Persistent Mode Operation of 1.5 Tesla MRI Magnet for Human Extremities

The Electromagnetic Applications & Instrumentation division has developed 1.5 Tesla conduction cooled superconducting magnet for Magnetic Resonance Imagining (MRI) applications. The warm bore size of the magnet is 300 mm which is cooled by low vibration two stage pulse tube cryocooler. The central magnetic field and the stray magnetic field measurements are carried out. A persistent current switch made of Nb-Ti filaments in Cu-Ni high resistivity stabilizer material is installed in the magnet. The magnet is energized with the help of four-quadrant power supply and after reaching 178 Amp is put into persistent mode. The spatial field uniformity is measured with Nuclear Magnetic Resonance (NMR) probes in a Diameter of Spherical Volume (DSV) of 160 mm. The spatial magnetic uniformity is measured to be 56 ppm. The temporal field stability has also been measured by measuring the drift in the magnetic field for 48 hours. The centering of NMR probe array is also carried out to minimize the errors. The temporal field stability is measured to be 0.05ppm/hour. In this paper persistent mode operation of the superconducting magnet is discussed in detail along with magnetic measurements using NMR sensors.