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Abstract
An analytical study is made for appropriate extraction system design. It is found that the extraction through two parallel plane electrodes gives a divergent beam. This divergence effect can be avoided by using a shaped electrode with appropriate geometry such as Pierce electrode which makes the beam nearly parallel. Lastly, a study is made on the spherical design of the extraction electrode. It is found that the beam is focused to a minimum radius (beam waist) located at a distance equal to the focal length of the spherical extraction electrode.
Estimation is made to find the accelerating field required to prevent beam against space charge expansion which is found to be proportional to the cube root of the gap distance. The influence of the accelerating voltage ratio applied on single gap accelerating column shows optimum voltage ratio for minimum output beam radius. Higher than the optimum voltage ratio the beam suffers crossover aberration.
In the experimental part of this work the plasma parameters (n, T) are measured with the use of Langmuir probe. It is found that the increase of the RF power increases the plasma density. This increase of the plasma density is due to the increase of the electron impact energy gained from the rf field between collisions. Also the increase of the pressure increases the plasma density, this increase of the plasma density decreases the plasma temperature and hence the ion beam emittance decreases.
The axial type RF source is modified by an electromagnet which makes a magnetic mirror distribution at the source axis; this confines the plasma near the source exit aperture. The extracted argon ion current has been investigated as a function of the discharge parameters. The magnetic field influences the discharge to resonate at certain value of the magnetic field which appears clearly at low discharge pressure. The use of the magnetic field increases the reliability of the ion source which could deliver higher currents without increasing the discharge pressure and eliminating the breakdown inside the source. The ion beam divergence is decreased by the use of a solenoid coil in the ion beam path which confines the beam resulting in an increase of the ion current.
The beam profile is traced with the use of ion beam scanner. The ion beam divergence is deduced from the beam profile and the transverse beam emittance is calculated. The ion beam current is extracted from the RF ion source with extraction voltage 10 KV at pressure 6.5 x 10-6 torr and the collector is at a distance 20 cm from the base of a focusing magnet. This ion beam?is focused and injected into the accelerating column. The output ion beam current reaches 115?µA with energy 45 KeV.