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Hybrid Magnet - Setup and mode of operation
The magnetic field will be created by a so-called Series-Connected Hybrid Magnet System. This is a new approach developed at the National High Magnetic Field Laboratory (NHMFL), Tallahassee, FL, USA. A normal-conducting inner coil is connected in series with a superconducting outer coil. The same high current of 20,000 Amperes flows through them, one coil after the other.
Running costs are considerably lower in this arrangement when compared to a purely normal-conducting (resistive) magnet. This is because a large fraction of the magnetic field is generated by the superconducting coil. The magnet is operated with direct current. In the final stage of completion the consumed electrical power of 8 Megawatt will require a voltage of 400 Volts.
The outer superconducting coil (about 60 cm inner and 120 cm outer diameter) generates a background magnetic field of approx. 13 Tesla. Superconducting strands made from a niobium-tin alloy (Nb3Sn) are employed as conductor. The superconducting coil operates with helium at 4 Kelvin (=-269°C) provided by a helium liquefier. The helium acts as a coolant and flows in the conduit through voids within the cable (the technology is called CIC = cable in conduit). The radiation shield and the high-temperature-superconductor current leads are cooled down to the liquid nitrogen temperature of about 80 Kelvin (= -193°C). The refrigeration power depends on the operating conditions.
Magnet with Bitter disks
A normal-conducting magnet is used for the inner section of the magnet. Here the magnetic field is the strongest; stronger than it could be achieved by a superconducting coil. At 4 Megawatt, this inner coil of approx. 60 cm diameter creates a field of 14 Tesla which is added to the field of the superconducting coil. As a result, about 27 Tesla are obtained at the sample position. The normal-conducting magnet consists of copper disks (of a copper alloy, so-called “Bitter disks”) stacked on top of each other to form a spiral. This stack is ideal to withstand high currents and forces. As a coolant water is pressed through holes penetrating the stack.
This heat has to be dissipated by a high-pressure cooling water circuit. The water circuit of the magnet transmits heat via a 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 is located in a bore with a diameter of 50 mm between two conical openings. These are designed as vacuum chambers in order not to attenuate the neutron beam by air on its way to the sample.
A new additional building accomodating all aggregates of the periphery has to be erected for the high field magnet, which is designed to be installed in neutron guide hall number 2. This so-called "Technikum“ will accomodate all facilities for the supply of cooling water and energy. For more details of these facilities please look at the animation.
A sophisticated control system protects the entire hybrid magnet against operating errors and technical failures. A major potential failure of the superconducting coil occurs when part of the winding exceeds the admissible operating temperature and looses its superconducting state. This results in a huge energy release by “Ohmic” heating which can damage the coil. For this reason, if such an event which is called a “quench” occurs, the magnet current has to be immediately lowered to zero (fast discharge). This is achieved by the magnet protection system. A main part of the system is a dump resistor which, in case of a failure, absorbs the energy stored in the magnet and releases it by getting hot while the power supply 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 reliable as possible.
Learn more about magnetism in the "Magnet Academy“ by NHMFL.
