The polarization dilution by molecular ions which are produced in the ECR primary proton source is discussed. The molecular component can be reduced to about 5&percent; by ECR source‐operation optimization. It is further suppressed by optimization of the extraction electrode optics and by the decelerating einzel lens in the 35 keV LEBT line. As a result, the proton polarization of the accelerated beam was increased to over 80&percent;, as measured in the 200 MeV proton‐deuterium polarimeter. The OPPIS upgrade for 6 2/3 Hz repetition rate operation is also discussed. © 2003 American Institute of Physics