The role of the electromagnetic field on the ECRIS beam production

In order to increase the beam intensity on the target it is mandatory to better understand the plasma generation, the beam extraction and the beam transport. It is well known that the plasma properties and the beam creation are modified by the microwave frequency and power. In this report we investigated how the ions beam generation in an ECR ion source is affected by the electromagnetic field excited inside the plasma chamber. For this purpose simulations and related experiments have been performed with the CAPRICE ECRIS [1].The injection flange of the ion source consists of a copper matching box where the WR62 waveguide line connects the Klystron generator and the source. The electromagnetic wave is transmitted into the plasma chamber through a coaxial line. In order to realize an impedance matching inside the copper box a movable plunger is present. This structure modification determines a volume change which affects the electromagnetic field inside the matching cube and inside the plasma chamber. This effect has been simulated with CST Microwave Studio [2]. A horizontal cut view of the matching box and of the plasma chamber together with the electric field excited inside is shown in figure 1. The hole and the shape of the plasma electrode have not been taken into account and a closed cylindrical plasma chamber filled with a medium with electric permittivity !r (!r=1) has been considered in the simulation. In the upper figure the plunger of 46 mm diameter located in the upper wall is moved 3.5 mm outside of the inner wall, in the middle and in the lower figures it is moved 0.5 mm and 4.5 mm inside, respectively.