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Experimental detection of relaxation to a helical state
 


One of the earliest predictions of a magnetic self-organization phenomenon was J.B. Taylor's hypothesis that magnetically confined plasmas would relax to the lowest energy state subject to the constraint that their magnetic helicity be conserved. For systems with low helicity, the relaxed state is axisymmetric, while at large helicity, the relaxed state is helically symmetric, or kinked. Taylor's relaxation hypothesis has been confirmed in the laboratory for axisymmetric states; the spontaneous toroidal field reversal central to MST and other Reversed Field Pinch experiments is the most prominent example. On the strength of this evidence, Taylor's hypothesis has been used to explain solar flares, coronal heating, and even the morphologies of extragalactic radio jets. However, it remains a hypothesis, with substantial theoretical support, but no rigorous proof. The long predicted helical state, which is expected for relaxation at large helicity, has now been observed in SSX. In the experiment, two compact, toroidal, axisymmetric spheromak plasmas with tunable magnetic helicity are injected at either end of a cylindrical volume. After a brief dynamical phase, the plasmas merge and achieve an equilibrium state. When the combined helicity exceeds the predicted threshold, this equilibrium state is observed to be helical, as shown below. Helical Equilibrium
This work was published as a PRL and selected by AIP in its electronic Physics: Spotlighting Exceptional Research: http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.103.215002 It is the first experimental description of non-axisymmetric magnetic self-organization in a compact toroidal plasma. It demonstrates the explanatory power of self-organization, may inspire further theoretical development, and should enhance confidence in the application of Taylor's hypothesis to space and astrophysical plasmas.

 

 
A National Science Foundation Physics Frontier Center,
established in coordination with the Department of Energy.


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