Effect of actual magnetic properties of steel on field quality in magnet systems


Precision 3D simulations of a field in the working zone demonstrate the necessity of taking into account the actual non-linear magnetic properties (B-H curves).

Completed orders:

Flerov lab. JINR (Dubna, Russia) – cyclotron DC72
Efremov Inst. (St.-Petersburg, Russia) – cyclotron CC18/9
Efremov Inst. (St.-Petersburg, Russia) – cyclotron CC12
Confidential (Genue, Italy) – MRI tomograph
ITER Team (Garching, Germany) - International Thermonuclear Experimental Reactor

The ferromagnetic saturation effect occurred in magnet systems necessitates magnetic measurements in a field range expanded up to 2.5-3.5 T. An original measurement technique is proposed which enhances the standard procedure for B-H measurements. Field calculations with the use of a finite-element model enable simultaneous consideration of several B-H curves to analyse behaviour of different materials used in the fabrication.

Numerical simulations of magnetic fields in electrophysic devices involve the use of some additional parameters derived from the B-H curve ( and saturation intensity) and are rather sensitive to the curve smoothness.

Due to technological and cost reasons, a cyclotron magnet system comprises components made of different ferromagnetic materials. This necessitates development of software tools capable of processing magnet data bases [3,4] and updating them via an input file describing B-H curves.

Magnetic measurements over the low and middle field range (0.05 kA/m to 15 kA/m, B<1.9 T) are performed on ring samples according to the Russian standard for magnetic measurements of non-retentive materials. For measurements in high fields (1kA/m<Í<600 kA/m, B>1.5 T), an original technique is proposed. The measurements are performed on barrel samples with the use of a computer-based permeameter [1,5]. A sample is placed between the poles of an E-shaped laboratory dipole magnet, which generates a longitudinal field inside the sample. The sample and the dipole magnet form a closed magnetic circuit. Magnetic properties are determined by the induction method [5]. Both for the ring and barrel samples, induction is determined by measuring emf generated in a measuring coil. A variable field is produced by current pulses in an exciting coil. The shape and frequency of the current pulses are taken so that to avoid eddy current and noise effects.

Readings from measuring devices (control current, measuring coil voltage, Hall voltage) are transferred via the data acquisition system to the processor, which also continuously controls measurements. This enables automated measurement at 10-20 points over each field range to generate a set of symmetric hysteresis loops with different field amplitudes. The peaks of the hysteresis loops form a B-H curve. It takes 2 to 20 min depending on the field level in the sample to obtain one point on a B-H curve. The full measurement period in both field ranges, including device testing and sample pre-magnetization, is 0.5 to 1.5 hr for one B-H curve.

As example, the computerised measurement system and measurement procedure were tested and validated during magnetic measurements on ferromagnetic steel for the magnet system of the cyclotron DC-72 developed and built at the Flerov Laboratory (JINR, Dubna, Russia) [5] for the Center of Nuclear Physics and Medicine (Metrology Institute, Bratislave, Slovakia). The measured data were used to simulate DC-72 field maps and optimise the magnet system design so as to provide required field distribution [6].


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