Results on intense beam focusing and neutralization from the neutralized beam experiment Page: 4 of 7
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inserted into the 7.6 cm beam tube. The thickness and
longitudinal length of the mesh were 2.2 mm, and 58.2
cm, respectively. Outer and inner diameters of the mesh
tube were 6.3 cm and 5.88 cm, respectively, thus
maintaining better than a 5 mm radial electrical isolation
from the beam tube wall. Figure 6 shows a pattern of
beam profiles corresponding to energies for vacuum
transport in (a) WARP calculation, the (b) 15 cm diameter
tube and (c) 7.6 cm diameter tube using the mesh bias of
positive 1 keV. Using the mesh bias, the measured beam
profile was in general agreement with WARP for vacuum
transport. Figure 7 shows the measured beam profile for
varying mesh bias. In Fig. 7(a), the lower line with solid
circles shows that a beam diameter of 2.4 cm was
measured with OV across the mesh bias for 255-keV beam
energy. A beam diameter of roughly 3.75 cm was also
measured by applying + 500 V across the mesh for the
same beam energy, shown by lines of solid diamonds and
cross symbols, respectively. The line with hollow circles
shows a measured beam diameter of 3.75 cm using a
mesh bias of +250V. A larger beam diameter of 4 cm was
measured with a mesh bias of + 1 keV for the same 255-
keV beam energy, as shown by lines of hollow diamond
and solid triangle symbols in the figure. The positive 250
V bias on the mesh provides a smooth trend of beam
shape, regardless of beam energies in the range of 245 to
Fig.6. Beam profile for vacuum transport for 240-310 keV
beam energies from (a) WARP calculations (b)
experimental measurements for transport through a 15 cm
diameter tube, and (c) experimental measurements for
transport through a 7.6 cm diameter tube using mesh bias
of +1 keV.
Beam diameter measurement by varying beam
energies was performed in a 15 cm diameter vacuum tube
separately, where the possibility of a wall-electron effect
was negligible. There was no mesh or plasma inside the
tube that could influence measurements of ion beam
transport in vacuum conditions. Figure 7(c) shows a
comparison of beam diameters for transportation through
the mesh embedded 7.6 cm diameter tube with a bias of
+250 V and 15-cm diameter vacuum tube. The dotted
lines with hollow circles and triangles represent beam
diameters that were measured in the x and y axis,
respectively, for a beam of energies 240 to 310 keV
transported through the 15-cm diameter tube. Diameters
of 4.53, 4.0, and 2.68 cm were measured in the x-axis for
the beam of energies 259, 268 and 298keV, respectively.
On the other hand, the lines with solid circles and
triangles represent beam diameters that were measured in
the x and y axis, respectively, for a beam of energies 244
to 290 keV transported through the 7.6 cm diameter tube.
Beam diameters of 3.76, 3.15, and 2.41 cm were
measured in the x-axis for the beam of 255, 268 and
287keV, respectively. These were end-to-end
measurements of a beam image, without the deduction of
any cut off value that was used for statistical error
reduction in Section-V. For a 255-keV beam, a difference
of 6 mm in beam diameter was measured between the two
cases. This difference was smaller for a more energetic
beam. For example, for a 288-keV beam, a difference of 2
mm in diameter was measured for the two cases. For a
higher energy beam (say 300 keV), the radial distance of
the beam from the wall was larger than the lower energy
beam (255 keV) and neutralization was not significant. By
using the mesh and an appropriate voltage across it, we
were still achieving a slightly smaller size than "expected"
for an un-neutralized beam. The difference in the two
cases, as we inferred, was due to the 58.2cm mesh liner in
the 7.6cm diameter tube was not long enough to cover the
entire 1-m long drift tube. As a result, partial
neutralization occurred beyond the ends of the mesh.
However, the mesh was a significant development in
overcoming uncontrolled neutralization of wall electrons.
240 250 260 270 280 290 300 31
Beam Energy (keV})
240 250 260 270 280 290 300 31
Beam Energy (keV)
Beam Energy (keV)
FIG.7. Beam diameters corresponding to beam energies
were measured in the (a) x-axis and (b) y-axis by varying
mesh bias, and (c) a comparison of beam size for a 255-
keV beam transported through a 15-cm tube (dotted lines
for the x-y axis) and mesh included 7.6 cm diameter tube
(solid lines for the x-y axis) with bias 250V.
Currents corresponding to positive and negative
voltages across the mesh were measured during 255 keV
beam pulse. Figure 8 shows experimental data of currents
measurements in the mesh. A negative current of 6.56mA
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Roy, P.K.; Yu, S.S.; Eylon, S.; Henestroza, E.; Anders, A.; Bieniosek, F.M. et al. Results on intense beam focusing and neutralization from the neutralized beam experiment, article, October 31, 2003; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc736749/m1/4/: accessed June 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.