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Abstract
Reflection of ions as neutral atoms at energies below 100 eV has important implications for fusion and other technologies, and can form the basis of a useful hyperthermal neutral beam. Experimental measurements of properties of such reflected atoms are presented. The apparatus developed for this work uses the acceleration of plasma ions across the sheath to a metal reflecting plate biased relative to the plasma potential. Ions are created by a coaxial RF (lower hybrid) plasma source with a 4 kG confining field. The plasma physics determining the characteristics of the ion current to the plate is discussed, and measurements of plasma parameters are presented.
Measurements of the energy distribution of reflected atoms were made with a mass spectrometer/cylindrical mirror analyzer in experiments with several gases (Ar, Kr, Ne, N, O) and metals (Ta, Mo, steel, Al), at several reflection angles. Peaked distributions were observed where the target atomic mass $m\sb2$ exceeded the projectile mass $m\sb1$, while only monotonic decreasing "tails" were seen where $m\sb2 < m\sb1$. The spectrum's peak energy increases with incident energy $E\sb{inc}$, while $E\sb{pk}/E\sb{inc}$ decreases for increasing $E\sb{inc}$ and increases with $m\sb2/m\sb1$ (but more slowly than for binary collisions). Measurements were also made of the absolute atomic flux for oxygen beams, and the angular distribution, which is forward-peaked.
Energy distribution measurements are compared to predictions of the Monte Carlo code TRIM, which uses the sequential binary collision model. The results differ from TRIM predictions that $E\sb{pk}/E\sb{inc}$ is nearly constant with $E\sb{inc}$ in the observed range and increases faster than observed with $m\sb2/m\sb1$.
The observed behavior of $E\sb{pk}/E\sb{inc}$ implies the projectile is reflected from a collective mass greater than $m\sb2$ which increases as $E\sb{inc}$ decreases. An n-body simulation was written to examine the effects of simultaneous interaction with multiple target atoms. The results indicate that projectiles at hyperthermal energies are reflected in a process which is not binary in character, with the effective mass of the target increasing as $E\sb{inc}$ falls and simultaneous interactions become more important. The process is not correctly described by a binary collision model.
