VectorFridgeMagnetPowerSupplyNonlinearity

Comparison of the linearity of different superconducting magnet power supplies, connected to the 9T magnet on our 9/5/1T vector magnet.

Magnetic field was measured with a Hall probe from wafer W0191 using an SR830 lockin amplifier and SRS560 voltage preamplifier, 100nA at 77Hz. Fridge is at ~460mK.

Comparison of Cryogenic Ltd SMS-120C PSU and Oxford Instruments IPS-120

In this experiment we sweep B from -15mT to +15mT at 2mT/min (except for the SMS-120C in USB mode, whose slowest sweep rate is 6.4mT/min). All switch heaters are on.

Cryogenic_vs_Oxford3.png

Note: When switching the power supply from Cryogenic SMS-120C to Oxford Instruments IPS-120, we forgot to ground the sample, which caused a change in the carrier density and thus the Hall resistance.

Cryogenic Ltd SMS-120C PSU in DAQ mode

Here we monitor the voltage across a shunt resistor (actually a transducer) in the PSU to get a less noisy idea of what the current supplied to the magnet is.

In this experiment we sweep B from -15mT to +15mT at 2mT/min. All switch heaters are on. DAQ4.png

  • Note the steps in shunt resisitor (actually the transducer) voltage and Rxy (not in B output reported by the software), increasing with field.This is due to the limited resolution of the DAC which sets the requested magnetic field.
  • Note the nonlinear shunt resistor (actually the transducer) voltage and Rxy around zero-field:
Zoomed-in:

DAQ5_ZoomIn.png

To see how linear the power supply is we can compare the Bz output from the software with the field converted from shunt resistor (actually the transducer) voltage:

DAQ_Shunt_vs_Bz.png

Now we plot the measured Hall voltage against the field determined from the shunt resistor (red), and also against the field reported by the Cryogenics software (blue)

DAQ_Rxy_Shunt_vs_Bz.png

Bottom line - the DAQ mode isn't great for low field measurements due to the limited DAC resolution and the non-linearity around B=0.

Cryogenic SMS-120C power supply in USB mode

This power supply has the 20-bit extended resolution option.

Note that the Cryogenic power supply only has 64 sweep rates in USB mode, and the lowest is 6.4mT/minute.

In this mode the sweep through B=0 is extremely non-linear:

USB2.png

Comparison of the Bz output with the field converted from the shunt resistor (actually the transducer) voltage:

USB_Shunt_vs_Bz.png

Plotting the measured Hall voltage against the field determined from the shunt resistor (red), and also against the field reported by the Cryogenics software (blue)

USB_Rxy_Shunt_vs_Bz.png

Plotting the measured Hall voltage against the field reported by the Cryogenics software before (red) and after (blue) changing the variable in "PSU Z Get Outputs.vi" to double (no improvement on the reported field resolution, which is around 0.3mT):

USB_B_resolution.png

Oxford Instruments IPS-120

Oxford.png
  • Used IPS 120 to power the magnet, Cryogenic power supplies for heaters.
  • 9T, 1T heater at 2.7V, 5T heater at 3.2V. All switches are above 10K.
  • Smooth sweep through B=0.
  • The sweep actually goes to and from the requested field.

Access Control

##* Set ALLOWTOPICVIEW = QedGroup

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(actually the transducer)
Topic revision: r6 - 09 Sep 2014, RoyLi
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