Development of a Plasma Pinch Photocathode
Descrição do Produto
S
~DEVELOPMENT
L.
0
OF A PLASMA PINCH PHOTOCATHODE
A
FINAL REPORT
~1
SEPT. 1985
TO
31 DEC. 1987
cvou :{ .... -"
-
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m89 ': 4-'' a I .,,i]
OFFICE OF NAVAL RESEARCH
NORTH QUINCY STREET
ARLINGTON, VA 22217-5000 CONTRACT: N00014-85-K-0598
BY JOHN F. ASMUS INSTITUTE FOR PURE AND APPLIED PHYSICAL SCIENCES, Q-075 UNIVERSITY OF CALIFORNIA, SAN DIEGO LA JOLLA, CA 92037
COTRCT
N004-5---9
BYS
'
i" r
V
1.0 INTRODUCTION ......................................... 3 2.0 FINCH PHOTOCATHODE STATUS ............................ 5 3.0 PUBLICZTIONS ........................................1 6
rccesion For N4TIS DT:
C; A&I LI
Li
13
2
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.~1°'
1.0 INTRODUCTION For developing
this
current,
only mgderate
emodies
and life)
for a
focused
source in
a liquid-jet
approacrI
(r
metal
pho ocathode
The principal advantage
vacuum.
to
simplicity and ita ability
- 'I with
The present laboratory pinch photocathode is operating
The peak pulse power exceeds
decane jet and a copper cathode.
100
of
produce high-power vacuum ultraviolet radiation.
efficiently
a
in
high-density plasma pinch formed
both its
this pinch over a laser is
advanced
cathode that will not poison
durability of an unsensitiZed
the
on
high-performance
ood vacuums 0f such systems.
illuminated by a high-Z,
that is
has
project
high-current photocathode technology for
a
(emittance,
from
years
such as ETA and ATA.-The need / is
LINACS
the
two
past
the
MW
a
at
repetion
rate
of
10Hz.
Photoelectron
current
densities as high as 60 A/sq cm have been attained in microsecond this point we feel that it would be
appropriate
pulses.
At
advance
to the next order of technical issues pertaining to
incorporation
of
the
into
device
an
the and
LINAC
operating
to
determining cathode life in such an environment. convenient and cost-effective opportunity has emerged for
A mounting
a full-scale demonstration and evaluation of the
pinch
has
begun
photocathode. assembling
Maxwell
Laboratories
of
San
Diego
a clone of an ETA module which will be available
for
we look forward to adapting
the
experiments by May 1988.
Thus,
pinch photocathode system to the ETA configuration and evaluating its will
performance be
From this
we
on
an
These data will be available then
to
on the nearby system at Maxwell.
able to measure beam emittance and cathode life
operating induction LINAC.
3
the
ATA
effort
for
comparison
with
the
from
results
the
thermionic dispenser cathode program that is presently
underway.
If
poisoning
the
thermionic
route
encounters
cathode
difficulties, the pinch photocathode may then offer an attractive alternative irrespective
with minimal lead time of the poisoning isue,
to installation on ATA. there is reason to believe
that the pinch photocathode will produce a low-emittance electron beam at higher current densities than possible with a cathode.
Thus,
the
pinch
photocathode
may
prove
thermionic to
be
of
-onsiderable importance in the scaling of FELs to higher powers. The rep-rate
next
two sections describe the status of
the
present
photocathode and our proposal to install it on the
induction LINAC module, respectively.
4
ETA
CH "PI
.
the
During
_i
the laser-guided
ource
converted
in 4n
.:,
,
,cond
avoided.
,s
a
to
undert.al-n
in
-:ailable
approach
widiqd
li
liquid-je.
for s.ve--,-ral
through
surface tension,
a
beam
reason s
rep-rate
iS
of the
lqid-
jet
of
the
pinch
version
lessened.
A
Padiation output may'7 be
liquid's
vapor
pressure,
and composition.
laboratory
experimental
pinch
photocathode schematically
In the right foreground of Figure 1 are
Figure 2.
longer
the
pictured in Figure 1 and illustrated
is
no
is
....
st
lc
not needed.
selection
density,
present
apparatus
for
vstem is
-as-transport
The
of 1h ig-
For these
form.
-
la
Background gas absorption of hard UV is
illuminator.
opTimized
variety
sense
maks
guide
a .h.nel-forming
a
nz naliy,
large
Thi-
pinch
gas-embedded
the necessity of interposed high-density background gas
First,
in
past year
t rav io le .
vacuu-u
STATUS
HOTOCATiODE
coax
the
inductors emerging from the PFN, below. They attach to the center of the spark-gap switch.
electrode is at
The liquid-jet pinch chamber
in the center and the liquid enters from the electrical the
top
center.
The cryogenic apparatus that
condensation of the fluid is
at the left.
valve
effects
Optical and electrical tube.
diagnostic instruments surround the perimeter of the pinch The
liquid-jet
nozzle
is a stainless steel insert
pinch-discharge cathode (Mallory metal).
the
within
the
Its flow aperture has a
diameter of lOOum. Figure 2 illustrates the overal arrangement in left
a highly schematic and simplified form (but drawn reversed, to
right,
pinch
from the perspective of Figure
discharge cathode is
located at the
5
1).
In Figure
2
the
intersection of the UV
F,3UE 1
FIGURE 2.
Fhuogriaph c~
the liqii-je-t
pinch photoca~thode device.
Schematic layout of the pinch photocathode apparatus.
and
re.tor ta.e) ol
the
liquid jet.
represent's
the
liquid trap
is
pinch
ory
-n .D cC
sup
from Wh'ich the fluid ihe trap. "' .... J"-" £
The cirl
HV
anode
(where
discharge
instaled)
It
drawn from the system and
n~ot-,raph-v.-
of the liquidH
is
movin= , peno mena aarP discernable. h
from left First,
evaporating, is
significantly.
form.ng around the jet
can be seen that the jet
Cleary,
band.
It
larer
pseudocolor photograph (Figure 3) displays
is
it are
than the liquid jet
plume region. sequen-ce (deitin source
intensity the visible
evident that the effective source size is
visible emission is
e
Second,
Only at the end
contours for the ratiation emerging from the pinch in
T
.romote
any indications of turbulence or hydrodynamic instability. Mi,4dle
the
to
of the pinch.
does not break up.
the
that. a tenuous
as required
proper uniform peheating and initiation
t...
ican
This verifies
The
coIlected
right and
fluid is
there
icq.id
scow-,:
'et is
with distance.
clou.d
the
has a reentrant
the diameter of the s-tre--m decreases
vp
(High
on
S
t:,.....
labeled
On
itself.
Thus,
it
is
many times
clear that much of
coming from the plasma generated
in
the
the other hand the bottom photograph the UV emiss ion) reveals a very
of the compact
of dimension comparable to that of the dense jet
itself.
observations
lead us to conclude that the
1V
source
is
behaving essentially as desired. Just
as the above spatial observations
the dynamics of the liquid-jet pinch, data. the
The pinch
yield insights as to
so ,too,
do the
temporal
top trace of Figure 4 displays the visible output as seen by the response of an S-20 calibrated
7
of
planar
idol3M
F.E
3.
v is ible
Eseudocolor
radiation
ul1traviolet
photographs
image
of
the
of the
plasma
liquid
jet
(center),
(top),
t he
and
the
image (bottom).
rV
FIGURE 4.
Visible radiation (top,
8
lus/cm), and photocurrent.
a
with the expectation 5V
and
rather
that
hard UV
tan
when
is
the
is
its
at
in
developed earlier
remaining
The
responds to hard hottes.
is
plasma
size
greatest
For
code
the program. two
the former we assembled
investigating
1700-element
potential.
linear
a
1/4m
spectrograph
CCD multichannel
analyzer.
together
with
Spectral
data were taken for a variety of radiation sources
for
These included our original gas-embedded pinch,
the
comparison.
jet
conventional
the
to
xenon flashlamp.
a
emerged that
It
pulsed laser,
classical
blackbody continuum.
liquid-
the
Figure
spectrum 6
is
a
of the spectrum of the pinch with that of the surface
comparicon spark.
flashover sparkboard,
surface
produced the hardest radiation as well as a
pinch
closest
and
a
pinch,
liquid-jet and a
a
pinch
issues addressed with the present
are those of spectral content and rep-rate
apparatus
the
conforms
This
of the PFN.
produced when
the plama
between
ratio
the metal photocathode
that
As expected,
photocathode.
ringing
late-time
the
indicates
figure
this
pinch versus time as calculated by the radiation-hydro
the
in
of
poise with a much higher
narrower
pulse and the
initial
trace
from the copper
current
photoelectron tis
bottom
The
photodiode.
From such
balanced
effectively
evidence we conclude
inertial,
magnetic,
the carefully
that
and
hydrodynamic
tailored
forces
stabilize and contain the plasma so that it radiates
as an optimal blackbody.
Thus, it is compact and has a very high
surface brightness so that it may be optically imaged to
produce
high-current photoemission. The
most recent issue addressed in 9
the experimental work is
I',,-_.
-
0,C-", .0 0 0 0"
'-.__2-
3CCOKDCC 1
I__
LC/ FIUR
5. Raito-
F
i r
C
C\-
coecluaino-h
output
Radiation-hydro
versus
oa
aito
50010C0150
TIME:
FIGURE 5.
(nanosecords)
code calculation
time for the liquid-jet
10
of the total radiation
pinch.
C
-a,,
P' - t-a o f t h.
1 iqIi
d - e t pinh
F~r~E 7.Schematic diagram of rep-rate
arnid S urf:-ac
pinch system.
r,
acap-a .i
otne
.e
... _t .
.
ity or
.
...
.h.
o
.. - - v,
I*
:e,.C 771
.
-Lz
s sel"
,,ie!,
!
.
.
e
-,
f il-afe 4,
.
rp
.
-
-
ran
.....
u-P_,l y
de-'grades
te
firs-
involving
the
re....
the
,by
vacuum
pump
wat er.
These
r
in
30k
pump.
a
for
terr;s.
r
prope-r
ectr :o
-
ch _to )d c._ho
Figure 8
quickly,
operated at a 1 H-
was
shows the optical
the
fast calorimeter. energy. However,
The the
We determined
the first
(The cryogenic
rep
of
thereafter. after
output
shots.
that
the
Evidently,
and the later are
debris of the preceding shots that has cold
trap
not
been
was not employed
in
experiment. )
.
indicated
~u i re
discharges are pinches
decane-jet
~'"
n,- C'i
_ock-on)
i
vacuum also degrades
...
this
ignitron
th
ocatnode
Fi gre ure 7
inch as recorded by a Gentec or tfhree pulses are of nominal
two:
p.:
o we r
in
p
"
configi r,_a.ion
_-,ih
......
p
T- ;
ore r--.,on .In
rep-rate
em--
thie pincn
_.
.s
m
acnema-,ica!y
~:Kc:yi'as
-'~
to
concep-
een and
larged and straigh7.tened the line to the
operated the cold
measures
improved the
by Figures 9 and 10.
'rap
with
discharge
chilled
flowing
performance
Figure 9 displays the
as
waveforms
for the first pulse of a ten-pulse train. The top two traces with a
1 ms sweep duration,
monitor the pulse charging.
The upper of
these shows the PFN voltage dropping below the baseline to a full charge of -30
kV.
The long time constant of the HV probe suggests
12
FIGURE 8.
Rep-rate optical calorimeter output of the pinch.
13
FIGURE 9.
FIGURE 10. Rep-pulse behavior
Pe-rformance of the
of the pinch photocathode
pulse-charging system.
14
that
this
fact
170 us.
is
taking 1 ms,
whereas
the charging duration
The 170 us charging current pulse is
speeds.
in
shown in
lower trace of both the upper and lower trace pairs at sweep
is
the
different
The upper trace of the lower pair labeled "photo-
current" is the output of the metal cathode photodiode. It occurs
~te e~. displays
of the charge cc le when the pinch is fired. Figure 10 the improved rep rate performance (to be compared
with
Figure 8) resulting from the higher vacuum pumping speed. In this figure
the
charge
voltage and photocurrent for three pulses during a single
upper pair of traces display
the
repetitive
pulse
three-second oscilloscope sweep. A three-sweep overlay displaying the
10 pulses of a 10-second run appears at the bottom
of
this
figure. The system now runs reliably and reproducibly at 1 Hz. It has now been operated at 10 Hz,
but with some erratic triggering
that is being resolved. A
computer
modeling
effort has been
underway
beginning of the pinch photocathode effort. incorporated and
energy
since
Initially,
energy and momentum balance,
Spitzer
loss through blackbody radiation.
the
the code
resistivity,
In the past
year
multiple ionization and radiation transport have been included in an approximate way. This capability will be utilized to assist in the
design
section. in
duration of 0.1 us. this
the
next
To date all of the experimental work has been performed
the 1-3 us range of pulse duration.
pulse with
of the ETA test photocathode described in
code
Consequently,
will be helpful in
However,
the ETA has
the computer
making
the
15
modeling
transition.
typical 1 us code result was shown earlier (Figure 5).
a
A
3.0 PUBLICATIONS 1.
gisrus,
J.F.
Proceedings of
and R.H.
Lovberg,
SFIE, 873 pp. 245-7-43
16
"Dense-Pinch Photocathode", (13-15 January
1E)
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