Deviation of H− beam extraction simulation model

Abstract

Negative hydrogen ion source extraction system development is dependent on accurate and fast simulation methods for modelling the behaviour of ion and electron beams. Traditionally this type of work has been done using ray-tracing extraction codes, such as IBSimu. The plasma extraction model in IBSimu has been observed to underestimate the charge density near the plasma sheath, leading to incorrect prediction of the current at which the system produces the optimum emittance. It is suspected that this deviation results from the approximations made by the model, neglecting the magnetic field and collisional effects near the sheath region. Results and comparisons to simulations are presented for three ion sources, for which two show similar variation and one is more accurately modelled. Conclusions with the currently available data can not be made and therefore further experimental and simulation campaigns to study the effect are suggested.

H- beam emittance analysis in a multicusp ion source

“…The emittance minimum drifts to lower bias current values as the gas flow increases. This is contrary to the behaviour observed in some other filament ion sources, as mentioned in [4]. It can also be seen that the shift in emittance minimum decreases as gas flow values increases.…”

“…As can be seen from the figure, the curve passes through an emittance minimum value at a certain I bias value, for all gas flows. The minimum corresponds to an optimum plasma sheath shape [4]. The emittance grows to either side of the minimum, indicating the plasma sheath becoming more convex or concave from the optimum shape.…”

“…Fig 2(B) considers only the negative ion charge density through the I bias values. In order to include the effect of electron densities, we can define an effective current at the plasma sheath, I eff , which corresponds to the sum of currents due to H − ions and electrons [4].…”

“…It can also be seen that the value of R ec increases with gas flow. This can be understood in terms of the effect of pressure on electron transport and collisions in the plasma [4]. But such high R ec values are difficult to comprehend as it represents very high electron density at the plasma sheath.…”

A study of the optical effect of plasma sheath in a negative ion source using IBSIMU code

“…For example, significant deviations in the beam emittance behavior between IBSimu simulations and experiments have been observed. 2 One of the possible reasons for this is the underestimation of charge density near the plasma sheath in the simulations.…”