What makes magnetic field stronger

All high-field magnets—that is, magnets that generate fields substantially greater than 2 T—are electromagnets. Magnetic fields are central to the operation of many devices crucial for the functioning of a modern society. For example, electric motors and generators of electrical power take advantage, respectively, of the force exerted by a magnetic field on a wire that carries an electric current and of the complementary process whereby electrons in a wire moving across a magnetic field will feel a force that can drive a current along the wire.

Other devices, such as read-out heads in magnetic disk memories, depend on magnetic-field-induced changes in the electrical resistance of certain materials, which are used to sense the orientations of the microscopic magnetic domains that encode digital information on the disk.

Magnetic resonance imaging MRI devices, which are now extensively used for medical applications, take advantage of a different aspect of the interactions between fields and matter. Here, the combined effects of ac and dc magnetic fields on the magnetic moments of the spinning nuclei particularly protons in the human body are used to obtain detailed information about the environment of the nuclei, which can distinguish between tissues of different types and can reveal changes due to pathological conditions.

For motors and generators and many other electromechanical devices, increases in strength of the magnetic fields employed could lead to important improvements. However, the fields used in these devices are generally limited by practical considerations of cost and weight and do not approach the strengths employed in specialized research applications.

By contrast, MRI devices require the highest available fields to achieve satisfactory resolution and sensitivity, and the magnets used in these devices are constantly pushing the limits of magnetic field technology.

Very high fields are necessary for many crucial research applications in materials science, chemistry, and biology. The success of these experiments may have major impacts on health care and technology. High magnetic fields in very large volumes are also required for accelerators in high-energy physics and in plasma research aimed at the realization of controlled nuclear fusion.

Research using high magnetic fields has proven to be a critical tool in solving problems of technological relevance. High field research has led to an increased understanding of matter, to the discovery of entirely new phenomena, and, subsequently, to the development of new devices and products of significant technological and societal importance.

And magnetic fields continue to be used to attack many problems of scientific and technological interest. One of the current problems being addressed with high field research is in energy generation and storage. As another example, ion cyclotron resonance ICR has proven to be an essential analytical tool for understanding oil-pipeline-clogging deposits and oil-spill pollution.

These are just a couple of examples, and more will be presented in the body of this report. Given the important role that high fields have played in past technological advances, it is a good bet that research involving high magnetic fields will continue to yield technological advances in the future.

One indication of the broad range of research dependent on the availability of high magnetic fields may be seen in Figure 1. The categories listed are condensed matter physics and materials, engineering materials, instrumentation, and magnet technology , biology and biochemistry, chemistry, and geochemistry. Figure 1. These topics are discussed in Chapters 4 , 5 , and 7. Scientists have been building electromagnets that deliver fields of ever-increasing strength since the nineteenth century.

Two issues have had to be confronted at every step of the way. First, the field of an energized electromagnet exerts forces. Second, if the electrical conductor of which the magnet is made is a normal metal, resistive heating produced by the electric currents can cause the magnet to fail. Beginning in the s, scientists learned how to build magnets using superconducting wires, which can carry a current without resistance at sufficiently low temperatures; however, superconductors have their own limitations, as will be discussed later in this report.

In particular, all superconducting materials have a critical magnetic field above which they can no longer support resistance-less current flow and cannot be used in magnet construction. A major goal of research in high magnetic fields is to learn how to create superconductors with higher critical fields and to learn how to make magnets out of these materials.

However, the highest magnetic fields attained to date have been produced by resistive magnets that are operated in a pulsed mode to minimize destructive heating effects. The construction of magnets that operate at high fields is, and has always been, an engineering challenge. The quantities that determine whether a magnet meets this definition are not just the magnitude of the field itself but include the total energy stored in the field, which is proportional to the integral of the square of the field over the volume affected.

Thus an MRI magnet having a maximum field strength of 8 T and a bore large enough to accommodate a human being is as much a high-field magnet as a smaller-bore magnet for an NMR spectrometer operating above 20 T. The highest field attained so far in a dc magnet is 45 T, while pulsed field magnets can operate at 60 T and above. In pulsed-field experiments, there is generally an inverse relationship between the field strength attained and the duration of the pulse.

The current record for a nondestructive pulsed magnet is T for a duration of about 10 ms. See Figure 1. The technological challenges involved in construction and operation of the various kinds of high-field magnets are discussed in Chapter 7 of this report, along with recommendations of goals for new magnet construction in the coming decade.

Magnets at the high-field frontier are necessarily complex and expensive, both to construct and to operate. High-field magnets require a highly skilled staff to keep them running and to maintain the instruments that make them useful. In addition, resistive magnets require a large infrastructure to supply needed electric energy and cooling power. For these reasons, it is natural that the highest field magnets should be concentrated at a very small number of national facilities.

It also plays a crucial role in the training of high-magnetic field scientists and in the development of next-generation magnets and magnetic materials.

The leading status of the United States in high-magnetic-field science is due in very large measure to the many contributions of NHFML.

There are, however, certain areas where it may be more advantageous to create distributed facilities, able to accommodate large numbers of users. An important example of this is in the field of chemical and biological NMR spectroscopy, where the committee envisions the establishment of several state-of-the-art user facilities.

Also, some important applications of high magnetic fields require the combination of high magnetic field with other expensive facilities, which may be best achieved by the deployment of a specialized magnet at an existing facility such as a synchrotron light source or a neutron source.

As mentioned above, the very highest magnetic fields for research are necessarily restricted to purpose-built facilities, which require significant infrastructure investments. For example:. These properties make electromagnets useful for picking up scrap iron and steel in scrapyards. The magnetic field around an electromagnet is just the same as the one around a bar magnet.

While the destructive nature of strong magnetic fields places a practical limit on how strong of a field earthlings can create, it does not place a fundamental limit. Magnetic fields that surpass about a billion Gauss are so strong that they compress atoms to tiny needles, destroying the ordinary chemical bonds that bind atoms into molecules, and making chemistry as we know it impossible. Each atom is compressed into a needle shape because the electrons that fill most of the atom are forced by the magnetic field to spin in tiny circles.

While such extremely strong magnetic fields are not possible on earth, they do exist in highly-magnetized stars called magnetars. A magnetar is a type of neutron star left over from a supernova. The intense magnetic field of a magnetar is created by superconducting currents of protons inside the neutron star, which were established by the manner in which the matter collapsed to form a neutron star.

Duncan summarized many of the theoretically-predicted exotic effects of magnetic fields that are even stronger:. At the most extreme end, a magnetic field that is strong enough could form a black hole. General Relativity tells us that both energy and mass bend spacetime.

Such actions are presumably not at work in white dwarfs, neutron stars, magnetars or any other compact objects. The fields are produced in their differentially rotating progenitors. Using the total width of the convection zone grossly overestimates the current filament width.

Clearly velocities are also much less than c. The comparably strong fields in neutron stars are subsequently produced in the rapid collapse of the magnetized heavy progenitor star not having had time within the time of collapsing to dissipate the magnetic energy which becomes compressed into the tiny neutron star volume. The classical electrodynamic estimate clearly fails in providing an upper limit on magnetic field strength that would match the observational evidence.

Other no less severe discrepancies are obtained from putting the neutron star magnetic field energy equal to the total available rotational energy both in the progenitor or in the neutron star assuming equipartition of rotational and magnetic energy—clearly a barely justified assumption in both cases. Magnetic energy cannot become larger than the originally available dynamical energy of its cause of which it is just a fraction. If much stronger fields were generated at all, it must have happened during times and in objects where magnetic fields could have been produced by processes other than classical dynamos.

One thus has to enter quantum electrodynamics respectively quantum field theory in order to infer about the principal physical limitations on the generation of any magnetic fields. The following investigation is motivated less by observations than by this fundamental theoretical question.

The physical interpretation of this solution was given much later in Aharonov-Bohm theory [ 5 ]. This length, which is the gyroradius of an electron in the lowest lying Landau energy level, can be interpreted as the radius of a magnetic field line in the magnetic field B. Field lines become narrower the stronger the magnetic field. On the other hand, rewriting Equation 3 yields an expression for the magnetic field.

Use of the Compton wavelength relates the limiting field strength in neutron stars to quantum electrodynamics. It raises the question for a more precise theoretical determination of the quantum electrodynamic limiting field strength accounting for relativistic effects. It also raises the question whether reference to other fundamental length scales may provide other principal limits on magnetic fields if only such fields can be generated by some means, i. Very formally, except as for inclusion of relativistic effects, Equation 4 provides a model equation for a limiting field in dependence on any given fundamental length scale l c.

Under this simplifying assumption the critical magnetic field B c scales simply with the inverse square of the corresponding fundamental length. Formally, this is graphically shown in Figure 1 under the assumption of validity of the Aharonov-Bohm scaling at higher energies. Figure 1. Log-Log plot scaling of the maximum possible magnetic field strength, B c , normalized to the fictitious Planck-magnetic field, B Pl , as function of fundamental length scales based on Equation 3.

Length scales l on the abscissa are normalized to the Planck length l Pl. Horizontal lines indicate the relation between other length scales and critical magnetic fields under the assumption of validity of the Aharonov-Bohm scaling. It is interesting that this limit coincides approximately with the measured [ 6 ] absolute upper limit on the electron radius vertical blue dashed line.

The dashed black curve indicates a possible deviation of the Aharonov-Bohm scaling near the quantum electrodynamic limit. The Compton limit to magnetic fields was known from straight energy considerations [cf. For this reason detection of magnetic fields exceeding the quantum limit by up to three orders in magnetars was an initial surprise.

However, more precise relativistic electrodynamic calculations including higher order Feynman graphs readily showed that the Compton limit can well be exceeded. To first approximation in the anomalous magnetic moment of electrons [ 9 ] the lowest Landau level shifts according to. It suggests a decrease of the lowest Landau energy level for increasing fields, obviously with violent non-physical consequences for astrophysical objects [ 10 ].