Neutral beam injector for active plasma spectroscopy

A diagnostic beam system has been developed for the RFX reversed field pinch, Padova, Italy. Currently the system is loaned to Alcator C-mod, MIT, Boston. The system is primarily used for measurement of the ion temperature by charge-exchange recombination spectroscopy and for magnetic field measurements via motional Stark effect. The system comprises an ion source, beam duct equipped with vacuum pumps and various diagnostics of the beam. The ion source provides 50 keV, 5 A hydrogen beam. Ions are extracted from a plasma created by an arc-discharge source and, after accelerating and focusing, are neutralized in a gas target. A plasma emitter, which is formed by collisionless expansion of a plasma jet on to the grids, has low perpendicular ion temperature. These results are in rather low (0.01 rad) angular divergence of the extracted ion beam. The grids of ion optical system are spherically curved providing geometric focusing of the beam at a distance 4 m. Current density at the focal plane reaches 100 mA/cm2. Arc-discharge plasma box provides highly ionized plasma, so that the extracted beam has about 80% of full energy specie. The injector provides 50 ms duration pulse each 5 min. In order to increase the signal to noise ratio the beam can be modulated with a frequency variable up to 250 Hz.