LVShim Process
LVshim provides an accurate method of collecting and analyzing scan data to determine and correct magnetic field inhomogeneity. This procedure has been updated to include the TwinSpeed (TRM) which has Whole-Body (WB) and Zoom (ZM) gradient coils. The calibration only needs to be done on the Whole-Body coil. LVshim consists of four procedures: Rough, Main, Gradient and Passive Shim. It is important when performing LVShim that all prerequisites are completed and that the proper order is followed.
When doing LV shim as the result of an upgrade, the recommendation is to re-enter the last set of shim currents into the magnet and start with Main LV shim. You should not have to do Rough LVShim if the initial system had a reasonable shim prior to the upgrade.
After ramping the magnet, the cryocooler coldhead must be turned off for 10 minutes before installing shim currents. This is due to the magnetic flux buildup within a superconducting shield around the coldhead sleeve during magnet ramping.
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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. Here are some methods for adjusting the vessel pressure.
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The objective is to improve the homogeneity to the point where Grafidy and other calibrations can be performed. Use the three-piece LVshim phantom and the LVshim PSD with a starting bandwidth range of 2000 - 5000 Hz. The Rough shim process continues until the sampling diameter has been increased to 45-cm DSV (40-cm DSV for CRM Body Coil) and the bandwidth decreased to 1000 Hz. In most cases this should take about 3 iterations.
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Passive Shim (S-V Magnets Only)
The S-V and SXc_2 magnets do not have shim coils to compensate for the xy (2, -2) and x2-y2 (2, 2) harmonics. The compensation is achieved by taking a 12-plane LVshim scan, running the LVshim tool and selecting "Passive" for the shim type. The LVshim program will calculate the amount of passive shim required and will show the appropriate drawer number and shim locations for steel placement.
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LVShim Software. General information on Calibration File Generator, Magnet Serial Number, Calibration File Name, Shim Type, Image Data, Operation Mode and Display Mag and Phase Diff Images
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Shim Power Supply Details. Shim power supply information by magnet type.
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Data Sheets