Static Fields
The hybrid electromagnet at LNCMI-Grenoble
Contact : pierre.pugnat [a] lncmi.cnrs.fr
Contact : luc.ronayette [a] lncmi.cnrs.fr
Contact : rolf.pfister [a] lncmi.cnrs.fr
Hybrid electromagnets combining resistive and superconducting technologies are currently the only technology to deliver static magnetic fields in excess of 42 Teslas.
This value is the maximum value achieved today by a purely resistive electromagnet at the High Magnetic Field Laboratory in Hefei, China (CHMFL). Its required 32.3 MW [1] to be provided at the magnet current leads.
In the frame of a CNRS-CEA collaboration a modular hybrid electromagnet has been designed and built at LNCMI-Grenoble to produce intense magnetic fields and fluxes. The LNCMI-Grenoble hybrid magnet reached 42 Tesla in a diameter of 34 mm on November 8, 2024 [6] with a power about 24 MW. The Tallahassee (US) and the Hefei (China) facilities are today the two places worldwide where 45 T DC field is available by Hybrid electromagnets but using a much higher power (~ 32 to 35 MW).
Fig. 2: a) Cross-section of the Grenoble hybrid magnet
Fig. 2: b) Photograph of the Grenoble Hybrid magnet.
Height is 5.4 m and mass around 54 tonnes.
One of the key feature of the LNCMI-Grenoble hybrid magnet (Fig. 2), is its modularity in terms of magnetic field and fluxes as summarized in Table 1. It then opens to a wide community of users and permit tp developp scientific themes at the LNCMI, such as the search for axionic dark matter [7] or high field Magnetohydrodynamics.
The hybrid magnet aims to achieve 43 T with an electrical power of 24 MW in a 34 mm room temperature diameter bore. For this it combines a polyhelix insert (25.5 T) two Bitter coils (9 T) and a 1100 mm-diameter superconducting coil (8.5 T).
The latter consists of a 9km Rutherford cable-on-conduit (RCOCC) conductor in Nb-Ti/Cu (Fig. 1) , developed to control induced-current losses. This technology, an alternative to the Cable In Conduit Conductor (CICC), has been produced by soldering a Rutherford cable onto a hollow rectangular Cu-Ag stabilizer. During operation it is cooled at 1.8 K by a superfluid helium bath pressurized at 1200 hPa (Bain Claudet technology developed at CEA-Grenoble [2]). The coil is made up of 37 stacked double-plate pankakes connected in series and housed in a helium vessel connected by a cryogenic line to an external cryogenic satellite.
A 35 K-eddy-current shield is inserted between the resistive and the superconducting coils to decrease the field variation with time seen by the RCOCC in case of a power cut of the resistive inner coils. A redundant superconducting coil protection system (MSS) has been developed for quench detection and energy extraction (80 MJ), together with a dedicated control and acquisition system. The hybrid magnet requires additional equipment (Fig. 2) such as a 5,000 m³/h pumping unit, a dedicated 150 l/h helium liquefaction unit, 7,500 A/±15 V power converters for the superconducting coil and the 12 + 18 MW power converters for the resistive coils,[3],[4],[5].
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RESISTIVE MAGNETS
Contact : francois.debray [a] lncmi.cnrs.fr
Magnetic fields up to 38 T on resistive magnets and 42 T on the new Grenoble Hybrid magnet are produced in a 34 mm diameter bore at room temperature. In both cases the resistive coils are made from in house developped copper alloys.
Dedicated rectifiers transform the high-voltage alternating current (AC) supplied to the laboratory in a two-stage process (225 KV to 15 KV, then 15 KV to 550 V) into a highly stable direct current (DC) that powers the high-field magnets. As understood two centuries ago by A.M. Ampère, the magnetic field produced by a coil is proportional to the number of turns of the coil and the intensity of the electric current. The maximum current available is 2 times 33,000 amperes. The maximum magnetic field can be reached in less than two minutes (on resistive magnets). it is an essential feature for researchers who scan the magnetic field to characterize the properties of a given material over the full range of magnetic fields available.
The polyhelix is the central part of the high field magnets of LNCMI. There are used both for purely resistive magnets and for the hybrid magnet.
Top wiew of a 14 helix insert. Helices are electrically connected in series and hydraulically cooled in parallel.The rings pass the electrical current from an helix to another.
Slits are performed in the rings to optimize the flowrate between helices.
For the High field magnet cooling , the High field facility is connected to the ‘Isere’ trough a dedicated pumping station shared with the nearby ILL Neutron facility and the ESRF Synchrotron facility. Magnetic field plateaus of several hours are then available for researchs which required to accumulate experimental data for obtaining high-quality scientific results.