More Flux

By Stephen McGill
Assistant Scholar/Scientist

Are you ready to witness an extreme makeover? Well, ready or not, a makeover is coming to the Mag Lab this year. Now don't expect to see Ty Pennington or his design team running up and down the halls with a film crew from ABC, because that's not the kind of makeover we are talking about.

There will be no large-scale demolition or a new building going up in one week's time. No, this makeover is one that has been years in the making, and it's being carried out by the designers and engineers in the lab's Magnet Science & Technology division. Soon their plans will take shape inside one of the magnet bays in the of the lab's DC Field Facility. The magnet they will build there throughout this year is unlike any other resistive magnet at the Magnet Lab, or even the rest of the world. We are about to experience an extreme magnet makeover: the construction of the Split Florida Helix magnet.

This computer generated model shows the coil deformations in the Split Florida Helix magnet. Click for larger image.

This new magnet will open up entirely new areas of research by enabling never-before-possible optics experiments. What's so special about optics experimentation? Scientists learn more about the intrinsic properties of materials by shining light on them. Looking at which kinds of light are absorbed or reflected at different angles gives researchers insight into the fundamental electronic structure of matter, and that's the kind of stuff that over decades has led to smaller and faster computers, and other quality-of-life enhancements.

Engineering the near impossible

Most of the resistive magnets at the Mag Lab produce a magnetic field by running current through a continuous metal strip shaped into a helix. (A helix is the curve formed when a material is wrapped around a round cylinder.) By increasing the current or the number of loops in that helix, it is possible to make the magnetic field stronger. Because of the engineering requirements of generating large magnetic fields, these magnets typically only have openings at either end of a long, small-diameter tube, called the magnet bore, which runs along the axis of the helix (the center). These two holes at either end of the bore provide the only access for placing a sample inside the magnet, and in addition provide the only access for instrumentation needed to make whatever measurements scientists require for their experiments.

If a measurement can be done by attaching wires to the sample, then the sample accessibility down the bore might seem luxuriously roomy since wires are small, flexible, and can be routed to the sample at the center of the magnet. However, what happens when optical access to a sample is suddenly required? How does a person route electromagnetic waves at optical frequencies from the far-infrared to ultraviolet down to and then back from a sample placed at the center of a long, narrow tube?

This early model of the Split Florida Helix magnet coil shows the traditional magnet bore (in the center), as well as the placement of the four ports that will be used to scatter light at the sample inside the magnet. Click for larger image.

In the case of visible-light optics, one answer has been to use optical fibers. Like a wire, an optical fiber is somewhat flexible and allows light to be similarly routed. However, though optical fibers do simplify the problems of light delivery and collection, they also severely limit the types of experiments possible. What one really wants to do is eliminate the optical fibers altogether and instead drill a hole through the side of the magnet to get direct optical access to the sample. This would eliminate the need for an optical fiber since light can now pass directly from a light source through a window in the side of the magnet and then directly onto the sample.

In fact, while we're drilling, let's just keep going straight through the bore and out the other side. Now, one can do an optical transmission measurement and collect light coming out through the hole and window behind the sample. Keep that drill warmed up because now we're going to drill yet another hole through the side of the magnet except that this one is perpendicular to the first. So, now we have optical access to the sample through four windows evenly spaced around the mid-plane of the magnet. Imagine next that these holes through the side are not small but big — really big — and then imagine that in addition, the entire magnet can actually tip over so the field axis is horizontal instead of vertical.

Now imagine doing this when the magnet itself is generating tremendous magnetic fields — so high the magnet wants to literally tear itself apart — in the very place the engineers carved out four holes. At this point, you should have a picture of what makes the Split Florida Helix magnet so unique.

So, who will use such a magnet and what will they use it for? Well there are a lot of answers to that question. There will be many different users and experiments planned for this magnet, not just optical ones. Obviously, however, this magnet will be revolutionary for magneto-optics because it provides close and direct optical access to samples. Only with this magnet is the door now open to allow optical measurements that up to this point were either very difficult, limited in scope, or just impossible.

Last year, a number of highly respected researchers specializing in various optical disciplines traveled to the Mag Lab to outline science they would like to pursue once the Split Florida Helix magnet becomes operational. The topics presented at the workshop covered plans for research in physics, chemistry and biology. The techniques presented by the various speakers were just as diverse. The success of these new techniques at high magnetic fields will mark the arrival of a new, versatile probe for studying fundamental states of matter that complements other existing techniques at the Mag Lab.

Significantly, these new high-magnetic-field capabilities will be unique to the Magnet Lab. With time, this new magnet combined with cutting-edge instrumentation and a Free Electron Laser could greatly expand the portfolio of research that can be done at high magnetic fields.

Hopefully you are beginning to see that the extreme makeover happening this year is not just limited to a new design of a magnet bay or a magnet itself. This makeover is transformative not just because of the technological breakthrough, but also because it touches the science in a way that opens the door to many new ideas and research goals.