Improvement of Beam Characteristics by Superimposing a Magnetic Field on a Field Emission Gun
Identifieur interne : 001C80 ( Istex/Corpus ); précédent : 001C79; suivant : 001C81Improvement of Beam Characteristics by Superimposing a Magnetic Field on a Field Emission Gun
Auteurs : Akio Takaoka ; Katsumi Ura ; Takeshi Tomita ; Yoshiyasu HaradaSource :
- Microscopy [ 0022-0744 ] ; 1989.
Abstract
We show analytically and experimentally how beam characteristics of a field emission gun (FEG) are improved by the superposition of a magnetic field. Especially, under the condition of the fixed position of the gun crossover, the relation between the radius of probe beam and the probe current on the specimen plane of a transmission microscope is investigated and the cases with and without the magnetic field are compared. The tested FEG is a tetrode FEG which operates in pulsed mode at 100 kV and the magnet is attached from the outside of the gun chamber. Owing to the structural restriction of the magnet size, the distribution of magnetic field becomes considerably broad, that is, the value of the full width at half-maximum is about 110 mm. When the peak value of the applied field is 17 mT, the measured current density at a high current region is improved by about twice in comparison with the case without magnetic field and this improvement factor roughly agrees with the calculated one. When the field intensity is increased up to 23 mT, the current density becomes further doubled analytically. The ray tracing shows that the reduction of the aberration results mainly from the confinement of trajectories to the axial region by the superimposed field. If the magnetic field with a peak field of 61 mT is concentrated in the cathode region, it is expected that the current density is improved by a factor of about 30 under the condition of the fixed current.
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DOI: 10.1093/oxfordjournals.jmicro.a050730
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<front><div type="abstract">We show analytically and experimentally how beam characteristics of a field emission gun (FEG) are improved by the superposition of a magnetic field. Especially, under the condition of the fixed position of the gun crossover, the relation between the radius of probe beam and the probe current on the specimen plane of a transmission microscope is investigated and the cases with and without the magnetic field are compared. The tested FEG is a tetrode FEG which operates in pulsed mode at 100 kV and the magnet is attached from the outside of the gun chamber. Owing to the structural restriction of the magnet size, the distribution of magnetic field becomes considerably broad, that is, the value of the full width at half-maximum is about 110 mm. When the peak value of the applied field is 17 mT, the measured current density at a high current region is improved by about twice in comparison with the case without magnetic field and this improvement factor roughly agrees with the calculated one. When the field intensity is increased up to 23 mT, the current density becomes further doubled analytically. The ray tracing shows that the reduction of the aberration results mainly from the confinement of trajectories to the axial region by the superimposed field. If the magnetic field with a peak field of 61 mT is concentrated in the cathode region, it is expected that the current density is improved by a factor of about 30 under the condition of the fixed current.</div>
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<abstract><p>We show analytically and experimentally how beam characteristics of a field emission gun (FEG) are improved by the superposition of a magnetic field. Especially, under the condition of the fixed position of the gun crossover, the relation between the radius of probe beam and the probe current on the specimen plane of a transmission microscope is investigated and the cases with and without the magnetic field are compared. The tested FEG is a tetrode FEG which operates in pulsed mode at 100 kV and the magnet is attached from the outside of the gun chamber. Owing to the structural restriction of the magnet size, the distribution of magnetic field becomes considerably broad, that is, the value of the full width at half-maximum is about 110 mm. When the peak value of the applied field is 17 mT, the measured current density at a high current region is improved by about twice in comparison with the case without magnetic field and this improvement factor roughly agrees with the calculated one. When the field intensity is increased up to 23 mT, the current density becomes further doubled analytically. The ray tracing shows that the reduction of the aberration results mainly from the confinement of trajectories to the axial region by the superimposed field. If the magnetic field with a peak field of 61 mT is concentrated in the cathode region, it is expected that the current density is improved by a factor of about 30 under the condition of the fixed current.</p>
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<abstract>We show analytically and experimentally how beam characteristics of a field emission gun (FEG) are improved by the superposition of a magnetic field. Especially, under the condition of the fixed position of the gun crossover, the relation between the radius of probe beam and the probe current on the specimen plane of a transmission microscope is investigated and the cases with and without the magnetic field are compared. The tested FEG is a tetrode FEG which operates in pulsed mode at 100 kV and the magnet is attached from the outside of the gun chamber. Owing to the structural restriction of the magnet size, the distribution of magnetic field becomes considerably broad, that is, the value of the full width at half-maximum is about 110 mm. When the peak value of the applied field is 17 mT, the measured current density at a high current region is improved by about twice in comparison with the case without magnetic field and this improvement factor roughly agrees with the calculated one. When the field intensity is increased up to 23 mT, the current density becomes further doubled analytically. The ray tracing shows that the reduction of the aberration results mainly from the confinement of trajectories to the axial region by the superimposed field. If the magnetic field with a peak field of 61 mT is concentrated in the cathode region, it is expected that the current density is improved by a factor of about 30 under the condition of the fixed current.</abstract>
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