- Email: [email protected]
RF windows used at S-band pulsed klystrons in the KEK linac
Vacuum/volume
Pergamon PII: SOO42-207X(96)00034-6
RF windows KEK iinac
47/numbers 6-8/pages 625 to 628/1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All riahts reserved 0042-207X196 $15.00+.00
used at S-band pulsed klystrons in the
S Michizono,“” Y Saito,” S Fukuda,” K HayashP and S Anami,” aKEK, National Laboratory Physics, Tsukuba, lbaraki 305, Japan; bMitsubishi Electric Corporation, CEW, Amagasaki,
The breakdown of the alumina RF-windows used in the development of klystrons. This breakdown bombardment and/or localized RF dissipation. In KEKB klysrrons, several kinds of alumina ceramics windows are being considered. An improved RF Copyright 0 1996Elsevier Science Ltd. Key words: Alumina,
ceramics,
klystron,
for High Energy Hyogo 661, Japan
in high-power klystrons is one of the most serious problems results from excess hearing of alumina due to mulripactor order to develop RF windows having high durability for the are being examined, and the breakdown mechanism of RF window installed in a KEKB klystron is also being rested.
RF window.
Introduction
Klystron failures and window breakdown
So far, 48 S-band pulsed klystrons (2856 MHz, 30 MW in maximum, 3.5 ,US,25 pps) have been operated at the KEK 2.5 GeV linac. Each klystron has an RF window installed in its output portion in order to isolate vacuum from the atmosphere, and to pass RF power. The window comprises a brazed aluminaceramic disk and a pill-box housing. Breakdown of the RF windows during RF operation is one of the most serious problems in long time RF-operation, and comprises a large majority of klystron failures.’ This breakdown is mainly due to localized surface melting of alumina, leading to punctures due to multipactoring and/or localized RF dissipation. Multipactoring involves a resonant multiplication (in the RF field) of secondary emitted electrons generated at the alumina surface, which results in luminescence of alumina.’ ’ This multipactoring can be suppressed to some extent by using thin-film coatings having low secondary-electron emission yields. such as TiN.’ Localized RF dissipation depends on the ceramic purity and structure.’ In order to prevent window breakdown, it is important to evaluate the alumina materials statistically and to study the breakdown mechanism. In this report. the cumulative status of RF window breakdown and R&D concerning window materials are summarized. The breakdown mechanism of RF windows is also described and the RF window used for a KEKB klystron a is examined.
The cumulative status of klystron failures is given in Table I .’ Failure due to internal arcing between the electrodes of the electron gun was most frequent for klystrons in which oxide cathodes were used. The use of barium-impregnated (BI) cathodes, which have been used since 1987, suppressed internal arcing due to less electrode contamination. Thus, a cumulative mean time before failure (MTBF) increases by more than four times (51,734 h; this corresponds to ten years of operation, since the operation time during one year was 5,120 h in 1993). However, the breakdown or RF windows still remained. The window-breakdown has to be studied not only for life time but also developments for a 50 MW-klystron used at the KEKB project. In order to understand the window-breakdown process, statistical research of the window-housing temperatures during RF operation was carried out at the linac, where each klystron output power is around 25 MW. Figure 1 shows the temperature increase from room temperature (AT) measured up to May. 1995. The temperatures of some windows became more than 45 C (room temperature was 27°C); AT of failed klystrons were larger than those of the living klystrons. The dependence of AT on the operation time was summarized (Figure 2) to investigate whether such a large AT existed from the beginning or not. AT increased with the operation time for those windows with a high AT; AT of klystrons ‘89507’ and ‘91512’ increased rapidly (about 5000 h). and AT of ‘895 12’ increased slowly (about 25000 h). Both of these windows showed localized surface melting (Figure 3) indicating that alumina ceramics gradually deteriorate along with the operation time due to some reasons which can probably be attributed to the alumina materials. Actually. as shown in Figure 4. AT (average surface heating)
*Corresponding author: S Michizono, KEK. National Laboratory for High Energy Physics, Tsukuba, lbaraki 305, Japan. Tel: 0298-64-l 171 (Japan). Fax: 0298.-647438 (Japan), E-mail: michizon(g kekvax.kek.jp
625
S Michizono
et a/: S-band
pulsed klystrons
in the KEK linac
Table 1. Failure analysis of klystrons. Unused klystrons are those which have never been used in the linac. Living klystrons are those which have been used (working) or had been used and can be used there again (stand-by)
Year of production
Cathode
1979-1987 oxide 1987-1993 BI
“0
Total No. of klystrons
No. of unused klystrons
No. of living klystrons
Average operation time (h)
No. of failed klystrons arcing
Causes window
111 104
2 28
5 52
8,558 18,719
106 24
13 18
5
12 0
others
Cumulative Mean age operation (h) (tube-hours)
MTBF (h)
21 6
10,783 11,176
11,187 5 1,134
1,185,777 1,241,618
IO
15 20 25 AT (K) Figure 1. Distribution of the temperature increase from room temperature (AT) during RF operation (-25 MW, 3.5 ps, 25 pps) until May, 1995. Both the failed and living klystrons are counted.
-
89507
-89510 + 89512 il -90513 + 91506 -a- -91512 l 92504
-m-
-. 0
5
40
rf o&&i
tirn?(tho&md
3hoours~
Figure 3. Photograph of the failed window (a). Many localized melted locations can be observed and (b) The melted surface of the window was observed with a scanning electron microscope.
Figure 2. Increase in AT with the operation time. The legend is the klystron number. Window materials and breakdown mechanism depends on the window material. Three kinds of ceramics (A, B and C) are used for RF windows. The AT of ceramics A and B were from 5 to 25°C; on the other hand, the AT of ceramics C ranged from 5 to 7°C. It is considered that the difference of AT between the window materials was related to the appearance of localized surface heating. 626
From the viewpoints of the structures and properties of alumina materials, high-power tests were performed. Ceramics A and B have larger grains with a diameter of more than 50 pm, including many voids (Table 2). Ceramic D has the smallest grains with a diameter of about 2 pm (dense structure); this is not used for klystron windows because of a metallizing difficulty. The dielectric constants (t) and loss tangents (tanb) are measured using an
S Michizono
et al: S-band pulsed klystrons
in the KEK linac
10
30-
KEKB klystron
n
0
0
0
4 41.1s.4Opps
25cl 0
0
c
zo-
S
15-
0
0
0
0”
0 0
0
00
00”
0
o”O
lo-
00 0
0
:/ 5:
8
,,,‘l,l’,,,‘,,,I,,,I,,,
0
ot,...‘....‘....“““....‘....““.“,..1 0 5
15
10
rf
operation
20
time
25
(thousand
0 30
35
A, B and C during RF operation at the KEK linac in May, 1995.
(i) Excess heating due to multipactoring or localized RF dissipation results in the production of the F-center oxygen defects (me-breakdown).
and results of high-power
Materials
Window
A B C
klystron klystron klystron
D
waveguide
type
6
during
10
12
RF operation.
(ii) Localized ohmic losses due to excited electrons in the Fcenter take place under an RF electric field. (iii) These ohmic losses enhance an increase in the F-center density. causes surface melting or (iv) Such a ‘runaway’ phenomenon dielectric breakdown. A ceramic having a dense structure and/or crystallized additives. and consequently having low tan& is durable for RF windows.
Windows for KEKB klystrons Since 50 MW klystrons capable of a 4 /ls pulse width at 50 pps are required for the KEKB project.’ the development of RF windows is necessary. Ceramic C has been adopted for the window material as well as TIN film coatings for multipactor suppression. The film thickness is controlled so as to suppress multipactoring and to avoid excess ohmic heating of the film.’ To check and verify the durability of ceramic C, the window installed in a 50 MW klystron was tested. and the AT of the window was measured (Figure 5). AT was suppressed by about 10% compared to an RF window with ceramic A. Also. AT of ceramic C at the linac had a small distribution. as previously described, though it is statistically not sigmlicant. It has been confirmed that ceramic C is suitable for the RF windows of the KEKB klystrons.
tests
tan d (10 “)
Pre-existmg defects
Defects after 200 MW operation
97.6 99.5 99.7
3.2 3.0 0.4
F+.F F’ F’
F+.F F+.F F’
99.0
1.3
F+
F’
Purity (%)
8
( z 25
RF cavity’ in order to estimate the RF dissipation. The rather high tam3 values of ceramics A and B are probably caused by impurities and their voids.’ Ceramic C is specially sintered for crystallizing additives of SiOz and MgO in order to reduce RF losses (tan@. High-power tests of these window materials were carried out using a resonant ring.‘.‘.’ Since all of the samples were coated with TIN films for multipactor suppression, multipactoring took place only at less than 10 MW of transmission power. The results are shown in Table 2. From a spectrum analysis of an alumina luminescence of less than IO MW, F-center oxygen defects (each oxygen vacancy with two trapped electrons) were observed at ceramics A and B. where localized surface melting took place. It has been reported that F-center defects are introduced by heating,’ and that excited electrons in the F-centers are capable of contributing to electric conduction.’ Localized RF dissipation possibly took place around the voids of ceramics A and B. which induced F-center defects (pre-breakdown). On the other hand. ceramics C and D were considered not to be liable to F-center defects due to crystallized additives and its microscopically dense structure (without porosities), respectively. Thus, the breakdown mechanism is considered to be as follows:
Table 2. Properties
1
average power (kW)
Figure 5. AT of the 50 MW klystron
hours)
Figure 4. AT of window materials MW, 3.5~~. 25 pps) measured
2
40
Grain
size
Comments localized surface melting localized surface melting no breakdown surface melting at uncoated area
627
S Michizono
et al: S-band pulsed klystrons
in the KEK linac
Conclusion
References
The breakdown of the RF window is due to localized excess heating. Actually, RF windows with a large AZ’ (temperature increase from room temperature) show localized surface melting, or sometimes punctures leading to vacuum leaks. Since breakdown is related to the occurrence of the F-center of alumina ceramics, ceramics having a low tan6 (such as ceramic C) are durable. At the KEKB project, since 50 MW klystrons were used, an improvement in the RF windows became necessary. An RF window with ceramic C was chosen for the KEKB klystron, and AT was reduced by about 10% compared to a window with ceramic A.
‘0 Shimomura (Ed.), Photon Factory Activity Report, KEK Pro,yress Report. KEK, 1994. ‘S Michizono, Y Saito. S Yamaguchi, S Anami, N Matuda and A Kinbara, IEEE Tram on Elecrr Insul, 28, 692 (1993). ‘S Michizono, A Kinbara, Y Saito, S Yamaguchi, S Anami and N Matuda, J Vat Sci Technol, AlO, I 180 (1992). “S Fukuda, S Michizono, K Nakao, Y Saito and S Anami, Pm o/‘thc 1994 Linac Conf; Tsukuba, Japan, I. 1994, p 427. “Y Saito, N Matuda, S Anami, A Kinbara, G Horikoshi and J Tanaka. IEEE Tram on Elrctr Insul, 24, 1029 (1989). ‘K H Lee and J H Crawford, Jr, Appl Phys Letr, 33,273 (1978). ‘K H Lee and J H Crawford. Jr, Phys Rw, B19, 3217 (1979).
628
Pergamon PII: SOO42-207X(96)00034-6
RF windows KEK iinac
47/numbers 6-8/pages 625 to 628/1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All riahts reserved 0042-207X196 $15.00+.00
used at S-band pulsed klystrons in the
S Michizono,“” Y Saito,” S Fukuda,” K HayashP and S Anami,” aKEK, National Laboratory Physics, Tsukuba, lbaraki 305, Japan; bMitsubishi Electric Corporation, CEW, Amagasaki,
The breakdown of the alumina RF-windows used in the development of klystrons. This breakdown bombardment and/or localized RF dissipation. In KEKB klysrrons, several kinds of alumina ceramics windows are being considered. An improved RF Copyright 0 1996Elsevier Science Ltd. Key words: Alumina,
ceramics,
klystron,
for High Energy Hyogo 661, Japan
in high-power klystrons is one of the most serious problems results from excess hearing of alumina due to mulripactor order to develop RF windows having high durability for the are being examined, and the breakdown mechanism of RF window installed in a KEKB klystron is also being rested.
RF window.
Introduction
Klystron failures and window breakdown
So far, 48 S-band pulsed klystrons (2856 MHz, 30 MW in maximum, 3.5 ,US,25 pps) have been operated at the KEK 2.5 GeV linac. Each klystron has an RF window installed in its output portion in order to isolate vacuum from the atmosphere, and to pass RF power. The window comprises a brazed aluminaceramic disk and a pill-box housing. Breakdown of the RF windows during RF operation is one of the most serious problems in long time RF-operation, and comprises a large majority of klystron failures.’ This breakdown is mainly due to localized surface melting of alumina, leading to punctures due to multipactoring and/or localized RF dissipation. Multipactoring involves a resonant multiplication (in the RF field) of secondary emitted electrons generated at the alumina surface, which results in luminescence of alumina.’ ’ This multipactoring can be suppressed to some extent by using thin-film coatings having low secondary-electron emission yields. such as TiN.’ Localized RF dissipation depends on the ceramic purity and structure.’ In order to prevent window breakdown, it is important to evaluate the alumina materials statistically and to study the breakdown mechanism. In this report. the cumulative status of RF window breakdown and R&D concerning window materials are summarized. The breakdown mechanism of RF windows is also described and the RF window used for a KEKB klystron a is examined.
The cumulative status of klystron failures is given in Table I .’ Failure due to internal arcing between the electrodes of the electron gun was most frequent for klystrons in which oxide cathodes were used. The use of barium-impregnated (BI) cathodes, which have been used since 1987, suppressed internal arcing due to less electrode contamination. Thus, a cumulative mean time before failure (MTBF) increases by more than four times (51,734 h; this corresponds to ten years of operation, since the operation time during one year was 5,120 h in 1993). However, the breakdown or RF windows still remained. The window-breakdown has to be studied not only for life time but also developments for a 50 MW-klystron used at the KEKB project. In order to understand the window-breakdown process, statistical research of the window-housing temperatures during RF operation was carried out at the linac, where each klystron output power is around 25 MW. Figure 1 shows the temperature increase from room temperature (AT) measured up to May. 1995. The temperatures of some windows became more than 45 C (room temperature was 27°C); AT of failed klystrons were larger than those of the living klystrons. The dependence of AT on the operation time was summarized (Figure 2) to investigate whether such a large AT existed from the beginning or not. AT increased with the operation time for those windows with a high AT; AT of klystrons ‘89507’ and ‘91512’ increased rapidly (about 5000 h). and AT of ‘895 12’ increased slowly (about 25000 h). Both of these windows showed localized surface melting (Figure 3) indicating that alumina ceramics gradually deteriorate along with the operation time due to some reasons which can probably be attributed to the alumina materials. Actually. as shown in Figure 4. AT (average surface heating)
*Corresponding author: S Michizono, KEK. National Laboratory for High Energy Physics, Tsukuba, lbaraki 305, Japan. Tel: 0298-64-l 171 (Japan). Fax: 0298.-647438 (Japan), E-mail: michizon(g kekvax.kek.jp
625
S Michizono
et a/: S-band
pulsed klystrons
in the KEK linac
Table 1. Failure analysis of klystrons. Unused klystrons are those which have never been used in the linac. Living klystrons are those which have been used (working) or had been used and can be used there again (stand-by)
Year of production
Cathode
1979-1987 oxide 1987-1993 BI
“0
Total No. of klystrons
No. of unused klystrons
No. of living klystrons
Average operation time (h)
No. of failed klystrons arcing
Causes window
111 104
2 28
5 52
8,558 18,719
106 24
13 18
5
12 0
others
Cumulative Mean age operation (h) (tube-hours)
MTBF (h)
21 6
10,783 11,176
11,187 5 1,134
1,185,777 1,241,618
IO
15 20 25 AT (K) Figure 1. Distribution of the temperature increase from room temperature (AT) during RF operation (-25 MW, 3.5 ps, 25 pps) until May, 1995. Both the failed and living klystrons are counted.
-
89507
-89510 + 89512 il -90513 + 91506 -a- -91512 l 92504
-m-
-. 0
5
40
rf o&&i
tirn?(tho&md
3hoours~
Figure 3. Photograph of the failed window (a). Many localized melted locations can be observed and (b) The melted surface of the window was observed with a scanning electron microscope.
Figure 2. Increase in AT with the operation time. The legend is the klystron number. Window materials and breakdown mechanism depends on the window material. Three kinds of ceramics (A, B and C) are used for RF windows. The AT of ceramics A and B were from 5 to 25°C; on the other hand, the AT of ceramics C ranged from 5 to 7°C. It is considered that the difference of AT between the window materials was related to the appearance of localized surface heating. 626
From the viewpoints of the structures and properties of alumina materials, high-power tests were performed. Ceramics A and B have larger grains with a diameter of more than 50 pm, including many voids (Table 2). Ceramic D has the smallest grains with a diameter of about 2 pm (dense structure); this is not used for klystron windows because of a metallizing difficulty. The dielectric constants (t) and loss tangents (tanb) are measured using an
S Michizono
et al: S-band pulsed klystrons
in the KEK linac
10
30-
KEKB klystron
n
0
0
0
4 41.1s.4Opps
25cl 0
0
c
zo-
S
15-
0
0
0
0”
0 0
0
00
00”
0
o”O
lo-
00 0
0
:/ 5:
8
,,,‘l,l’,,,‘,,,I,,,I,,,
0
ot,...‘....‘....“““....‘....““.“,..1 0 5
15
10
rf
operation
20
time
25
(thousand
0 30
35
A, B and C during RF operation at the KEK linac in May, 1995.
(i) Excess heating due to multipactoring or localized RF dissipation results in the production of the F-center oxygen defects (me-breakdown).
and results of high-power
Materials
Window
A B C
klystron klystron klystron
D
waveguide
type
6
during
10
12
RF operation.
(ii) Localized ohmic losses due to excited electrons in the Fcenter take place under an RF electric field. (iii) These ohmic losses enhance an increase in the F-center density. causes surface melting or (iv) Such a ‘runaway’ phenomenon dielectric breakdown. A ceramic having a dense structure and/or crystallized additives. and consequently having low tan& is durable for RF windows.
Windows for KEKB klystrons Since 50 MW klystrons capable of a 4 /ls pulse width at 50 pps are required for the KEKB project.’ the development of RF windows is necessary. Ceramic C has been adopted for the window material as well as TIN film coatings for multipactor suppression. The film thickness is controlled so as to suppress multipactoring and to avoid excess ohmic heating of the film.’ To check and verify the durability of ceramic C, the window installed in a 50 MW klystron was tested. and the AT of the window was measured (Figure 5). AT was suppressed by about 10% compared to an RF window with ceramic A. Also. AT of ceramic C at the linac had a small distribution. as previously described, though it is statistically not sigmlicant. It has been confirmed that ceramic C is suitable for the RF windows of the KEKB klystrons.
tests
tan d (10 “)
Pre-existmg defects
Defects after 200 MW operation
97.6 99.5 99.7
3.2 3.0 0.4
F+.F F’ F’
F+.F F+.F F’
99.0
1.3
F+
F’
Purity (%)
8
( z 25
RF cavity’ in order to estimate the RF dissipation. The rather high tam3 values of ceramics A and B are probably caused by impurities and their voids.’ Ceramic C is specially sintered for crystallizing additives of SiOz and MgO in order to reduce RF losses (tan@. High-power tests of these window materials were carried out using a resonant ring.‘.‘.’ Since all of the samples were coated with TIN films for multipactor suppression, multipactoring took place only at less than 10 MW of transmission power. The results are shown in Table 2. From a spectrum analysis of an alumina luminescence of less than IO MW, F-center oxygen defects (each oxygen vacancy with two trapped electrons) were observed at ceramics A and B. where localized surface melting took place. It has been reported that F-center defects are introduced by heating,’ and that excited electrons in the F-centers are capable of contributing to electric conduction.’ Localized RF dissipation possibly took place around the voids of ceramics A and B. which induced F-center defects (pre-breakdown). On the other hand. ceramics C and D were considered not to be liable to F-center defects due to crystallized additives and its microscopically dense structure (without porosities), respectively. Thus, the breakdown mechanism is considered to be as follows:
Table 2. Properties
1
average power (kW)
Figure 5. AT of the 50 MW klystron
hours)
Figure 4. AT of window materials MW, 3.5~~. 25 pps) measured
2
40
Grain
size
Comments localized surface melting localized surface melting no breakdown surface melting at uncoated area
627
S Michizono
et al: S-band pulsed klystrons
in the KEK linac
Conclusion
References
The breakdown of the RF window is due to localized excess heating. Actually, RF windows with a large AZ’ (temperature increase from room temperature) show localized surface melting, or sometimes punctures leading to vacuum leaks. Since breakdown is related to the occurrence of the F-center of alumina ceramics, ceramics having a low tan6 (such as ceramic C) are durable. At the KEKB project, since 50 MW klystrons were used, an improvement in the RF windows became necessary. An RF window with ceramic C was chosen for the KEKB klystron, and AT was reduced by about 10% compared to a window with ceramic A.
‘0 Shimomura (Ed.), Photon Factory Activity Report, KEK Pro,yress Report. KEK, 1994. ‘S Michizono, Y Saito. S Yamaguchi, S Anami, N Matuda and A Kinbara, IEEE Tram on Elecrr Insul, 28, 692 (1993). ‘S Michizono, A Kinbara, Y Saito, S Yamaguchi, S Anami and N Matuda, J Vat Sci Technol, AlO, I 180 (1992). “S Fukuda, S Michizono, K Nakao, Y Saito and S Anami, Pm o/‘thc 1994 Linac Conf; Tsukuba, Japan, I. 1994, p 427. “Y Saito, N Matuda, S Anami, A Kinbara, G Horikoshi and J Tanaka. IEEE Tram on Elrctr Insul, 24, 1029 (1989). ‘K H Lee and J H Crawford, Jr, Appl Phys Letr, 33,273 (1978). ‘K H Lee and J H Crawford. Jr, Phys Rw, B19, 3217 (1979).
628