The concept as well as the design of a linearly extended microwave electron cyclotron resonance heating (ECRH) plasma source is introduced. A magnet system inside the plasma chamber is used to simultaneously generate the resonant magnetic field, shape and accelerate the plasma. After a discussion of microwave absorption by a low pressure plasma and the influence of magnetic fields of the closed loop type on the motion of charge carriers subjected to a microwave field the design of a so called elongated-mirror-geometry (EMG)-plasma source is introduced. Basic plasma parameters, performance characteristics as well as first results on the gas phase deposition of silicon oxide films are reported.