240 GHz EPR Spectrometer Manual

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240 GHz EPR Spectrometer Manual

Information about the design and operation of the EPR spectrometer optimized for g=2 at a <a href="Magnet12Tesla" class="TMLlink">magnetic field</a> of 8.6 Tesla. The spectrometer has, loosely speaking, three modes of operation: CW EPR, pulsed EPR with a ~50 mW solid state source, and pulsed EPR with the FEL (FELEPR).


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EPR with VDI source

 

FELEPR

 

 

EPR with Solid State Source

 

Operation

This section details the steps to run the 240 GHz EPR Spectrometer. Taken with modifications from "240 !GHz EPR Spectrometer" by Devin Edwards and Louis Claude Brunel.

Preparing Probe

For this spectrometer the probe corresponds to the waveguide, sample, modulation coil and any other equipment inside the bore of the magnet (i.e. optical fibers etc). The following is a list of considerations, rather than an explicit list, as there are substantial sample-to-sample changes in setup.

  • No magnetic materials should be placed in the magnet! In addition to the obvious risk of becoming stuck, these materials also can strain the magnetic field coils in the magnet thus reducing homogeneity, damaging or even quenching the magnet.
  • Because any metal can screen the oscillating field from the modulation coil, metal should be avoided within the modulation coil. This means a thin mirror is highly prefered.
  • In general the field of the modulation coil is measured at the edge of the coil. If the sample is within the coil, it will see a higher field, and placing your sample far from the modulation coil reduces the field.
  • A magnetic field maxima occurs at the surface of the mirror. If not placed on the mirror, a thin sample should be carefully positioned at a magnetic field maxima.
  • Anything passing to the sample must be installed so as to ensure the probe head remains vacuum tight for cryostat operation. Several sets of leads are already installed for general electrical needs. Additionally, a port is available for optical fibers.

A general procedure is as follows:

  1. Check the waveguide, modulation coil, mirror and other probe parts are clean, free of condensation and at room temperature.
  2. Mount modulation coil on end of waveguide.
  3. Place sample on mirror or in sample holder. It is important to center the position as much as possible.
  4. Secure mirror and sample in modulation coil.
  5. Check that all connections are in good contact, plugged in, correctly secured to the waveguide frame to prevent noise, and installed correctly to maintain vacuum (for cryostat operation).
  6. Lower the waveguide into cryostat SLOWLY. A pair of blue marks on the front of the setup will guide you to the correct orientation (this has the primary lead connector facing towards you).
  7. Plug in the primary lead connector. It is wise to check all leads. The modulation coil can be tested by increasing the voltage on the SRS830 Lockin Amplier and observing a rise in current on the Keithley 199 DVM. (At ~5V, current should be in 80 mA range. If it is much lower, the coil is likely partially shorted) Check that the Lakeshore temperature controller is now reading both A and B channels.

Liquid Samples

If a liquid sample is being used in cryogenic conditions the following has been previously used for ~8 μL samples.

  1. First precool the cryostat to 230K following the Cryostat directions (Section 2.2) with the probe outside the cryostat replaced with a blank on top(the o-rin willd enable suficient vacuum to purge and cool the cryostat). Be sure to purge the cryostat well.
  2. Attach the modulation coil and any other parts to the waveguide except the mirror and the sample holder. Purge the waveguide with dry nitrogen for at least five minutes. Check all connections.
  3. Prepare sufficient liquid nitrogen to submerge the sample.
  4. Pipette the sample into the sample holder. Bang or knock the sample holder until the liquid sample is flat inside. This may be done more easily a few µL at a time.1 Once the cryostat has stabilized at 230K, close the small valve on the pump manifold to bring the cryostat to atmospheric pressure.
  5. Dunk the sample into liquid nitrogen and wait several minutes to allow it to thermalize. Ensure the cryostat is at atmospheric pressure before removing the sample from nitrogen.
  6. Quickly place the sample on the mirror and secure it to the modulation coil.
  7. Place the waveguide in the cryostat as normal.
  8. Secure the screws and connect oxford connector.
  9. Proceed to the desired temperature

The faster the above is completed the better, and more likely the sample was frozen rapidly and remained frozen throughout.

Cryostat

The cryostat and transfer line should be pumped out with a turbo-pump (down to ∼1e-6) roughly once a month to ensure good performance. Poor temperature performance or icing/condensation on the outside of the cryostat or legs of the transfer line may indicate poor vaccuum.

Starting

  1. Close the large and small valves on the pump manifold and start roughing pump next door(big green button).
  2. On the dewar leg (long leg) of the He transfer line, move O-Ring and nut to ∼1inch from end of dewar. Also check the conditition of this o-ring as well as the o-ring on the cryostat port.
  3. Open needle valve at top of dewar leg ∼1/4 a turn.
  4. Make sure 3 psi pressure release valve is in place. Vent port and top port on the dewar should be closed, release port should be open.
  5. May Require Two People: Take transfer line and place dewar leg on top of the dewar port. Lightly tighten nut and open the top port.
  6. Slowly lower leg into dewar until tank pressure jumps (indicating the leg is in helium) and tighten nut fully to prevent the leg from sliding into the dewar.
  7. Monitor cryostat (short) leg of transfer line for flow. If no flow is achieved open needle valve until a gentle flow occurs.
  8. Once any flow is observed, insert the cryostat leg into the crostat until it gently bumps the end and secure the nut.
  9. Open large valve on the pump manifold and evacuate cryostat until gauge stops moving. Wait ∼2 minutes and close valve. Watch pressure rise until gauge stops moving to return to atmospherice pressure.
  10. Repeat twice to completely purge cryostat and return to atmospheric pressure.
  11. On pump manifold open smaller valve completely. Enter set temperature to !LakeShore332.vi (can be opened from !PESRMenu.vi) it is wise to start with a temperature ≥5K even if you want to go lower. Medium heater is a good choice for temperatures <30K, high is better above 30K, but this should be adjusted as needed to converge.
  12. Cooling rate may be altered by opening and closing the needle valve on the transfer line. A cooling speed of 200-400mK/minute is a good compromise of speed and helium usage. The cooling speed often changes during the course of cooling and the cooldown should be monitored/checked occasionally.

Changing Temperatures

  1. Select the new temperature on !Lakeshore332.vi
  2. Select the correct heater setting. Above 30K use High, otherwise start with Medium and adjust. In general, use the lowest heater selection possible. If the heater is running above ∼90% increase the heater setting, if running below ∼10% decrease the heater setting.
  3. If operating in continuous flow mode below ∼4K, you must open the large valve (in addition to the small valve) on the pump manifold.
  4. If moving the temperature dramatically, or the system is not heating or cooling well you ought to adjust the needle valve on the dewar leg of the transfer line. Closing the valve (turning counterclockwise) will reduce the flow and therefore reduce the cooling power and opening (turning clockwise) increases the flow and therefore increases cooling power. The increased flow will be reflected in an increase in pressure read from the gauge on the vacuum manifold.
  5. Observe both Temperature A and B. Temp A reflects the temperature near the heater and is the temperature the Lakeshore 332 is controlling. Temp B is located on the waveguide near the sample and better reflects the temperature at the sample and both should stabilize before continuing with experiments. It is common for Temp B to be several hundred milli-Kelvin to 1 Kelvin lower than Temp A. Lower helium flow rates tend to lead to a homogeneous temperature in the cryostat.

One Shot Mode

In order to operate between ∼1.5 K and ∼2.3 K you must operate in single shot mode.

  1. On !Lakeshore332.vi turn the heater to Off.
  2. Open the needle valve on the dewar leg of the transfer line substantially.
  3. Open the large valve (in addition to the small valve) on the pump manifold.
  4. The temperature should drop to ∼2.3 K and remain flat. During this period you are accumulating liquid Helium in the cryostat. Wait at this point for roughly thirty minutes.The one shot mode lasts until the accumulated helium is exhausted. This is of order an hour, but can be increased by waiting longer at this step.
  5. Close the needle valve securely. The temperature should begin to drop slowly and stabilize at ∼1.5 K. This is the base temperature.
  6. To raise the temperature, use the !Lakeshore332.vi with the heater set to Low, if the heater won’t raise the temperature, partially close the large valve on the pump manifold to change the temperature.

Starting Spectrometer

The setup for CW and pulsed measurement is largely similar. This will detail starting the spectrometer. Note- parameters in italics are updated parameters. Both sets seem to work.

  1. Start the power supplies. These are located on the back of the EPR optical table. There are 4 power supplies at chest/eye level (VDI, 2 x Lambda, HP) and 3 up high.
  2. Check the fan on the source begins to turn and that the quasioptics are all in place (including mirror before detector horn).
  3. Extend the Quasi-Optical Bridge.
  4. Adjust the dial attenuator near the detector horn to 4.2.
  5. Adjust the dial attenuator on the far right of the 10GHz stage (suspended below the detector) so the TOP DVM reads -30 mV.
  6. Adjust the dial attenuator at the back of the 10GHz stage to set the SECOND highest DVM to 1.3 V. Note, 1.29-1.3 V is OK.
  7. Adjust the dial attenuator at the left of the 10GHz stage to set the BOTTOM DVM to -50 mV.
  8. Open PESRMenu.vi (link on the desktop).

CW Measurements

Setup

  1. Connect the 0 and 90 degree outputs of the 10GHz stage (front two BNC connectors) to the A inputs of the two SR560 Pre-Amplifiers. Both amplifiers should be set for DC coupling of the A inputs with high and low pass filtering set at 1 kHz and 100 kHz respectively (for 20 kHz modulation) at 6 dB rolloff with x10 amplification and low noise.
  2. Connect the 600 Ω outputs of the SR560s to the A inputs of the SR830 Lock-In Amplifiers. Both SR830s should be DC Coupled to input A with Floating ground. A time constant of 300 ms and a roloff of 18 dB are typical. Channel 1 and Channel 2 should be set to X and Y respectively.
  3. The modulation BNC cable (hanging from ceiling near the Lockin Amplifiers) should be connected to the Sine Out of the bottom SR830 using the internal clock set to 20 kHz (typical). The amplitude 0-5 V should be increased and ensure a current is read out on the Keithley 199 DVM located on top of the magnet power supply. If no current is present check the primary lead connector is plugged into the probe.
  4. The TTL output from the back of the bottom SR830 should be connected via BNC to the Ref In on the reference panel of the top SR830, which should be set for External triggering with a Pos Edge. This will allow coherent detection on both channels of the spectrometer.
  5. Using !AttnSet.vi (available as Digital Attenuator in PESRMenu.vi) lower the attenuation value until the 3rd HIGHEST DVM reade -25 mV. If at 0 dB the DVM still does not read >-8 mV:
    • Check the power supplies are still on.
    • check the source is running.
    • Check the cable labeled TTL hanging near the SR830s is unplugged.
    • Check the optical path.
    • Check attenuator near detector horn is at 4.2.
    • Reset the digital attenuator by opening NI Measurements and Automation Explorer (NI MAX) and try again.
    • Get Help!
  6. Open Sweep.vi (as CW EPR in PESRMenu.vi).
    1. Click AutoStop to ON
    2. Select the Sweep Rate (mT/s) (0.2 mT/s is a good start for lines >5 G)
    3. Enter 200 as the Time Step (ms)
    4. Check that Round Sweep is set to OFF
    5. Click Configure Devices and input/confirm the following:
      • #1 I/O:
        Setting: Time Stamp
      • #2 I/O:
        Setting: Field (sweep coil)
      • #3 I/O: GPIB 8
        Setting: SR830 - Ch1
      • #4 I/O: GPIB 8
        Setting: SR830 - Ch2
      • #5 I/O: GPIB 12
        Setting: SR830 - Ch1
      • #6 I/O: GPIB 12
        Setting: SR830 - Ch2
      • #7 I/O: GPIB 14
        Setting: Lakeshore332-A
      • #8 I/O: GPIB 14
        Setting: Lakeshore332-B
    6. Click Save Settings
    7. Click Test Devices and check there are no errors. single sample results are shown in the yellow bar below the Show/Hide Trace buttons.
    8. Select to Hide Trace on the first two (Time and Field) and the 6th and 8th (Lakeshore Temp A and Temp B) readout buttons. This means only the 3rd-6th (quadrature detection from the SR830s) are plotted.
    9. At the bottom-center of the graph, set the X-axis to Field.

Taking CW Spectra

  1. Set Attenuation.vi to maximum attenuation. This is good practice before making any changes.
  2. Check the value of the wiregrid polarizer (located on the source side of the quasi-optical table). If you don't know which power to use, start with full power corresponding to 0 degrees.
  3. Set the modulation field by increasing the voltage on the reference panel of the BOTTOM SR830 and matching the current reading on the Keithly 119 DVM to the desired field modulation. If you don't know what modulation to use, start with 30 mA.
  4. To set your starting field, you will manually move the field of the sweep coil. Open <a href="Magnet12Tesla" class="TMLlink">IPSFrontPanel.vi</a>. Check that the PS Selector (bottom of the panel) is set to "IPS120-20 sweep' (NOT IPS 120-20 Main). In the sweep control panel press Settings and enter your desired starting field (between -0.06 and 0.06 T) as Target Field and for Sweep Rate enter 0.05. Select OK and under the sweep control panel select Go to set. Wait as the field is changed.
  5. Under Sweep.vi input the Target Field[T] to move the sweep coil to (between -0.06 and 0.06 T)
  6. Check the sensitivity of the SR830s is set with room for signal. A good starting point is 1 mV.
  7. Retune Attenuation.vi so that the DVM that is 3RD FROM THE TOP reads -25 mV.
  8. Input a filename into Sweep.vi as the Log File. If the file already exists it will NOT be overwritten, rather the datapoints will be appended to the end of the existing file.
  9. In Sweep.vi, in the tan area above the filename, enter header information which may include: Sample Name, Modulation Amplitude, Wiregrid Setting, Digital Attenuator Setting, SR830 settings, Sweep Range, Sweep Rate, and Time Stepsize etc. This will be appended to the top of the datafile. If you add-on to an existing file, the header will NOT be changed.
  10. Check settings, then in Sweep.vi press the green Start button to start the scan.

Repeat the last steps to collect data.

Spectra are saved as textfiles, with a header several lines long. Data is presented in 8 columns, separated by commas, as follows: Time,Field,X1,Y1,X2,Y2,TempA,TempB.

Rapid Passage EPR

 

Pulsed EPR

Setup

  1. Connect the 0 and 90 degree outputs of the 10 GHz stage to the two coax SMA cables leading to the 50 Ohm inputs on the NI PXIe 1076. Convention is, 0 degree -> Ch1, 90 degree -> Ch0.
  2. Connect the BNC labeled 'To Scope' from the !PulseBlasterESR card to the BNC labeled 'Scope Trigger'.
  3. The BNC labeled 'Pulsed Signals' from the !PulseBlasterESR card should be connected to the input of the comparator box. The output of the comparator box should e attached to the cable hanging near the DDA 120 labeled TTL. This carries the TTL signal to the 15 GHz switch which sculpts the pulses from the source.
  4. Open !SpinEcho_with_digitizer.vi (available as Spin Echo in PESRMenu.vi). Start the vi, and go to the Data tab. You should see traces appear in the Raw signals window. If you do not:
    • Press Exit, then stop the vi. When the vi has stopped, start the vi again. If no traces appear, repeat this process one more time. (This is an issue with the vi incorrectly exiting the previous run). If you have restarted the vi twice and still see no traces:
    • Check the cable labeled 'To Scope' is connected to the cable labeled 'Scope Trigger'
    • Check the connection between the cable labeled 'Scope Trigger' and the digitizer
    • Get Help!
  5. Go to the PESR tab. Check that Rep Period is set to 100m. Select Hahn Echo from the menu to the right
  6. Set pulse lengths. A good place to start is 470n for P1, 1u for tau_s, 10u for tau_e, 100n for tau_step, and 550n for P2.
  7. Push Set pulses.
  8.  

 

FELEPR

 

Detection System

 

Quasioptics

 

Intermediate Frequency Stage

 

Detection

 

Scope

 

Digitizer

-- %USERSIG{BlakeW - 2015-02-10}%

 

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