A significant improvement in the performance of the BNL Mark III magnetron negative ion sources has been achieved by putting cylindrical grooves on the cathode to geometrically focus surface produced H−and D−ions into the emission slit in the anode. The addition of these grooves has allowed us to reach desired negative ion emission current densities with an order of magnitude lower arc current than was previously required. An additional improvement in the operation of the magnetron has resulted from increasing the anode‐to‐cathode spacing in the back of the source, which facilitates ignition of the discharge at low pressures. The net result of these modifications has been an improvement in the gas and power efficiencies by at least a factor of three, and a reduction in the emittance of the beam.