Develpoment of a one-meter plasma source for heavy ion beam chargeneutralization Page: 3 of 5
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polarization reversal. However, plasma
emission is observed and is simply
explained by electron emission from the
vacuum micro-gaps between the
dielectric surface and the edge of the
metal electrode surface. For this
configuration, the value of the dielectric
constant is the key factor. Commonly
used ferroelectric materials have
extremely large dielectric constants:
BaTiO3 has a dielectric constant in the
range of 1000-3000 and Pb(Zr,Ti)O3 has
a constant in the range of 3000-6000.
Once the threshold voltage is reached
plasma is formed over the entire surface
of the dielectric. Typical current density
yields are 0.5 A/cm2. Plasma emissions
from these dielectrics have been
characterized for BaTiO3. There is a
sharp fall off in electron density from the
dielectric surface as a function of
distance. The velocity of the plasma
moving away from the dielectric surface
is ~ 1 cm/ s. In this study 8 - 16 kV,
0.25 s pulses were applied to the
electrodes. These measurements were
completed at a pressure near 10-5 Torr.
The fact that the plasma is essentially all
metal means that neutrals sticks to the
walls of the vacuum system and does not
result in a pressure rise.
The features of this plasma source are
exactly what are required for the charge
neutralization on NTX. Furthermore, its
ability to make the plasma emitting layer
arbitrarily long is important for the 1-
meter long plasma source. The source
will be mounted on the walls of NTX
drift tube just past the last focusing
quadrupole magnet. The drift tube is
approximately 3 in. in diameter. This
small tube diameter will allow the
density to be ~ 1012 cm-3 on axis. The
approach taken is to build a source with
cylindrical ferroelectric pieces stacked
together to form a 1-m long ferroelectric
cylinder. Initially, a 20 cm long
ferroelectric source made of 1" long and
1%" thick ferroelectric cylinders was built
for evaluation (Fig. 2). The front surface
electrode was made of 36, 10 mil.
stainless steel wires strung along the
length of the cylinder. The wires are
mounted at each end of the source with
an aluminium ring with 36 set screws.
Each ring is mounted in a Delrin
insulating sleeve to isolate it from the
outer surface of the ferroelectric
cylinders. The wires are pulled tight and
actually hold the ferroelectric cylinders
firmly together. Figure 2 clearly shows
the wires mounted on one of the
aluminium rings and the black Delrin
insulating sleeve behind the rings. Not
shown in the photograph is the copper
wire that is wound around the outer
surface of the ferroelectric cylinder to
provide a good electrical contact.
Electrically, the high voltage pulse is
applied to one of the aluminium
mounting rings and the copper wire
wound around the outer surface of the
cylinder. All of the stainless steel wires
are at the same potential through the
The power supply for this pulsed source
is a standard capacitor bank with a pulse
forming network to match the
impedance of the source and maintain
the microsecond pulse shape. A
schematic of the supply is shown in Fig.
3. As presently configured the pulse
forming network is matched to 4 ohms
and has a maximum output of 8 kV and
2 kA. Thyratrons control the discharge
of the charging capacitors. The output
of the power supply is two 1-
microsecond pulses with an adjustable
time delay between the pulses.
[Location of Figure 3.]
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Efthimion, Philip C.; Gilson, Erik P.; Grisham, Larry; Davidson, RonaldC.; Yu, Simon; Waldron, William et al. Develpoment of a one-meter plasma source for heavy ion beam chargeneutralization, article, January 18, 2005; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc779199/m1/3/: accessed June 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.