Progress in the development of a large RF negative ion source for fusion

A large area RF source for negative ion-based injection systems has been developed and for the first time H−-current densities have been extracted which are comparable to those produced by conventional arc sources. This paper describes the design of the source and gives the results of H−-extraction experiments. A feature of this RF source is an increase of extracted ion current by up to a factor of four if noble gases are added to the hydrogen discharge. By this effect, a current density of 9 mA/cm2 at 0.65 Pa has been achieved in volume production. With caesium seeding up to 20 mA/cm2 could be extracted at 1 Pa source pressure with the addition of argon and a hot plasma grid. Recently, the RF source has been modified for long pulse operation by the installation of a Faraday screen.