New mechanism found to limit electron flow in plasma-based systems

Many frontier technologies of societal benefit—from plasma thrusters for spacecraft propulsion to tokamak fusion reactors for harnessing nuclear fusion as a clean energy source—contain plasma-facing surfaces that emit electrons. Understanding how many electrons can flow from cathodes through plasmas is a key problem in fundamental and applied physics impacting the design, efficiency and lifespan of various plasma-based technologies. 

Scientists at Lawrence Livermore National Laboratory (LLNL) have identified a new mechanism that controls the flow of electrons from electron-emitting cathodes through plasmas. The research by LLNL physicist Michael Campanell and student interns Cindy Wang and Kyle Nguyen reveals a phenomenon termed the “backflow saturation effect,” providing fresh insights that challenge a century-old assumption.

Since Nobel laureate Irving Langmuir’s pioneering work, the plasma physics community has traditionally assumed that the maximum electron current was limited predominantly by a “space charge effect,” where a buildup of electron charges creates a potential barrier—known as a “virtual cathode”—that prevents additional electrons from entering the plasma. The new research shows that the backflow saturation effect can set even stricter limits than the space charge effect.

Backflow saturation happens when electrons previously emitted into the plasma reverse course and flow back toward the cathode; but it doesn’t just limit electron flow, it also changes the plasma’s state and its interactions with electrodes. This effect could potentially be harnessed by researchers to reduce the impact of ion energy on the cathode, thereby mitigating surface erosion caused by high-energy ion collisions, which can lead to processes like sputtering, where atoms are physically ejected from the material due to the energy of the impacting ions.

To uncover this effect, the researchers developed and applied an innovative simulation code that models the entire plasma diode system—a device that uses plasma to move electrons between two surfaces, creating an electric current—including the cathode sheath, the interior plasma and the anode sheath. This approach produced cleaner data that simplified the analysis and interpretation of results. The code utilized in this study builds upon a framework Campanell developed a decade ago when he was a Lawrence Fellow, in collaboration with LLNL physicist Maxim Umansky.

With these new insights, the researchers emphasize the need for basic experiments to further demonstrate and differentiate the backflow saturation effect from the space charge effect.

–Physical and Life Sciences Communications Team