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Monday, August 10th, 2015
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| 12:00a |
A small, modular, efficient fusion plant It’s an old joke that many fusion scientists have grown tired of hearing: Practical nuclear fusion power plants are just 30 years away — and always will be.
But now, finally, the joke may no longer be true: Advances in magnet technology have enabled researchers at MIT to propose a new design for a practical compact tokamak fusion reactor — and it’s one that might be realized in as little as a decade, they say. The era of practical fusion power, which could offer a nearly inexhaustible energy resource, may be coming near.
Using these new commercially available superconductors, rare-earth barium copper oxide (REBCO) superconducting tapes, to produce high-magnetic field coils “just ripples through the whole design,” says Dennis Whyte, a professor of Nuclear Science and Engineering and director of MIT’s Plasma Science and Fusion Center. “It changes the whole thing.”
The stronger magnetic field makes it possible to produce the required magnetic confinement of the superhot plasma — that is, the working material of a fusion reaction — but in a much smaller device than those previously envisioned. The reduction in size, in turn, makes the whole system less expensive and faster to build, and also allows for some ingenious new features in the power plant design. The proposed reactor, using a tokamak (donut-shaped) geometry that is widely studied, is described in a paper in the journal Fusion Engineering and Design, co-authored by Whyte, PhD candidate Brandon Sorbom, and 11 others at MIT. The paper started as a design class taught by Whyte and became a student-led project after the class ended.
Power plant prototype
The new reactor is designed for basic research on fusion and also as a potential prototype power plant that could produce significant power. The basic reactor concept and its associated elements are based on well-tested and proven principles developed over decades of research at MIT and around the world, the team says.
“The much higher magnetic field,” Sorbom says, “allows you to achieve much higher performance.”
Fusion, the nuclear reaction that powers the sun, involves fusing pairs of hydrogen atoms together to form helium, accompanied by enormous releases of energy. The hard part has been confining the superhot plasma — a form of electrically charged gas — while heating it to temperatures hotter than the cores of stars. This is where the magnetic fields are so important—they effectively trap the heat and particles in the hot center of the device.
While most characteristics of a system tend to vary in proportion to changes in dimensions, the effect of changes in the magnetic field on fusion reactions is much more extreme: The achievable fusion power increases according to the fourth power of the increase in the magnetic field. Thus, doubling the field would produce a 16-fold increase in the fusion power. “Any increase in the magnetic field gives you a huge win,” Sorbom says.
Tenfold boost in power
While the new superconductors do not produce quite a doubling of the field strength, they are strong enough to increase fusion power by about a factor of 10 compared to standard superconducting technology, Sorbom says. This dramatic improvement leads to a cascade of potential improvements in reactor design.
The world’s most powerful planned fusion reactor, a huge device called ITER that is under construction in France, is expected to cost around $40 billion. Sorbom and the MIT team estimate that the new design, about half the diameter of ITER (which was designed before the new superconductors became available), would produce about the same power at a fraction of the cost and in a shorter construction time.
But despite the difference in size and magnetic field strength, the proposed reactor, called ARC, is based on “exactly the same physics” as ITER, Whyte says. “We’re not extrapolating to some brand-new regime,” he adds.
Another key advance in the new design is a method for removing the the fusion power core from the donut-shaped reactor without having to dismantle the entire device. That makes it especially well-suited for research aimed at further improving the system by using different materials or designs to fine-tune the performance.
In addition, as with ITER, the new superconducting magnets would enable the reactor to operate in a sustained way, producing a steady power output, unlike today’s experimental reactors that can only operate for a few seconds at a time without overheating of copper coils.
Liquid protection
Another key advantage is that most of the solid blanket materials used to surround the fusion chamber in such reactors are replaced by a liquid material that can easily be circulated and replaced, eliminating the need for costly replacement procedures as the materials degrade over time.
“It’s an extremely harsh environment for [solid] materials,” Whyte says, so replacing those materials with a liquid could be a major advantage.
Right now, as designed, the reactor should be capable of producing about three times as much electricity as is needed to keep it running, but the design could probably be improved to increase that proportion to about five or six times, Sorbom says. So far, no fusion reactor has produced as much energy as it consumes, so this kind of net energy production would be a major breakthrough in fusion technology, the team says.
The design could produce a reactor that would provide electricity to about 100,000 people, they say. Devices of a similar complexity and size have been built within about five years, they say.
“Fusion energy is certain to be the most important source of electricity on earth in the 22nd century, but we need it much sooner than that to avoid catastrophic global warming,” says David Kingham, CEO of Tokamak Energy Ltd. in the UK, who was not connected with this research. “This paper shows a good way to make quicker progress,” he says.
The MIT research, Kingham says, “shows that going to higher magnetic fields, an MIT speciality, can lead to much smaller (and hence cheaper and quicker-to-build) devices.” The work is of “exceptional quality,” he says; “the next step … would be to refine the design and work out more of the engineering details, but already the work should be catching the attention of policy makers, philanthropists and private investors.”
The research was supported by the U.S. Department of Energy and the National Science Foundation. | | 11:00a |
A new look at superfluidity MIT physicists have created a superfluid gas, the so-called Bose-Einstein condensate, for the first time in an extremely high magnetic field. The magnetic field is a synthetic magnetic field, generated using laser beams, and is 100 times stronger than that of the world’s strongest magnets. Within this magnetic field, the researchers could keep a gas superfluid for a tenth of a second — just long enough for the team to observe it. The researchers report their results this week in the journal Nature Physics.
A superfluid is a phase of matter that only certain liquids or gases can assume, if they are cooled to extremely low temperatures. At temperatures approaching absolute zero, atoms cease their individual, energetic trajectories, and start to move collectively as one wave.
Superfluids are thought to flow endlessly, without losing energy, similar to electrons in a superconductor. Observing the behavior of superfluids therefore may help scientists improve the quality of superconducting magnets and sensors, and develop energy-efficient methods for transporting electricity.
But superfluids are temperamental, and can disappear in a flash if atoms cannot be kept cold or confined. The MIT team combined several techniques in generating ultracold temperatures, to create and maintain a superfluid gas long enough to observe it at ultrahigh synthetic magnetic fields.
“Going to extremes is the way to make discoveries,” says team leader Wolfgang Ketterle, the John D. MacArthur Professor of Physics at MIT. “We use ultracold atoms to map out and understand the behavior of materials which have not yet been created. In this sense, we are ahead of nature.”
Ketterle’s team members include graduate students Colin Kennedy, William Cody Burton, and Woo Chang Chung.
A superfluid with loops
The team first used a combination of laser cooling and evaporative cooling methods, originally co-developed by Ketterle, to cool atoms of rubidium to nanokelvin temperatures. Atoms of rubidium are known as bosons, for their even number of nucleons and electrons. When cooled to near absolute zero, bosons form what’s called a Bose-Einstein condensate — a superfluid state that was first co-discovered by Ketterle, and for which he was ultimately awarded the 2001 Nobel Prize in physics.
After cooling the atoms, the researchers used a set of lasers to create a crystalline array of atoms, or optical lattice. The electric field of the laser beams creates what’s known as a periodic potential landscape, similar to an egg carton, which mimics the regular arrangement of particles in real crystalline materials.
When charged particles are exposed to magnetic fields, their trajectories are bent into circular orbits, causing them to loop around and around. The higher the magnetic field, the tighter a particle’s orbit becomes. However, to confine electrons to the microscopic scale of a crystalline material, a magnetic field 100 times stronger than that of the strongest magnets in the world would be required.
The group asked whether this could be done with ultracold atoms in an optical lattice. Since the ultracold atoms are not charged, as electrons are, but are instead neutral particles, their trajectories are normally unaffected by magnetic fields.
Instead, the MIT group came up with a technique to generate a synthetic, ultrahigh magnetic field, using laser beams to push atoms around in tiny orbits, similar to the orbits of electrons under a real magnetic field. In 2013, Ketterle and his colleagues demonstrated the technique, along with other researchers in Germany, which uses a tilt of the optical lattice and two additional laser beams to control the motion of the atoms. On a flat lattice, atoms can easily move around from site to site. However, in a tilted lattice, the atoms would have to work against gravity. In this scenario, atoms could only move with the help of laser beams.
“Now the laser beams could be used to make neutral atoms move around like electrons in a strong magnetic field,” added Kennedy.
Using laser beams, the group could make the atoms orbit, or loop around, in a radius as small as two lattice squares, similar to how particles would move in an extremely high magnetic field.
“Once we had the idea, we were really excited about it, because of its simplicity. All we had to do was take two suitable laser beams and carefully align them at specific angles, and then the atoms drastically change their behavior,” Kennedy says.
“New perspectives to known physics”
After developing the tilting technique to simulate a high magnetic field, the group worked for a year and a half to optimize the lasers and electronic controls to avoid any extraneous pushing of the atoms, which could make them lose their superfluid properties.
“It’s a complicated experiment, with a lot of laser beams, electronics, and magnets, and we really had to get everything stable,” Burton says. “It took so long just to iron out all the details to eventually have this ultracold matter in the presence of these high fields, and keep them cold — some of it was painstaking work.”
In the end, the researchers were able to keep the superfluid gas stable for a tenth of a second. During that time, the team took time-of-flight pictures of the distribution of atoms to capture the topology, or shape, of the superfluid. Those images also reveal the structure of the magnetic field — something that’s been known, but never directly visualized until now.
“The main accomplishment is that we were able to verify and identify the superfluid state,” Ketterle says. “If we can get synthetic magnetic fields under even better control, our laboratory could do years of research on this topic. For the expert, what it opens up is a new window into the quantum world, where materials with new properties can be studied.”
Going forward, the team plans to carry out similar experiments, but to add strong interactions between ultracold atoms, or to incorporate different quantum states, or spins. Ketterle says such experiments would connect the research to important frontiers in material research, including quantum Hall physics and topological insulators.
“We are adding new perspectives to physics,” Ketterle says. “We are touching on the unknown, but also showing physics that in principle is known, but at a new level of clarity.”
This research was funded by the National Science Foundation, the Air Force Office for Scientific Research, and the Army Research Office. | | 11:59p |
A bipedal robot with human reflexes Deep in the basement of MIT’s Building 3, a two-legged robot named HERMES is wreaking controlled havoc: punching through drywall, smashing soda cans, kicking over trash buckets, and karate-chopping boards in half. Its actions, however, are not its own.
Just a few feet away, PhD student Joao Ramos stands on a platform, wearing an exoskeleton of wires and motors. Ramos’ every move is translated instantly to HERMES, much like a puppeteer controlling his marionette. As Ramos mimes punching through a wall, the robot does the same. When the robot’s fist hits the wall, Ramos feels a jolt at his waist. By reflex, he leans back against the jolt, causing the robot to rock back, effectively balancing the robot against the force of its punch.
The exercises are meant to demonstrate the robot’s unique balance-feedback interface. Without this interface, while the robot may successfully punch through a wall, it would also fall headlong into that wall. The interface allows a human to remotely feel the robot’s shifting weight, and quickly adjust the robot’s balance by shifting his own weight. As a result, the robot can carry out momentum-driven tasks — like punching through walls, or swinging a bat — while maintaining its balance.
Ramos says the interface takes advantage of a human’s split-second reflexes, which give the robot much faster reaction times than robots that adjust their balance based on visual feedback from onboard cameras.
“The processing of images is typically very slow, so a robot has difficulty reacting in time,” says Ramos, of MIT’s Department of Mechanical Engineering. “Instead, we’d like to use the human’s natural reflexes and coordination. An example is walking, which is just a process of falling and catching yourself. That’s something that feels effortless to us, but it’s challenging to program into a robot to do it both dynamically and efficiently. We want to explore how humans can take over complex actions for the robot.”
Ultimately, Ramos and his colleagues envision deploying HERMES to a disaster site, where the robot would explore the area, guided by a human operator from a remote location.
“We’d eventually have someone wearing a full-body suit and goggles, so he can feel and see everything the robot does, and vice versa,” Ramos says. “We plan to have the robot walk as a quadruped, then stand up on two feet to do difficult manipulation tasks such as open a door or clear an obstacle.”
Ramos and his colleagues, including PhD student Albert Wang and Sangbae Kim, the Esther and Harold E. Edgerton Center Career Development Assistant Professor of Mechanical Engineering, will present a paper on the interface at the IEEE/RSJ International Conference on Intelligent Robots and Systems in September.
Balance and feedback
To give the human operator a sense of the robot’s balance, the team first looked for a way to measure the robot’s center of pressure, or weight distribution, which indicates its balance and stability. The researchers worked with HERMES, a 100-pound biped robot designed by the team, along with the interface, for disaster response. They outfitted the robot’s feet with load sensors that measure the force exerted by each foot on the ground.
Depending on the forces measured, the researchers calculated the robot’s center of pressure, or where it was shifting its weight. They then mapped out a polygonal area, the edges of which represent each of the robot’s feet. They determined that if the robot’s center of pressure strayed toward the edges of this support polygon, the robot was in danger of falling.
Ramos and Wang then built the balance-feedback interface: a large polygonal platform equipped with motors, and an exoskeleton of metal bars and wires that attaches to a person’s waist — essentially, the human body’s center of mass. With computer software, the researchers translated the robot’s center of pressure to the platform’s motors, which apply comparable force to the exoskeleton, pushing a person back and forth as the robot shifts its weight.
“The interface works by pushing harder on the operator as the robot’s center of pressure approaches the edge of the support polygon,” Wang explains. “If the robot is leaning too far forward, the interface will push the operator in the opposite direction, to convey that the robot is in danger of falling.”
Leaning in
In experiments to test the interface, Wang repeatedly struck the robot’s torso with a hammer. Ramos, standing on the platform, was unaware of when the hammer would strike. As Wang struck the robot, the platform exerted a similar jolt on Ramos, who reflexively shifted his weight to regain his balance, causing the robot to also catch itself.
The team also tested whether the robot kept its balance while punching through drywall. Ramos, in the exoskeleton, mimed the action, and the robot simultaneously carried it out. The platform pushed forward on Ramos as the robot made contact with the wall. In response, Ramos rocked back on his heels, causing the robot to do the same.
“These experiments show the versatility of the human operator. In one test, the robot unexpectedly got its arm stuck in the wall. But, because the human was in the loop, the operator could arrive at a creative solution which was translated directly to the robot,” Wang says. “Our next goal is to try more complex coordinated movements such as swinging an axe or opening a spring-loaded door. These actions are difficult for many robots. If the robot stands stiff while pushing on a door, it tends to tip over. You have to lean your body weight into it and catch yourself as the because it’s so natural to humans, you can have the human do it.”
Jonathan Hurst, associate professor of mechanical, industrial, and manufacturing engineering at Oregon State University, says the new balance interface is an intuitive platform for operators, as an operator can use it without thinking.
“This interface likely won't even distract a person,” says Hurst, who was not involved in the research. “It's normal to keep your balance while focusing on a task. But perhaps more important than just a way to control a robot in the absence of knowing how to do it autonomously is being able to observe and collect data from the robot. Given hours of data recording the details of human strategies for balance and pose adjustment, I'd be willing to bet they will discover some relatively simple approaches for autonomous strategies.”
This research was funded, in part, by the Defense Advanced Research Projects Agency. |
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