Develpoment of a one-meter plasma source for heavy ion beam chargeneutralization Page: 2 of 5
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The cold plasma ion motion is neglected,
and electrons from the cylindrical
plasma move into the beam channel,
reducing the net positive beam charge
over the larger volume of the plasma
channel. For NTX, ion beam densities
are ~ l08 - 109 cm-3. Calculations
require the plasma to be in the range of
0.1- 2 meters in length with an electron
density comparable to 1-100 times the
beam density. The operating pressure
for the plasma needs to be ~ 10-6 - 10-5
Torr to prevent neutrals from stripping
the beam ions to higher charge states.
2. PREVIOUS RF PLASMA SOURCE
The heavy ion beams for fusion will
have pulse lengths in the sub-
microsecond range. To achieve the low
neutral gas pressure and plasma density
required for charge neutralization on
NTX, the volumetric plasma source was
designed to operate in a pulsed mode,
where a gas valve and RF power are
triggered simultaneously . The idea is
to create plasma in the source when the
transient gas pressure is ~ 0.1-0.5 mTorr.
With the plasma sound speed 1-2 orders
of magnitude larger than that of the
neutral gas, the plasma will arrive in the
ion beam drift tube before the neutral
gas. Since the ion beam pulse duration
is ~ 1 s, the conditions required for the
charge neutralization can be achieved
transiently for the duration of the beam
pulse. This can be seen from the source
characteristics for a net forward power
of -3.5 kW vs. time in Fig. 1. Before t =
3.75 ms, the plasma density is less than
the sensitivity of the Langmuir probe (-
107 cm-3), and the neutral pressure is
below the sensitivity of the dynamic
pressure measurement (10-6 Torr). At
t=3.75 ms the electron density is 1011
cm3 and simultaneous the neutral
pressure is low. The ionization fraction
for t = 3.75 - 4 ms is in the range of 50-
100%. At later times the power density
is not sufficient to sustain the ionization
fraction, and the neutral density rises
faster than the electron density. During
the experiments at LBNL, the pulsed
plasma source was effective in charge
neutralizing the ion beam. The plasma
was approximately 10 cm in length
along the NTX drift tube. However, the
neutral pressure in NTX would rise due
to the plasma source's gas puff and the
vacuum system took a few minutes to
pump down again. This experience
forced the consideration of a different
plasma source type for scaling the
plasma into a 1-meter column with a
reduced neutral pressure rise.
[Location of Figure 1]
3. FERROELECTRIC PLASMA
Ferroelectric materials have been
intensively examined as high current
density electron emitters [5-7]. They
have been projected to serve as large
surface area, high current density
cathodes. An electrode mesh-like
structure is mounted to the emitting side
of the ferroelectric material and the back
surface has a metal plate electrode. A 3-
15 kV potential is applied to the
electrodes depending upon the thickness
of the ferroelectric material. For ultra
thin film ferroelectric materials, the
applied voltage results in spontaneous
polarization reversal on a nanosecond
timescale, and a high electric field.
Spontaneous polarization reversal yields
a noncompensated charge at the surface
and a high electron emission across the
entire thin film.
For millimeter thick ferroelectrics, the
electric fields are too small to produce
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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/m1/2/: accessed September 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.