A key to producing good fusion reactions is controlling rotation of the ultra-hot plasma gas that swirls inside tokamaks and fuels the reactions. At PPPL, physicists Mario Podestà and Ron Bell have developed a real time velocity (RTV) diagnostic that could become part of a system for actively controlling the velocity of the rotation. "Control of rotation is critical for optimizing plasma stability against a range of instabilities," said Stan Kaye, deputy program director for NSTX-U. Such stability is essential for fusion reactions to take place.

The new device monitors four locations in a plasma, enabling the diagnostic to make rapid calculations of how the velocity profiles of the atomic nuclei, or ions, inside the plasma evolve over time. The diagnostic gathers information by observing what happens when a beam of neutral atoms is injected into the plasma and interacts with the ions to produce a photon of light. Detecting the light enables the instrument to calculate the velocity of the ions by taking into account the Doppler effect — the same process that causes the pitch of sirens to sound higher when speeding toward someone and lower when rushing away.

The small number of measurements required is crucial to the real-time calculation speed. "It's like the difference between building a road car and a race car," said Podestà. "When you build a race car, you strip out everything that's not necessary and push to increase performance. Similarly, these four measurements give the minimum amount of information to infer the plasma's velocity as the plasma discharge evolves."