Princeton's NSTX reactor  (Source: Princeton/William Heidbrink)

Researchers at Princeton have devised a breakthrough in fusion plasma containment dubbed the "snowflake divertor", which would extend reactor component lifetimes and lower costs.  (Source: V. Soukhanovskii, Lawrence Livermore National Laboratory)

Tests show the snowflake divertor achieves a lower temperature flux on the divertor walls and has a larger radiating region. This is good because it means that the walls will last longer without melting and requiring replacement.  (Source: V. Soukhanovskii, Lawrence Livermore National Laboratory)
U.S. scientists devise a way to make fusion reactors safer and hardier, without compromising energy production

Taming ultra-hot plasma and sustaining a fusion reaction is no easy feat.  Scientists have been trying for some time now to accomplish the delicate dance to get a fusion reactor to produce a surplus of energy -- mimicking the processes occurring in the Sun.

Currently, hopes for productive fusion rest on the International Thermonuclear Experimental Reactor (ITER), a multibillion-dollar international research reactor being constructed in Cadarache, France by an international team from the United States, China, the European Union, India, Japan, the Republic of Korea and the Russian Federation.

Scientists at the National Spherical Torus Experiment (NSTX) at the Princeton Plasma Physics Laboratory and Dr. D.D. Ryutov from Lawrence Livermore National Laboratory believe that they have devised a critical solution for the new reactor, utilizing their much smaller test reactor.

From Russia With Love

The Tokamak is a reactor design in which super-hot plasma is contained within a protective magnet field, forming a donut-shaped torus.

The plasma containment design is a fruit of the Cold War research race and one of the Soviet Union's greatest triumphs in that race.  Soviet physicists Igor Tamm and Andrei Sakharov worked out the theory behind the device in the early 1950s; and then in 1956 Lev Artsimovich and his team of researchers at the Kurchatov Institute, Moscow actually built the world's first working Tokamak.

The device allowed the Soviets to obtain electron temperatures of over 1000 eV by the late 1960s.  That claim was met with initial skepticism, as researchers in the United States and Europe were far behind it.  It was validated, though, by later testing.

Taming the Tokamak

Today the Cold War is a distant memory, but the Tokamak has become a cornerstone of modern fusion research.  Rival designs (such as the U.S. invented Stellarator design) have thus far failed to offer equivalent efficiencies.

However, to achieve efficient fusion ITER must push the plasma temperature to new heights.  The optimal temperature for the fusion processes of the reactor's radioactive hydrogen fussile fuels -- deuterium and tritium -- is an incredible 100 million degrees Kelvins -- about 360,000 times room temperature.

Scientists will heat the plasma using ohmic (current based) heating, neutral beam injection, and radio frequency (RF)/microwave heating.  The plasma will be composed of approximately 0.5 g of the radioactive hydrogen mixture, and will fill most of the 840 m3 reactor chamber. 

The critical problem once the reaction is started is how to harvest it and protect the system from the high-energy byproducts.

Neutrons from the fusion reactions will escape the field as they are no longer charged.  This is a good thing, as it will eventually be how power is harvested from a commercial fusion reactor.  However, like all aspects of the design, it offers significant challenges.

These high-energy neutron particles must be absorbed by protective blankets and used to breed tritium fuel from lithium.  They must also transfer energy into coolant, lowering their kinetic energy to safe levels.  Officially ITER is not designed to produce power, though it targets producing ten times the heat needed to start and sustain the reaction (roughly 500 MW of net power, sustained for 1,000 seconds).  In a real plant the heated coolant would be used much like the steam in a coal power plant to produce work and electricity.  In ITER the heat will be disposed of, as the system is only a demonstration device.

While neutron absorption necessitates some clever engineering, an even trickier problem is what to do with the small fraction of destructive charged plasma particles that escape the containment field.  These particles, which spill into the "scrape-off" layer surrounding the magnetic shell are funneled via a secondary magnetic field into a "divertor chamber".

The problem is that even with new super-strong steels being devised, the divertor may not be hardy enough to weather the hellish heat from the diverted plasma.  As a result the components would likely degrade quickly, necessitating frequent replacement, and reducing the likelihood that the technologies might be commercially viable.

Typically diverted plasma flows in through an X-shaped field, named, unsurprisingly, the X-divertor.  The Princeton team reported at the 52nd annual meeting of the APS Division of Plasma Physics that, for the first time, they have successfully employed a new kind of field -- a "snowflake divertor" which is named as such due to its resemblance to a snowflake or starburst. 

With the new field, rather than striking the divertor walls and releasing all their heat, the super-hot particles manage to cool better and heat the walls less.  Princeton's test Tokamak -- the NSTX saw greatly reduced heat flux on the divertor walls using the design. 

And more importantly, the new design did not negatively impact the high performance and confinement of the high-temperature core plasma, and even reduced the impurity contamination level of the main plasma.  And the design only needs two or three existing magnetic coils, so inclusion in reactors like ITER should be relatively straightforward.

In all, this means that the field creates a hardier, safer fusion device, without compromising the critical fusion processes.  That's very good news for the Tokamak project and for the hopes of commercial fusion.

Looking Ahead

The ITER reactor is expected to cost around $23B USD to complete.  Construction began two years ago and will finish in 2018.  The finished design will consist of nine torus segments each weighing between 390 and 430 tons.  The full device will be approximately twice as large and sixteen times as heavy as any fusion device to date. 

Its reaction goals far exceed the current world record holder -- the UK's JET reactor.  JET achieved a 16 MW net power fusion reaction for less than a second.

In order to complete Tokamak and optimize its design, ongoing breakthroughs like the "snowflake divertor" will be critical. 

Research will continue at the NSTX, DFIII-D (General Atomics, San Diego, Calif.), TFTR (an older design, also at Princeton), Alcator C-Mod (MIT, Cambridge, Mass.), LDX (also at MIT), and the MST (University of Wisconsin Madison).  Unfortunately a couple other high profile fusion projects were recently scrapped -- the NCSX (a stellator design at Princeton) and the UCLA ET (a larger reactor design at UCLA in Los Angeles, Calif.).

Despite those losses some of the world's brightest minds are working hard towards the goal of completing ITER by 2018.  If all goes well, these researchers will be able to employ ITER from 2018 to 2038 to gather the knowledge necessary to deploy affordable fusion power to the world.

A critical challenge in the U.S., though, will be funding of the research and direct costs necessary to build ITER.  The Department in Energy and other federal departments currently provide billions to study the topic of fusion, which most scientists agree holds the key to the world's clean energy future.  However, in a struggling economy and growing calls for budget cuts, there's much fear that fusion research will be put on the chopping block.

"It's okay. The scenarios aren't that clear. But it's good looking. [Steve Jobs] does good design, and [the iPad] is absolutely a good example of that." -- Bill Gates on the Apple iPad
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