage, using 1k and 1uF and a square wave at 10 kHz. Here are the results, which look reasonable to me (firts is the TI, ringing at about 0.5 MHz, second is the whitening, almost no ringing):

So now I'm quite confident that the electronics is working. In the first trace you can see some intermittend background noise, due to the ambient light leaking into the QPD.

More investigations will follow.

2017-08-10

• The attached plots compare the new and old geometries with .5 mm and 1 mm sample gaps. They are the same plot on linear and logarithmic axes respectively

2020-06-09

• 11:00 am in chamber
  • GaAs in CR1
  • S1600805 in CR2
• 11:04 am roughing pumps on
• 11:15 am turbo pump CR1-4 on

2018-03-05

• 10:45am in chamber:
  • AK_01 in CR1
  • AK_02 in CR2
  • AK_03 in CR3
  • AK_04 in CR4
• 10:55am roughing pump on
• 11:05am turbo pump on

2017-06-27

• I built a new model of the ESD to determine whether or not the spikes in the electric field at the corners was affecting the results enough that it had to be accounted for in further models. To create the model, I created a 2D profile of the arm used in my original model and filleted the corners at a radius of .05 mm, since the electrode model is .1 mm thick, this made completely rounded edges. In creating this model I caught an earlier mistake in the original one, I only set one half of the surface of the electrodes to have a potential or to ground, the "bottom" was left with no charge. I fixed this mistake and then compared the two models at a potential of 1000 V. For speed of computation I ran both models with a finer mesh size and then calculated the electric field at approximately the middle of the ESD, 1mm above the fourth electrode arm. For the rounded electrodes the field had a value of 84024 V/m and for the rectangular electrodes the field had a value of 80728 V/m, which is less than a 4% difference in field magnitude. Furthermore, the field shapes appear nearly indistinguishable; I am confident from this test that I can proceed modelling the arms of the ESD as rectangles.

• checked that the high voltage paths are working:
  • the DAC signal is properly received and amplified by the Trek 2220 HV amplifier
  • the HV switch box is working as expected: I can switch on and off each path individiually using the DAC signals
• installed four samples (refer to the optical drawing for the numbering)
  • S1600472 in bay 1
  • S1600478 in bay 2
  • S1600473 in bay 3
  • S1600480 in bay 4

​

• balanced all samples and centered all optical levers. There is a problem with the fifth connector in the ADC interface board (or maybe with one ADC channel). So one of the quadrants of QPD4 was not working. I moved the connector to number 6 and updated the model. Everything looks fine now.
• the power in the four QPD is quite diverse: this is not unexpected, since it depends on the interference of the reflection from the two surfaces of the disk

• installed the picomotor controllers on the bench, connected them to the net using the switch on the top of the clean room. For the moment being they are using DHCP.
• connected all picomotors to the controllers.
  • Bottom controller, from left to right: QPD1x, QPD1y, QPD2x, QPD2y
  • Top controller, from left to right: QPD3x, QPD3y, QPD4x, QPD4y
• tested the remote control and motion of all picomotors

​

Today I installed the four GeNS systems into the chamber, connected the ESD to the HV feedthrough and put the periscopes in place.

Note 1: the lens holders have been remachined to solve a dimension problem, by removing completely the two lips against which the lens was supposed to be sitting. To recover a reasonable centering of the lens, I added two small shims inside each holder. They are made out of wrapped aluminum foil.

Note 2: I added some small shims (again made of wrapped aluminum foil) below the base plate, to make it as close to horizontal as possible

Finally, I managed to align the optical level beam for the QPD1, using as usual a small container with water to get the horizontal reference.

Some pictures below.

2017-07-14

• I have completed a rough, but functioning script that calculates the modal force profiles. The force values are still coming out incorrect (on the order of 10^14) but the script can take in my model as a .m file and return an array with a force value per mode. I am attaching both the .m file and the matlab script
• I have done very little work with the numerical integration itself, based on the 2D numerical integration code I received I just appended a z component and left it at that so when I return from Livingston I will  fix that component

2017-07-05

• I sorted out my mathematical lapse in logic and computed the correct force profiles in the perpendicular direction in both disks. The issue is that now the force profiles don't match up. The fact that there is a measured force distribution for the E^2 case outside the disk is only an artifact of the numerics because it is being calculated only from the electric field data which is defined outside the sample. It can be easily removed for final plots once the force distributions are matched by either redefining the cut plane or putting a data filter specifically on the E^2 plot. The jumps in the E^2 plot suggest that the meshing is still too large, I will try to fix this first, hopefully it will help resolve the difference.

2017-07-05

• In order to improve my data I shrunk the region of the finer meshing slightly and made the mesh even smaller and then recalculated the force profiles. This time I tried sampling regions inside the disc rather than immediately at the surface. The attached graphs were sampled at the center of the disc. These two techniques vastly improved the data, now the profiles appear the same, but the magnitudes differ by a factor of 2 again. Previously this was due to an error in my calculation of the force, now I do not believe this to be the case. I will leave my work here for the purposes of my first report, it is an interesting result. I also restricted my data set to the finely meshed box which resolved the earlier data display issue. 

The cymac3 internal clock was off by about 10 seconds. When I tried to start the NTP service, I found out that the cymac3 couldn't reach any external server. It turned out that the gateway in /etc/network/interfaces was set to the wrong address. I fixed it and rebooted. Now NTP is working and the time is correct.

This fixed a small issue with diaggui, which always complained about a data receiving error when starting a measurement (although after the complian the measurement could continue)

Today I started programming part of the user interface that will be used to perform the measurements. Not much implemented so far, but you can get an idea of the look:

Buttons on the left sidebar will allow the user to perform some basic tasks. The main panel has a plot (which will show spectra or ring down measurements) and a log section.

Turbo pump controller (new chamber) was configured. Need to reduce the frequency or setup a standby mode. First pump down: E-7 range reached within about an hour. See plot: blue - old crime chamber, pink - new crime chamber.

The new laser is still acting up a bit, with a wandering line moving sometimes inside the band. Nevertheless, I could perform the first complete measurement of four samples in parallel.

Everything went as expected, there is no visible cross coupling between modes of different disks. The MATLAB code for the analysis is ready and working.

Here are the results:

Everything is working pretty well. This morning the pressure was about 1.2e-6 Torr. I connected the high voltage amplifier and I could drive the disk without problems.

I measured the beam shape and size at the QPD. We have about 50 uW, we see a TEM01-like mode due to the interference of the two disk surfaces (this is normal). The beam is about 3 mm in diameter. using this information and the estimated optical lever length of 1.2 m, I calbrated the QPD NORM signals in units of angular motion of the disk surface. The computation posted in CRIME_Lab/60 is actually wrong. I'll post the correct one later.

Injecting broadband white noise I could excite all the modes that are visible up to about 30kHz. I tuned the COMSOL model, by changing the thickess of the disk to 1.017 mm, to fit the frequency of the first few modes. Here are the modes I could measure:

Take a look at the attached PDF file for the shape of all the modes, including all that are not visible. We see all the modes we expect to be able to excite with the central suspension of the disk.

The roughing pump is making a lot of non stationary low frequency noise. I turned it off, and the pressure stayed constant at 1.2e-6 Torr over about 1.5 hours. Here's the difference in the QPD spectrum:

It turned out that I have enough excitation authority to knock the disk out of the right place. So I had to vent to recover the situation. I'll open the chamber tomorrow and see what happened.

Here's a first bird eye look at the ring downs. We see beating of the two almost denegerate modes in some cases. Fits will follow, using the procedure I used for the LMA measurements.

This afternoon we started the pump-down with all the system installed into the chamber. Unfortunately the IGM vacuum gauge isn't working, so we can't be sure what the pressure is. To be fixed

We can't generate any arbitary signal with the real time model, since awg is not working properly yet. For the moment being I added a uniform random number generator in the model (only option I found for noise) and send it into the ESD filter bank. In this way I can generate band-passed noise.

I plugged in the DAC output to the HV amplifier input, and I could send white noise to the electrostatic drive. Behold: I was able to excite quite a few modes. In the following trace blue is a reference and red is right after I sent white noise (3 V peak to peak) to the disk for a while (less than 1 minute). Excitation stopped at 4:56pm LT.

Using COMSOL and tuning the disk thickness at 1.018 mm I could hit the frequency of the first butterfly mode (1109 Hz) and get a reasobly good estimate of the other modes.

After about half a hour of ring down, most of the modes are gone but the two lowest are still going strong.

Note that the flattish background noise seems to be generated by some sort of glitches. I tried to swap the laser and the power supply, without change. More investigations are needed.

Note that the roughing pump was still on during the test.

Using the first ring down of the day (GPS 1155754513 + 3600 seconds), I computed the amplitude of each of the modes already identified, using a short FFT spectrogram (each FFT is 1 second long, overlap of 0.5 s).

Then I used the same code I developed at LMA to fit the ring down, including the beat between the unresolved mode pairs. The fit is versy sensitive to the initial conditions, so I had to fine tune them for each of the 20 modes. Still, all fits were successful with 30 minutes of work.

Here are all the fits:

And in summary all the measured Qs, which turned out to be larger than what I was expecting, considering that the disk is not annealed.

The analysis code in MATLAB is attached.

Here's the first spectrum of the QPD X and Y signals, acquired with the digital system. Roughing and turbo pumps are still on.

The noise floor seems quite non stationary. To be investigated.

I finished the first version of the automation software to measure the ring down of the disk modes. I tested it with the new substrate that was installed yesterday. Here are some screenshots and a brief explanation of how it works.

It is based on a Python/Tk GUI, that can be launched on the workstation with the command ~/CRIME/crime.py

The main screen is similar to the following. Once a baseline spectrum is acquired, it is shown in the main panel:

The user should specify the folder and prefix of the result files, and other parameters related to the excitation. The when the "Excite and ring down..." button is pressed, here's what happens

1. If a baseline spectrum (before excitation) is not available, one is acquired with the specified parameters
2. A broadband white excitation is applied with the selected amplitude and duration

1. Another spectrum is taken. This is then whitened by dividing it with the baseline. This could be used directly to select the modes that have been excited. However, some parts of the noise floor are non stationary, so a second whitening is performed: the noise background is estimated by removing all lines, and it is then again divided out from the spectrum.
2. All lines above a SNR threshold are then selected and shown in the main window together with the whitened spectrum: 

At this point the amplitude of the peaks are continuosly monitored (every second) and thei amplitude shown in a new window. The user can select a subset of the modes for the plotting.

There are some wandering peaks in the spectrum, so some of the peaks aren't actually modes that get excited. This is easily fixed in the post processing of the results. 

All peak amplitudes are saved to files in real time, so if you stop the GUI you'll have some partial results.

The following table shows the lowest eigenfrequency (Hz) for different sizes of disks

Here are the nominal parameters of the disk with flats

A COMSOL simulation gives the frequencies and mode shapes shown in the attached PDF file. Following the list of frequencies and a classification of the mode family (numer of radial nodes, number of azimuthal nodes in a half turn):

2017-08-23

• Excitations

  • Quiet time before excitation: 1187577794
    Excitation broadband: 1187577826
    Quiet time after excitation: 1187577848

  • Quiet time before excitation: 1187585078
    Excitation broadband: 1187585111
    Quiet time after excitation: 1187585133

  • Quiet time before excitation: 1187592363
    Excitation broadband: 1187592395
    Quiet time after excitation: 1187592417

  • Quiet time before excitation: 1187599647
    Excitation broadband: 1187599679
    Quiet time after excitation: 1187599701

  • Quiet time before excitation: 1187606931
    Excitation broadband: 1187606964
    Quiet time after excitation: 1187606986

  • Quiet time before excitation: 1187614217
    Excitation broadband: 1187614249
    Quiet time after excitation: 1187614271

The results show very low Q values, so we suspected some issues. Yesterday we openend the chamber and moved the ESD up, as high as possible. New excitation followed:

• Quiet time before excitation: 1187676942
  Excitation broadband: 1187676974
  Quiet time after excitation: 1187676996

• Quiet time before excitation: 1187684226
  Excitation broadband: 1187684258
  Quiet time after excitation: 1187684280

• Quiet time before excitation: 1187691510
  Excitation broadband: 1187691542
  Quiet time after excitation: 1187691564

• Quiet time before excitation: 1187698794
  Excitation broadband: 1187698826
  Quiet time after excitation: 1187698848
   

   

New set of excitations after raising the ESD:

• Quiet time before excitation: 1187676942
  Excitation broadband: 1187676974
  Quiet time after excitation: 1187676996
• Quiet time before excitation: 1187684226
  Excitation broadband: 1187684258
  Quiet time after excitation: 1187684280
• Quiet time before excitation: 1187691510
  Excitation broadband: 1187691542
  Quiet time after excitation: 1187691564
• Quiet time before excitation: 1187698794
  Excitation broadband: 1187698826
  Quiet time after excitation: 1187698848

Q values are now reasonable

. 

The excitation is working fine. However, there is a large cross coupling in the excitation between channels: so if I switch on only one of the ESD paths, I actually excite all other disks too.  It might be due to the common ground or to the fact that the cables run close to each other. This needs some investigation, but it's not a big issue for the moment being.

On the other hand, the optical levers are very well decoupled: each one sees a different set of modes. So there is no measurable cross coupling between the disks or the readout.

There seems to be something fishy with the Y picomotor of QPD3: it doesn't always move in the same direction for the same set of steps. Some investigations needed here too. Autocentering might fail, but it's still possible to center it manually.

Excitation at 4:56pm (quiet time before excitation 1170032165, quiet time after excitation 1170032234).

Another excitation at 8:30am (2017-02-02) (quiet time before excitation 1170088185, quiet time after excitation 1170088373)

The pressure is at abour 3e-6 Torr. I centered the QPD and started an excitation. The HV amplifier manual states that the driver can source both positive and negative voltage, so this time I didn't add any offset, but simply drove with 1000 V peak to peak. After the excitation the QPD was slightly miscentered in X and I had to manually recenter it.

Good data starting from

PDT: 2016-09-20 16:38:09.330642 PDT
UTC: 2016-09-20 23:38:09.330642 UTC
GPS: 1158449906.330642

NOTE: it's a good idea to take a look at both the X and Y signals for each mode. Some of them look stronger in Y than in X. So far I only used X.

New excitation (2000V) at about 8:06am. Had to recenter the QPD again after the excitation.

Engaged the 500Hz high pass filter on the ESD filter bank. New excitation ended at 8:11am. Amplitude 1000 V. Recentered the QPD at 8:11:35am

Installed the etched disk: using manually the centering ring allowed me to get the beam on the QPD. A couple of taps to the disk were enough to get the beam centered.

Pump down started at 8:52am

I was looking at some past trend data and discovered that EPICS values were not written to the frames. I added the following two lines to /opt/rtcds/tst/x3/target/fb/master to fix this:

/opt/rtcds/tst/x3/chans/daq/X3EDCU_CR1.ini
/opt/rtcds/tst/x3/chans/daq/X3EDCU_TST.ini

Now EPICS values are saved to frames, but they are all zero! I noticed that we always had the same problems with the cymac2 too.

So for the moment being I set up daqd to save X_NORM_IN1 and Y_NORM_IN1 at 32 Hz. In this way I can monitor the QPD centering.

/opt/rtcds/tst/x3/chans/daq/X3EDCU_CR1.ini
/opt/rtcds/tst/x3/chans/daq/X3EDCU_TST.ini

Apparently, there was a mismatch in the configuration, and DAQD was adding a wonderful 16 Hz comb all over the spectrum.

I stopped the processes, but couldn't restart x3cr1. It turned out that I can't save a channel to frames with a sampling frequency lower than 256 Hz. I changed the model, recompiled and restarted. Now the 16 Hz is gone.

/opt/rtcds/tst/x3/chans/daq/X3EDCU_CR1.ini
/opt/rtcds/tst/x3/chans/daq/X3EDCU_TST.ini

Using the already installed high voltage feedthrough, I cabled one of the electrostatic actuators (1mm gap between electrodes) and installed it into the chamber. One of the electrodes is connected to the feedthrough cenral pin, the other is grounded on the bottom of the chamber.

The electrostatic actuator is mounted at about 1 mm above the disk, see pictures.

As a preliminary test, I checked that switching the HV amplifier on and off with about 1.5kV produces a visible motion (~2-3 mm) of the optical lever beam. So the actuator is working.

I ran a series of COMSOL simulations to compute the dilution factors of a coated disk with the dimensions we are currently using (75mm diameter, 1mm thick, 1um of coating).

The mesh is generated as follows:

1. a free triangular mesh with defined maximum element size is generated on the coating top surface
2. the triangular mesh is swept across the coating, generating a defined number of layers
3. the same mesh is swept through the substrate, generating a defined number of layers

The plots below shows the effect on the dilution factor convergence of the three parameters above. It turns out that the size of the surface trinagular mesh is the most relevant parameter, followed by hte number of layers in the coating. Instead, the number of layers in the substrate is not particularly relevant.

The disks coated at Montreal are hold with three small clips. Therefore there are three small regions close to the edge that are not coated. See the picture below to see one of the samples with the clips.

To check the effect of the clip fingerprints on the dilution factor, I set up a COMSOL simulation. For simplicity, I started with only two small clips as shown below:

The result is that they have a very small effect. The first plot below compares the dilution factor (energy in the coating over total energy) with and without the fingerprints:

Another way to look at it is given below: the plot shows the percentage difference in the computed dilution factor. It's always smaller than 1%, so completely neglegible. In conclusion: we don't need to model the clips.

ntroduction

A set of substrates have been coated by the Colorado State University Fort Collins group, with ~500 nm tantala and various ion assist beam parameters. Here's a table summarizing the depositions parameter, by Le Yang

Coating losses before annealing

The plot below shows the measured loss angle for all modes of all samples, before annealing. The error bars for the datapoints are from the 95% confidence intervals computed from 8 measurements each. The red line is the average value over frequencies, and the shaded red area gives the 95% confidence interval of the mean value. The loss angle seems reasonably indipendent of frequency.

The following pot then shows the averaged loss angle as a function of the serial number, for reference

There are three main parameters that are changed in the deposition: the assist beam voltage, the assist beam current and the content of oxygen in the assist beam. The plots below show the losses as a function of those parameters. The x axis changes in each of the four panels, and for each plot, the color code is linked to one of the process variables:

Conclusions

Quoting Le Yang and Carmen Menoni

1. under certain conditions as oxygen flow increase and Ar flow decrease, the loss angle becomes worse
2. with existence of oxygen ions the loss is mitigated by increase of beam voltage
3. relatively small particle size of oxygen compared with argon the caused the less effective interaction between assist ions and coating adatoms on the surface
4. with increase of ion dose, the mechanical loss drops

Introduction

The same set of samples described in the previous entry have been annealed at 500C for 9 hours. Then the loss angles have been measured again.

Results

The plot below shows the measured loss angle for all modes and all samples. After annealing all loss angles are significantly decreased, and they also show an increasing trend with frequency. As before, the blue points are the measurement points (averages of 8 ring-downs each) and the error bars are computed from the statistical error of the measurments. The red line shows the average of the loss angles for frequencies below 15 kHz, weigthed with the data points uncertainties. The red shaded area shows the 95% confidence interval of the mean.

If we plot the frequency-averaged loss angle as a function of the serial number, we see that there isn't much of a spread in the values:

We can again plot the loss angle as a function of the process variables. There are three main parameters that are changed in the deposition: the assist beam voltage, the assist beam current and the content of oxygen in the assist beam. The plots below show the losses as a function of those parameters. The x axis changes in each of the four panels, and for each plot, the color code is linked to one of the process variables:

This time I can't see much of a trend anywhere in those plots.

Linear fit

Since the loss angles show a clear increasing trend with frequency, instead of computing the mean value, I fit each dataset with a linear dependency on the frequency. To improve the fit I restricted the computations only to frequencies below 12 kHz. The results are shown below

The following plot shows the fitted loss angle at 1 kHz, as a function of the serial number. There is more spread in the results than when using the simple average:

And again, the dependency of the loss angle at 1 kHz on the process parameters:

The lowest loss angle is obtaine on sample S1600525, which was deposited without oxygen, low current and low voltage. But it's also the one sample that was deposited in a precedent separate run, and annealed twice at 500C.

S1600439 has been measured as received (before annealing, elog 137) and after annealing (elog 144). 

Q values are significantly increased for almost all modes, see the plot below for a comparison. Only modes with low Q are not improved.

I made a COMSOL model that can compute the distribution of elastic energy for each mode, dividing it into:

• bulk and shear energies (integrated over the entire volume)
• edge energy (integrated over the edge surface only)
• surface energy (integrated over the top and bottom surfaces)

Then I used the measured Q values for the MO_101 disk and tried to see if I could reproduce it with the energy distribution. The first plot here shows that the loss angle of the disk (inverse of the Q) has a trend that is already quite well reproduced by the ratio on edge energy over total energy:

In particular the edge energy distribution is enough to explain the splitting of the modes in families. This fit is obtained assuming that the edge losses are uniform along the entire edge, and frequency independent. If we assume a "thickness" of the edge of the order of 1 micron, the loss angle is about 3.5e-3, which seems resonable to me since the edge is not polished.

Then I tried to improve the fit by adding also bulk, shear and surface losses. It turns out that shear is not very important, while bulk and surface are almost degenerate. The following plot shows a fit using only edge and surface losses:

The result is improved, expecially for the modes with lower loss angle. Again, assuming a surface thickness of 1 micron, the main surfaces have a loss angle of 1.3e-5, while the edge is 2.3e-3.

Including all possible losses gives a fit which is basically as good as the one above:

However, the parameters I got are a bit differentL: the surface losses are reduced to zero, while bulk dominates with a loss angle of 1.4e-4, and shear is not relevant. 

In conclusion, I think the only clear message is that the Q of our disks are indeed limited by the edge. The remaining differences are difficult to ascribe to a paritcular source. Since th disks are thin, I tend to ascribe them to the surface, which would imply that we are far from being able to see the bulk/shear losses. If I use only edge and surface losses, I found as expected that the polished main surfaces have much lower loss angle by a factor 200 or so.

2017-08-18

• With both electrodes driven at a positive voltage, the results are still an improvement over the original design, but by smaller factors, particularly in 3 of the higher frequency modes at around 2 and 3 kHz. With another parametric sweep I may be able to find a better design. The opposite voltage was useful because it could couple force in opposite directions to adjacent anti-nodes with opposing signs. An adjusted configuration could probably be found to interact with anti nodes with the same signs. An alternative option would be to leave the radial position of the more central ESD constant but rotate it relative to the sample by some amount, though this would also require an additional parametric sweep as well as a much larger ESD. 

2017-08-18

• We created a prototype of PCB for the ESD design. Unfortunately the orientation of the two combs was flipped, so it will require some creative mounting to get right. The final design had a 6 mm offset between the two combs, .5 mm traces and 1 mm gaps between them The vertical traces are 12 mm long and there is a 3.75 mm gap between the end of the vertical traces and the opposite horizontal one. The ESD will arrive on Tuesday to be installed Wednesday and we will see how the new design works out.

We first measured the distance of the ESD from the disk in the test chamber (CR0). We had to remove the retaining ring to have reliable measurements

• distance between top of the ESD and mounting plate: 12.30 mm
• distance between top of the disk and mounting plate: 10.53 mm
• ESD thickness: 0.55 mm

So initially the distance between disk and ESD is 1.22 mm

We re-aligned the optical setup to a horizontal reference, and moved down the ESD as much as we could. It's not completely clear if the ESD is touching the disk. We'll see after pump down. The new distance from the top of the ESD to the mounting plate is about 11.80 mm, so we should have moved the ESD 0.5mm closer to the disk.

Pump down started at ~1:30pm

Excitations

• Old configuration, 1.2mm distance ESD-disk:
  • GPS before exc. 1186417492 
  • GPS after exc. 1186417546
• New configuration, 0.7mm distance ESD-disk
  • GPS before exc. 
  • GPS after exc.

The plots below compare the SNR and peak amplitude of all excited modes, in the new and old configuration. The new confgiuration is worse than the old one. This is unexpected, since the distance between ESD and disk is smaller.

However, yesterday we found out that setting the ESD so close to the disk is very tricky, and we might have some touching.

Additionally, the measured Q values of all modes are signfiicantly lower (by factors of >3), so it seems there is some additional friction. The mode frequencies are still compatible with the expected values, so it's unlikely that the ESD is touching the disk. One possible explanation for the worse Q can be residual gas damping in the area between the ESD and the disk: basically the gas moelcules that are left in the enclosed region between disk and ESD can create a viscous damping, which gets larger when the distance gets smaller [PhysRevLett.103.140601, arxiv:0907.5375]. I'll try to do some computations later today.

A preliminary design of the ESD board is available on the DCC: D1600214

2017-08-16

• I placed created a very narrow ESD placed along the edge of the sample. The thought behind this is that it will not cross over into any other modes that will cancel out the force. However, it does not appear to couple force into enough of the area of the disk to cause a worthwhile improvement, as can be seen in the plot, more modes lost amplitude than gained and some were worse by as much as a factor of 1000.

2017-08-14

• I did my best to increase the excitation in the higher order modes. By making the ESD narrower (a 6mm electrode overlap) the higher order excitation is improved drastically, by factors of between 10 and 30 for most modes.  I also created a double ESD (see image) that excited the modes by a factor of 3 or more better than the thinner drive. The plotted ratios are relative to the original geometry, but both of these geometries do better than previous geometries by factors of 2 or 3. 
• After a lot of experimentation, I think that there are non-trivial numerical artifacts from the force projection method. I have noticed that in the modes that are almost entirely unchanged by modifications, both the mode and its doublet have equal regions of positive and negative antinodes directly above the ESD force profile. This can be more clearly seen in the attached mode plot, the rectangle represents the region of the ESD. As a result of this, when the mode shape and force profile are multiplied and integrated the resultant force is very small. I expect this does not appear in the lab because the modes could be rotated at a different angle relative to the ESD. I am not sure how to effectively resolve this, perhaps checking other rotations of the mode shapes could be productive though I am unsure how to effectively accomplish this.