Double pulse experiment with a velvet cathode on the ATA injector Page: 4 of 6
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DOUBLE PULSE EXPERIMENT WITH A VELVET CATHODE ON
THE ATA INJECTOR*
G. Westenskow, G. Caporaso, Y. Chen, T. Houck, and S. Sampayan,
Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA 94551 USADouble pulse transport experiments were conducted on
the front end of the ATA accelerator to obtain data on the
capability of a velvet cloth cathode to produce two
successive pulses. Pulses of approximately 3 kA were
extracted from the cathode with interpulse spacings varying
from 150 ns to 2.8 ps using an anode-cathode voltage of
about 1 MV. Analysis of the current and voltage waveform
data from the injector indicate that the effects of cathode
plasma on the second pulse of a two-pulse burst is minimal.
I. BACKGROUND
The ATA injector has typically used velvet
cathodes [1] to obtain its 10 kA operation at 2.5 MV.
Although the precise mechanism by which these cathodes
operate is uncertain, it is believed that there is field
emission from the tips of velvet fibers that extend into the
gap. The emitted current is believed to lead to the creation
of a plasma layer at the cathode surface. Velvet cathodes
have been used occasionally at ATA for short bursts at
repetition rates up to 1 kHz. Velvet sources were employed
for a short time on another high repetition rate. injector.
Above a 50 Hz rate the emission current would die out, but
recover as the repetition rate was lowered. This suggested
that the source of the plasma from which the electrons were
extracted might be adsorbed gasses on the fiber's surface
which were totally depleted above a certain repetition rate.
The rate of deposition of gas onto a .clean surface is
proportional to.the pressure, and there is an equilibrium
thickness of adsorbed gasses on a surface for a given
ambient pressure.
These inferences suggest that there might be a suitably
large amount of adsorbed gasses on the velvet surface to
permit multiple-pulse operation.
II. INJECTOR CONFIGURATION
A schematic of the injector used in the Double Pulse
Experiments is presented in Fig. 1. The ATA injector is
composed of two sections, each with five 250-kV induction
cells in series, that in normal operation provides a nominal
2.5-MV, 70-ns FWHM pulse across the anode-cathode
* The work was performed under the auspices of the U.S.
Department of Energy by Lawrence Livermore National
Laboratory under contract W7405-ENG-48.(A-K) gap. During the experiments a 5.25-inch diameter
velvet cathode was used as the electron source. The
separation between the anode and the cathode was about
14 cm during most of the experiment. At an anode-cathode
voltage, Vak, of about 2.5 MV (normal operation)
approximately 8 kA of current is drawn from the cathode.
At V ak = 1.0 MV, as used in the double pulse experiments,
approximately 2.5 kA of current is drawn from the cathode.
The average emission current density to provide 2.5 kA of
current from.a 5.25 inch cathode is 18 A/cm2
velvet
cathode anode
bucking -13 cm
coil - - r -
--07 1. 14cm (normal)
Solenoid
Figure-1. Electrode configuration used for double pulse
experiment.
.III. DOUBLE PULSE OPERATION
For these experiments the triggering arrangement of
the ATA injector was modified so that the even and odd
cells of the injector could be fired independently. Extra
lengths of cable were added to the high voltage trigger of
the odd cells so their firing could be delayed in fixed
increments. Since for each pulse only half the cells were
involved, the anode-cathode voltage was about half of its
normal value. The electrode package was not modified for
the reduce voltage operation. We expect that the beam
brightness would decrease for the lower voltage operation.
When the A-K gap was shortened by about 3 cm in the
latter part of the experiment, there was a further decrease in
the beam brightness.
Figure 2 shows data for a 2.2 s time separation
between the first and second shot. It also shows a source of
error involved with trying to determine changes between
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Westenskow, G.; Caporaso, G.; Chen, Y.; Houck, T. & Sampayan, S. Double pulse experiment with a velvet cathode on the ATA injector, report, April 24, 1995; California. (https://digital.library.unt.edu/ark:/67531/metadc620153/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.