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Origin of beam non-uniformity in a large Cs-seeded negative ion source
M. Hanada, T. Seki, N. Takado1, T. Inoue, T. Morishita, T. Mizuno1, A. Hatayama1, T. Imai,
M. Kashiwagi, K. Sakamoto, M. Taniguchi, K. Watanabe
Japan Atomic Energy Research Institute, Naka, Ibaraki-ken,311-0193, Japan
1Faculty of Science and Technology, Keio University, Hiyoshi, Yokohama 223-8522, Japan
Abstract: Origin of the longitudinal beam non-uniformity, that is one of the key issues in large
Cs-seeded negative ion sources for fusion application, was experimentally investigated in the
JAERI 10 A negative ion source. After a sufficient caesium of ~0.3 g was seeded in the
negative ion source to enhance the negative ion production, the longitudinal distribution of
the beam intensity was measured. The distribution of the beam intensity was non-uniform,
and was absolutely different from that without Cs. From the correlation between the beam
intensity and the plasma parameters, it was found that the beam non-uniformity was due to
the localization of the plasma caused by B x ∇B drift of the fast electron from filaments.
Here, B is the magnetic field near the filaments whose strength is the sum of the magnetic
filter field and the confinement magnetic filed around the negative ion source.
To improve the beam non-uniformity, the B x ∇B drift of the fast electrons from
filaments was suppressed by modifying the filament configuration. The beam non-uniformity
was dramatically improved. The root-mean-square deviation of the beam intensity from the
averaged value deceased to a half of that before the modification while the beam intensity
integrated along the longitudinal direction was kept to be constant. Thus it was
experimentally confirmed that the beam non-uniformity was improved by suppressing the B x
∇B drift of the fast electrons from the filaments.
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