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A technique is described which controls the energy spread of an ion beam by simple electrical means. There is also provided means for rapid control over the current density in the beam.
English (United States)
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Reduction in Ion Beam Energy Spread by Space Charge Control
A technique is described which controls the energy spread of an ion beam
by simple electrical means. There is also provided means for rapid control over
the current density in the beam.
The essence of the technique is to make use of the space charge potential
developed in a high current density ion beam. The space charge is used to
control the ion flux in the beam itself.
Control over this space charge effect is provided by a grid or mesh structure
placed across the beam. By varying a single voltage applied to this mesh, the
space charge effect can be minimized or maximized, causing a rapid and distinct
change in the transmitted beam characteristics.
In particular, by appropriate choice of dimensions and voltages, the
transmitted beam current density and energy spread can be switched between
two widely different values by a small change in control voltage applied to the
Shown in Fig. 1 is an example of the technique. An incident ion beam of
current density j(o) and energy E(o) (with spread Delta) is directed at a grid
structure which is used to control the beam. The grid array consists of two
grounded grids (fine mesh) with a center grid which can be electrically biased by
a control voltage V(c).
For small positive values of V(c), the incident ion beam is fully transmitted,
with its current density and energy spread unchanged.
However, when the positive control voltage V(c) reaches a critical value
V(crit), the transmitted ion current density abruptly drops to a fraction of the
incident value, and the energy spread narrows sharply.
These changes are shown in Fig. 2. When V(c) exceeds the ion energy
E(o), no ions are transmitted.