Extraction and Acceleration of High-Current Ion Beams of Multiply Charged Ions

I. Litovko, E. Oks*
Institute for Nuclear Research NASU 47 pr. Nauki, Kiev 03028,
*High-Current Electronics Institute, pr. Academicesky 2/3, Tomsk 634055, Russia
Abstract. The characteristics of the ion beam extracted from high current ion sources type MEVVA were investigated by
using computer code Kobra3-INP. It has been used to evaluate the extraction and acceleration systems in attempt to
generate an ion beam with high current, high brightness and low divergence. The results show that the shape of aperture
plays important role in beam formation. The simulation can provide the basis for optimizing the extraction system and the
acceleration gap for ion source.
Keywords: ion beams, charge state distribution, ion sources
PACS: 52.65.-y
The computer simulation of the extraction of a charged particle beam from a plasma source and its transport has
used about 50 years [1]. The aim of these simulations is always to investigate the ion beam quality, which makes
optimization for a specific application possible. As rule, the applied numerical method based on solving the Poisson
equation with the unknown space charge term and then the result is used for the solution of motion equations. A
repeated iteration allows to achieve of self-consistent solution.
In this work for investigation high current ion beams with different charge state was used 3D computer code
Kobra [2, 3]. It allows translate the geometry information into mesh information and take into account plasma source
geometry and acceleration gap geometry as well as physical condition for beam formation. The finite difference
method (FDM) is used for the discretisation of equations. For solution of the set of equations an iterative point-topoint
over relaxation method (SOR) is applied. The first step is solution of the Laplace equation with using seven
point differential schemes. Equation of motion is solving by exact integration. By repeated solving of Poisson
equation, motion equations for particles and re-determination of the space charge distribution a self-consistent
solution can be found. The existing boundaries between regions with space charge and region with plasma condition
are taken into account. plied The ion source type MEVVA [4] has been used as experimental setup. The plasma is usually fully ionized and
the ions confined in the plasma are of different charge states. The production of higher charge states is a process
which takes place close to the cathode [5]. In it has been shown that the charge state distribution is frozen at a very
short distance from the cathode, because of the charge state distribution can be influenced by controlling the
discharge condition close to the cathode. The ion charge state can be enhanced by increasing the discharge current or
applying a strong magnetic field [6].
In this work has been made calculation for different geometry extraction system as well as acceleration gap for
purpose system optimization in attempt to generate the steady beam and to study the behavior and influence of
different factors on beam parameters.
In the simulation all available information from experiment has been used. By varying unknown parameters it
was tried to reproduce the experimental beam properties by the simulation. Inside the plasma a homogeneous ion
density distribution is assumed. The starting energy is given by a direct ion drift energy which is determined by the
physics of plasma formation and the ion temperature. Corresponding data for plasma source have been taken from
experiment. The space charge inside the plasma is compensated by electrons with Boltzman density distribution. The
full space charge compensation is assumed for drift section evaluation. The accel-decel system is used for saving the
space charge compensation of the extracted ion beam. The calculation has been made for phosphor ions with
different ion charge state (45% P+1, 45%P+2, 10%P+3 on current fraction).
The evaluations have been made for different geometry of extraction system for purpose.

The results of calculation confirm that geometry extraction system, shape of apertures and its diameter plays very
essential role in beam formation and its intensity. One can find from comparing different geometry that one can be
used to reduce the divergence of the ion beam (1a), but other can provide biggest beam intensity (1c).
The characteristics of the phosphor ions beam with different charge state extracted from ion sources and its
transport was investigate by using the Kobra computer code. The results show that the geometry of extraction
system plays important role in beam formation. The beam intensity and divergence is essential depended from
aperture shape and diameter of hole. Simulation also indicate that beam intensity depend from emission current
density and plasma potential and potential on screening electrode. The variations of the potential allow change
charge state fractions in extracted ion beam. All computational results are in a good agreement with experimental
data and can provide the basis for optimization the extraction system and acceleration gap for the ion source.
1. S. A. Self, Phys. Fluids, 6, 1762 (1963)
2. P. Spaedtke, “Computer simulation of extraction”, in: The physics and technology of ion sources, edited by I.G.Brown,
Berlin, Wiley-VCH, 2004
3. P. Spaedtke, H. Emig, J. Klabunde, et.al, Rev. Sci. Instrum. 67(3), 1146 (1996)
4. I. G. Brown, B. Feinberg, J. E. Galvin, J. Appl. Phys, 63,4889(1988)
5. F. J. Paolini, I. G. Brown, Rev. Sci. Instrum. 66, 7 (1995)
6. A. Anders, I. Brown, M. Dickinson, R. MacGill, Rev. Sci. Instrum., 67, 1202-1204 (1996)

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