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SLAC-174 UC-28 (4
NEW MATERIALS AND TECHNOLOGY FOR SUPPRESSING
MULTIPACTOR IN HIGH POWER MICROWAVE WINDOWS
KIM0 M. WELCH
STANFORD LINEAR ACCELERATOR CENTER
STANFORD UNIVERSITY
Stanford, California 94.305
PREPARED FOR THE U. S. ATOMIC ENERGY
COMMISSION UNDER CON’jXACT NO. AT(O4-3)-515
August 1974
Printed in the United States of America. Available from National Technical Service, U. S. Department of Commerce, 5285 Port Royal Road, Springfield Virginia 22151. Price: Printed Copy $6.45; Microfiche $1.45.
ABSTRACT
High power microwave window development work at the Stanford Linear
Accelerator Center for the period June 1973 through January 1974 is given.
Emphasis was on development and testing of window coating materials and pro-
cedures used to suppress mull&actor in microwave windows. These windows were fabricated from Ae,O,, type AL-300 ceramic. Coating materials tested included Cr203, oxides of titanium and TiN. The latter material was discovered and evaluated during this program, and was found to have excellent multipactor
suppression properties. Also, a new vacuum coating procedure, featuring
crossed field diode array sputtering, was developed’ and successfully used to
coat a number of windows. Windows were also evaluated which were coated
with TM using vacuum evaporation techniques. Windows were also coated in
partial pressures of air, oxygen and argon. Windows featuring these coating materials and techniques were evaluated by testing in a high power microwave
storage ring at a frequency of 2.856 GHz. The ring, windows permitting, will
operate at peak power levels on the order of 100 Mw at a duly cycle of -9 x lo-?
-j,i-
.
TABLE OF CONTENTS
Page Introduction. ............................. 1
Window Coating to Suppress Multipactor ............... 1
Program Objectives, ......................... 4
Microwave Ring Test Facilities ................... 4
Comparison of Cr203 Coated Window with Standard SLAC
Windows ............................. 9
A New Approach to Window Coating ................. 14
Second Generation Apparatus. .................... 16
Third Generation Coating Apparatus ................. 37
Conclusion .............................. 37
References .............................. 44
8.
LIST OF FIGURES
Page
SLAC klystron microwave window . . . . . . . . . . . . . . . . 2
Power dissipated in a typical Cr203 coated window as
a function of RF storage ring power . . . . . . . . . . . . . . . 3
RF test ring window cooling configurations . . . . . . . . . . . 5
High power microwave storage ring test facility . . . . . . . . ‘7
Kurtosis as defined in report. . . . . . . . . . . . . . . . . . . 8
Power dissipated in windows with standard TixOy ac
sputtered coating before and after 625’C vacuum bakeout. . . . 10
Power dissipated in windows without coating . . . . . . . . . . 11
Power dissipated in windows coated with Cr203 compared
with standard Ti 0 XY
coated windows prior to vacuum
bakeout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Power dissipated in windows coated with Cr203 compared
with standard Ti 0 XY
coated windows after 625’C vacuum
bakeout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Microwave window positioned in first generation sputtering
apparatus. . . . . . . . . . . . . . . , . . . . . . . . . . . . . 15
Power dissipated in the same window with standard TixOy coat, with and without Cu plate on window sleeve
and with first TiN coating . . . . . . . . . . . . . . . . . . . , 17
Power dissipated in window with and without copper plating
on sleeve and with standard coat and first TixOy coating
using first generation apparatus . . . . . . . . . . . . . . . . . 18
Linear Y~lf” diode sputtering array. . . . . . . . . . . . . . . 19
Linear sputtering array apparatus for L-band window
coating feasibility studies . . . . . . . . . . . . . . . . . . . . 21
Filament evaporation coating apparatus . . . . . . . . . . . . . 22
Window aging effects during RF testing . . . . . . . . . . . . . 23
Power dissipated in windows coated with titanium nitride
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
in second generation apparatus compared with standard
TixOy coated windows prior to vacuum bakeout . . . . . . . . . 24
- iv -
18.
19.
20.
21.
22.
23.
24.
25.
26. 27.
28.
29.
30.
31.
32.
33.
Page
Power dissipated in windows coated with titanium nitride
in second generation apparatus compared with standard
TixOy coated windows after 625’C vacuum bakeout . . . . . .
Prebakeout comparison of power dissipated in standard TixOy coated windows with windows coated with titanium
nitride by evaporation filament . . . . . . . . . . . . . . . . . Post bakeout test results of windows evaporation coated
with titanium nitride compared with standard Ti 0 XY
26
27
coated windows . . . . . . . . , . . . . . . . . . . . . . . . . Power dissipated in window number 15 as a function of
sequential coating with different materials . , . . . . . . . .
Power dissipated in window number 17 as a function of
sequential coating with different materials. . . . . , . . . . .
Power dissipated in window number 17 on post bakeout test
as a function of coating with different materials . . . . . . . .
Power dissipated in window number 19 as a function of
sequential coating with different materials. . . . . . . . . . .
Power dissipated in window number 19 in post bakeout test
a; $function of sequential coating with different materials . .
& window coating apparatus with the window removed . . bt(NF
%-be&window coating apparatus with window installed . . . . Window mounting bell jar plate assembly . . . . . . . . . . .
Third generation window coating apparatus . . . . . . . . . .
Third generation coating apparatus showing diode
sputtering array. . . . . . . . . . . . . . . . . . . . . . . . . Third generation sputtering apparatus with solenoid
magnet removed. . . . . . . . . . . . . . . . . . . . . . . . .
Power dissipated in titanium nitride coated windows as a function of calculated film thickness . . . . . . . . . . . . . . Power dissipated in windows as a function of titanium nitride
coating thickness and ring peak microwave power . . . . . . .
28
30
31
32
33
34
35
36 38
39
40
41
42
43
-v-
-l-
INTRODUCTION
High power microwave window development work has been an ongoing
endeavor at SLAC for many years. The gradual upgrading of klystron efficiency and output power has made this essential. High dividends have resulted from
this work as reflected in klystron manufacturing yield, operating reliability and
extended tube life. Few SLAC klystrons have failed on the accelerator in recent
years due to window problems.
High power microwave windows are particularly subject to multipactor
problems. 2-7 Multipactor is an electron bombardment heating phenomenon
unique to RF vacuum devices. It results from unfortunate combinations of
optimum surface orientation with respect to the electric field, high secondary
electron emission coefficient of material surface(s), electron energy, and RF
frequency. For a number of technical reasons AGO3 and He0 appear to be the
best candidates for window material to date. 7 For reasons of safety Af203 is preferably used over the toxic De0 window material. On the other hand Af203
has a high secondary electron emission coefficient (on the order of 10 at the
maximum of the energy-yield function). This high secondary emission coefficient and dimensional constraints imposed on RF window design compound the problem
of multipactor. A typical SLAC high power klystron window is shown in Fig. 1.
A transformation from the TEIO rectangular waveguide mode to a TEll circular
waveguide mode creates very complicated fringing fields in this structure. Problems of multipactor do not necessarily occur just at high power levels. Re-
gions of multipactor shift in the window structure or completely disappear with
changing power levels. This effect is illustrated in Fig. 2, which shows calori-
metric power measured in the window water cooling jacket as a function of trans-
mitted power in a microwave storage ring.
WINDOW COATING TO SUPPRESS MULTIPACTOR
In order to suppress multipactor, thin films of oxides of titanium have been
used to coat windows for a number of years. This material has been shown to
have a much lower secondary electron emission coefficient than AJ203. 8 Both
evaporation6 and sputtering techniques 9,lO have been used in applying the
material. Scientists at SLAC actively participated in much of this early work 7,g,11,12 .
A. High Purity AP203 Disk (_ 84 cm dia)
B. Cupronickel Sleeve, Brazed
C. Stainless Steel Jacket D. Water Cooling Channel E. Vacuum-RF Flange F. Crushed Copper Gasket G. Rectangular Waveguide (3A X 7.2 cm)
FIG. l--STAC klystron microwave window.
-2-
0 10 20 30 40 50 60 70 80
Storage Ring Peak Microwave Power - Megawatts ,,,,*,,
I I I I I I I I -
. - R’
High multipactor region
tangent and resistive
Frequency: 2.856 GHz Duty cycle: -9 x 10s4
(Window 4a prior to vacuum bakeout)
I I I I I I I
FIG. 2--Power dissipated in a typical Cr203 coated window as a function of RF storage ring power.
-3-
-4-
Subsequent success at SLAC has been largely due to rigorous control of
procedures used in window coating. The procedures and associated equipment
were the outgrowth of empirical work, and the technology did not readily trans-
late to differing configurations. Careful control of film thickness was necessary, as films which were thick would result in resistive heating problems, and films
which were too thin would not totally suppress multipactor. Also, there is some
indication that properties of the reactively sputtered oxides of titanium may
change with bakeout. For these reasons microwave window research work con- tinues in the industry13 and at SLAC. I’ I4
PROGRAM OBJECTIVES
Due to much higher power l$$ons in the offing and the need to develop
coating technology for a large w klystron window in the early development
stages, an independent window research program was commissioned to address
the following tasks :
1. Refine RF ring testing procedures to yield quantitative data for “fine
structure” comparison of window performance.
2. Test microwave windows offered by outside vendors and which feature
promising new coating materials.
3. Investigate new possible multipactor suppression materials and appli-
cation procedures.
4. Translate technology of conventional or newly developed materials and
procedures for application to a generalized window configuration.
MICROWAVE RING TEST FACILITIES
Facilities did not exist to make calorimetric measurements of window
losses during ring testing. Such measurements have been shown to be of value
in evaluatingavindow.coatings. 13 The ring (microwave storage ring) was modi-
fied to include these provisions. Three possible water cooling configurations
could be used to cool the window during tests. These are illustrated in Fig. 3.
As shown, configuration “A” essentially involves cooling of the window assembly
through conduction over long lengths of waveguide. Configuration “B” amounts
to water cooling of the copper waveguide in close proximity to the window flanges.
Lastly, configuration %” amounts to direct water cooling of the window assembly
as well as the nearby waveguide, as in “B”i When using the “C” cooling config-
uration, power dissipation in the window water cooling jacket was measured
Combination thermometer
Configuration I’ C”
/ and thermocouple wel I
Window flange out e=q$
\ Rectangular wavegiude
” A” water cooling configuration
1 m
typical both ends Klyst:on window
“A” includes only end cooling
“6” includes “A” and ” B” cooling configuration
” C ” includes ” A ‘I, ” B ” and ” C ” cooling configuration
FIG. 3--RF test ring window cooling configurations.
-5-
-6-
using both mercury thermometers and thermocouples, while water flow was
measured with a Brooks flowmeter (500 cc/mm full scale). Due to thermal
losses from the system, calorimetrics serve only as a relative measure of
window merit. However, we achieved excellent repeatability and were able to predict probable window failure and cause of same using this technique. In over
1000 simultaneous thermocouple and thermometer water temperature measure-
ments the difference in window power dissipation indicated by the two tempera-
ture measuring methods amounted to 2.6 watts (a = h2.5 watts) and was random.
Figure 4 shows this microwave ring test facility.
Provisions existed for measuring temperature of the center of the window
through view ports in the ring using an lR detector. However, temperature cali- bration problems occur with changes in the emissivity of window materials, view
port transmission, and instrument adjustments. Also, there are problems of
interpretation of indicated temperatures for different water flow rates and cool-
ing configurations. A method was established for normalizing “ouV the effects
of changes in emissivity of the various windows. This involved integrating the
temperature function in the manner illustrated in Fig. 5. Coordinates are trans- lated for the data of each scanning so that the temperature minimum at the window edge is taken as zero. The integral then represents only the area shown
%ashedfl in the above figure. The effects of change is spectral emissivity with
temperature variations across the window are second order in this normalized
integral. Also, changes in the value of the normalized integral with emissivity
settings of the IR detector amount to only 16% over the range of 0.2 - 0.6. The normalized integral of the temperature function qualitatively describes the degree
of “peakedness”, or kurtosis of the function. It has been shown that relative
kurtosis is an excellent indication of window quality. 14 When used in conjunction
with calorimetric data, it is particularly useful in finding the cause of window
heating. A kurtosis value 2 50 ‘C-cm at 50 kW average power is acceptable.
Kurtosis values 1 150 to 200 ‘C-cm indicate serious window problems at 50 kW
average power or less. The storage ring was calibrated by direct insertion of a special flanged bi-
metal waveguide section. 15 The bimetal section was first calibrated up to an
average power of N 50 kW by calorimetric measurements on insertion in the out -
put waveguide of a high power klystron with a water load. It was discovered that
FIG. 4--High power microwave storage ring test facility.
I R indicated window temperature OC
Test # 1, configuration “A”
Relative kurtosis of test #2 greater than that of test # 1
Independent of absolute temperature, normalized integral ( i.e. hoshed area) is defined as ‘I relative kurtosis”
100 Test # 2 configuration “C”
Edge
0 Window Center Edge
FIG. 5--Kurtosis as defined in report.
-8-
-9-
ring power levels were actually -17% lower than originally thought. The new
value is probably correct to within *5%.
COMPARISON OF Cr203 COATED WINDOW WITH STANDARD SLAC WINDOWS
The first three months of the program were spent refining window testing methods, establishing ?iormal” window behavior as measured with these new
testing methods, and evaluating a number of windows coated with Cr203 (sub-
mitted for evaluation by Litton Industries, Inc. , San Carlos).
The statistics of “normaltt standard SLAC coated windows, both before and
after the typical 625’C vacuum bakeout, are shown in Fig. 6. This data repre-
sented an adequate statistical sample to serve as a “bench mark” in all future
work. Window data taken during hot test of a klystron are also shown in this figure. Window degradation due to bakeout is suggested in the data.
Having established “normal” window behavior, we sought to quantify
extremely “abnormal” window behavior with the tools at hand. Tests were there-
fore conducted on one window with no coating and a second window with standard
coating only on the cupronickel sleeve (see Fig. 1). These test data are shown
in Fig. 7. They graphically demonstrate the merit of window coating to suppress
multipactor.
Windows coated with Cr203 films of varying thickness C’fog”, 0.025 mm, and
0.075 mm) were RF tested before and after bakeout. Detailed results of these extensive tests are reported elsewhere. 14 However, typical performance data
before and after bakeout are shown in Figs. 8 and 9 respectively. These data do
not agree with results reported elsewhere, 13 where, (1) direct measurements of the secondary electron emission coefficients indicated Cr203 and oxides of
titanium were essentially identical, and (2) it was concluded that Cr203 “adequately
suppressed multipactor”, based on test data. Results of data in Figs. 8 and 9 would indicate to the contrary. This result may be largely due to differences in
the geometry of the windows tested. This is further indicated by the compari-
tively small difference in performance of uncoated windows and Cr203 coated
windows reported at that time. Test results at SLAC on windows coated with this material indicated the
following: (1) there is inadequate suppression of multipactor at low power levels
independent of window coating thickness, (2) there is poor reliability or quality
control in window coating as suggested by failure at the offing of one fogged, one
100
I “Standard” TixOy cooted windows subsequent to 625 ‘C vacuum bakeout (8 windows tested*)
Single preboke data points (4) beyond 00 MW
c Three data points taken on klystron in post bakeout hot test
-“Stondord” Ti,Oy coated wmdows prior to vacuum bakeout (13 windows tested*)
Frequency: 2.856 GHz
Duty cycle: -9xlO-4
*Distribution in doto represents real performance vonotlons +one standard deviation. One standard devlotion in calorimetric data on co&poring - IO3 simultaneous thermocouple and thermometer readings is 2.5 watts
I I I I I I I I I I I
0 IO 20 30 40 50 60 70 80 90 100 IIV I20
STORAGE RING PEAK MICROWAVE POWER - MEGAWATTS 2615ClS
FIG. g--Power dissipated in windows with standard TixOy ac sputtered coating before and after 625OC vacuum bakeout.
- 10 -
g 20
- I 0 t
0 L
0 I .o 2.0 3.0 4.0 5.0 \ Storage Ring Peak Microwave Power - Megawatts ,,,,,,,
. Uncoated window 8a (failed a 5.7MW)
Window I3a only window sleeve Frequency: 2.856 GHz
Typical ” standard ” coated window (avg of I3 sample)
\ FIG. ‘I--Power dissipated in windows without coating.
- 11 -
90
‘I
Ti,Oy cooied windows fir on performance of thirteen i
lindow umber
20
30
40
50
60
120
preboke samples
Frequency: 2.856 GHz
Duty cycle: -9 X 10e4
:ooting Coating noierial thickness Comments
I Cr203 0.075 mm
Cr203 0.025mm Window crocked
C’P3 “Fog”
Cr203 0.025mm
Cr203 0.075mm Window folled of - 2.5 MW !
20 30 40 50 60 70 80 90 100 110 IZW STORAGE RING PEAK MICROWAVE POWER-MEGAWATTS ZISIC,P
FIG. B--Power dissipated in windows coated with Cr203 compared with standard TixOy coated windows prior to vacuum bakeout.
- 12 -
r 1
“Standard” Ti,Oy coated windows f 1 o on performance of eight samples after 625°C wxuum boke
P” I I I I I 0 IO 20 30 40 50 60 70 80 90 100 110 120
Frequency: 2.856 GHz
Duty cycle: -9~lO-~
5b Cr203 0.025mm See preboke data (50)
70 0203 0.025mm No preboke test
90 Cr203 0.075mm No preboke test
I I I I I I
STORAGE RING PEAK MICROWAVE POWER - MEGAWATTS 255IC20
FIG. g--Power dissipated in windows coated with Cr203 compared with standard TixOy coated windows after 625OC vacuum bakeout.
- 13 -
- 14 -
0.025 mm, and one 0.075 mm thick coating window, (3) there appears to be a
deterioration in degree of multipactor suppression subsequent to
bakeout, (4) the chromium oxide coating is not reduced to a partial metallic
state as a consequence of high temperature vacuum baking, (5) the loss tangent
properties of the material are excellent as evidenced by negligible differences in
power dissipated in windows with three different film thicknesses (pre-bakeout
data), and lastly (6) the dielectric strength of Cr203 is not adequate for high
power window application as evidenced by craters and “worm-like” eruptions
which existed to varying degrees on all of the windows tested.
A NEW APPROACH TO WINDOW COATING
It occurred to the writer that titanium nitride was thermally much more
stable than oxides of titanium. Therefore, if it proved to have satisfactory
secondary emission properties as well as thin film resistivity comparable to oxides of titanium, it might then prove to be a superior window coating material.
Experiments really became l’mixedl’ when it was decided to use a newly invented
coating apparatus to deposit the TiN films as well as films of TixOy (i.e. , com- plex oxides of titanium) on test windows. This coating apparatus was the out-
growth of research in low pressure crossed field electrical discharges at SLAC
over a number of years, ” 16-lg and it was coined a crossed field diode sputter-
ing array. A double array of diodes was constructed for a “first try” at (1) use of a
new crossed field sputtering scheme in deposition of thin films, and (2) reactive
sputter coating of TiN films for the suppression of multipactor. A window was placed between these arrays for coating as shown in Fig. 10. The assembly, referred to as the first generation apparatus, was placed in a bell jar and
evacuated to N low7 Torr. The window was then coated using an anode potential
of +3.5 kV and a magnetic field of -0.06 T for a period of 5 minutes at an indicated
pressure of -5 x lo3 Torr. A gas mixture of 90% 02, 10% A was used in this initial coating. The cathode plates were removed from the diode arrays, new
plates installed (a precaution taken in all subsequent experiments), and a second
window coated. During this coating operation a gas mixture of 90% N2, 10% A
was used. Inspection of both windows indicated significant nonuniformity in thickness.
The anode array pattern was transposed, to some degree, onto the surface of the
- 15 -
2555A2
FIG. lo--Microwave window positioned in first generation sputtering apparatus.
- 16 -
window. The minimum film thickness was on center with each anode cylinder,
suggesting some degree of sputtering of the substrate by the magnetically focused
ion beams. Also, the coating was very visible. The “rule of thumb” in the past
has been that if one can see the film, it is too thick. A thick film will cause
resistive heating problems in RF operation, particularly after bakeout. Roth windows were therefore subsequently recoated. under the same conditions, but
for a l-1/2 minute period. Figures 11 and 12 show RF ring test results for the titanium nitride and
oxide of titanium coatings respectively. The windows herein discussed were
tested by insertion in a high-power microwave storage ring. The above figures
show power dissipated in the windows for conditions of standard SLAC coating,
standard coated windows with copper plating on the cupronickel sleeves (see Fig. 1; this plating operation was a simultaneous development activity), and
windows subsequently coated with the first generation apparatus. There is some
suggestion that the window coated with Ti, Oy in the first generation apparatus .
exhibited slightly higher dissipation losses than the standard window. However,
the TiN coated window appeared equivalent to the standard coated window in
terms of dissipation. Initial results, though far from conclusive, were most encouraging.
SECOND GENERATION APPARATUS
Schedules required that an apparatus be developed in the near future for thin
film coating of the inside surface of a cylindrical, Af,O,, UHF klystron out-
put window ( N 15 cm dia. x 15 cm long). Test facilities did not exist to evaluate different coatings prior to commitment of the window to the klystron assembly.
In that new equipment had to be constructed to satisfy the above requirement, we
used this opportunity to further explore use of the crossed field diode sputtering
scheme and TiN in suppression of multipactor. A linear diode array was constructed, and used to coat the typical S-band
klystron windows (Fig. 1) which could be tested in the RF storage ring. This linear diode array is shown in Fig. 13. A rotary vacuum feedthrough was used
to permit eventual coating of the inside of the UHF window, and to facilitate motion for ion beam scrubbing and more uniform film deposition on the S-band
windows. When coating the disk windows, the diode array was spaced from the
surface of the window a distance equivalent to that spacing which would exist on
I I I I I I I I I I I I I I
F Frequency: 2.856 GHz I- $0 - I
Duty cycle: -9x10-4
0
Copper plated
Ti.0,. standard coat
P .‘F
Ti,Oy Standard “benchmark” window
tLd
Standard window with Cu plated sleeve /'
T&O, Window lb tested 25 doys later
D-B Id Ti,Oy TIN First generotion apparatus coat (e/31/73) -
of TIN onto window lc after cleaning I I I I I I I I I I I I I _
0 20 40 60 SO 100 120 140 STORAGE RING PEAK MICROWAVE POWER- MEGAWATTS 111,111
.-
FIG. IL-Power dissipated in the same window with standard TixOy coat, with and without Cu plate on window sleeve and with first TiN coating.
- 17 -
100
90
80
r 5 70 I
5 0 60 z 3
z 50
@ % v, 40 cl7 0
6 B 30
20
IO
0
Independent tests of standard recoated window (13c,d) by different technicians on different working shifts I test of “Standard” coating
w (13b). (Low level Multipocl sts window coating too thin1
technicians on different shifts)
Frequency: 2.856 GHz
copper plate on window sleeve Duty cycle: - 9 x 10e4
0 20 30 40 50 60 70 00 90 100 110 120
STORAGE RING PEAK MICROWAVE POWER MEGAWATTS 251x2,
FIG. 12--Power dissipated in window with and without copper plating on sleeve and with standard coat and first TixOy coating using first generation apparatus.
- 18 -
,
ANODE
TITANIUM CATHODE 2555A3
FIG. Is--Linear %aIfTf diode sputtering array.
- 20 -
insertion of the array in the cylindrical output window. Figure 14 shows the
smaller klystron window and linear diode array in a vacuum bell jar. The windows were also attached to a rotary feedthrough to permit sequential coating
of each side without venting of the vacuum system.
A second bell jar sealing plate was also constructed to facilitate evaporation
coating of thin films of TiN and TixOy onto S-band windows, and with the same
flexibility for possible subsequent use in coating the UHF windows. This apparatus is shown in Fig. 15. Numerous window coating experiments were
conducted using the above apparatus with the previously mentioned gas mixtures,
as well as laboratory air and pure argon. During early RF tests on the many standard SLAC windows, an aging effect
was noted. It was observed that there were large variations in power dissipa-
tion in any particular window from hour to hour and even day to day. We sub- sequently determined that stability could be achieved in window performance by
aging for one hour at -50-60 MW peak power and with water cooling configura-
tion “B”. This effect is demonstrated in Fig. 16. We thereafter performed this
aging operation on all windows on initial testing. The Cr203 windows previously discussed, and statistics generated on the standard windows reflect data taken after this aging ritual where window performance permitted. We did not deter- mine if this was required with the TiN coated windows. Data in Figs. 11 and 16
are good indications of the repeatibility we achieved in our measuring technique. Repeated tests of the same window by different technicians were done without
their knowledge to evaluate the overall statistics of our measurement technique.
Results were gratifying and a tribute to the care exercised by these technicians
in measurement technique.
Test results of the first four windows coated using the linear diode sputter
array, and nitrogen are shown in Fig. 17. Calculations were made in an effort
to predict window coating thickness. These calculations predicted a coating
thickness of several hundred angstroms on the first window which was coated using the linear array (window No. 14b). In an effort to qualitatively evaluate
loss tangent properties of the material, we increased the coating thickness of the
subsequent window by a factor of -3.7. As shown iu Fig. 17, losses were
slightly higher in this window (1%~). To make sure that this variation was not
due to intervening variables, window 17~ was cleaned and recoated with sligbtly
:LYSTRON 1 WINDOW
2555A4
I
G. 14--Linear sputtering array apparatus for L-baud window coating feasibility studies.
-2l-
FIG. 15--Filament evaporation coating apparatus.
- 22 -
I I I I I I I I
Frequency: 2.856 GHz 80 - Duty cycle: -9X10e4
P l 13g
0
i . Initial independent* tests of B’ stondard cooted window (13 f,g) ’ with copper plote on sleeve
F\ .
n .-.
Final window (l3h) test after aged - 1 hour
./y.‘powei level if -60 ,W with woter coohng conflguration “B” at
*Tested by different technicians on different shifts
I I I I I I I I I I I
0 20 40 60 80 100 120
STORAGE RING PEAK MICROWAVE POWER- MEGAWATTS 1111.11
FIG. Is--Window aging effects during RF testing.
-23 -
0 0 IO 20 30 40 50 60 70 00 90 100 II0 120
STORAGE RING PEAK MICROWAVE POWER-MEGAWATTS 2LIIC2,
FIG. l’l--Power dissipated in windows coated with titanium nitride in second generation apparatus compared with standard TixOy coated windows prior to vacuum bakeout.
Test 17~
TiN coated wmdows
Frequency: 2.656 GHz
Duty cycle:-9xU4
Coating time Electrode Magnetic number tow per side-min potential Fteld -
l4b 4.OXlO-5 N2ondA 2.5 3.0 kV 0.081 T
‘IL-Y l7C 4.5XlO-5 N2ond A 7.5 3.0 kV 0.087 T _
M l7d 4.5~ lO-5 NZand A 2.0 2.5 kV 0.067 T
H l5d 4.OXlO-5 N2 and A 2.5 2.5 kV 0.061 T
*Thuckness bosed on measurements in third generation apparatus. Fig. 32 and 33
- 24 -
- 25 -
less coating (3% by appropriate scaling factors) than used with window 14b. The
recoated window is numbered 17d, and performed similar to 14b, as also shown
in Fig. 17. A third window was coated and verified the repeatibility and merit
of the initial coating parameters (15d).
Three things were made evident by the above tests: (I) we were very lucky in our initial choice of coating parameters; (2) window performance does depend
on TiN coating thickness; and (3) the coating procedure suggested good repeati-
bility.
Test data on these windows subsequent to bakeout are shown in Fig. 18. It
was determined that window 15d had been contaminated with diffusion pump oil
during bakeout. However, data are included for tbe value of this lesson alone.
Experimental results of windows coated with TiN and using evaporation techniques are shown in Fig. 19 (prebakeout) and Fig. 20 (post bakeout). These
windows appear to run hotter both before and after bakeout than the windows
coated with TiN and using the diode array system. It is much more difficult to control film thickness when using evaporation techniques, and one might there-
fore suspect that the coating was too thick. Results of window No. 16 in Fig. 19
suggest to the contrary. The low power multipactor peak in window 16~ suggests
the coating is too thin. On subsequent recoating the low level peak disappeared
(window No. 16d, of Fig. 19) until after bakeout (window No. 16e, of Fig. 20).
Therefore, in all of the above cases, there is some suggestion that the window
coating was not sufficiently thick.
Table I lists the various combinations subsequently used in coating windows
with the linear diode array system and evaporation filament. Subsequent sputter-
ing with the linear diode array system and the 02-A gas mixture gave very
unsatisfactory results - to the point where calorimetric data could not be taken
due to ring matching problems. This was inexplicable considering initial per- formance with the first generation apparatus. Windows were not coated by
sputtering in argon with the linear array system but, all other coating combina- tions are included in Table I, with figures containing corresponding data.
100
90
80
Window 15e (15d post bokeout) coniaminoted with diffusion / pump oil during bakeout / \
Window 14~ (14b post bakeout)
Window 17e (17d post bakeout)
IO
0
Ti,Oy coated windows +_ 1~ on performance of eight samples after 625 ‘C vacuum bake
Frequency: 2.856 GHz
Duty cycle: m 9 XIOm4
I I I I I I I I I I I I 0 IO 20 30 40 50 60 70 00 90 100 110 120
STORAGE RING PEAK MICROWAVE POWER - MEGAWATTS 25dlC22
FIG. B--Power dissipated in windows coated with titanium nitride in second generation apparatus compared with standard T&Oy coated windows after 625OC vacuum bakeout.
- 26 -
“Standard” Ti,Oy coated windows
/ ,I’ n *1 (r an performance of thirteen prebake samples
Frequency: 2.856 GHz
Duty cycle: -9 X lO-4
i
Window Filomeni Pressure number power TOU
Gas Cqoting time mtnutesfside Comments
@--a 16~ 630W 4.5XlO-5 N2-A 15 First TiN evap. coat RF tested (10/15/73l
E-43 16d 732 W 4.5XlO-5 N2-A 15 Overcoat of window 16~ above
Y--Y 14d 732 W 7.5XU5 N2-A IO
A?.-& 17f 732 W ELOXIO-~ N2-A I5 i I I I I I I I I I I I
0 IO 20 30 40 50 60 70 80 90 100 II0 120 STORAGE RING PEAK MICROWAVE POWER - MEGAWATTS ?511C21
FIG. 19--Prebakeout comparison of power dissipated in standard TixOy co&ted windows with windows coated with titanium nitride by evaporation filament.
- 27 -
100
90 Window 179 (17f post bakeout)/J’
80
t??vcm\~’ , Window 14e (14d post bakeout)
/kQsssx\\\\\v- - Window l6e (l6d post bakeout)
IO
0
Ti,Oy coated windows -+lv on performance of eight samples after 625 “C vacuum bake Frequency: 2.856 GHz
Duty cycle: -9x (OS4
0 IO 20 30 40 50 60 10 00 90 100 110 I20
STORAGE RING PEAK MICROWAVE POWER - MEGAWATTS 2bSJC24
FIG. Z&-Post bakeout test results of windows evaporation coated with titanium nitride compared with standard T&Oy coated windows.
-28..
- 29 -
TABLE I
Window No.
15a
15c
15d
17a
l7b
17d
17e
17f
1% 19a
19b 19c
19d
19e
19f
T Coal N2 -A
X
X
X
X
X
X
X
X
X
X
X
X
ium Air
T
X
X
X
Appar us Sputter Evap.
(Standard)
X
(Standard)
(Standard)
X
X
(Standard)
(Standard)
X
X
X
X
X
X
X
T Bake ut Before After
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
T Figure
21 21
21
22
23
22
23
22
23
24
25 24
25 24
25
Based on data thus far given, indications are that windows coated with TiN and the linear diode array gave the best results. There is some suggestion that a
window sputter coated in a partial pressure of air would perform very satisfac- torily both before and after bakeout. However, this was based on tests of only
one window (19c and 19d). It was proposed by the writer that sputtering of titanium nitride be used in future applications due to the high control evidenced
by the data, and overall satisfactory window performance.
The linear diode array was subsequently used to coat the cylindrical UHF
window with TiN. A swivel magnet was constructed to permit proper orientation
of the magnetic field with the diode array when coating the inner surface of the
cylindrical window. The apparatus is shown in Fig. 26 with the UHF window
removed from the system. This same apparatus is shown with the window
installed in Fig. 27. The window served as the vacuum bottle during coating.
Windows were also subsequently coated with the evaporation filament shown in
Fig. 15.
A I I I
/' I I I
/ /
Window 15 d; titanium nitride coat with second generation apparatus (third coating)
Frequency: 2.856 GHz Duty cycle: -9X 10e4
0 I I I I I I I I I I I
0 20 40 60 80 100 120
STORAGE RING PEAK MICROWAVE POWER- MEGAWATTS II1IISV
FIG. 21--Power dissipated in window number 15 as a function of sequential coating with different materials.
- 30 -
Window 17 f: titanium nitride ’ P Y coat using evaporation
Window 17 d; titanium nitride coat using second generation apparatus (third coating as second coating too thick)
Window 17 a; standard coat (first coating) 1
Frequency: 2.856 GHz Duty cycle: -9XlO-4
1 *Before vacuum bakeout *Before vacuum bakeout
0 n I I I I I I I I I I I I I I I I I I I I I I
0 20 40 60 80 100 120
STORAGE RING PEAK MICROWAVE POWER - MEGAWATTS ,11,.,0 "0 20 40 60 80 100 120
STORAGE RING PEAK MICROWAVE POWER - MEGAWATTS ,11,.,0
FIG. 22--Power dissipated in window number 17 as a function of sequential coating with different materials.
- 31 -
80
0
f3 I I I I I I I I
_ Window 17b; standard ,’ coat (first coating)*
Window I7 e; titanium nitride coat using second generation apparatus (third coating)
Window I7 9; titanium nitride coat using evaporation (fourth coating)
i
Frequency: 2.656 GHz Duty cycle: -9X10e4
*Possible contamination during bokeout
I I I I I I I I I I I 0 20 40 60 80 100 120
STORAGE RING PEAK MICROWAVE POWER-MEGAWATTS 1”“”
FIG, 23--Power dissipated in window number 17 on post bakeout test as a function of coating with different materials.
- 32 -
’ I ’
Window I9 e; Ti,Oy probable coat using evaporation in partial pressure of argon
Window 19 c; Ti,O,,TiN coat using air with second generation apparatus (second coating)
Window 19 a; standard coat (first coating)
Frequency: 2.856 GHz Duty cycle: -9X10m4
*Before vacuum bakeout
I I I I I I I I I I I
0 20 40 60 80 100 120
STORAGE RING PEAK MICROWAVE POWER- MEGAWATTS 11,111,
FIG. 24--Power dissipated in window number 19 as a function of sequential coating with different materials.
- 33 -
a g 20
x
indow 19 d; T&O,, TIN coot using air with second generation apparatus (second tooting )
Frequency: 2.856 GHz Duty cycle: -9X10e4
0 20 40 60 80 100 120
STORAGE RING PEAK MICROWAVE POWER- MEGAWATTS 1111.11
FIG. 25--Power dissipated in window number 19 in post bakeout test as a function of sequential coating with different materials.
- 34 -
2555A6
FIG. 26--M window coating apparatus with the window removed.
- 35 -
of-l?= FIG. 27--35d~m~l window coating apparatus with window installed.
-36 -
- 37 -
THIRD GENERATION COATING APPARATUS
Our next objective was to construct a window coating system which could serve as an alternative to the standard coating system in the event of difficulties
in the future, and which could be used in future studies with TiN thin films and
perhaps other new materials. Features of this new apparatus are shown in
the following figures. Figure 28 shows a window mounted on the top bell jar
plate swivel assembly. This swivel assembly permitted rotation of the window
for sequential coating of each side, and if desired, edge heating of the window
with the discharge array ion beams. A solenoid coil was used to provide the magnetic field at the discharge array. The complete apparatus with magnet coil is shown in Fig. 29. The diode sputtering array is shown in Fig. 30. The
window is tilted in this figure to provide a better view of the array. The sputter
array may be moved in a direction transverse to the system axis by manipulation
of a bellows at the bottom of the bell jar base plate assembly as shown in Fig. 31. This permitted ion beam “scrubbing” of the total window assembly and made
possible more uniform film deposition during the coating operations. With the
given configuration, the klystron window assembly could be electrically biased
with respect to the anode and cathode of the array. As a rule windows were
edge heated with the array ion beam prior to coating, but at times subsequently
coated while biased at anode potential.
A series of eight windows were coated in the third generation apparatus with progressively thicker films until the scaled second generation apparatus optimum
parameters were achieved. This corresponded to a total optimum film thickness
of -75 % as subsequently measured with a crystal film thickness monitor.
Results of subsequent RF tests of these windows are shown in Figs. 32 and 33. Calorimetric daiawerenot taken on the first two windows (14f ( 1) and 17h) due to
excessive multipactor and a desire not to push them to destruction. Test per-
formances of the last two windows featuring “optimum parameter” coatings
suggested that good control and repeatibility is achieved with this coating system, and also that the technology can be easily translated from one apparatus to another.
CONCLUSION
A new material has been reported for use in the suppression of multipactor in microwave windows. This material, titanium nitride, was subsequently used
at the writers suggestion, with considerable success in eliminating serious
A A
KLYSTRON ,WlNDOW
/
FIG. %--Window mounting bell jar plate assembly.
- 38 -
2555AP
FIG. 29--Third generation window coating apparatus.
- 39 -
c
FIG. 30--Third generation coating apparatus showing diode sputtering array.
- 40 -
FIG. 31--Third generation sputtering apparatus with solenoid magnet removed.
-4l-
80
f! i- s
I I I I I l-
Frequency: 2.856 GHz Dutv cvcle: -9XW4
0
e-
@-@ 17i. 26 it A-A l5f 43 x Y-V I6 f 58 ii
*Coated using third generation sputtering apparatus IF I I I I I I I I I I I I L 0 20 40 60 80 100 120 140
STORAGE RING PEAK MICROWAVE POWER- MEGAWATTS 11n.1.
FIG. 32--J?ower dissipated in titanium nitride coated windows as a function of calculated film thickness.
-42-
II0
100
90
80
70
60
50
40
30
20
IO
0
68 MW peak power
56.7 M W peak power
,34 MW peak power
4.2
Il.3 MW peak power E
I I I I I I I
0 IO 20 30 40 50 60 70 Calculated Coating Thickness- Angstroms
FIG. 33--Power dissipated in windows as a function of titanium nitride coating thickness and ring peak microwave power.
- 43 -
-44-
-+ multipactor problems in large aluminum e e storage ring RF cavities. ‘O Initial findings in window tests indicate that TiN may be superior to the less stable oxides of titanium.
A new method has been reported for sputtering thin films. It was success-
fully used to reactively sputter films of titanium onto windows for the suppres-
sions of multipactor. Sputtering rates are predictable and films of uniform thickness were obtained by manipulation of the substrate and discharge array.
This coating scheme shows considerable promise in future research and indus-
trial applications. 21
ACKNOWLEDGMENTS
The author is most thankful to Dr. J.V. Lebacqe for providing the oppor-
tunity to conduct this research program and making possible the completion of
this paper. Many members of the klystron group at SLAC contributed their
energy and talent to this program. Paul Lee was exceptional in his contribution
in the construction of apparatus and in the preparation and coating of windows.
Bob Byers and his staff of test technicians including Rod Curry, Dave Soule and
Ted Evans performed magnificantly in their duties, and always provided the
requested data. My thanks to Joe Cordova and Chris Flegal for some of the basic Penning discharge research work they accomplished, and to Joe Cordova
who constructed the first generation apparatus with a minimum of direction. Lastly, thanks are also extended to Dick Muffley for his excellent photography.
- 45 -
1.
2.
3.
4.
5.
6.
7.
8.
9.
REFERENCES
Kimo M. Welch, “Low Pressure Crossed Field Sputtering of Thin Films for .
Multipactor Suppression Using a Simple Diode Array ,‘I Stanford Linear
Accelerator Center preprint (Submitted to Jour. Vat. Sci. Tech. ).
D. H. Preist, “Multipactor Effects and Their Prevention in High-Power
Microwave Tubes,” Microwave Jour., Oct. 1963, cf 55. (This paper
contains a useful list of early references on multipactor. )
D. H. Priest, R. C. ‘Ialcott, “On the Heating of Output Windows of Microwave
Tubes by Electron Bombardment ,I’ IRE Transactions on Electron Devices,
July 1961, p. 243.
E.F. Vance, “One-Sided Multipactor Discharge Modes ,I’ J. Appl. Phys.
34-, 3237 (1963).
J. R. M. Vaughan, “Some High-Power Window Failures ,I’ IRE Transactions
on Electron Devices, July 1961, p, 302 c. f.
R. C. Talcott, “The Effects of Titanium Films on Secondary Electron
Emission Phenomena in Resonant Cavities and at Dielectric Surfaces,”
IRE Transactions on Electron Devices, ED-g, 405-410, Sept. 1962.
R. B. Neal, gen. editor; R. W. Bierce, J. Jasberg, contributing authors,
The Stanford Two-Mile Accelerator (W. A. Benjamin, Inc. , New York,1968),
p. 320 c.f.
L. Holland, L. Laurenson, ‘IThe Secondary Electron Emission Characteris-
tics of Clean andcontaminated Titanium,” Second International Symposium
on Residual Cases in Electron Tubes, Milan, 470485, March 1963.
W. P. Schulz, “Apparatus for Coating Substrates by Cathode Sputtering,”
U.S. IPatent No. 3,293,168 (December 20, 1966).
10.
11.
12.
13.
14.
15.
16.
- 46 -
R. Hayes, %esearch on Microwave Window Multipactor and its Inhibition,”
Final Report Contract No. DA 36-039 5C-90818, Department of the Army,
Task No. lG6-22001-A-055-04, U.S. Army Electronics Laboratories, Fort
Momnouth, New Jersey. For the period 1 July 1962-30 June 1964.
J.H. Jasberg, J.V. Lebacqz, “High Power Microwave Windows ,I’ Stanford
Linear Accelerator Center Report No. SLAC-PUB-49 (1964), 5e Congres
international, Tubes pour Hyp&frequences, Paris, 1964.
R. W. Bierce, W. R. Fowkes , J. H. Jasberg, “Window Materials Design and
Properties for Use in High Power Klystrons ,‘I Stanford Linear Accelerator
Center Report No. SLAC-PUB-92, March 1965.
L. Lesensky, L. Tisdale, “New High-Power Broadband Coated A1203
Windows ,I’ Final Report Contract No. DAAB07-71-C-0316, DA Project
No. 7x2 6330 4D215, U. S. Army Electronics Command ,Fort Momnouth,
New Jersey. For the period 1 July 19’71- 1 May 1972.
Kimo M. Welch, %Iicrowave Storage Ring Tests of High Power S-Band
Klystron Windows Coated with Cr203 Films for the Purpose of Suppressing
Multipactor , ” Stanford Linear Accelerator Center Report No. SLAC-TN-74-10
(1974).
This bimetal section, invented by Dr. J. V. Lebacqz, amounts to a short
section of flanged copper waveguide with a stainless steel disk brazed into
the E-plane. The measure of resistive heating of the disk is correlated to
transmitted power.
Kimo M. Welch, “Performance of a Penning-Type Ionization Gauge in High
Gamma Radiation Fields, ” Report No. SLAC-TN-66-34, Stanford Linear
Accelerator Center (1966).
17. Kimo M. Welch, “Brief Comments on Noise Measurement Findings on
18.
19.
20.
21.
- 47 -
Large Ionic Vacuum Pumps,” Report No. SLAC-TN-67-15, Stanford
Linear Accelerator Center (1967).
Kimo M. Welch, “Diode Sputter bn Pump Speed for Hydrogen and Nitrogen
as a Function of Axial and Transverse Magnetic Fields up tc 1.2 Tesla ,I’
Report No. SLAC-TN-72-10, Stanford Linear Accelerator Center (1972).
Kimo M. Welch, “Properties and Applications of Low-Pressure Penning
Discharges,” Stanford University Special Studies Course EE391, under
direction of Dr. Sidney A. Self, May 1973, 83 pg.
M.A. Allen, P.B. Wilson, “RF Systems for High-Energy e-e+ Storage
Rings , ” Proceedings of the IXth International Conference on High Energy
Accelerators, Stanford Linear Accelerator Center, June, 1974.
Kimo M. Welch, ‘Speculation on Applications of a New Crossed-Field Thin
Film Sputtering Device ,I1 (in preparation).
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