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Customized power amplifier for realizing tiny magnetic fields

It’s a World Record – realization of a unique magnetically shielded laboratory with a damping factor of more than one million

Whether for the direct measurement of brain waves or for the determination of neutron-torque, one needs a reliable degaussing of the magnetic shields in order to achieve the necessary low magnetic field strengths. A basis for this is a current-controlled HERO Power Precision Power Amplifier from Rohrer Mess- und Systemtechnik in Munich, tailored for the specific application.

Extreme requirements call for special, customized solutions.

The human brain still remains a mystery. Therefore it's a goal of the researchers to measure the location of brain activity directly and clearly. Previously, one could only estimate values by means of other variables, for instance by deducing from an increased blood oxygen content to a locally increased brain activity, which is not very accurate. “One solution would be to measure directly by MRI the extremely small magnetic fields in the nano-Tesla range that produce the brain waves. In addition, the neural effects occur in the millisecond range, i.e. at frequencies of less than 1 kHz”, summarizes Dr. Rainer Körber, employee at the PTB (Physikalisch-Technische Bundesanstalt), Department of Biosignals.

This method to localize brain activity by using low field MRI would then enable an exact representation in the anatomical image and avoid the erroneous estimation of the EEG. The problem: The Earth's magnetic field (50 μT) and the various magnetic fields of electrical and electronic systems interfere and distort the measurements. Solution: a strongly magnetically shielded chamber. The PTB scientists have now developed a method which works with magnetic field strengths of less than 50 μT (millionths of a tesla). Via this method the hydrogen nuclei are selectively electromagnetically excited, which then give a signal until the return to the ground state. If one uses low field strengths, one can simultaneously detect brainwaves. Such a combination is not possible with conventional high-field MRI.

PTB have built a magnetic shielding cabin: it consists of two layers of Mu-metal, a soft magnetic nickel-iron alloy of high magnetic permeability, ideally suited to shield low frequency magnetic fields. A specially designed coil system to the Mu-metal layers creates an excellent degaussing of the corresponding chamber, with residual magnetic fields of less than 1.5 nT at a gradient of less than 20 pT/cm. The cabin installed in Berlin was built by the company Vacuumschmelze (Hanau). The degaussing is done with the amplifier by Rohrer.

The photo shows the inner layers of the magnetic shielding with Prof. Dr. Peter Fierlinger (left) and doctoral candidate Michael Sturm. (Source: TU München)

The photo shows the inner layers of the magnetic shielding with Prof. Dr. Peter Fierlinger (left) and doctoral candidate Michael Sturm. (Source: TU München)

Nuclear physicists at the Technical University of Munich are also planning high-precision measurements. The researchers want to determine whether neutrons react to electric fields. If neutrons have an electric dipole moment, this might help explain what happened after the Big Bang, within the first seconds of the universe. “In our universe, there is matter, but nearly no antimatter. Both matter and antimatter were probably created in equal parts with the Big Bang and then immediately annihilated again to energy. Apparently, during this process some matter was left – the stuff of which we and the universe around us consists. “We want to appraoch the answer to the question of why there’s an asymmetry between matter and antimatter with the high-precision study of the properties of neutrons”, summarizes Tobias Lins, member of the research group at the TU Munich.

By measuring the electric dipole moment of a neutron, theories can be tested that try to extend the Standard Model. According to the Standard Model, the electric dipole moment of a neutron is far too small to be measured by means of today's technologies. Pursuant to theories that extend the Standard Model, there must be a much larger electric dipole moment, which is in a range that should be able to be demonstrated experimentally.

Once more, the magnetic field of the Earth and those of transformers, motors, electric appliances or even metal doors cause problems. The suitable magnetic shield was developed at the Technical University of Munich by the research group led by Prof. Peter Fierlinger. At the setup, inter alia, PTB, IMEDCO (shield) and Rohrer (amplifier) are involved.

The result is a unique magnetically shielded laboratory with a damping factor of more than one million against external interference in the micro-Tesla range at frequencies in the millihertz area – a world record. “This is a great achievement, because attenuation in the low frequency region is especially difficult”, says Tobias Lins. The magnetic field inside the chamber is only a few 100 pT and is smaller than the average magnetic field between the stars of the universe.

The new, almost magnetic free space allows to improve the accuracy of recent measurements of the electric dipole moment of the neutron by a factor of 100 and thus to advance into the dimension of the theoretically predicted size of the phenomenon.

The outer part of the Munich laboratory consists of a magnetically shielded room (MSR) of two shells Magnifer, a highly magnetizable alloy. Each of these shells consists of 2 x 1 mm thick heat-treated Magnifier-sheets. An 8 mm thick aluminum shell in between is supposed to shield the higher frequencies. The shield absorbs the field lines of external magnetic fields and thus prevents their penetration in the interior of the experiment.

Whether for the direct measurement of brain waves or for the determination of the neutron-torque – in order to achieve these low magnetic field strengths a reliable degaussing of the magnetic shields is required. A basis for this is a current-regulated amplifier, tailored for the specific application.

High attenuation factors can only be created by large amounts of magnetizable alloys, however, to achieve the smallest possible residual field is a challenge. This requires a carefully designed shielding and an efficient degaussing. The term 'degaussing' is misleading in this context, since the shielding does not demagnetize, but perfectly adapts the magnetic field to the environment, called equilibration. For this purpose, the shielding material is alternately magnetized in opposite directions, with the strength of the magnetization decreasing continuously until the desired state of equilibrium is reached. To achieve this balance a sophisticated configuration of coils, transformers, amplifiers, digital and analogue technology is needed. Critical is the amplifier, the noise of which must be small and the zero line extremely stable and accurately adjusted.

The combined technology with amplifiers by the company Rohrer and shielding by IMEDCO now allows to achieve such low residual fields with massively reduced material usage for applications in bio-magnetism, material tests or lowest frequency NMR.

For this equilibration, the HERO POWER amplifiers have proven to be ideal. Specific requirements in this respect are low ripple and accurate zero crossings. In particular, the input filter, a balanced combination of high- and low-pass, and the fine tuning of the DC offset to 10-6 and the extreme symmetry of the output stages contribute to this.

A measuring setup with PA614B current-controlled HERO Power Precision Power Amplifier from Rohrer Mess- und Systemtechnik in Munich for the precise measurement of the magnetic field.

A measuring setup with PA614B current-controlled HERO Power Precision Power Amplifier from Rohrer Mess- und Systemtechnik in Munich for the precise measurement of the magnetic field.

High currents are required for the equilibrating alternating field and it requires amplifiers with sufficient power to fully magnetize, but also with high precision at low currents. The residual magnetization depends on the residual current before switching off. It is therefore important that the amplifier guarantees a sine curve at low currents and that it can be lead linearly to zero before being turned off. It is useful to additionally use a transformer in order to avoid potential noise due to resonance, realizing an exact as possible zero-crossing, for which perfectly symmetrical characteristics in all four quadrants are a prerequisite.

A measuring setup with PA2088B current-controlled HERO Power Precision Power Amplifier from Rohrer Mess- und Systemtechnik in Munich for the precise measurement of the magnetic field.

A measuring setup with PA2088B current-controlled HERO Power Precision Power Amplifier from Rohrer Mess- und Systemtechnik in Munich for the precise measurement of the magnetic field.

The amplifiers used are specially made by Rohrer based on the HERO POWER, since there are no comparable systems from stock. The amplifier was built according to the specifications of the scientists so that they match perfectly with the necessary equilibration process. Moreover, the performance has to be high enough to fully magnetize the plate in a first step.

“Every customer gets what it wants. The HERO amplifier is the basic product which is equipped according the demands of each customer,” says Helmut Rohrer, CEO of Rohrer GmbH – Measuring and Systems Engineering. Which of the possible techniques is used depends on the customer's requirements.

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