This figure comes from my doctoral work on a very practical fusion-materials problem: how do you remove oxygen from the first wall of a tokamak?
Tore Supra needed a reliable way to clean oxygen from plasma-facing walls, and our group had a contract to help understand the physics behind that cleaning. We worked with carbon-fiber composite, pyrolytic graphite, and stainless steel - the materials and material families that mattered for the wall. It was not a one-person project: students, PhD students, and senior researchers ran long irradiation and thermal-desorption campaigns on several plasma and TDS devices, then stitched the results together into reports, papers, and a practical cleaning scenario.
The figure shows one of the key graphite results. We first loaded the samples with oxygen in deuterium-oxygen plasma, then tried to remove that oxygen by deuterium glow-discharge cleaning. The original oxygen-loading energy mattered much less than one might expect. Once cleaning started, the material response was controlled mainly by the energy of the cleaning ions themselves.
Low-energy deuterium cleaning was slower in clock time, but gentler: oxygen stayed closer to the surface, so a thinner layer had to be sputtered away. High-energy cleaning looked faster, but it had a trap. The same energetic deuterium ions that sputtered the surface also drove oxygen deeper into the carbon-fiber composite by collision cascades. The wall was being cleaned and mixed at the same time.
That was the useful lesson: "higher energy" is not automatically "better" for tokamak wall conditioning. In graphite composites, energetic cleaning ions can bury the impurity you are trying to remove. The practical optimum is a compromise between sputtering rate, cleaning time, and ion-driven oxygen transport into the wall.