Anbo Wu

An Active Quench Protection System for MRI Magnets

An active quench protection system is developed to safely protect a 1.5 T superconducting MRI magnet. The protection system includes a digital quench detection controller, a set of quench heaters and voltage taps, a battery backed DC power supply and solid-state switches. The quench controller takes the voltage signals from the voltage taps over superconducting coils and monitor a quench onset. To distinguish a quench signal from operational signals, the quench controller uses a pre-determined moving average voltage over 50 ms as the threshold signal for quench detection. The threshold signal is more than 10 times larger than any of the normal operation signals to avoid false quenching. Once detected moving average voltage exceeds the threshold, the quench controller will close solid-state relays and power quench heaters to accelerate the quench propagation over the entire magnet. The system has been tested in a MRI magnet test module.

Development of a Large Bore Long Pulse Magnet at GE Global Research Center

A 3.5 Tesla pulsed magnet with a 1.35 meter bore wound with copper strips has been developed, successfully tested and used at GE Global Research. The magnet has energy of 14 MJ at the peak field. The magnet is composed of 4 pancake coils in series pre-cooled with liquid Nitrogen and powered by a 750 V, 8000 A power supply. This paper describes the design of the magnet, power supply, control circuit and the test results of the system

Cold Storage and Release Characteristics of a Thermal Battery Unit Using Solid Nitrogen and Solid Nitrogen Impregnated in Metal Foam

Large cold storage capacity is very important to maintain the stable working condition of a superconducting magnet. Solid nitrogen is frequently used for cold storage at cryogenic temperatures. In this paper, we report the experimental investigation of the cold storage and cold release characteristics of a thermal battery unit using solid nitrogen, as well as the combination of solid nitrogen and copper foam under different conditions. In order to enhance the heat transfer without significantly compromising the cold storage capacity, the copper foam is adopted to enhance the effective thermal conductivity of solid nitrogen. It is found that the copper foam can effectively enhance the cold release process, making the cold energy stored effectively utilized.

Stability Test of a Pair of Conical Coils for Open-Bore MRI Magnet

To improve the openness of MRI scanners and reduce potential patient claustrophobia, a novel magnet concept with conical-shaped outermost main coils was investigated. This concept may also improve the coil stability because the Lorentz force acting on conical coils is mostly perpendicular to the coil/former interface, which may reduce friction-slip induced quench risk. A pair of conical superconducting coils has been tested to evaluate its stability. The test coil has 16° conical angle with a 50-cm diameter at the smaller diameter end. Both coils are supported by an aluminum former embracing coil OD surfaces. The pair of coils was ramped in a vertical Dewar with instrumentation measuring coil temperature, current, voltage, and strains. Two training quenches were observed at 74% and 81% Io/Ic before the coil current reached 100% of the short sample Ic value.