The beam brightness of the J-PARC (Japan Proton Accelerator Research Complex) cesiated rf-driven Hˉ ion source has been increased by using several unique measures, including low plasma electrode temperature, slight water feeding in hydrogen plasma and so on [1-3]. The J-PARC source enables high energy linear accelerators to eject a beam with 60 mA peak intensity. The conditions to minimize the co-extracted electron current lower than the Hˉ ion current of 66 mA and the beam qualities attained with the conditions are presented in this paper. The low co-extracted electron current will be necessary for a higher beam duty factor operation.
 A. Ueno, New J. Phys. 19 (2017) 015004
 A. Ueno et. al., "Emittance improvements of cesiated RF-drivenH− ion source to enable 60 mA operation of high-energy and high-intensity LINACs by plasma impurity controls" to be published in AIP Conf. Proc. NIBS 2016
 A. Ueno et. al., "Transverse RMS emittance evaluation based upon explicit and reasonable definitions of 100% and 95% beams" to be published in AIP Conf. Proc. NIBS 2016