Just as health-food manufacturers work on developing the best possible sodium
substitutes for low-salt diets, physicists at the National
Institute of Standards and Technology (NIST) have acquired new knowledge
on a promising sodium alternative of their own. Sodium-like tungsten ions could
pepper-and conveniently monitor-the hot plasma soup inside fusion energy devices,
potential sources of abundant, clean power.
Sodium-like tungsten ions (blue) are far smaller than neutral sodium atoms (orange)—the ion's 11 electrons are pulled in very tightly by the 74 protons in the tungsten nucleus, making their energy jumps far more expensive than in neutral sodium and causing them to emit high-energy ultraviolet wavelengths of light, rather than visible light, as is the case with ordinary sodium. Credit: Talbott, NIST
Tungsten-having the highest melting point of any metal-will be
used in some high-strength structural components in the experimental ITER fusion
reactor under construction in France (see “NIST Light Source Illuminates
Fusion Power Diagnostics,” NIST Tech Beat, Oct. 11, 2007.). When ITER
cooks up its hot, dense fusion plasma, it could erode trace amounts of tungsten
from its structures and strip away many of its electrons in the process. When
63 of tungsten’s 74 electrons are removed, it becomes chemically analogous
to sodium atoms, which have 11 electrons as well.
Ordinary sodium gas radiates bright yellow-orange light, which has proven useful
for everything from mundane streetlamps to exotic atom lasers. Sodium radiates
approximately 99 percent of its visible light in two shades of orange, which
scientists have termed the “D” spectral lines.
Sodium-like tungsten ions emit intense light in analogous “D” spectral
lines, but they are at far higher energy levels than sodium, and so are shifted
out of the visible spectrum to the extreme ultraviolet. Measuring the wavelengths
and relative intensities of lines in the spectrum of light released by a population
of tungsten ions in the plasma can provide information about the fusion plasma
conditions, such as its temperature, density and magnetic fields. Yet it has
been challenging to measure light in this portion of the electromagnetic spectrum.
NIST’s John Gillaspy and his colleagues have now provided the first measurement*
of both “D” lines in sodium-like tungsten, confirming theoretical
predictions of their energies and intensities. The NIST scientists further checked
their knowledge by measuring the spectrum of light from other sodium-like ions
of hafnium, tantalum and gold. The researchers used NIST’s electron beam
ion trap (EBIT), which employs an electron beam to make, catch and study highly
charged ions. To measure the spectra, they used an extreme ultraviolet (EUV)
spectrometer, originally developed to study 13.5 nanometer wavelength light
emitted from plasma sources for next-generation microelectronics applications,
but they discovered they could push it to detect radiation as low as about 2
nanometers, where tungsten’s lower-wavelength “D” line resides.
With this experimental knowledge of tungsten’s lines, researchers may
now have a robust new ingredient for measuring fusion reactor conditions.
* J.D. Gillaspy, I.N. Draganic, Y. Ralchenko, J. Reader, J.N. Tan, J.M. Pomeroy
and S.M. Brewer Measurement of the D-line doublet in high-Z highly charged sodiumlike
ions. Physical Review A, Published online 8 July 2009. doi/10.1103/PhysRevA.80.010501.