Magnet Control

• 1: Superconducting Magnets
• 1.1: SpecMan Magnet Control GUI
• 1.2: Heater Control
• 1.3: Operating the Magnet

1 - Superconducting Magnets

Overview

Superconducting magnets used in Bridge12 EPR spectrometers come with an integrated heater switch so the magnet can be operated in a driven-mode or in persistent mode. If the magnet switch is closed, the magnet is in persistent mode, similar to superconducting magnets. While the magnet will have the lowest drift rate in persistent mode, generating a very stable field, the field can not be swept/changed. To sweep the magnetic field, the heater has to be activated to open the switch.

A typical scenario to put the magnet in persisten mode at e.g. 5 T would look like this:

1. Assuming the magnet is at 0 T activate the heater to open the (superconducting) switch. Wait until the heater reaches its target temperature (about 11 K).
2. Ramp the field to the desired value, here 5 T. During ramping the field the magnet voltage will increase.
3. Wait for the field to settle. This is indicated by the lowest voltage read by the power supply. When the magnet is at field, the voltage will never be 0 V, since this voltage also includes the voltage drop across the magnet leads.
4. Deactivate the heater to close the switch. Wait for the heater to return to its base temperature.
5. Ramp down the current in the magnet leads.

At the end of this procedure this the magnet is in a 5 T persistent field state and the drift will be very low. To change the field, first the magnet leads need to be ramped up to match the current in the magnet.

1.1 - SpecMan Magnet Control GUI

The superconducting magnet can be completely controlled through the SpecMan4EPR software using the. From the SpecMan magnet GUI the user can activate the heater switches, move/sweep the magnet field, and can put the magnet in persistent mode.

Launching the Magnet GUI

To launch the magnet GUI:

1. Go to the DDI menu
2. Select Set Field from, FLD from the menu. This will open a separate window with all magnet controls.

The following window will appear:

The magnet control window has three different buttons to select the functionality:

1. Heater Control: Click this button to access the heater controls to enable/disable the heater switch (see figure above, left).
2. Field Control: Click this button to access the field controls to change/set the magnetic field strength (see figure above, right).
3. Polarity Control: Click this button to change the polarity of the magnetic field (currently not used) (not shown/used).

1.2 - Heater Control

To change the magnetic field first the superconducting heater needs to be activated. This will connect the magnet power supply to the main coil and will allow the user to increase or decrease the current in the main coil and therefore moving the magnetic field.

The value of the magnetic field is shown in two different colors, depending if the heater switch is activated or not:

• Yellow : The heater switch is off. If the field is at a value of 0 T, the main coil is completely discharged. If the value is not 0 T it means the magnet is in persistent mode, and the field is “parked” at that value. Before activating the switch, the current in the magnet leads not to ramped back to the persistent mode value (see figure above, left).
• Green : The heater switch is activated (on) and the field can be moved (see figure above right).

Operating the Switch

Opening the Switch at 0 T

To open the superconducting switch:

1. If the magnetic field is at a value of 0 T, click the icon to open the switch in the magnet GUI (see figure above, left).
2. Confirm that you want to open the switch by clicking Yes in the dialog box.
3. Wait for 10 - 20 seconds for the switch to fully open.

Opening the Switch when Magnetic Field is not at 0 T

To open the superconducting switch:

1. Enter the value of the persistent field to charge the current in the leads to match the current when the switch was closed.
2. Wait for the power supply to reach the field.
3. Click the icon to open the switch in the magnet GUI (see figure above, left).
4. Confirm that you want to open the switch by clicking Yes in the dialog box.
5. Wait for 10 - 20 seconds for the switch to fully open.

Closing the Switch

To close the superconducting switch:

1. Click the icon to close the switch in the magnet GUI (see figure above, right).
2. Wait about 10 - 20 seconds for the switch to close

1.3 - Operating the Magnet

Depending on the type of experiment the magnet field is either:

1. Moving For example in a echo-detected field-sweep experiment. Here, the field is swept while recording the echo intensity.
2. Static For example in a DEER/PELDOR experiment. These type of experiments are performed at a static magnetic field value. If the user does not change the field for a long time (e.g. an overnight experiment), it is recommended to put the magnet in persistent mode.

Typically, when switching on the spectrometer, the user would set a field value close the start value. This is not strictly required but will speed up operations when the field is moved a lot during an experiment session.

Once an initial value is set the field is entirely controled by SpecMan4EPR.

Setting an Initial Field Value

To prepare for an experiment:

1. Open the SpecMan4EPR Magnetic Field GUI.
2. Click on the heater control icon.
3. Activate the superconducting heater switch. After you clicked the icon to activate the switch wait for about 10 - 20 s for the switch to completely open.
4. Click the icon to control the magnetic field.
5. Enter the desired value for the magnetic field and press the ENTER key.
6. Wait for the magnet power supply to reach the desired magnetic field value.
7. (Optional) If the EPR experiments are performed at a static magnetic field, close the superconducting magnet switch to put the magnet in persistent mode. Enter a value of 0 T for the magnetic field to ramp down the current in the magnet leads.

Ramping Down the Magnet

It is completely fine to leave the magnet at field, if the spectrometer is not used for a short time. However, if the spectrometer is not used for a while, the magnetic field should be discharged. To ramp down the magnet:

1. (Optional) If the magnet is in persistent mode:
   1. Enter the field value, matching the magnetic field value of the magnet and hit the ENTER key.
   2. Wait for the power supply to reach the value.
   3. Open the superconducting magnet switch. After you clicked the icon to activate the switch wait for about 10 - 20 s for the switch to completely open.
2. Enter a value of 0 T for the magnetic field strength and hit the ENTER key.
3. Wait for the magnet to reach a field of 0 T.
4. Once the magnet reached 0 T, close the superconducting magnet switch.

Changing the Ramp Rate

When sweeping or changing the magnetic field, the rate, at which the magnetic field is changed, can not exceed a specific value. This value for the sweep rate is given by the magnet manufacturer. It is common for superconducting magnets to have different sweep rates for different field value regimes.