Production of intense ion beams at 80 keV with a three electrode extraction system

The characteristics of a 3 electrode extraction system with circular apertures, designed and used at FAR laboratory to accelerate ions, for neutral beam injectors, at energies of 80 keV and above, are presented. This system is compared to 4 electrode accelerators with post-acceleration which were thought to be the only alternative at high energies. Higher current densities can be extracted at the optimum perveance and similar beam divergence angles and transmitted power fractions along the beam line can be obtained. The required grid transparency for long pulse operation is identical for both system. The 3 electrode system offers the additional advantage to be simpler as a mechanical point of view. In particular, the problem of cooling the thin gradient grid is removed. Experimental results on the 3 grid system on the Periplasmatron test beam line are presented at the level of 40 A extracted current at 80 keV beam energy. Numerical simulation results on the 3 grid and 4 grid systems are also given