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. 

2017-08-15

• It appears to me that the major factor limiting the improvement of the modes that either remain equal or diminish is that the original design was already quite good at exciting those modes. As can be seen in the attached plots, the original design is about as well centered over the 5 modes as a rectangle on the x axis can be. As a result, maintaing a constant potential on the ESD it would be difficult to improve the coupled force without specifically tailoring the force profile to a certain mode.
• In order from left to right, the frequencies of the given modes are 14.2 kHz, 16.2 kHz, 16.3 kHz, 23.8 kHz, and 27.4 kHz.

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.

2017-08-16

• I created a model with a drive offset in the middle. It improved one of the modes by a factor of 14 or so, but overall, it diminished the vast majority of the modes by as much as a factor of 100. 

2017-08-16

• I created a model with two ESD's, essentially a combination of my previous two attempts with one ESD on the edge and one closer to the center of the disk. This test was quite successful compared to previous trials, the improvement seems to be on an average of a factor of 10. No modes are weakened by this design. I am going to run a sweep adjusting the central ESD and see what placement is best.
• Attached is an overlay of the force profile and all of the modes. Note that this image is very large, and is useful as a digital reference or very large print only.

2017-08-17

• I ran a sweep of the position of the middle ESD to determine the optimal arrangement. From the original geometry I offset the central ESD between -2 mm and 11 mm. From the plots below I conclude that the optimal geometry is the one that is shifted 2 mm to the right of the original design. 
• The first plot is the sweep data as a ratio to the data of the current geometry for all modes. The second plot is the root mean square of the eight high frequency modes that change the most over the course of the sweep, those are modes 9, 11, 13, 14, and 17-20. The frequencies of those modes in order are 9.5kHz, 11 kHz, 14 kHz, 16 kHz, 19 kHz, 20 kHz, 21 kHz, and 23 kHz. There is a very apparent peak at 2 mm which is the driving force behind my conclusion that it is the optimal design. The next two plots are the 2mm shifted design and the original design as ratios to the original geometry. The 2mm shifted geometry is much more consistent than the original design, there is a very noticeable minimum in the original design (improvement by a factor of 2) for the 23 kHz mode that is resolved in the shifted design to a factor of 9.
• The final plot are ratios of the two designs relative to each other. I found this plot useful to convince myself that the 2 mm shifted design was better than the original, particularly in the region above 1.5 kHz.

2017-08-17

• I made a drawing of the Optimal ESD design. The bottom combs (right hand in the rotated image) are set to ground and the 5 arm comb is set to a positive voltage and the 2 arm comb is set to a negative voltage. 

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

• I ran a sweep of the central position in the two ESD setup with both set at the same voltage. There were two designs that maximized excitation by different metrics, the design with a 3 mm shift from the original design maximized excitation overall, but the 20 kHz mode was worse by a bout a factor of 5. The design with a -2 mm shift maximized the high frequency modes, particularly the modes most affected by the shift.
• MATLAB crashed shortly after the sweep so I will have to recreate the RMS plots of the dynamic modes later.

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.

The two spectra below show basically no difference (blue roughing pump on, red, roughing pump off)

Instead, below is another comparison: blue same as before, standard condition, red with one of the two clean air filters momentarily off. There is some clear improvement. The second filter is too hard to switch off!

This morning I installed temporarily a second QPD to monitor the input beam. The goal was to understand where the vibrations at frequencies below 2kHz couple from. As shown in the photo, the second QPD was close to the first one.

The signals in the two QPDs were quite different, and the coherence between them wasn't great. So I concluded that the main coupling path is not through input beam of QPD vibration, but more likely real motion of the disk.

I removed the additional QPD and restored the setup to its nominal configuration. The readout infrastructure is still in the model.

The plot below compares the QPD signal spectra in different configurations (roughing pump on/off, air on/off).

The noise below ~<2kHz is making very hard to measure the Q of the first mode at 1100 Hz

The main source of noise are the clean air filters. I switched them to minimum for the moment being.

In normal conditions the RMS of the QPD signals is dominated by the 58 Hz line generated by the roughing pump. Also, when the modes are excited, they exhibit large sidebands at +- 58 Hz that are an annoyance for the analysis.

I improved a bit the level of the 58 Hz in the QPD signals by putting the roughing pump on top of a "Very Useful Box":

Despite the fact that this advanced vibration isolation is already a little bit effective, it might be good to try to build some better suspension and maybe add an acoustic isolation around the pump.

I suspended the roughing pump with four springs. The reduction of the 58 Hz peak is similar to what I got when the pump was sitting on a box. So most of the coupling is due to acousting noise.

The wandering line I mentioned in my previous elog, which is spoiling most of the sensitivity, turns out to be power noise of the laser. 

I used a Thorlabs PDA100 and a SR785 to measure the power noise out of the laser directly, and saw a huge forest of peaks above 20kHz. Among them, a couple of peaks are moving up and down in frequency very fast. The plot below compares two different times of the Thorlabs HNL210L laser (the new one, 21 mW) with the old JTSU laser we are using for the test setup:

The noise of the new laser is cleary much larger (even after the laser has been on for some time) and non stationary. This is a big issue for us. I will contact Thorlabs to inquire if this behavior is normal.

The attached video file shows the peaks dancing around on the SR785 screen.

The turbo pump of the CR0 chamber runs at 833 Hz. It causes vibrations that pollute the measurements in the CR1-4 chamber. In particular CR1 is extremely sensitive and the line is highly up-converted. It's not clean why CR1 is more sensitive than CR2-3-4.

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 opened the chamber and took the etched disk out. Inspection of the electrostatic drive does not show any sign of burn or damage.

So it seems that the problem we had previously was due to contamination of the chamber (in the first case) or of the ESD (in the second case)

I moved the roughing pump out of the clean room, adding an extension hose.

This reduced a lot the vibrations induced by the pump. In the past when the pump was running we often saw very large noise, see the red trace in the figure below. Now, in the same conditions, we get the blue trace, which is much better.

The plot below shows a comparison of different configurations:

• blue: what I sometimes manage to get tewking the pump position and keeping it on top of a plastic box
• red: now, with the pump far away
• green: with the pump off

We are quite close to the pump off condition.

I looked into a couple of turbo pump switch on periods, and in both cases when the pump speed hits 58 Hz, a resonance is excited. I'm not sure what's resonating.

Today I let the roughing pump reduce the pressure to a level lower than usual, and this seems to mitigate the effect. The disk wasn't shaking as much as it did in previous pump-downs.

I added three 1"x1" viton pads below the base plate, and realigned the entire optical system to the horizontal reference

The pads seem to reduce a bit the vibrations in the QPD_X direction, but significantly improve the situation in the QPD_Y direction, see below:

Here's a trend of the QPD signals when the IGM was turned on:

Turning it off does not bring the disk back.

The new chamber was build. The first attempt to pump down was unsuccessful because of dislocated lid. After the lit was placed properly, the chamber was pumped down to 0.1 torr within 8 min. In future a sky hook will be used to help placing the lid properly. The clamps should not be placed on the lid before the roughing pump turned on.

The turbo pump controller failed. Error code 698 – call the vendor.

The SkyHook has been put in place and bolted down to the floor.

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.

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

I changed the set point of the test chamber turbo pump to 666 Hz. This was done by setting the "standby rotation speed" to 80% and enabling the standby condition.

I failed to interface in a reliable way the datalogger with Linux. So i hooked up the four analog outputs of the vacuum gauge controller to the fast ADC. The voltages are read and saved to disk at a reduced rate (16 Hz, throught the epics channels):

X3:CR0-VACUUM_VAC1_OUTMON
X3:CR0-VACUUM_VAC2_OUTMON
X3:CR0-VACUUM_VAC3_OUTMON
X3:CR0-VACUUM_VAC4_OUTMON

A python script running on the workstation (~/pycrime/automation_scripts/vacuum.py) reads the voltage every second and update four epics channels:

X3:CR0-VACUUM_CG0 = convection gauge of test chamber (high pressure gauge)
X3:CR0-VACUUM_IGM0 = ionization gauge of test chamber (low pressure gauge)
X3:CR0-VACUUM_CG1 = convection gauge of main chamber (high pressure gauge)
X3:CR0-VACUUM_IGM1 = ionization gauge of main chamber (low pressure gauge)

See below for an example during pump down (the test chamber is at a too high pressure for the IGM to work, so it returns a bogus number).

Here's an example of venting and pumping down

After venting the vacum chamber (CR14) a few times, checking for leaks and trying to tune settings to the gauges controller, I gave up. I removed the low pressure gauge from the newer vaccum system (CR14). Inspection did now show any obviouse depositions around the electrode (due to some burns). I will pack the gauge ans send it to the manufacture for an RMA. Took the same gause from the older vaccum system (CR0) and installed it on CR14. Started pumping down. The low pressure gause turned on just fine. Will check the preassure in an our before starting a measurement.