Development of a Plasma Pinch Photocathode

Share Embed


Descrição do Produto

S

~DEVELOPMENT

L.

0

OF A PLASMA PINCH PHOTOCATHODE

A

FINAL REPORT

~1

SEPT. 1985

TO

31 DEC. 1987

cvou :{ .... -"

-

'

i800

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

/

.~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)

Lihat lebih banyak...

Comentários

Copyright © 2017 DADOSPDF Inc.