Development of an ultra low field-ripple electromagnet

We developed an ultra low field-ripple magnet by adopting a mineral insulation cable (MIC) which is used under high radiation field, and of which conductor is surrounded by MgO insulation layer and copper sheath. Magnetic field due to an alternating current in conductor is canceled by the induced current in the sheath. We made a solenoid magnet by MIC, and observed this cancelation effect in alternating currents from 50 Hz to 2.4 kHz. This effect has a tendency which is inversely proportion to the AC frequency, and the field ripple is suppressed by 10−2 in 50 Hz and 3×10−4 in 2.4 kHz. Namely, a power supply with the current ripple of 10−4 is expected to generate a magnetic field with the ripple of 10−6 or lower.