The possibility of He Neutral Beam Injection, especially 3He at 150–173 160keV, would extend the JET operating parameters and improve the physics interpretation from diagnostics. Extensive Fokker-Planck simulations have been made of NBI and combined NBI/RF heating, considering beam-plasma, beam-beam, and plasma-plasma fusion for D-3He, D-D, and triton burnup. High energy, monoenergetic He beams should allow access to ion temperatures of 25-50keV, and D-3He fusion power of several hundred kW. Pumping of 3He has been demonstrated in a full JET Neutral Beam box, and powers of 8-10MW at 155keV of neutral 3He are possible, although He metastable formation may enhance trapping at the plasma edge. Since the neutron production is due only to thermal D-D reactions, and beam-plasma reactions are measurable from the D-3He gamma ray branch, much clearer measurements are possible of the thermal ion distribution function. Charge-exchange with the He beam allows new possibilities for diagnostics with ex recombination spectroscopy, and active and passive ex particle detection. Detailed experiments, diagnostic measurements, and physics are proposed: • Standard operation with 3He NBI • Fusion product and charge-exchange measurements of ion velocity distributions • Hot-ion-hot-plasma experiments at reactor temperatures, 25-50keV • Synergistic effects of combined NBI/RF heating • Simulation of alpha particle thermalization and particle diffusion • Effects of velocity distributions on sawteeth, fishbones, and Alfven instabilities • High ion temperature plasmas with reduced fusion neutron emission using H, 3He and 4He NBI