High current, low emittance, steady state electron guns with plasma cathodes Page: 4 of 8
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I (within about 7%). I, measurements by DCCT and beam dump agreed within about 2%
for currents of up to 2.5 A, and within 5% for currents of 2.5-3 A.
For the conditions of Figure 2, the arc voltage (not shown) displayed V-I
characteristics that are typical for an HCD,9-10 where the arc voltage has a very weak
dependence on the arc current. However, as it was noted in Reference 8, the arc voltage
has a very strong dependence on the source pressure where it almost tripled as the pressure
was decreased by a factor of three. An interesting feature of this figure is the almost linear
dependence of the power supply load, which corresponds to the total current exiting the
source enclosure, on the arc current (up to power supply saturation at 25 A). The weaker
raise in I, with Ia is probably due to space charge effects at low extraction voltage.
Affects of variations in the repelling grid bias on the extracted currents (I, on beam
dump and total extraction from source enclosure IW and on the arc voltage VI are displayed
in Figure 3. The Figure 3 data was taken at a source enclosure pressure of 8 x 10- Torr and
an arc current of 35 A. As it can be seen from this figure, the arc voltage is almost
independent of the repelling gird bias, while the extracted current does depend on the
repelling grid voltage exhibiting dramatic changes at a bias close to that of the arc voltage.
Resistivity of a highly ionized plasma like an HCD has a very strong temperature
dependence. Based on the Figure 3 data, all indications are that the plasma resistivity, and
hence the plasma temperature, does not change as the repelling voltage (and hence, the
extracted current)) are varied, since the arc voltage almost does not change (the arc current
was kept constant at 35 A). In Figure 3, the arc voltage seems to decrease slightly as the
repelling grid voltage (absolute) decreased below about 10 Volts. At this repelling grid bias,
a significant portion of the bulk electrons can be extracted since their energy exceeds the
grid bias. However, their extraction causes changes in the plasma potential,' which in turn
affects the arc voltage.
At a source enclosure pressure of 8 x 10- Torr, the neutral particle density is about
3 x 10"cm. The plasma density inside the anode is not significantly different. Therefore,
the mean free path of the fast electrons is a few meters, i.e., much larger than the
experimental apparatus. In an HCD with a solid anode, these superthermal electrons would
travel the length of the discharge and be lost to the anode. Therefore, extraction of
superthermal electrons through the hollow anode in the Figure 1 electron gun should not
affect any plasma parameters.
Maximum contribution of the bulk electrons to the total extracted current is given
by
I = - envA (1)
4
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Hershcovitch, A. High current, low emittance, steady state electron guns with plasma cathodes, article, December 31, 1995; Upton, New York. (https://digital.library.unt.edu/ark:/67531/metadc664442/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.