Cancellation of the Ion Deflection due to Electron-Suppression Magnetic Field in a Negative-Ion Accelerator

The magnetic configuration inside a high voltage negative ion accelerator is crucial for achieving adequate accelerator efficiency and tolerable heat loads on grids and beam-line components (by suppressing co-extracted and stripped electrons) and good beam optic quality (by compensating undesired ion beamlet deflection). A simple permanent-magnet arrangement has been conceived and optimized for the accelerator extraction grid, which produces an asymmetric local magnetic field on the upstream and downstream sides of this grid. Such arrangement allows to "intrinsically" cancel the magnetic deflection of the ions, while improving at the same time the suppression of co-extracted and stripped electrons, thanks to the "concentration" of the magnetic field on the upstream side of the grid. The concept is being applied to MITICA, which is a full Heating Neutral Beam (HNB) Injector, being built at Consorzio RFX in Padova with the purpose of validating the injector design and optimizing the operation of the HNB [1] to be used in the ITER fusion reactor. MITICA shall produce a focused beam of neutral particles (H or D) constituted by 1280 individual beamlets having a total energy of about 1 MeV and a total power of 16 MW for 3600 s continuous operation. As a preliminary test, the new magnetic configuration has been implemented and experimentally mapped on the 1:1 scale Multi Channel Prototype grid, showing good uniformity and agreement with the calculated magnetic field distribution. The paper presents the features of this recent magnetic design improvement, together with a discussion of the experimental results.