Resonance broadening as a consequence of strong focussing of electron cyclotron wave beams

It is shown that for strongly focussed electron cyclotron resonance heating wave beams the power deposition profile is broadened and can be shifted as a consequence of resonance broadening caused by the finite beam width. This resonance broadening is in fact seen to be identical over the entire beam path, both near the waist as well as in the far field. For high optical depths and injection from the low field side the effect is found to be independent of the plasma temperature. In the case of modest optical depths, significant effects are observed when the resonance broadening exceeds the absorption line width, which for close to perpendicular propagation leads to the criterion: ΔNparallel,eff = 2/k0w0 > vt/c. For a 110 GHz wave beam as used in the numerical examples, significant effects of resonance broadening are observed once the beam is focussed to a waist parallel to the equilibrium field of w0 = 1 cm or below