REFLECTION OF LOW-ENERGY PLASMA IONS FROM METAL-SURFACES

Reflection of low energy (< 100 eV) ions from surfaces plays an important role in areas of plasma technology such as in the edge region of fusion devices. We have developed a beam source based on acceleration and reflection of ions from a magnetically confined coaxial rf plasma source. The beam provides a large enough ion flux (over 4 A) to allow the characteristics of the reflected neutrals to be measured despite the inefficiency of detection. The energy distributions are measured by means of an electrostatic cylindrical mirror analyzer coupled with a quadrupole mass spectrometer. Energy distributions have been measured for oxygen, nitrogen and inert gas ions incident with from 10 to 70 eV reflected from amorphous metal surfaces of several compositions. The characteristics of particle reflection are commonly predicted in plasma modelling by use of simulation codes based on data from much higher incident energies. Measurements on this beam source allow the assumptions and approximations used in such codes to be tested. The predictions of the widely used TRIM code are compared to measured reflected energy distributions at low incident energies; significant differences are observed, particularly for lower target/projectile atomic mass ratios. The adequacy of the sequential binary collision approximation (used by TRIM) for modelling a collision process which at low energies is actually a simultaneous interaction between several atoms is tested by comparison with n-body trajectory calculations.