SLAC-174 UC-28 (4 MULTIPACTOR IN HIGH POWER … · MULTIPACTOR IN HIGH POWER ... U. S. Department of Commerce, 5285 Port Royal Road, Springfield ... coating technology for a large

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 translated 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


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


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)


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 subsequently 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


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


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 performance with the first generation apparatus. Windows were not coated by

sputtering in argon with the linear array system but, all other coating combinations 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


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


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


- 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


0 20 40 60 80 100 120

STORAGE RING PEAK MICROWAVE POWER- MEGAWATTS 11,111,


- 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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