Microwave power absorption and intense collimated beam production is studied in the pulsed high-power microwave ion source at RIKEN by varying the radius of the plasma chamber (circular waveguide) in integer multiples of a quarter wavelength of the wave (nλ/4), where n is an integer (n=2–5) and λ is the vacuum wavelength of microwaves. Optimum wave coupling and power absorption due to a change in the microwave power density and the electric field distribution are investigated from measurements of the total achievable current density and the optical intensity, emitted by the pulsed argon plasma in the visible region at a wavelength of 404.3 nm. Results indicate a nonlinear absorption of microwave power. Depending upon the pressure, a decrease in the chamber radius leads to an enhancement of the electron temperature. Favorable beam conditions, i.e., highest density and temperature are obtained for the chamber with radius ≃λ. © 2002 American Institute of Physics.