LCC Vessel Pressure Control

An LVshim scan should only be performed when the LCC magnet is at its normal operating pressure of 4.0 psi (± 0.3 psi for rough shim and ± 0.1 for Main LVshim). Large pressure changes within the helium vessel will result in large variations in shim homogeneity and harmonic error.

note:

If you have a magnet other than LCC magnet, then you may directly go to the Rough or the Main LV Shim as necessary.

Here are some methods for adjusting the vessel pressure.

1 After Ramping An LCC Magnet And Shim Lead Is Already Engaged

note:

This method will only be useful after ramping when there are no shim currents in the shim coils.

The most rapid method to restore operating pressure at the completion of magnet ramping is to use the shim switch heaters. Make all electrical connections for shimming, then turn on all three shim switch heaters (Axial, T1 and T2). This method should restore about 2.0 psi of pressure per hour. Monitor the pressure in the magnet continually during this process to ensure that pressure does not exceed acceptable limits and cause venting of helium through the 5 psi relief valve. When the normal operating pressure is reached, begin the process of entering the ATR currents into the shim coil.

2 In Between Shim Iterations

  1. For low starting pressure:

    Close the shim lead vent valve and wait for the pressure to build up. The combination of the seated shim lead and the activation of the pressure controller should result in a pressure gain of 0.5 psi every 30 minutes.

    note:

    Turning off the shield cooler to try to build pressure faster is not recommended as this adds heat to the first stage shield. The shield will continue to radiate heat to the helium vessel until the cryocooler is able to cool it back down to its normal temperature. This action may result in venting of helium gas to maintain pressure within the operating limits.

  2. For high starting pressure:

    Open the shim lead vent valve and allow the lead to vent until the pressure drops to below 4.1 psi.

  3. In Between Shim Sessions

    The magnet’s helium vessel pressure should have been operating at 4.0 psi. The shim lead will be pre-cooled before insertion and engaging into the sav-con connector. Monitor the pressure during this process. Follow the same steps as listed in the method In Between Shim Iterations for raising or lowering the helium vessel pressure.