The figure showing a sawtooth crash was generated with modern measurement techniques and brings up the question, “How does one measure anything inside a fusion plasma?” At temperatures over a million degrees, nothing put into the plasma will survive. Plasma diagnostics is a whole field in itself, but here is a brief summary. There have to be enough windows, or “ports” to get light or other beams

After

Fig. 7.6 ‘erature distribution before, during, and after a sawtooth crash (top right and

colored squares). Yellow or light color is hot, and blue or dark color is cold. The pictured region at the q = 1 surface of the tokamak cross section is shown at the left. The small graph at the bottom shows the temperature distribution before and after the crash, showing that hot plasma has been moved out of the q = 1 surface [3] into and out of the plasma. The scattering of a laser beam can give information about the electron temperature and density of the plasma. Crossed laser beams can be used to measure the ion temperature. The transmission or emission of electro­magnetic radiation in the microwave, X-ray, or infrared range can show oscilla­tions inside the plasma. Sawteeth were first observed from the fluctuations in the soft X-ray radiation emitted by electrons at twice their cyclotron frequencies. Since the frequency depends on the magnetic field, and the field varies with position, this also tells where the radiation is coming from. Diagnostic beams of neutral atoms or heavy ions can be injected into the plasma since they can penetrate the magnetic field. In beam emission spectroscopy (BES), a beam of neutral hydrogen atoms is injected and reaches the plasma interior. There, it col­lides with the ions and is ionized by the electrons. In the process, it emits light whose spectrum can be analyzed by computer. This light carries information about the density and velocities of the ions and even the strength and direction of the local magnetic field. Heavy ion beam probes (HIBPs) work even better and can
even measure the internal electric fields. These are the main ways, developed over many years, to probe inside a fusion plasma and get the knowledge we now have on how a magnetized plasma behaves.