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NEREUS, A 250 kV,
80 kA ELECTRON BEAM GENERATOR*
K. R. Prestwich
Sandia Laboratories
Albuquerque,
New Mexico
Summary
A 250 - 400 kV, 80 kA, 39 ns electron
beam
generator
has been developed.
The machine consists
of
a water-dielectric
transmisa 600 kV Marx generator,
It was
sion line,
a diode,
and a beam drift
chamber.
necessary
to minimize
prepulse
for stable
diode operaSeveral
cathodes
were tested
during
development
tion.
Diodes with 26 and 8 nH
and results
are reoorted.
inductance
are described.
Introduction
Nereus is a compact,
250 kV, 80 kA, 30 ns electron
beam generator
developed
at Sandia Laboratories.
Four
Nereus machines
have beenfabricated
and are in use in
the followinq
experimental
areas:
rapid
energy deposition in materials,
electron
beam physics
and diagnostics,
diode,
and plasma-electron
beam interaction.
Nereus
The 1.5 kJ, 600 kV, oil-insulated
Marx generator
has twelve
50 kV, 0.10
PF stages arranged
in an n=2,
plus-minus
charged
confiquration.'
Its output
capacitance is 8.3 nF and the system inductance
is 6.0 PH.
The Marx generator
charges
the 1.9 n, 35 ns
transmission
line
to 500 kV in 0.45 us.
The line
electrodes
are stainless
steel
pipes with 8.63 and
The center
cylinder
is canti11.5 inch diameters.
levered
from a lucite
sheet that forms the oil-water
To minimize
resistive
losses
during
charqinterface.
ing, the resistivity
of water dielectric
must be
At 500 kV the peak electric
greater
than 1 M&cm.'
field
in the transmission
line
is 160 kV/cm.
The diode,
switch,
and prepulse
resistor
are shown
in Fig. 3. The switches
are either
l/B inch or 3/16
inch sheets
of low-density
polyethylene
that are
stabbed with an array
of 45 sewing machine needles.
The stab depth controls
the switch
breakdown
voltage.
Seventy
shots with l/8 inch sheet stabbed
0.020 inches
deep had a mean breakdown
voltaqe
of 434 kV with a
standard
deviation
of 29 kV.
The inductance
of the switch
and its electrodes
is
of this diode was measured to
11 nH. The inductance
be 26 nH. Thus for a 4 o diode imoedance.
the 10 - 90
current
risetime
is 19 ns.
Prepulse
While the transmission
250 pF switch
capacitance
supported
Cathodes
-Six of the cathodes
tested
Fiq. 4 and some of the operational
tabulated
in Table I.
Description
A complete
Nereus facility
consistinq
of the
generator,
its control
panel,
water and oil
storage,
vacuum system,
and a switch
stabber
are shown in
Fig. 1. The five ma,jor components
of Nereus are:
a
a water-dielectric,
coaxial
transmisMarx generator;
sion line;
a polyethylene
switch;
a diode;
and an
These are shown in
electron
beam drift
chamber.
Fig. 2.
*This work
Comission.
Thus 13% of
capacitance
of the diode and its base.
the transmission
line
voltaqe
(prepulse
voltaoe)
is
across
the 0.25 cm anode-cathode
qap and the mean
electric
field
in the gap is 260 kV/cm for 500 kV
In this case electron
emission
will
occur from
charge.
about l/16 of the cathode
area and the diode will
short
For reliable
diode operaout durinn
the main pulse.
tion,
it was necessary
to reduce the prepulse
to one
percent
of the Marx voltaqe.
This reduction
was accomplished
by connecting
a 30 n sodium-chloride
solution
resistor
in parallel
with the diode and insertino
a
5/16 inch lone section
of lucite
in the cathode
shank
to form a prepulse
switch
as shown in Fis.
3.
is
by the
line
is beino charged,
the
in series
with the 1600 pF
U.S.
Atomic
Energy
Table
Nereus
in llereus
are
parameters
shown
are
in
I
Cathode
Operational
d
V
I
2
J*
--(cm)
(kV)
(kA)
(n)
--(kll;
1.27
1.27
.152
.190
200
320
65
74
3.1
4.4
12
14
Flame-SprayedTunqsten
1.27
1.27
.190
.254
340
250
65
130
5.3
1.9
16
4
Tunosten
1.27
1.27
.254
.254
260
330
3.3
3.3
4
A
Two-Ring
1.27
1.27
.254
.254
260
330
66
67
4.0
4.9
4
4
Four-Rina
2.54
2.54
.254
.254
210
319
120
87
1.8
3.E
t
2.54
2.54
.762
.635
420
400
49
57
8.5
7.0
E
F
Cathode
Plain
Plasma
Brass
Wires
Parameters
v
For a given anode-cathode
spacing,
and at a given
voltaqe.
the current
from the plain metal and the flamesprayed-tungsten
cathodes
was aoproximately
20% less
than the current
from the tungsten
wire and the two-rino
cathodes.
Pinhole
x-ray
pictures
of the anode indicated
that the plain
metal cathode
emitted
electrons
from less
than l/16 of its area.
The flame-sprayed-tunqsten
cathode,
similar
to the one used on the Coqen machine,Zy3
emitted
uniformly
unless the spacing
was less than
0.15 cm; in this case it emitted
only from the edge.
The tunqsten
wire cathode
emitted
from a small number of
wires.
A higher
wire density
would qive better
results.
The epoxy ring cathodes
emit from the epoxy areas and,
for the two-ring
unit with a 0.25 cm anode-cathode
spacin the diode.
ing, a pinch always occurred
Fiqure
5 shows voltage,
current,
and impedance waveforms for one shot with the two-rinq
cathode
and an
anode-cathode
spacing
of 0.25 cm. Significant
current
493
starts
flowing
when the anode-cathode
voltage
is 70 kV.
The diode-voltage
monitor
is located
at the vacuumwater
interface
and measures the voltage
across
the
diode inductance
plus the anode-cathode
gap voltage.
At times shortly
after
the current
starts,
the inducThe diode impedance decreases
tive
voltage
is 120 kV.
from an open circuit
to 5 o in 20 ns, remains
constant
at 5 R for 20 ns, and then decreases
to about 1.0 R in
the next 10 ns.
At higher
impedances,
such as that obtained
with
the plasma cathode
and anode-cathode
spacing
greater
than 0.E cm, pinching
does not occur.
Analysis
of the
impedance at peak current
of this cathode
for 32 shots
with an anode-cathode
spacing
of 0.64 cm gave the following
relationship
between impedance and anodecathode
voltage:
z I
5.5 V-O.24
where Z is
voltage
in
in ohms and V in megavolts.
The range
this data was from 0.31 to 0.43 MV.
of
Diodes
As can be seen in Fig. 5, the peak voltage
across
the vacuum-water
interface
can be 20% higher
than the
peak anode-cathode
voltage.
In the diode shown in
Fig. 3, this
interface
is four lucite
insulators
with
interspersed
metallic
grading
rings.
The total
insulator
length
is 2 inches.
This tube has operated
with
electric
fields
greater
than 200 kV/in.
Figure
6 is a
sketch of an 8 nH diode designed
and fabricated
for
Nereus with a single
radial
vacuum-interface
insulator.
The electrodes
are shaped so that a uniform
voltage
gradient
exists
across
this
insulator
and the equipotential
lines
form a 45O anqle with the insulator,
as is desired
for maximum flashover
voltage.
The
lower inductance
decreases
the inductive
voltage
overshoot,
gives a faster
current
risetime,
and thus more
output
energy'per
pulse.
References:
1.
Unit,
2.
Inc.,
R. S. Clark,
Nereus Marx Generator,
Sandia report
to be published.
J. Kraemer
Brookline,
and W. Crewson,
Mass.,
April
3. J. Clark and S. Linke,
and Entrance
Conditions
of
Electron
Beam Accelerator,
LPS 23, August 1969.
Cogen Final
1968.
/
WplTEl ITORlCl
11761
\
oEto*mER
\
“AC”“I(
EG&G,
I. Smith, "Pulse Breakdown of an Insulator
4.
in a Poor
Vacuum," Proc. of the International
Symposium on
Insulation
of High Voltage
in a Vacuum, Boston,
Oct. 1964.
SYSTLY
NEREUS
Figure
Report,
kJ
Operational
Characteristics
a High Current
Relativistic
Cornell
University
Report,
NEREUS ELECTRON
BEAM FACILITY
OR” ISR
a 600 kV, 1.5
1.
Figure
494
2.
IOO7Y150sg200-
NEREUSDIODE
Figure
3.
$i 250300TIME
BRASS
ss
PLAIN
1
,I
EPOXY
METAL
0
FLAME
(nsec)
anode-cathode
voltage,
Figure 5. Diode
and impedance versus time
tage, current
nereus two epoxy ring cathode.
-SPRAYED
TUNGSTEN
TUNGSTEN
WIRES
FOUR
RING
DARK
AREA
XY
RASS
TWO
WA;ER
PLASMA
RING
Figure
EPOXY
4.
Nereus
Figure
cathodes.
495
6.
Nereus
low-inductance
diode.
volfor