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Instrument Operation and Commissioning
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Ramp Rate: The value in A/min that the power supply will ramp at while in To Set or To Zero mode. This may not exceed the following values:
| Allowed Ramp Rate | Output Current |
|---|---|
| 2 A/min | < 40 A |
| 1 A/min | 40-63 A |
| 0.5 A/min | > 63 A |
Target Current: The current the power supply will attempt to provide to the leads while in To Set mode. This may not exceed a value of 76.9 A.
Target Field: The value of the field the user wishes to achieve from the magnet, based on calibrations from the UVA cooldowns in January and June to convert this field into a desired current. To examine the drift of this field calibration, a box is available in the upper right corner which reads "For Drift Calibration." If the user occasionally enters the value of the target signal's frequency displayed from the NMR system, the control will automatically log this information and update the Target Field calibration.
Target Frequency: The frequency which the user wishes to see the target signal at, based on most recent calibrations. The box above all numerical controls allows the user to switch the target species between Proton and Deuteron, which updates what the calibration is that converts target frequency into target field (and subsequently, into target current.)
The power supply can be in one of three modes while it is turned on: Hold, To Set, and To Zero. When the power supply is first turned on, its output will be clamped for safety, which is signified by the 'Hold' light blinking yellow. The lights below each relevant button otherwise indicate what mode the power supply is in.
Hold: The default state of the power supply. In this state the current in the leads will be locked at whatever their current value, the value displayed under 'Output Current', is. In case the user is unsure what they are doing or if an unexpected event occurs, they should always switch the power supply to Hold until it is safe to continue ramping.
To Set: While in this state, the power supply will ramp until it reaches the current value displayed in the "Target Current" field. Once it reaches this value, it will switch to Hold mode automatically. We have observed the behavior of the power supply is such that after switching to hold, it takes a minute or so for the current to stabilize at the set point, as it overshoots and undershoots several times as it nears the set point. If the switch heater is off, the power supply will ramp very fast, if it is on, it will ramp at the rate set in "Ramp Rate."
To Zero: While in To Zero state, the power supply will ramp the leads towards a value of zero current. On reaching an output current of zero, it will switch to Hold mode automatically. If the switch heater is off, the power supply will ramp very fast, if it is on, it will ramp at the rate set in "Ramp Rate."
Finally, the power supply's Switch Heater can be turned on using the square button marked Switch Heater. If the switch heater is off, the power supply will be essentially shorted, and can ramp the leads up or down as it pleases, to actually ramp the magnet, it is necessary for the switch heater to be on. On clicking the square button, the switch heater will be turned on if it is off, or off if it is on. The switch heater takes time to open or close, so on hitting the button, a 30-second timer will be displayed on the screen, locking out other actions until the switch heater has completed changing state. The light next to the button indicates a read back from the power supply as to whether the switch heater has successfully opened or not. If the light is green, the switch heater is on, and if it is red, the switch heater is off.
If the switch heater is on, then ramping the output current by using "To Set" or "To Zero" will ramp the current in the magnet itself. If the switch heater is turned off while the current has been driven to some nonzero current, the magnet will be left in persistent mode. The current value displayed in "Persistent Current" logs the last known value of the current before the switch heater was turned off. Allowing for small field drifts, this should be roughly the current in the coils at any future time. It is unsafe to turn the switch heater on while the current in the leads and the current in the coils differ, so the user is locked out from changing the state of the switch heater unless the current and persistent current match within error bars.
The user is also unable to turn the switch heater on or off unless the power supply is in "Hold" state.
Shim Control
The USB Relay can also be used to control the state of the shim coils, using this same control. Pressing the Shim 1 button will open the switch for Slot 1 on the USB Relay, turning on Shim 1 if everything is connected properly. Pressing it again will turn Shim 1 off. Similarly, the Shim 2 button can be used to turn the second shim coil on or off. Note that in this case, the light next to the buttons only indicates the last command the computer knows it has sent to the shim coils, so it will be on if it last told them to turn on, and off if it last told them to turn off. If there is a cable connection that is lost or damaged, there is presently no way for the computer to determine that the shim is not actually on.
The state of the shim coils is also changed automatically by using the other features of the power supply control. When the Switch Heater is turned on, both Shim 1 and Shim 2 are also turned on, and when the main Switch Heater is turned off, both Shim 1 and Shim 2 are both turned off. Further, to prevent moving the field without both shim coils being on, if the user switches the power supply to "To Set" or "To Zero" while the switch heater is on, Shim 1 and Shim 2 will both be turned on, and locked from being turned off until the power supply is back in Hold mode. This means the shim coils may only be off at any point while the power supply is in Hold mode, and otherwise will be automatically turned on.
Safety Features
