FMCG/Operating procedure

This section will cover everything up to the noise measurement, and will assume everything from the "Experimental Setup" procedure has been completed.

FPGA and Arduino "Reset"

Both the FPGA and Arduino microcontroller tend to behave somewhat strangely after a computer restart, so it is a good idea to "reset" both of them before beginning.

1. Open up the Arduino code. The file is in the upper-right corner of the desktop and is named Decoupling Code (with LCD). Click the right arrow (under the "Edit" menu) to upload the code to the Arduino.
2. Once you receive an "Upload Complete" message in the program, the code is ready. The LCD screen should read all zeroes (or whatever values the user specified). However, because of a feature of the code, the outputs of the Arduino (and consequently, the current outputs of the current supplies) are all sent to the rail.
3. To truly set the currents to zero, turn any one of the Field Adjust knobs on the current supply box by one click. Then turn the knob back one click to return all supplies to zero.
4. Now open up LabVIEW 2014 using the shortcut on the bottom taskbar. Open the FPGA Magnetometer.lvproj project. LabVIEW will then load a bunch of programs which takes ~60 seconds.
5. Once the project is opened, inside the Main Programs folder, open the Set_AO_Zero_(Host).vi program. When the program is run, it will send a zero (reset) signal to all of the outputs of the FPGA. If an error is output when this code is run, something caused the computer to lose connection with the NI chassis and the computer will have to be restarted.

Absorption Scan

Before proceeding with a noise measurement, it is a good idea to measure the rubidium absorption in each of the cells.

1. On the array, switch the detectors to SUM mode. This is accomplished by moving both the 4-socket leads from the detector and the 2-socket leads that carry signals out of the room to the set of header pins denoted by a Σ or + label.
2. If not done already, turn on the TEC and LD for the probe laser diode (the right-hand SRS LDC501 controller). We have historically and recently run at 170.00 mA of laser diode drive current on the probe. With this drive current, the D2 absorption line occurs at a "temperature" of roughly 7.05 kΩ and we generally do our magnetometry at a temperature of 8.5-9.5 kΩ.
3. Now return to the rear of the rack and, after verifying the probe tapered amplifier TEC 2000 temperature controller is enabled, enable the probe tapered amplifier output by pressing the output button on the Newport Model 560B driver. The drive current on the TA sets the eventual light intensity on the detectors. Historically, we have worked with currents as high as 1200 mA, though recently we have worked at much lower values around 750 mA, which delivers ~500 μW of laser power through a hot cell to the detector.
4. Once the TA is enabled and light is being sent to the detectors, adjust the gain settings on the I-V converters appropriately. 100-200 uA/V is usually appropriate for the laser powers mentioned above.
5. At this point, the detector sum signals should be visible on the scope. The signals should be pretty constant over time. If you notice non-periodic "noise" on this signal, it's possible the beam alignment between the TA and the fiber input is poor. While monitoring the signal, go behind the rack and walk the beam using the two mirrors on the second shelf from the top.
6. Once the coupling is optimized, the laser can be scanned across the resonance to measure absorption. The detuning can be scanned by varying the temperature of the diode in one of three ways:
   1. The temperature can be set manually by pressing Set on the TEC half of the controller and entering a numerical value with the keypad followed by Enter.
   2. After Set is pressed, pressing the Live Entry button will allow the user to slowly ramp the temperature by turning the knob next to the Live Entry button.
   3. The controller can be controlled using the GPIB connector on the back. This is done using a LabVIEW program, which is what will be described below.
7. In the FPGA Magnetometer.lvproj project, open the Laser Scan v1.1 program under the Main Programs folder. By default, the program is set up to take an absorption scan with the probe beam:
   1. Differential should not be enabled.
   2. The light next to Scan Probe should be bright green.
   3. With the D2 resonance at roughtly 7 kΩ, ramping the temperature from 5 kΩ to 10 kΩ will more than cover the resonance.
   4. 200 points with a 100 ms dwell time at each point will allow the actual laser temperature to "keep up" with the set temperature as the ramp runs.
   5. A PreScan Dwell of 10 s will allow the laser temperature to stabilize at the start temperature before the ramp begins.
8. Clicking the LabVIEW Run button will begin the ramp. Results for each trace will be plotted in the XY Graph window. A .dat file containing all of the data collected (Thermistor resistance and 4 channels of voltage data) is recorded on the file in Path Out.

Balancing the detector

Before magnetometry can be performed, the detector on each channel should be balanced, ie adjusted so equal light is incident on each photodiode. Make sure the pump laser is off when the following is being performed.

1. On the array, switch the detectors to DIFFERENCE mode. This is the middle set of header pins, and is indicated by a Δ or - symbol. The signal being sent out of the room to the current to voltage converters is now the difference in currents from the two photodiodes.
2. Return to the I-V converters and dial the gain way up to increase sensitivity. For ~500 μW of total laser power, 1-2 μA/V is appropriate. If the detectors have not been balanced in some time, the imbalance of laser light will likely cause the output of the I-V converter to rail.
3. Connect a long BNC cable (the one passing through the port of the MSR is idea for this) to either the output of the I-V converter or to the green "monitor" BNC port of the detector you'd like to manage.
4. Take a DMM into the room and plug the other end of the BNC cable into it. Switch it on to DC-voltage mode. Now, carefully adjust the balance of the detector by rotating the body. This can be VERY TOUCHY. At this level of gain, I usually try to get the voltage level down to ±0.2 V.
5. Repeat on the other detectors as needed.

Enabling the magnetometer

Once the detectors are balanced, the magnetometer is "activated" by turning on the optical pumping laser.

1. Close the door to the MSR, either using the automatic switch, or manually using BRUTE STRENGTH.
2. If it hasn't been done already, enable the TEC and LD current on the pump LDC501 (left hand side). The D1 resonance line of the pump is near 8.4 kΩ when the laser is being driven at 170.00 mA.
3. Open up the TA-7600 pump tapered amplifier controller application on the computer (shortcut on the taskbar). Take note of the status message in the upper right corner.
   1. Not Connected: The USB cable is not properly plugged into the computer or the controller (black box on the rear of the rack.
   2. Key Locked: The enable key on the controller is in the disabled position. Turn the key 90 degrees clockwise to enable.
   3. Amplifier Off: The amplifier is ready is turned off, but ready to be activated.
4. Check the Input Power (mW) display. The input power must be at least 10 mW, but with good coupling, we usually see closer to 20 mW of input light. If the input power is less than 15 mW or so, adjust the coupling from the pump diode into the fiber (top shelf of rack, nearer set of optics).
5. The output power of the TA can be adjusted using either the slider or the input box above the two power guages. 1000-1200 mA is a good drive current to start at.
6. Once you are satisfied with the settings, the pump light can be applied to the magnetomers by clicking the Amplifier On/Off button. After a brief countdown, the TA will activate.
7. Once the TA is active, you should see the difference signal (henceforth referred to as the "magnetometer signal") swing as the device begins to actively sense fields.

Field Nulling

SERF regime demands very small residual magnetic fields, so any large fields remaining inside of the room must be "shimmed" or "nulled". One detector will be nulled using the "global" room coils, and the other one(s) with the "local" shell coils. Nulling one channel with the room coils will bring the fields at the other channels near zero, and the remaining fields at each sensor can be shimmed using their local coils.

The following will assume the room coils are connected to the single-channel "Wyllie Supplies" and each individual set of shell coils is connected to the appropriate port on the back of the main control box.

The process of nulling the fields is somewhat difficult to explain in words, but gets easier with practice. The procedure below is not to be followed word-for-word, but can hopefully be useful as a guide.

Board layout of a single-channel Wyllie current supply. (click for details)

1. Since we'll be using the Wyllie supplies with the room coils to null the first sensor (Channel 1 recommended, but not necessary), make sure there's a wide output current range on these supplies. Make sure the DC voltage ranges (1) on all three supplies are set to "full" and the output resistors (2) are set to 1 kΩ. This amount of current is generally enough to generate the nulling fields for the room.
2. Connect the the two channels of the BK precision function generator to the X and Z AC inputs of the Wyllie supply. To start, set them to each output 20 Hz, .1Vpp sine waves. The amplitudes can be adjusted later to suit a the specific situation. The buttons above the BNC ports enable the outputs.
3. Reduce the gain on the I-V converter until the output no longer rails. At this point, I've found it easier to go to a "scan" mode on the oscilloscope (time divisions of either 100 or 250 ms). This will allow you to easily see the characteristic dispersive lineshape for the magnetometer.
4. Try sweeping the y field and search for the dispersive lineshape.
   1. If the lineshape is observed, park right in the middle of it.
   2. If after scanning the entire range of the potentiometer, the lineshape is not observed, try to park the y at the "steepest" point (maximum of dV/dB).
5. Enable the output on the BK function generator to apply a field to the Z coils. If it's not possible to see the magnetometer's response to this transverse signal, increase its amplitude on the function generator. Adjust the DC value of the X supply and try to reduce the response to the X signal.
6. When the response to Z has been minimized, or if you seem to be unable to reduce the response at all, disable the BK output to Z and apply a modulation signal to X. Use the DC Z coils to minimize the magnetometer's response to the X signal.
7. Nulling notes
   1. During the above two steps, the DC value of the magnetometer signal will likely swing wildly. Use the Y current supply to attempt to keep the DC value close to zero. Actually, it's a decent idea to keep sweeping the Y field anyway...as the X and Z fields get closer to zero, the dispersive shape when scanning the y field should become more and more apparent.
   2. It is possible for the response to [X, Z] fields to decrease even if the [Z, X] fields are not approaching zero. In fact, if the transverse fields grow huge, the response to the transverse fields will grow small simply because the magnetometer gets worse at detecting **any** field. During the nulling procedure, when you're approaching the "true null point", the response to the transverse field will actually "slowly" get larger as you approach the optimum point and then "quickly" shrink to a very small value, only to begin quickly increasing when the optimum point is passed. If instead, you're approaching a "false null point", the response will "slowly" get smaller but never quite reach the "very small response value" of the true null point.
   3. As you continue to iterate and bring the X and Z fields get closer to their null points, the process will get easier and the wild DC swings associated with adjusting the transverse fields will get smaller (obviously, as the magnetometer is also becoming less sensitive to DC transverse fields). You can probably begin increasing the I-V gain and decreasing the applied field (on the BK fungen).
8. Once the optimum (X,Y,Z) fields are applied, check to see if larger output resistors can be used. Use a DMM to measure the DC voltage on the monitor outputs (white BNC ports). The voltage range on the monitor output is roughly -25 V to +25 V on each channel. So, for example, if 1 kΩ resistors are being used and the monitor voltage reads 2 V, a switch to the 5 kΩ output resistors is possible as long as the DC voltage is increased to 10 V (maintaining the same current). Larger output resistors have historically given us lower magnetic noise (leading us to believe we are voltage-noise limited on the current supplies).