The influence of the permanent magnet configuration, electromagnetic excitation mode, and input microwave power on the operation of a multipolar electron cyclotron resonance (ECR) source is investigated. A 12.5-cm i.d. multipolar ECR plasma source was operated in argon gas at 0,77 mTorr with a flow rate of 20 sccm. The input power was varied from 240 to 750 W. The microwave coupling efficiency, the ion density and uniformity, and the ion production efficiency were determined for three different multipolar magnetic field configurations (8 pole/8 magnet, 4 pole/8 magnet, and 4 pole/4 magnet) and two electromagnetic modes (TE211 and TE311. The 4 pole/8 magnet configuration operated in the TE211 mode had the best efficiencies and uniformity. In particular, it had a microwave coupling efficiency as high as 98% and an ion production efficiency with 240 W of input power of 290 eV/ion. A numerical analysis of the ECR excitation indicated that this configuration excited with the TE2n mode also had the most uniform electromagnetic excitation of plasma by the microwave fields. © 1992, American Vacuum Society. All rights reserved.