What Happens if You Focus a 5W Laser With a Giant Magnifying Glass? Negative Kelvin Temperature!
A group of physicists from Reis University has developed a technology for cooling photoionized neutral strontium plasma.
In order to avoid the difficulties associated with the high temperature of such an aggregate state, the researchers used a neutral plasma created by photoionization of an ultracold atomic gas. After 135 microseconds of cooling, the ion temperature decreased by almost four times., before 50 mK.
The technology works through rate-dependent scattering and momentum exchange between near-resonant photons and ions, molecules or atoms. Immediately after the formation of the plasma, the opposing polarized beams illuminated it, forming a one-dimensional optical flow for laser cooling along its direction. The peak intensity of a single beam reached 100 mW / cm2. The decrease in the cooling efficiency caused by the coherent coupling of states and the resulting magnetically induced transparent regions was minimized due to rapidly changing collision velocities in the plasma.
Experimental scheme. Cooling (408 nm) and repulsive (1092 and 1033 nm) lasers were used in opposite configurations with the indicated polarization. The 422 nm light for LIF was generated with a slit to illuminate the central slice of the plasma.
Refrigeration was effective only in the central region and in areas for which the expansion rate along the laser axis remained less than or comparable to the range of velocity trapping for a noticeable time. However, due to the high frequency of collisions in the plasma, the effect spread in all directions.
According to physicists, in addition to lowering the temperature, the technology allows creating rather sharp velocity gradients and measuring shear viscosity. These same optical powers also slow down plasma expansion, opening up pathways for containment and other manipulations..
Experiments with classical hot plasma are also progressing well. China’s experimental superconducting tokamak managed to increase the temperature «artificial sun» up to 100 million degrees °FROM.
text: Ilya Bauer, photo: Rice University, inanemathgeek