A prototype compact electron cyclotron resonance ion source with all permanent magnets named Kei3 and based on Kei2, has been developed to produce various ions. Kei3 has an outer diameter of 280 mm and a length of 1120 mm. The magnetic field is formed by the same permanent magnet as Kei2. We investigated the basic performance of the Kei3 source at a previous experiment. The maximum beam current of C~(4+), N~(5+), O~(6+) and Ne~(7+) were 565 μA, 185 μA, 99 μA and 50.5 μA, respectively. In order to increase the beam current of heavy ions, such as argon, we modified the microwave injection. The rf shield in the plasma chamber was used as a tuner of microwaves. The rf shield was installed at the position of a mirror peak of the injection side, and it can be moved 30 mm to an upstream side from there. As a result, we found optimal position of the rf shield for the production of Ar ions.
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