Passive Diagnostics

Abstract

Magnetically confined hot fusion plasmas emit electromagnetic radiation in a wide spectral range extending from the radio frequency range to the X-ray region. It covers a wavelength range of almost ten orders of magnitude. Each range has its own diagnostic potential. Three generation mechanisms need to be distinguished: (i) the acceleration of electrons and ions in the confining magnetic field, (ii) the acceleration of electrons in the field of ions, and (iii) the electron impact excitation of not fully ionized impurity atoms in the plasma.Ad (i): The acceleration by the Lorentz force causes the electrons and ions to gyrate around the magnetic field lines, resulting in what is called ion cyclotron emission and electron cyclotron emission, ICE and ECE, respectively. The frequency range is determined by the local confining B-field and the aspect ratio of the device. In modern fusion experiments, the ICE lies within 10-100 MHz, while the ECE lies within 50-500 GHz. Only ECE has gained diagnostic significance. Ad (ii): The continuous emission connected with electron ion collisions is called bremsstrahlung. If the electrons and ions are free before and after the encounter, the emission is called free-free radiation. If the electrons are captured by ions into a bound state, the emission connected with the process is called recombination radiation, or free-bound radiation. Both terms, free-free and free-bound radiations, are synonymously used with the term bremsstrahlung. Bremsstrahlung covers the spectral range from the microwave to the soft X-ray region. Most important diagnostic applications are in the visible and the soft X-ray regions. Ad (iii): If impurities are present in the plasma, they can be ionized by electron impact. The degree of ionization depends on the local electron temperature. While atoms with lower atomic numbers in low ionization states can be found at the low-temperature plasma edge, in the hot-plasma center, only ions with high nuclear charge can exist in high, but not fully ionized states. Spectral lines characteristic for the ion species and the ionization state are emitted after electron impact excitation. The spectral range extends typically from the optical into the X-ray regime. Spectroscopy of line emission from the plasma is the classical plasma diagnostic method in the optical, UV, VUV, and X-ray regions. It allows for the determination of a number of important plasma parameters; in particular, it allows for the quantification of the plasma impurity content. While the ion species and the ionization state of the impurities are uniquely determined by the wavelength of the emitted spectral line,