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A theory outlining the mechanism of the low pressure hydrogen plasma used in radio frequency proton sources is given. Taking into account secondary emission from surfaces of plasma vessel it is shown that the electron density is maximum at the tube's center. This is confirmed experimentally, and a source is designed with its extraction electrode positioned radially near the plasma center. Compared with axial extraction used in conventional r.f. sources, radial extraction yields beam currents over 20% higher, and allows the application of higher extraction voltages. Beam currents up to 150 Ma. can be extracted, of which more than 85%, is proton current. Two methods are used for the measurement of the emittance of the accelerated beam.