A simple, novel technique was developed for generating a large-diameter, uniform electron beam with appropriate current density for spacecraft charging studies

Adamo, R. C.

Nanevicz, J. E.

English

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I. Background 549
2. Multlpactor Breakdowll 550
3. MUltipactor Electron Source 552
References 5S5
9. A Rugged Electron/Ion Source for
Spacecraft Charging Experiments
J. E. NaneviczandR. C. Ademo
ElectromogneticSciencesLoborolory
StanfordResearchInstitute
1. BACKGROUND
For the past several year.s, SRI has been involved in a laboratory investiga-
tion of the behavior of materlals under simulated spacecra/1 charglrig _onditions.
These tests require that a sample be installed in the vacuum chamber, the chamber
pumped dOwil, the test run, and a new sample installed with the minimal delay.
To carry out such a program, it has been neCes,lary to have a inlgged, reliable
electrOn/tim source available. A simple, novel technique has been developed for
generating a lar_e-dlameter, uniform electron beam with appropriate current
density for spacecraft charging studies.
Our experimental work started usin_ tliermi_electric sources. The following
difficulties were experienced:
(1) C_ntamtmttton -the cathode became contaminated by produets evolved
i fi_om the test sample. (Diffei'eiltiai pumping was not used in out_ system.)(2) Beanl uli|torriiity - achieving a large-diameter beilm with ilnltorm curi'eilt
density proved ditti_ult.
(3) Beam diameter -expanding the beam fi'om a thei-mlonic cathode to a
diameter of 8-10 in. tn a tiist_inc_ ot a toot _vas difficult.
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(4) Beam characteriStiCs depend or1acdeleratt_g voltage - it was found that
With simple lens arran_efnents, Changing beam energy also changed other beam
= parameters.
(5) Complex electrostatic lenses reqtlired- progressively more complex
lenses were needed to achieve desired beam properties.
(6) Light from filament- in photocondttctivity e_periments, the light from the
= filament can alter material behavior.
- Accordingly, itwas decidedthatan alternateelectronsource shOuldbe devised. ...
- 2. MULTIPACTORBREAKDO_/N
:: PaSt experience withrfvoltage-breakdownmechanisms inlow-pressure sys-
tems indicatedthata process was availablewith thepromise of generatingan
_ electron beam Without the disadvantages of thermionic cathode systems. As indi-
=_: cated in Figure I, when an rf signalisappliedto a pair.of parallelplateelectrodes
:_'i and the ambient pressure is reduced monotonically,one findsthatthe voltage
: required tOproduce breakdown decreases untila minimum is reached (ata pres-
:_ sure of = 50 miCcons Hg inthe Figure)and then increasesagain (alongthe dashed
_' lineinthe Figure).1 Itthe electz'odespacing iScorrect, however, one findsthat
" the breakdown voltagebecomes independentOfpressure and folloWSthe solidcurve
at lOW pressures:2,3 In thisca_e, a ne_ typeOf bt-eakdoWncalled"multipaCtor"
=_ occurs.
= The mechanism of multipactor discharge iS illustratecl in Figure 2. If rf
frequency and spacitlg are correct, an initial electron occurring near the lower
\,
=_, electrodewillbe acceleratedacross the gap, strikethe upper electrode,and
: generate one or more secondary electrons just as the field changes polarity. The
secondary electrons, in turn, are accelerated across the gap and generate addi-
tional secondarieswhen they strlk_thelower electrode. In thi_way, the number
of electrons in the breakdown cascades until variou_ loss mecitanL_ms come into
!i play and limit further growth in the number of electrotm parJjcipattng in the
_ breakdoS#n.
iI Thus, the multipactOr breakdown may l_e thought of as a sheet of electrons
_i. oscillating between a p_ir of electrodes iri synchronism With rite applied rf field.
it should be noted that there is no _equh.ement for the presence of gas molecules
to sustaitl the breakdown, in tact, if there are at_- molecules present, they will
=,' intimatelybe st/-uckby an eleCtrottand ionized. These ionizedgtismolecules are
_" t-esponsible for the slight glow visible in the breakdown. The multipactor pi'oc_eds
vei'y well, however0 evett tit hai'd vdcuUms.
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' EXPERIMENTAL
100
8O
' 50 80 100 200 400 (;00 t000 • •
fd _ MHz--cfn
Figure :_. Multip_ctor Breakdown Voltage as a Furiction of
FreqoenCy and GAP Width _orParallel-PlaneElectrodes
3. MULTIFACTORELECTROS SOURCE
AS wag indicatedearlier,thenumber of electronsparticipatinginthe break-
down increasesuntillossmethane.sinsbecome important. These includediffusion
of electronsfrom bet#veenthegaps as the resultofCoulomb forces,deviation
from synchronism of some of the electrons,etc. Past experience intryingto
• avoid multipactorindicatedth,,.tconsiderableadditionalosscan be toleratedwith°
out extinguishingtO. discharge. This in turnindicatedthatitshouldbe possible
-i' todeliberatelyextracta sizab1'electt-otlcurrent from thedischarge.
The first approach at devisin_ a scheme for electron extzactiorl is shown in
_ FtglJre 4, Holes were _imply drilled in one ot the electrodes to permit part of the
_ electron sheet to pass through the electrode once per rf cycle. |( was round that
thi_ scheme worked remarkably well. It wa,,_ possible to drill a sufficient number
of holes i/1 the plate that at a distance of 12 to 18 in. from the multipac_,r source,
there was rio pattern evidet|t in the beam Wht._flit iiltlminated a phosphor target.
...... O0000006-TSF09
! _ _ MI_ TAL
'i!_ SOURCE _ ELECTROOES
i=_ _ _t'IOLES TO ALLOW EXTRACTION
:._ OF _LECTRONS
Figure 4. Multipactor Discharge Electrode Modified to Permit Extraction of _=..,Electrons
;;v
_: As the result of this initialsuccess, and because an electron source was badly
ii,i: needed for a series of experimertts that were gettingunder way. there was no
_. further,experimentation to devise alternate schemes for electron extraction.
The setup presently in use for the study of insulator photoconductivity is shown
!_:i in Figure 5. A control grid (Consisting of a second metal sheet drilled with the
... same h01e pattern as the lower multlpactor electrode plate)was added to the ._ource
i ,5 tO permit simple control of beam current. For these experiments, the accelerat-
!_ ittgVoltage is applied between the gun and the target. By simply adding a grounded
-_" grid above the target, itis possible to obtain the same beam cOrrent while main-
m'_:l' talninga region ot zero fieldabove the target. Radio frequency power requirements
i _ to feed the source are modest (under 10 W). Ceramic capacitors are used in series
with t_iecoaxial cable from the rf source to provide isolation for the 0-20 kV .................
_il accelerating voltage used. with the system.
._, The multlpaCtor source currently in use provides a beam 8 in. in diameter at
_. the source, and somewhat larger at the ta_-get. In the limited experimentation
if: ')
_-,.. ... carried otlton the source so far, electron beam current densities of up to 5 ,A/era"
°i:_ have been achieved.. It is not clear that this represents the highest current actliev-
_;! able with this sotirce. It is also likely that the present source design does not
_. represent the optimum scheme for generating maximum current, however, the
o_. presently attainable beam current density is almost 3 orders of mak_nitude higher
-=_:. than typical substorm currents.
_,
-;iil _ larger Beam can be achieved by using larger diameter plates in making the
_-':_ _ource. An advantage ot this approach over trying to spread the beam via lense_
i_i is that, with the presettt setup, bealn size is virtually entirely independent (,r
_,:. aeceieratii_g voltage.
<<;,. The characteristics of the mutllpactor source can be summarized as follows:
-°_" (i1 Contamiisatton re_listaht
o......_ (21 Physically simple
: b :,_
_:i;; 553
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(3} Produee_ large diameter bearii dL_eetly
(4) Beam character/sties independent of accelerating voltage
(5) Stability good
(6) No light OutpUt
ItS operation, in the ye_z" since it was first assembled, has bee_ highly satis-
fadtory, and it is recommended as an electron source tot systems that must
operate uhder conditiofls where normal good vacuum practice regardinl_ long term
cleanliness rflust be ignored. ""
/ ±
/ //,'1/ L**T .I,L__
"":'o:=:'-...... F"l b" '+,I_
+ As
GAID
_U_CE DIAMETER ,, 8 in.
MAXIMUM ELECI'I_I_IN I_URRI_NTDENIJlITV - S _AIb_ |
TARGET ...,_ ....... ":I/_, ,; ' 2
V//I//_,,,///z// / _,_' // ////;
Figure 5. Schematic or Multipactor Electt.on Source
_54
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References ................................
1. Chown, J..B., Scha_lmar L W.E., and Mot_tta, T. (1959) Voltage breakdown
characteristics o£ miCrvwave antonflas_ Proc. IRE 4_(Nb:8):1331:1_3"_.
2. Chown0 J.B.. Nanevicz, J.E.0 and Vance, E.F. (1970) _HF Breakdown on a
Nlhe-Cajun Rocket. j PL Technical Memorandum 33-447.
3. AugUst, G. 0 and Chown, J.B. (1970) Reductioti or Gas Breakdown Th='e_holds _"
irl the Ionosphere Due to Multtpacting, ,_PI, Tecttntcal ]_emorandum 3_]-447,
/
555
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A rugged electron/ion source for spacecraft charging experiments

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