Transport of negative ions across a double sheath with a virtual cathode

Transport of negative ions across a double sheath with a virtual cathode

Transport of negative ions across a double sheath with a virtual cathode 150 150 UKAEA Opendata

Transport of negative ions across a double sheath with a virtual cathode

A 1d analytical model of the sheath in a negative ion source, such as those proposed for heating and diagnostic beams on present and future fusion devices, has been developed. The model describes the transport of surface produced negative ions from a cathode, across the sheath to a plasma containing electrons, positive ions and negative ions. It accounts for the situation where the emitted flux of negative ions is greater than the space charge limit, where the electric field at the cathode is zero, and a virtual cathode is formed. It is shown that, even in the presence of a virtual cathode, there is a maximum current density of negative ions that can be transported across the sheath into the plasma. Furthermore, for high rates of surface production the virtual cathode persists regardless of the negative bias applied to the cathode, so that the current density transported across the sheath is limited. This is a significant observation and implies that present negative ion sources may not be exploiting all of the surface production available. The model is used to calculate the transported negative ion flux in a number of examples. The limitations of the model and proposed future work are also discussed.

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01/01/2011