Characteristics of multiantenna rf ion source

A multiantenna rf ion source is studied. The new antenna system is developed to reduce the rf voltage on the antenna compared to that of the conventional loop antenna, which can reduce high voltage rf breakdown on antennas. The antenna was installed in a 1/6 scale ion source for the neutral beam injector (NBI) (H-) used in large helical devices. The characteristics of the ion density current of the plasma produced by using the segmented antennas were studied. A dense plasma with ion saturation current density of 92 mA/cm2 was generated at 45 kW of rf power at 9 MHz for the two-antenna system. At around 10 kW of rf power the plasma density jumped and became a high density mode. The threshold power for the density jump seems to be shifted by the antenna segments and the rf frequency. The current density of the positive ion beam with a small accelerator voltage is 26 mA/cm2 for 10.3 keV of extraction energy, 45 kW of rf power, and 0.16 Pa of plasma source pressure of hydrogen. This multiantenna rf source could potentially be scaled up to the large NBI source