Hybrid Magnet - Setup and mode of operation
The magnetic field will be created by what is known as a ”Series-Connected-Hybrid Magnet System”. This is a new approach developed at the National High Magnetic Field Laboratory (NHMFL), in Tallahassee, FL, USA. A resistive inner coil is connected electrically in series with a superconducting outer coil. The 20,000 A DC current flows through both coils.
Operating costs are considerably lower for this configuration when compared to a single resistive electromagnet of the same field. This is because a large fraction of the magnetic field is generated without power consumption by the superconducting coil. This magnet system is operated with direct current.
The outer, superconducting coil (about 50 cm ID and 120 cm OD) generates a background magnetic field of approx. 13 Tesla. Superconducting strands made from a niobium-tin compound (Nb3Sn) are employed as the conductor. The superconducting coil is operated at 4 Kelvin (-269 °C) using helium as a coolant provided by a helium refrigerator. The helium flows in the wire conduit through voids within the cable (the technology is called CIC = cable in conduit). The thermal radiation shield around the coil and the high-temperature superconducting current leads are cooled down using helium gas of about 40 Kelvin. The refrigeration power consumed depends on the operating conditions.
A resistive coil is used for the inner section of the magnet. Its magnetic field is the strongest, stronger than could be achieved by a superconducting coil in the existing background field. At 4 Megawatts of electrical power, this inner coil of approx. 60 cm diameter creates a field of 14 Tesla, which is added to the field of the superconducting CIC coil. As a result, about 26 Tesla are obtained in the coil centre. The resistive coil consists of copper-alloy disks called Bitter plates stacked on top of each other to form a spiral.
This stack can withstand high currents and forces. Cooling water is pumped through holes penetrating the stack. Heat generated in the resistive coil has to be dissipated by a high-pressure cooling water circuit. The pure water circuit of the magnet transfers heat via a heat exchanger to the cooling water circuit of the cooling facilities. Cooling towers, chillers, and a water storage tank of approx. 300 cubic meters are located in this circuit.The sample for the neutron scattering experiment sits in the horizontal 50 mm diam. central bore of the magnet system located between conical openings at both sides of the magnet. These chambers are evacuated in order not to attenuate the neutron beam by air scattering.
A new additional building to accommodate all the peripheral systems had to be erected for the high-field magnet. This "Technikum“, as it is known, will accommodate all facilities for the supply of cooling and energy. The magnet system will be installed in Neutron Guide Hall 2 at the Extreme Environment Diffractometer EXED.
A sophisticated control system protects the entire hybrid magnet against operating errors and equipment failures. A major potential failure of the superconducting coil could occur if part of the winding exceeds the admissible operating temperature and looses its superconductivity. This could result in a high production of energy due to ohmic heating, which can damage the coil. For this reason, if such an event (called a “quench”) occurs, the magnet current has to be immediately reduced to zero (fast discharge). This is achieved by the magnet protection system. A main component of the system is a dump resistor that, in case of a failure, absorbs the energy stored in the magnet, while the connection from the power supply to the magnet is interrupted by circuit breakers. Independent detection electronics with an uninterruptible power supply have the sole task of detecting such a quench as fast and reliably as possible.
Learn more about magnetism in the "Magnet Academy“ by NHMFL.