The theory of long waveguide structures radiating lower hybrid waves into a plasma

It is shown that the radiation into a plasma from the structures composed of many waveguides is well described by the theory of the infinite structures. This theory results in the efficient numerical code which is appropriate for the study of the nonlinear response of the plasma ill front of the grill. Both the infinite conventional grill and the structure built up from the infinite series of identical N-waveguide multijunction sections are investigated. It is proved that the spectrum of the last structure is the superposition of N spectra of the infinite conventional grills with the specially selected phase shifts. The theoretical results for three long structures (24-waveguide conventional grill on ASDEX and two multijunction arrays, namely a 32-waveguide grill on JET and a 30-waveguide grill proposed for T15) are compared with the predictions of the theory for the corresponding infinite structures. and the agreement is good.