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Exploding Plasmas

Phys. Rev. Lett. 99, 155001 (2007) S. Laha, P. Gupta, C. E. Simien, H. Gao, J. Castro, T. Pohl, and T. C. Killian

Exploding plasmas expand into surrounding space under the influence of many factors, such as thermal energy and inertia of the plasma constituents, particle interactions, and magnetic fields. This expansion is important in inertial confinement fusion experiments, x-ray lasers, or production of energetic (>MeV) ions through irradiation of solids or foils, but understanding it often requires complex numerical simulations that may not shed much light on the underlying physics.

Professor Killian's research group has taken advantage of the unusual properties of ultracold neutral plasmas to study the expansion as never before. Ultracold neutral plasmas are formed by photoionizing laser-cooled atoms, and they have temperatures that are orders of magnitude colder than normal plasmas. This allows precise sculpting of the particle density distribution in order to realize the initial conditions of an analytic solution of the fundamental equations used to describe ionized gases. This exact solution was recently predicted by theorists, and it emphasizes the importance of electron thermal energy in driving the expansion. For a plasma with a Gaussian density profile, the plasma will remain Gaussian as it expands, as shown in the figure below.


Plasmas1

Figure Caption

The Gaussian fits (solid red lines) to linear cuts through the density profile (blue data) show that the expansion preserves the Gaussian shape. The time indicated in each frame is the evolution time since plasma creation. The right-hand axes show that the differences between data and fit are small.