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Energetic neutral particles from neutral beam heating systems are widely used for active spectroscopic measurements of key plasma parameters in fusion experiments. Both the plasma discharges and the neutral beam systems are normally operated with hydrogen or deuterium. Helium beams are used in dedicated diagnostic beam lines as they offer deeper penetration and are subject to less background radiation and enable resonant double charge exchange with alpha particles. Neutral beam systems using pure helium either require specialized helium gas pumping with a pumping speed in excess of 1000 m3/h or are restricted to short pulses (normally less than 1 s). A doped hydrogen/helium beam combines the requirements for plasma heating and diagnostics without the need for sophisticated helium pumping. A small flow of helium gas is injected into the plasma source for the time helium particles are required. The helium current is typically 10% of the total extracted current. The reduction in heating power of the doped beam can be kept below 5%. The small amount of helium gas does not cause an excessive pressure rise along the beam line and does not reduce the reliability of the beam heating system. Doped deuterium/helium beams have been successfully tested and routinely used at JET. The Hel beam emission spectra obtained with a doped deuterium/helium beam produce sufficiently strong visible lines for spectroscopic applications. Furthermore, the simultaneous availability of helium and hydrogenic particles in the beam allows us to extend spectroscopic measurements to another atomic system and hence cross-check results from helium beams with those from hydrogenic beams. The only investment required is an additional helium gas inlet system into the ion source. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]