What wall probes really measure: how the probe shapes the signal in electron cloud experiments

What is it about?

We study what a capacitive / wall-sector probe actually records when an electron cloud moves inside a trap. Using simple models, we compute the charge that appears on a metal sector of the wall as the plasma drifts. Even when the plasma follows a clean circular path at steady speed, the probe signal is not a perfect sine wave. It picks up extra harmonics because the probe response changes with distance and angle. If the plasma speeds up near one side of its path, or if the cloud is bigger or elongated, these extra harmonics and phase coupling get stronger. Common shortcuts like using the ratio of the second harmonic to the first to estimate displacement can then be misleading. We show why that happens and how the bispectrum can light up even when the plasma motion itself is simple. We also outline how using several probes lets you reconstruct the path of the plasma and tell instrument effects apart from real plasma behavior.

Featured Image

Photo by Egor Komarov on Unsplash

Photo by Egor Komarov on Unsplash

Why is it important?

Capacitive / sector probes are cheap, fast, and non destructive, so many labs rely on them. But if the signal is shaped by the probe as much as by the plasma, you can misread the physics. Our results give clear warnings and a practical way forward: use multiple probes, recover the trajectory, and only then infer dynamics. This leads to better diagnostics and more reliable conclusions in devices like SMARTEX-C and similar devices.

Perspectives