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PPPL’s dynamic diagnostic duo

Kenneth Hill and Manfred Bitter are scientific pioneers who have collaborated seamlessly for more than 35 years. Together they have revolutionized a key instrument in the quest to harness fusion energy — a device called an X-ray crystal spectrometer that is used around the world to reveal strikingly detailed information about the hot, charged plasma gas that fuels fusion reactions.

“Ken and Manfred are consummate diagnosticians,” said Michael Zarnstorff, deputy director for research at DOE’s Princeton Plasma Physics Laboratory(PPPL), where the duo has worked for nearly four decades. “Over the years they have developed highly innovative and uniquely capable tools for analyzing the results of fusion experiments.”

These tools record key plasma parameters on fusion facilities in the United States, China, Japan and South Korea. They are being designed for a new German facility and will play a key role on ITER, the huge international experiment under construction in France to demonstrate the feasibility of fusion power.

New applications for the spectrometers are rapidly expanding. Prospective new uses range from medical and industrial applications to the study of high energy-density physics. “An abundance of contexts is opening up,” Zarnstorff said.

Low-key physicists

Behind all these efforts are two low-key physicists.  “I have known and worked with Ken and Manfred for over 30 years and have always admired their scientific work and polite demeanor,” said Philip Efthimion, who heads the Plasma Science and Technology Department at PPPL.

The two divvy up tasks based on “whatever one of us is interested in and needs to do,” said Hill. “We have to try to check each other and make rational decisions instead of emotional ones.” Bitter puts it this way: “We are in this business together some 35 years. Everything that comes up is discussed between us.”

The physicists first joined forces at PPPL in the late 1970s when the Princeton Large Torus, the Lab’s major experiment at the time, was reaching temperatures of more than 10 million degrees Celsius. That blistering heat stripped light-emitting electrons from the hydrogen atoms in the plasma, eliminating light as a source of information about the atomic nuclei, or ions, in the plasma and creating the need for a new diagnostic tool.

Enter the X-ray crystal spectrometer, which gleans vital data from the X-rays that ions emit. At the heart of this tool is a hair-thin crystal that separates the X-rays into their wavelengths, or spectrum, and sends them to a detector. Shifts in the wavelengths reveal the temperature of the ions and other key data through a process called Doppler broadening — the same process that causes sirens to sound higher when speeding toward someone and lower when rushing away.

Bitter and Hill worked on early X-ray spectrometers under Schweickhard von Goeler, who headed diagnostics and whom everyone called “Schwick.” Von Goeler and Hill introduced the first such device, whose lower resolution — or ability to distinguish between wavelengths in the spectra — was not yet sufficient to measure Doppler broadening. Responding to this challenge, von Goeler and Bitter built an improved spectrometer with higher resolution for Doppler measurements.

Astonishing solar scientists

The new PPPL device produced results that astonished solar scientists. The spectrometer revealed far more details of the X-ray spectrum for iron, an element used for diagnostic purposes in the plasma, than instruments aboard satellites that studied the spectra of iron in the sun had been able to show.

But the new spectrometer, which PPPL also installed on the Tokamak Fusion Test Reactor (TFTR), the Laboratory’s key fusion experiment in the 1980s and 1990s, had a severe limitation. The cylindrically curved crystal provided only a single line of sight through the donut-shaped plasma and could record only the temperature of ions found at points along that line of sight. “What you really want to know is how hot it is at many points throughout the plasma,” said Hill.

To increase the number of sightlines, PPPL put five X-ray spectrometers on TFTR. “They were large,” Hill said of the devices, “and you couldn’t imagine many more. So Manfred came up with the idea for a single crystal and a 2D [or two-dimensional] detector that would give you a continuous profile of the plasma.”

Bitter’s concept, now a worldwide standard for fusion research, was simple and elegant. He envisioned a crystal whose spherically curved surface collected X-ray spectra from the entire plasma and imaged them onto a detector that recorded both the spectra and the location of the ions they came from. The revolutionary result: A complete picture of the plasma’s ion temperature, captured with just one X-ray spectrometer.

Bitter and Hill first tested this design in 2003 on Alcator C-MOD, the fusion facility at MIT. While this trial showed that the concept worked, the 2D detector used at the time couldn’t record all the spectra that flowed in from the crystal. “The count-rate limit of this detector was very low,” recalled Hill. “You couldn’t see how the temperature evolved over time.”

Like comparing an airplane to a bicycle

This problem led to a search for a better detector, which Bitter found on a trip to Europe. While there in 2005, he learned of a device that the European Organization for Nuclear Research (CERN) had developed that could record spectral images in far greater detail than the detector he had been using. “It was like comparing an airplane to a bicycle,” Bitter said of the new detector, which made the spherically curved crystal spectrometer fully operational.

MIT became the first to use the new spectrometer when the university’s Plasma Science and Fusion Center installed it on Alcator C-MOD in 2006 in a collaboration between MIT and PPPL. “It’s been a really great leap forward,” said John Rice, the principal research scientist at the MIT facility. “The original detector [on the 2003 spectrometer] had all sorts of problems and with this new system we can image the complete plasma.”

Other fusion laboratories quickly followed. PPPL-designed spectrometers are now essential tools on the Korea Superconducting Tokamak Advanced Research (KSTAR) facility in Daejon, South Korea; the Experimental Advanced Superconducting Tokamak (EAST) in Hefei, China; and the Large Helical Device (LHD) in Toki, Japan.

Still to come are spectrometers planned for ITER in Cadarache, France; Wendelstein 7-X (W7-X) in Greifswald, Germany; and the upgraded National Spherical Torus Experiment (NSTX-U) at PPPL. For these projects, Bitter and Hill are providing expert guidance.

“The highlight of my time here has been working with Ken and Manfred,” said physicist Novimir Pablant, who led the design of the LHD spectrometer and is developing the devices to be installed on ITER and W7-X. Joining Pablant on the ITER project is physicist Luis Delgado-Aparicio, who is developing the NSTX-U spectrometer and has likewise been inspired by Bitter and Hill. “They are incredible to work with,” said Delgado-Aparicio. “The degree of certainty to which they want to test their ideas is acute.”

Bitter and Hill are still collaborating on new spectrometers. Among them are devices to study laser-produced plasmas at the University of Rochester and the Lawrence Livermore National Laboratory. What keeps the two scientists going? “X-ray spectrometry is a field that I find fascinating,” said Bitter. “It has so many applications and it’s very interesting to design new diagnostics.” Hill fully seconds those sentiments. “There’s just a lot of interesting physics in this field,” he said. “And there are broad applications and interest for this technology.”

U.S. Department of Energy
Princeton Plasma Physics Laboratory is a U.S. Department of Energy national laboratory managed by Princeton University.

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