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Friday, March 10, 2006

Breathless

I read this breathless account of scientists at Sandia achieving high temperatures in a plasma with a bit of incredulity. The press releases, with the scientists apparently egging them on, make it sound like these high temperatures have special significance -- "a temperature beyond that of a star's interior."
The plasma, caught in the grip of the very strong magnetic field accompanying the electrical current, is compressed to the thickness of a pencil lead. This happens very rapidly, at a velocity that would fly a plane from New York to San Francisco in several seconds.

At that point, the ions and electrons have nowhere further to go. Like a speeding car hitting a brick wall, they stop suddenly, releasing energy in the form of X-rays that reach temperatures of several million degrees -- the temperature of solar flares.

The new achievement -- temperatures of billions of degrees -- was obtained in part by substituting steel wires in cylindrical arrays 55 mm to 80 mm in diameter for the more typical tungsten wire arrays, approximately only 20 mm in diameter. The higher velocities achieved over these longer distances were part of the reason for the higher temperatures.
In fact, if the plasma emits radiation with a short enough wavelength, the equivalent "color" temperature can indeed get quite high. The statistical Boltzmann to Planck photon equivalence comes from the following relationship:
E ~ kT ~ hv
which gives Temperature ~ 0.01/wavelength. For visible blue light wavelength of 500 nm, this gives a temperature of about 20,000 K. To get temperatures in the "billions" range, you need short wavelength x-rays bordering on the gamma range territory -- i.e. wavelengths shorter than 0.01 nm.

So they have created what looks like a concentrated source of high-energy x-rays. I certainly hope they have invested in high quality lead liners. The photo below has a bit of marketing panache behind it; at least they didn't go so far as having a guy peering over the railing.

1 Comments:

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5:45 PM  

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