Surface Production of Negative Ions by Positive Ions and Atoms in the Electron Suppressor Region

Abstract

A simple model of the production of negative ions by positive ions and atoms from the source plasma striking a caesiated surface is presented. The model combines calculated flux rates for the ions and atoms at a surface with negative ion yields to determine the surface production rates. The model takes into account the relative potential between the plasma and the electron suppressor in calculating the surface production rates. By making the assumption that the surface production rates can be directly compared with the extracted beam current then the model is compared with data where the beam current has been measured as a function of electron suppressor bias and knowledge of the plasma parameters of the source also exists. At high bias voltages relative to the plasma potential only volume produced negative ions can be extracted as negative ions created on the surface cannot cross the sheath. The model reproduces features of the ion current dependence on electron suppressor bias. At high bias voltages where negative ions cannot leave the surface there is still an enhancement of the non‐caesiated performance which may be attributed to volume production enhancement due to the presence of caesium in the plasma. This has implications for the replacement of caesium with other low work function materials.