May 30, 2008

New Iron based superconductors might resist magnetic fields over 100 Tesla


Researchers at the National High Magnetic Field Laboratory at Florida State University have discovered that the new iron based family of superconducting material kept superconducting all the way up to 45 tesla. 45 Tesla is the most powerful magnet sustained field in the world. The researchers did not find the upper limit for magnetic field resistant superconductivity for the new material. Scientists are calling the material “doped rare earth iron oxyarsenides".

The new superconductors seem like they will be able to make improved MRI machines and research magnets, a new generation of superconducting electric motors, generators and power transmission lines.

Tesla is a unit of magnetic field strength; the Earth’s magnetic field is one twenty thousandth of a tesla.

A high tolerance for magnetic field is one of three key properties researchers hope for in superconductors. Also desirable are the abilities to operate at relatively high temperatures and in the presence of high electrical currents. Superconductors are used to make MRI and research magnets, and now they are being tested in a new generation of superconducting electric motors, generators, transformers and power transmission lines. Today, the most powerful superconducting magnet generates a field of about 26 tesla. If a superconductor could be found that tolerates a higher current and field, it may make possible more powerful magnets, opening up vast new research areas to scientists and power applications.

Based on both theoretical calculations and what we’re seeing from the experiments, it seems likely that this is a completely different mechanism for superconductivity. If it’s found that these materials can support high current densities, then they could be tremendously useful.



The hybrid 45 tesla magnet site



The Magnet Lab’s 100-tesla multi-shot, currently the most-powerful reusable magnet in the world. Designed to operate at 100 teslas, the multi-shot has so far been kept to 89.9 teslas, still a world record.

100-tesla multi-shot magnet, with its central insert coil being loaded.

All of the highest-field magnets are pulsed: a single swift current pulse sent through the assembly creates a field that rises, peaks, and decays (typically) within a few thousandths of a second. The short duration limits the heat and stress on the materials, so the highest fields can be contained without destroying the magnet.



The Los Alamos team is currently investigating how the Fermi surface evolves as the cuprate's composition changes. In comparing all the data (including the controversial results from an experiment conducted at Los Alamos in 1991), one sees dramatic changes in the Fermi surface as the materials get closer to the number of dopants that is optimum for the highest superconducting temperature. The Magnet Lab is continuing its quest to produce higher fields. Indeed, Mielke is spearheading a new electromagnet design, the "single-turn," named for its single loop of copper. The single-turn has already produced pulsed fields as high as 240 teslas. The field lasts but a few millionths of a second, and then—the magnet explodes! Remarkably, the magnet's design allows a sample to survive the explosion intact.

Mielke is planning to use the single-turn to measure the Fermi surface of plutonium and to investigate superconductivity in the heavy-electron metals, but he needs to refine his measurement techniques. "A changing magnetic field can generate an unwanted voltage—electromagnetic interference (EMI)—in the measurement probe," he explains. "It's hard enough to measure small signals in the 100-tesla magnet, where the field goes from nothing to everything in a few thousandths of a second. When the field ramps up in the single-turn's millionth of a second, the EMI is much higher, and the measurement becomes that much harder."


Pulsed fields of microseconds up to 300 Tesla are possible and the 100 tesla multi-shot can get 90 tesla for 25 milliseconds.

FURTHER READING
Preprint of the research paper

From Nature the research paper, Two-band superconductivity in LaFeAsO0.89F0.11 at very high magnetic fields

Researchers report resistance measurements of LaFeAsO0.89F0.11 at high magnetic fields, up to 45 T, that show a remarkable enhancement of the upper critical field B c2 compared to values expected from the slopes dB c2/dT 2 T K-1 near T c, particularly at low temperatures where the deduced B c2(0) 63–65 T exceeds the paramagnetic limit. They argue that oxypnictides represent a new class of high-field superconductors with B c2 values surpassing those of Nb3Sn, MgB2 and the Chevrel phases, and perhaps exceeding the 100 T magnetic field benchmark of the high-T c copper oxides.

The new iron based superconductors could open up a new path to room temperature superconductors

MRI magnets are usually in the 0.5 to 3.0 Tesla range

The most powerful MRI has a 9.4 Tesla field

Researchers and physicians hope that the 9.4T will usher in a new era of brain imaging in which they will be able to observe metabolic processes and customize health care.

Oncologists, for example, may one day be able to tailor radiation therapy based on a brain tumor's real-time response to treatment. Currently, physicians often must wait weeks to see if a tumor is shrinking in response to therapy. With the 9.4T, it will be possible to see if individual cells within the tumor are dying long before the tumor has begun to shrink.

It offers physicians a real-time view of biological processes in the human brain.


Magnetic field strength is an important factor in determining image quality. Higher magnetic fields increase signal-to-noise ratio, permitting higher resolution or faster scanning. However, higher field strengths require more costly magnets with higher maintenance costs, and have increased safety concerns

Better magnet material (such as the iron based superconductors) could also allow a magnet field strength at current levels with smaller magnets. Some MRI devices how weigh hundreds of tons.

Superconducting magnets at wikipedia


Current superconductors have enabled a 36.5 MW prototype engine for the navy that is three times smaller than the same power engine using ordinary wiring. The new iron superconductors might make engines even smaller or more powerful (if they can also carry a lot of current.)

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