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Updated: May 24, 2026

Photoelectron Imaging of Anions Illustrated by 310 Nm Detachment of F−
Published on: July 27, 2018
Sheath structure in negative ion sources for fusion (invited)
R McAdams1, D B King, A J T Holmes
1EURATOM∕CCFE Association, Culham Science Centre, Abingdon, Oxfordshire OX14 3DB, United Kingdom. roy.mcadams@ccfe.ac.uk
Abstract:
In fusion negative ion sources, the negative ions are formed on the caesiated plasma grid predominantly by hydrogen atoms from the plasma. The space charge of the negative ions leaving the wall is not fully compensated by incoming positive ions and at high enough emission a virtual cathode is formed. This virtual cathode limits the flux of negative ions transported across the sheath to the plasma. A 1D collisionless model of the sheath is presented taking into account the virtual cathode. The model will be applied to examples of the ion source operation. Extension of the model to the bulk plasma shows good agreement with experimental data. A possible role for fast ions is discussed.
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