A new-type microwave plasma source has been developed for materials processing. The plasma reactor employed a launcher of azimuthally symmetric surface waves at a frequency of 2.45 GHz and also magnetic multicusp fields around the reactor chamber walls. This configuration yielded high-density(Ne≳1011cm−3)plasmas sustained by surface waves even at low gas pressures below 10 mTorr, following easy plasma ignition by electron cyclotron resonance (ECR) discharges. Electrical and optical diagnostics were made to obtain the plasma properties in Ar. It was shown that a transition occurs from ECR exited to surface-wave excited plasmas under conditions where the plasma electron density exceeds a critical value ofNe∼1×1011cm−3.