Li+ alumino-silicate ion source development for the Neutralized Drift Compression Experiment (NDCX) Page: 4 of 8
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I 'ja IN+1FIG. 5. (Color online) A typical beam extraction voltage
and current waveforms. A 0.64 cm diameter lithium alumino-
silicate source was operated at 1260 0C - 1275 0C.
where, qV is the ion kinetic energy with potential V
across the electrostatic deflector plates.
III. EXPERIMENTAL SETUP TO
CHARACTERIZE A FABRICATED ION SOURCE
Figure 3 shows the ion source test stand setup used for
0.64 cm diameter lithium alumino-silicate sources. The
length of the beam diagnostics column is < 0.5 m, accom-
panied with a 0.1 mm slit, E x B filter, scintillator, and a
Faraday cup. To meet the NDCX-II ion beam brightness
requirements, the lithium alumino-silicate source operat-
ing temperature was about 1250 0C-1300 0C. A tungsten
filament was used to heat the emitter to this temperature.
In order to minimize heat loss, the Pierce electrode was
made of stacked molybdenum plates but still maintained
a 60 opening angle normal to the source surface (Fig.
3a). The heater package was encased by heat shielding
materials (molybdenum foils and ceramics). The beam
diagnostics column consisted of a 2.9 cm long (4 cm di-
ameter aperture) Faraday cup to measure beam current;
an ExB filter with YAP scintillator to detect the beam;
and a pyrometer to measure the source surface temper-
ature. Temperature in this study was measured using
a disappearing filament-type brightness pyrometer with
null-balance, lamp-current measuring circuit, made by
Leeds and Northrup Co, (8632-C series) and calibrated
with emissivity 1.0. The pyrometer is sensitive to the
brightness at A 0.65 microns. The 'brightness temper-
ature' measured using the pyrometer is affected by the
emissivity of the alumino-silicate material, and we note
that the emissivity of the alumino-silicate at A=0.65 mi-
crons may not be 1.0. The Faraday cup is temporarily
removed from the beam axis, without breaking vacuum,
when doing the temperature measurement with the py-
rometer (Fig. 3b). A source surface temperature of 1250
C was measured when 110 to 120 watts rms electrical
heating power was applied.
When using the ExB filter, a 0.1 mm slit was installed
at the entrance to enable accurate species identification
at the detection plane (scintillator). The location of thefilter with respect to the ion source and the scintillator is
shown in Fig. 3(c). A higher extraction voltage, such as
20 kV, was used to transport the beam to the scintilla-
tor location. A gated, 2 ps gate width, image intensified
CCD camera was used to view light emission from the
scintillator. Figure 4 shows slit beam images on the scin-
tillator with different values of E and B fields using the
EXB filter. When a magnetic field of 168.5 G (0.83 A)
and electrostatic potential of 150 V (in total 300 V)
with the dipole of the filter were applied for a 20 kV
beam, the beam was returned to its original field free
position on the scintillator (E=0, B=0); this identified
the element emitted as Li7. To illustrate the sensitivity
to contamination from other alkalis, we note that Na+
would appear at the field-free position for 1.5 A (bot-
tom trace). At this magnetic field strength there was no
signature of the beam at the field free location.
IV. LITHIUM BEAM CURRENT DENSITY
MEASUREMENTS
Figure 5 shows a 5-6 ps beam extraction voltage and
beam current waveforms when the source surface tem-
perature was at 1270 7 0C. The beam current was
recorded by the Faraday cup (+300 V on the collector
plate and -300 V on the suppressor ring).
While keeping the source surface temperature con-
stant, the lithium ion beam current was measured by
varying the extraction voltage, up to 10 kV. Figure 6
shows measured beam current density vs. V3/2, mea-
sured at 1240 0C, 1260 0C, 1270 0C and 1275 0C. A space
charge limited current density of 1.1 mA/cm2 was mea-
sured with extraction voltage of 2.5 kV at 1275 0C tem-
perature, after allowing a conditioning (surface cleaning)
time of about 12 to 30 hours. At the same temperature,
the current density was raised to 1.47 mA/cm2 when the
extraction voltage was increased to 10 kV. Beam emission
stability with these current density levels was observed
for more than 72 hours for a pulse repetition rate of 0.033
Hz. However, the beam current density decreased grad-
ually after this period, leading to the data shown as dot-
ted lines in Fig. 6. Thus the maximum space-charge
limited beam current density was reduced to 0.6 - 0.7
mA/cm2 corresponding to an applied extraction voltage
of 1.88 kV (shown as the dotted line with hollow circles
in Fig. 6). The same data line (still at 1275 0C) shows an
emission-limited current density of 1.37 mA/cm2 with 10
kV extraction voltages due to electric field enhancement.
The solid line with hollow circles, at the left, colored in
red, represents the space-charge limited current density,
calculated using the Child-Langmuir law (Eq. 1) for the
gun geometry. Beam current density at lower extraction
voltage follows the space-charge limited (SCL) Child-
Langmuir law. At a higher extraction voltage, there are
not enough ions, at a given temperature, on the source
surface to extract, and thus extracted current fall below
the Child-Langmuir law. It is preferable to run an injec-
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Roy, Prabir K.; Greenway, Wayne G.; Kwan, Joe W.; Seidl, Peter A.; Waldron, William L. & Wu, James K. Li+ alumino-silicate ion source development for the Neutralized Drift Compression Experiment (NDCX), article, October 1, 2010; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc834531/m1/4/: accessed April 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.