soft X-ray imaging is a diagnostic that views the hot core of a plasma through its soft X-ray emission, using filtered detector arrays to make fast movies of the core's temperature and impurity structure.
The qualifier does the work: "soft" X-rays are the lower-energy end of the X-ray range, easily absorbed by matter, as against the "hard," penetrating X-rays of radiography. That absorbability is the diagnostic's selectivity. A 📝plasma core at 📝fusion temperatures glows in soft X-rays through 📝bremsstrahlung and impurity line emission, while the cool edge emits almost none — so a thin metal foil, classically 📝beryllium, placed in front of each detector blocks visible and ultraviolet light and passes only the radiation of the hot interior. Arrays of filtered diodes view the plasma along many chords, and tomographic inversion turns their signals into a two-dimensional map of core emissivity.
What the technique trades in spectral detail against 📝spectroscopy it repays in speed: diode arrays resolve microsecond timescales, which makes soft X-ray imaging the standard instrument for watching the plasma core move. The periodic collapse of a 📝sawtooth oscillation, the rotating structure of a growing 📝magnetic island, and the drift of accumulated impurities toward the axis all appear first, and most clearly, in the soft X-ray cameras.