17 September 2009

Sodium tungsten ions to monitor fusion reactors

Sodium-like tungsten ions could soon monitor fusion energy devices, potential sources of abundant, clean power.

Tungsten, which has the highest melting point of any metal, will see use in some high-strength structural components in the experimental ITER fusion reactor under construction in France, said physicists at the National Institute of Standards and Technology (NIST).

When ITER cooks up its hot, dense fusion plasma, it could erode trace amounts of tungsten from its structures and strip away electrons in the process. When 63 of tungsten’s 74 electrons whittle away, 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% of its visible light in two shades of orange, which scientists term the “D” spectral lines.

Sodium-like tungsten ions emit intense light in “D” spectral lines, but they are at far higher energy levels than sodium, and so shift 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. The problem remains that it is a challenge to measure light in this portion of the electromagnetic spectrum.

There is now a way to measure “D” lines in sodium-like tungsten, confirming theoretical predictions of their energies and intensities, said NIST’s John Gillaspy.

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, which employs an electron beam to make, catch, and study highly charged ions. To measure the spectra, they used an extreme ultraviolet 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.

For related information, go to www.isa.org/manufacturing_automation.