Negative hydrogen ion production in fusion dedicated ion sources

Graphical abstract: In RF sources the acceleration of positive ions to a few tens of eV by the plasma potential difference between the driver and the extraction regions can have an important effect on negative ion production by enhancing the negative ion yield from caesiated surfaces and by charge exchange reactions with caesium atoms. The presence of energetic positive ions can have other implications: modifying the virtual cathode in front of the plasma grid, ionizing caesium atoms. Highlights: ► The physics of volume and surface production of hydrogen negative ions is reviewed. ► Effects of positive ion acceleration by plasma potential difference are investigated. ► Caesium ionization in extraction region by electrons and charge exchange are compared. ► Charge exchange with energetic positive hydrogen ions dominates caesium ionization. ► Negative ion production by charge exchange of positive ions with caesium is discussed. - Abstract: A brief description is given of the basic processes in negative ion sources dedicated to fusion. It is considered that in these sources negative ions are produced by ions and atoms interacting with a caesiated surface, but this mechanism is not unique: the volume production, based on dissociative electron attachment to rovibrationally excited molecules, is also active. We suggest that in RF sources the acceleration of positive ions to a few tens of eV by the plasma potential difference between the driver and the extraction regions can have an important effect on negative ion production by enhancing the negative ion yield from caesiated surfaces, and by charge exchange reactions with caesium atoms. The presence of energetic positive ions can have other implications (modifying the virtual cathode in front of the plasma grid, participating in caesium ionization).