Design of a visible tomography diagnostic for negative ion RF source SPIDER

The ITER heating neutral beam injector, based on 1 MV D− ions, will be tested and optimized in the SPIDER source and MITICA full injector prototypes, using a set of diagnostics not available on ITER. Beam intensity uniformity is required to stay within ±10%, thus beam profile is measured with a complementary set of diagnostics. Among them, visible tomography measures the line of sight (LOS) integrated Hα or Dα radiation generated by the collisions between fast particles and neutral background molecules, and emitted on a plane perpendicular to the beam. A sufficient number of well arranged LOSs allows a tomographic reconstruction of the 2D beam emission profile, which is proportional to the beam density. On SPIDER the system is equipped with about 3000 LOSs, grouped in 15 fans, and the tomographic algorithm is based on the pixel method. The design of the diagnostic is presented, with description of layout and main components and test of the prototype linear CCD camera.