Over the past few years, magnets with different peak fields and waveforms have been developed. They are listed below.

65 T user fields with fast cooling

The fast cooling user magnets are designed with 10 layers of CuNb micro-composite wire developed by the China Northwest Institute for Non-ferrous Metal Research. Between the inner and outer sections, G10 rods with cross section 4 mm × 5 mm were inserted as spacers around the entire circumference, parallel to the coil axis. The liquid nitrogen thus flows freely into the channels from the groves on the bottom flange and comes out from the groves on the top flanges. This magnet is driven by the two 1.2 ~ 4 MJ capacitor bank modules. For a typical 60 T pulsed magnetic field with 21 mm bore and pulse duration of 6~60 ms, the cooling time between subsequent pulses is 15 ~ 45 minutes. The service life is about 800 ~ 1200 pulses.

Fig.1  Cooling curve of the 65 T user magnet

50 T long pulse field with 100 ms flat-top

The pulse magnet for 100 ms flat-top field consists of two coaxially nested coils and has a bore of 22 mm bore. Each of these two coils is driven by a rectifier connected to the pulse generator. Fields up to 50 T with 100 ms flat-top with 0.5 % ripples have been generated. At the end of the flat-top, both rectifiers are switched into the inverter mode by gradually changing the trigger angles from 90° to 120° to take the energy away from the magnet.

60 T short pulse with flat-top

The ripples from the rectification of the pulse generator power supply may disturb the sensitive measurement. An innovative method for generating a flat-top pulsed magnetic field by means of capacitor banks is developed. The system consists of two capacitor banks as they are normally used to generate a pulsed field. The two discharge circuits (the magnet circuit and the auxiliary circuit) are coupled by a pulse transformer such that the electromotive force (EMF) induced via the transformer in the magnet circuit containing the magnet coil is opposed to the EMF of the capacitor bank. At a certain point before the current pulse in the coil reaches its peak, the auxiliary circuit is triggered. With optimized parameters for charging voltage and trigger delay, the current in the magnet circuit can be approximately kept constant to obtain a flat-top. Fields up to 64 T with 10 ms flat-top have been obtained with a conventional user magnet used. As there is no switch on or off of electronic devices during the pulse, the flat-top is absolutely smooth without noises.

Fig.2 Typical magnetic field waveforms

90 T dual-coil system

Up to now, all the magnets for more than 80 T field are multi-coil system. However, strong electromagnetic coupling exists among the coils in the multi-coil magnet. When the inner coil starts working, the fast rise of the magnetic field generated by the inner coil induces high EMF (electromotive force) in the out-er coil. The EMF has opposite direction to the polarity of the voltage on the power supply of the outer coil. Thus, the current in the outer coil is forced to drop, resulting in a large drop of the magnetic field generated by the outer coil. Take the 90.6 T system at the WHMFC as an example, the magnet consists of two coils. At 50 ms, the magnetic field reaches 47 T and the current in the inner coil starts rising. With the rise of the current (magnetic field) in the inner coil, the current (magnetic field) in the outer starts falling. At 54 ms, the inner coil current reaches the maximum value, as well as the total magnetic field, the magnetic field generated by the outer coil reduces to 43 T. It is obvious that the outer coil contributes only 43 T when the total magnetic field reaches 90.6 T. However, the outer coil must be designed mechanically strong enough to bear the magnetic stress at 47 T. The magnetic stress at 47 T is almost 20% higher than that at 43 T. As mentioned above, the magnetic stress is the biggest obstacle in the design of the high field pulsed magnet. Making the magnet 20% stronger is an apparently challenging is-sue. In order to solve this problem, a compensation method to reduce the electromagnetic coupling among the coils is proposed for at the WHMFC and is applied to a dual-coil magnet. The dual coil magnet with a bore size of 10 mm consists of the inner coil and the outer coil, which are driven by the 1.6 MJ / 25 kV module and by the twenty 1.2 MJ modules respectively. The inner coil consists of eight layers of 2.8 mm × 4.3 mm CuNb wire. The outer coil was wound directly on the inner coil with twelve layers of 4 mm × 8 mm soft copper. Peak field up to 94.8 T has been obtained. Of course, more energy is required for the decoupling design. Anyhow, the energy is easier to be solved than the design of the coils for 10% higher magnetic field. The compensation seems to be promising.

Fig.3  Model of dual-coil maget

In 2023, the Upgrade of the Pulsed High Magnetic Field Facility (PHMFF-U, Stage II) funded by the Chinese National Development and Reformation Committee, was officially approved in October, 2023. The pulsed magnet aiming at peak value up to 110 T in 10 mm diameter and 70 T with 10 ms flat-top in 14 mm diameter will be built.

Fig.4  Coil winding