Introduction

We know from classical electrodynamics that accelerated charged particles are sources of electromagnetic radiation. Particles accelerated in electric or magnetic fields radiate with distinct characteristics. Electric micro-fields present in the plasma result in bremsstrahlung emission by plasma electrons. External radiation fields interacting with the plasma give rise to scattered radiation. Charged particles moving in magnetic fields emit cyclotron or synchrotron radiation, depending on the energy range of the particles.

The interaction of radiation with plasmas in all its aspects – emission, absorption, scattering and transport – is a key to understanding many effects in both laboratory and natural plasmas. Laboratory plasmas in particular do not radiate as black bodies so that an integrated treatment of emission, absorption and transport of radiation is usually needed. Core plasma parameters such as electron and ion temperatures and densities as well as plasma electric and magnetic fields may all be determined spectroscopically, in the most general sense of the term. Rather arbitrarily we shall confine our discussion to radiation from fully ionized plasmas thus excluding line radiation on which many diagnostic procedures are based. To some extent alternative spectroscopic techniques, in particular light scattering, have replaced if not entirely supplanted measurements of line radiation as preferred diagnostics of some key parameters in fusion plasmas (see Hutchinson (1988)). In the course of this chapter we shall outline the basis of some of these diagnostics, notably those that rely on bremsstrahlung and cyclotron radiation as well as those involving light scattering. We shall limit our discussion of radiation to plasmas in thermal equilibrium, with few exceptions. Non-thermal emission, while an important issue in practice, is in many instances still relatively poorly understood.