Reckoning the Ratios

The Scientist: Chris Klepper

What He Measures: Isotopic ratio of fusion fuel

The Technique: Residual Gas Analysis

There are three isotopes of hydrogen. The most abundant, typically referred to as hydrogen and also called protium, is the lightest; deuterium and tritium are, in turn, heavier. Each can be used in fusion, depending on plasma conditions. Typically a nearly 50/50 mix of deuterium and tritium (DT) yields the most fusion energy. JET, the world’s most powerful tokamak fusion reactor, is also the only one that can currently run with DT fuel. DT experiments have been rare, in part because tritium is limited in supply and tricky to work with. JET’s recent experimental campaign marked the first time it has used DT fuel since 1997.

How do scientists know exactly how much deuterium and tritium is in their plasma? They use residual gas analysis (RGA), which measures gas ions as they leave the tokamak through a component called a divertor.

ORNL physicist and RGA expert Chris Klepper likens measuring this “plasma exhaust” to a vehicle emissions test. Working with various collaborators over the decades, including ORNL’s Don Hillis and Ted Biewer and US ITER’s Kurt Vetter, he has developed instruments to analyze the DT ratio in this exhaust using Penning gauge optical spectroscopy and quadrupole mass spectrometry.

His career and JET’s lifetime have overlapped: He contributed to both the 1997 and 2021 DT experiments. Klepper and Vetter devised a solution, now in use on JET, for protecting RGA diagnostics from the higher radiation environment of DT plasmas. The instrument used on JET, a version of which is also in use on the German fusion machine Wendelstein 7-X, is a prototype for the ITER tokamak, which is now under assembly in France.

As fusion machines advance, his job gets tougher, said Klepper, who is leading RGA diagnostics for US ITER.

“Fusion plasma diagnostics are challenging and become increasingly so as one approaches fusion reactor-like conditions,” said Klepper. “This is not only because of harsher conditions, but also because the access [to the tokamak] becomes more difficult.”

Taking the Temperature

The Scientist: Ephrem Delabie

What He Measures: Temperature of the fuel ions

The Technique: Charge Exchange Spectroscopy

Klepper’s colleague Ephrem Delabie agrees: Plasma diagnostics are demanding. His job is to find out the temperature of the fuel ions.

“It’s a basic thing that you need to know,” explained Delabie. “But at the same time, it’s quite a difficult thing to measure.”

Fusion can occur only at extremely high temperatures. Knowing how hot fuel ions are — and how the temperature changes as ions migrate from the core to the edge of the plasma — is critical for understanding how energy is transported out of the plasma, aiding prediction of how much fusion will occur under different conditions.

That’s why charge exchange spectroscopy (CXS), a diagnostic technique with deep roots at ORNL, is important. CXS uses a beam of particles injected into the plasma to neutralize and excite the plasma ions themselves so that they emit light; that spectrum reveals the ion temperature. 

ORNL has been involved for decades in JET’s CXS diagnostics, which have evolved along with the tokamak itself. Biewer, and Hillis before him, worked on earlier iterations of the technique, and Delabie, based at JET, implemented a variation known as main ion CXS.

Over the past year, JET experiments have been run with protium, deuterium, tritium, and a DT mix. With that CXS data, scientists like Delabie are probing how the temperature and movement of those ions vary by isotope. The heavier tritium, for example, retains the plasma energy better, boosting fusion performance.

CXS also sheds light on how plasmas go from low- to high-power modes, a transition, Delabie explained, driven by a heat flux from the core to the cooler edge. “To understand how much of that heat flux is in electrons and how much is in the ions, you need to know the ion temperature,” he said.

Exhaustive Measurements

The Scientist: Bart Lomanowski

What He Measures: Electron temperature and density of plasma exhaust

The Technique: Passive Divertor Spectroscopy

Plasma exhaust poses one of the biggest challenges for future tokamak-based fusion power plants. At millions of degrees, intense heat from the plasma could damage the divertors, even when — like at JET and ITER — they are made of heat-resistant tungsten.

“In these cases, you have extremely high heat fluxes in a very small footprint,” explained ORNL experimental physicist Bart Lomanowski, referring to the amount of heat per square meter deposited on the divertor surface. “And to develop solutions to this heat exhaust challenge, we need to intersect plasma physics with materials science, technology and engineering.”

A former mechanical engineer, Lomanowski has been working on this problem for five years, mostly on site at JET. With JET’s diagnostics team, he helped develop passive divertor spectroscopy techniques to reveal the emission spectrum of plasma exhaust—a process not unlike how astronomers study the Sun. From these spectra, scientists can infer the density and temperature and study different ways to dissipate the heat before it reaches the divertor.

That plasma power is intense — even greater than what space shuttles experienced on reentry to Earth. Among other things, scientists like Lomanowski are trying to understand how that heat flux and dissipation methods vary depending on the specific hydrogen isotopes used for the plasma.

Scientists and engineers at JET have continuously upgraded diagnostics over the machine’s four-decade lifetime, and this system is no exception. This spring, Lomanowski is slated to upgrade the instrument so it can access previously hidden pockets of the plasma.

“I want to make sure that we take full advantage of JET’s unique capabilities as the largest fusion experiment in the world, and, in particular, JET’s relevance to the ITER project,” Lomanowski said. “The new measurements of the divertor plasma density and temperature will help predict ITER performance and inform the design of next-step fusion devices.”