Multi‐antenna RF Ion Source at a High RF Power Level

A multi-antenna radio-frequency ion source with a Faraday shield is newly tested at a high RF power level in a large area negative ion source of 1/5th scale of the Large Helical Device-NNBI ion source. Inductively coupled dense hydrogen plasmas were generated uniformly over an area of 25 x 25cm(2) at an RF input power up to 300 kW for a 10ms pulse duration. A large negative plasma potential for the non-Faraday shielded antenna was remarkably reduced by introducing a Faraday shield. The positive ion saturation current density measured by Langmuir probe reached 148 mA/cm(2) at 174kW around the center of the plasma. The optimal hydrogen filling pressure ranged around 0.13 Pa- 0.4 Pa for the positive ions. Ion beam extraction with a single hole (phi 0.5cm) extractor has been studied systematically. A maximum H- ion beam current density of 1.6 mA/cm(2) was obtained preliminarily. It was confirmed that the plasma profile was controllable by both the number and configuration of the antennas.