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Coaxial HOM coupler for the 500 MHz RF damped cavity
Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 91–94
Coaxial HOM coupler for the 500 MHz RF damped cavity T. Kosekia,*, M. Izawab, T. Takahashib, Y. Kamiyaa, K. Satohc, H. Ogatac a
Synchrotron Radiation Laboratory, The Institute for Solid State Physics (ISSP), University of Tokyo, 5-1-5 Kashiwanoha, Kashiwashi, Chiba 277-8581, Japan b Photon Factory (PF), High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan c Toshiba Corporation, Tsurumi, Yokohama 230-0045, Japan
Abstract We have developed a new higher-order modes (HOMs) coupler of coaxial waveguide type for the 500 MHz damped cavity. An SiC ceramics is adopted as microwave absorber. Two prototype models of the HOM coupler have been fabricated and tested. The detailed design of the coupler is described in this paper. The results of RF characteristics measurement and high power conditioning are also presented. # 2001 Elsevier Science B.V. All rights reserved. PACS: 29.20.D Keywords: RF cavity; Higher-order mode; Microwave absorber; SiC
1. Introduction We developed a 500 MHz damped cavity for high brilliance synchrotron radiation sources. The cavity has a large diameter (140 mm) beam-duct made of silicon–carbide (SiC) microwave absorber. The HOMs excited in the cavity are guided out to the beam duct and dissipated in the absorber [1]. The cavities of this type have been already adopted and operated well in the PF ring of KEK [2] and the NEW SUBARU ring of Himeji Institute of Technology [3]. The cavity also aims at being installed in the VSX storage ring, the ultra-low emittance synchrotron light source,
*Corresponding author. Tel.: +81-471-36-3565; fax: +81471-34-6041. E-mail address: [email protected] (T. Koseki).
which is being proposed by the University of Tokyo [4]. The HOMs with frequencies lower than the cutoff frequency of the beam-duct cannot propagate the beam duct, and some of them remain in the cavity with rather high Q-values. Such dangerous ‘‘trapped modes’’ are TM011, TM020, TM021, TE111, TM110 and TM111 modes. To suppress the instabilities due to them, the frequency-detuning method using fixed tuners is applied in the PF ring successfully [2]. By choosing suitable block lengths of the fixed tuners, frequencies of the trapped modes are well detuned so that the instability can be avoided. However, for a ring with a larger circumference such as the VSX ring, the method becomes less effective, because of low revolution frequency. In such a case, it becomes more preferable to reduce Q-values of the trapped modes.
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 2 4 9 - 2
92
T. Koseki et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 91–94
We have been developing a coaxial HOM coupler with an SiC microwave absorber. It can be attached to the horizontal and vertical fixedtuner ports of the cavity. Recently, two prototype models of the HOM coupler have been fabricated and tested.
2. Coaxial HOM coupler Fig. 1 shows the schematic view of the HOM coupler. It has a rod-shaped coupling antenna with a coaxial waveguide. A microwave absorber is attached to the end of the coaxial waveguide in order to absorb the extracted HOM power. The waveguide are terminated by the stainless flange to seal vacuum. The fixed-turner block mounted on the ICF-flange is used for detuning frequencies of a few trapped modes, unable to couple to the rod antenna. The fixed-tuner block and the rod antenna are made of OFHC copper and cooled by water. The dimensions of the rod-antenna and fixed-tuner block were determined by the measurement using cold models of the HOM coupler [5]. The length of the tuner block indicated as LB in Fig. 1, depends on the parameters of the ring in which the cavity is installed. For a beam test of the HOM coupler at the PF ring, LB was chosen as 79.9 mm, that is suitable for detuning the TM110 mode.
As the microwave absorber, we adopted the SiC ceramic, CERASIC-B (‘‘pressureless sintered SiC’’, Toshiba Ceramics Co. Ltd.), which is the same material as the absorber in the beam-duct. The SiC ceramic gave satisfactory results in various studies on the HOM absorber for damped cavities [6–9]. The SiC absorber is made in a taper shape to reduce the reflection coefficient of microwave. It is divided in the azimuthal direction into four pieces and brazed on the inner conductor. Heat load on the SiC absorber is removed via the inner conductor. A disk spacer made of macor is put in the waveguide to support the inner conductor. The shapes and positions of the SiC absorber and macor spacer were determined by the electromagnetic simulation code, HFSS. In this design, VSWR obtained is less than 1.7 in the frequency range between 0.7 and 1.4 GHz [10]. The simple structure and compact size are notable features of the HOM coupler. Specially, the size of waveguide on the outside of ICF flange are adequately small. It is advantageous to avoid interference with other components of the storage ring, such as synchrotron radiation beamlines.
3. Low power measurements We attached two HOM couplers to the cavity and measured their RF characteristics at low
Fig. 1. Schematic view of the HOM coupler.
T. Koseki et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 91–94
power levels. The S-parameters of the cavity were measured using an HP8510C network analyzer in both cases with and without HOM couplers. In the case without coupler, the fixed-tuner of 81 mm block length was attached to the port. All data were taken after adjusting the frequency of the accelerating mode to be 500.1 MHz by the tuner and taken at atmospheric pressure. The measured frequencies and Q-values of all trapped modes are summarized in Table 1. The required Q-values Qc , below which the coupled-bunch instabilities are not induced at the nominal operation mode for the VSX ring, are also given in the table. For the TM011, TM021, TE111 and TM111 modes, the reflection responses (S11 ) measured by a small rod antenna at monitor port are shown in Fig. 2. The Q-values of all trapped modes except a few modes are reduced drastically by the coupler whereas the Q-value of the accelerating mode is not affected. Only three modes, TM110H, TM110V and TM020, cannot couple to the rod antenna and still have high Q-values. For the TM110 modes, the resonant frequencies strongly depend on the length of fixed-tuner blocks [5]. It is expected that they can be detuned easily so as not to induce the instabilities. For the TM020 mode, however, the frequency shift by the fixed-tuner is not so large
93
that the detuning method is less effective [5]. If a strong instability due to TM020 occurs at the VSX ring, the other method such as bunch-by-bunch feedback and Landau cavity, should be applied to suppress the instability.
4. High power test We carried out a high power conditioning of the HOM couplers at the RF test bench of the Photon Factory. A detailed description of the bench is given in elsewhere [11]. The cavity was evacuated by a 300 1/s turbo molecular pump through the beam duct. The vacuum pressure before applying RF power was 4.8 10 6 Pa. Levels of input RF power was gradually increased so as to keep the vacuum pressure below 5 10 4 Pa and then it reached 60 kW after 4 h from the start. The temperatures of the cavity wall, cooling water channels and the HOM couplers were monitored by thermocouples. The measured temperatures of the HOM couplers were almost the same as those of the cavity wall through the conditioning. Thus it implies that the HOM coupler can be used at the input RF power much higher than 60 kW. The input RF power was, however, restricted to 60 kW
Table 1 The frequencies and Q-values of all trapped modes and the accelerating mode. Qc means the critical Q-value, i.e., target values of mode damping for the VSX ring Without couplers
With horizontal coupler
With vertical coupler
With horizontal and vertical couplers
Frequency (MHz)
Q
Frequency (MHz)
Frequency (MHz)
Frequency (MHz)
Longitudinal TM010 500.1 TM011 792.8 TM020 1312.1 TM021 1371.2
37000 22000 9000 10000
500.1 37000 789.7 190 1312.2 8600 Not measurable
500.1 37000 790.3 230 1312.3 8300 Not measurable
Transverse TE111H TE111V TM110V TM110H TM111H TM111V
36000 35000 8600 42000 11000 10000
Not measurable 706.9 35000 789.2 8600 793.1 40000 Not measurable 991.2 9000
702.7 34000 Not measurable 789.2 8600 793.1 40000 988.5 9500 Not measurable
701.0 706.5 788.9 792.8 985.2 990.3
Q
Q
500.1 786.9 1312.4
789.2 793.1
Q
Qc
37000 120 7600 Not measurable
150 520 520
Not measurable Not measurable 8600 45000 Not measurable Not measurable
3700 3700 100 100 60 60
94
T. Koseki et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 91–94
Fig. 2. Measured trapped modes without and with the HOM couplers: (a) the TM011 mode. A small peak on the ‘‘with coupler’’ line is the TM110H mode which does not couple to the rod antenna, (b) the TM021 mode, (c) the TE111 mode and (d) the TM111 mode.
at maximum in this test, as a trouble occurred at the input coupler of the cavity.
5. Conclusions and future plans The HOM coupler strongly couples to six trapped modes in the cavity and reduce their Qvalues drastically without affecting the accelerating mode. With two HOM couplers, input RF power up to 60 kW was applied to the cavity successfully, and unusual heating was not observed on the couplers. Although the input power of 60 kW is a sufficient level for low energy (1.0 GeV) operation of the VSX ring, more RF power is required for high energy (1.6 GeV) operation in order to obtain much longer Touschek lifetime [12]. Thus we will continue the high power test and try to increase the input power up to 100 kW. Furthermore, we will carry out actual beam test of the HOM couplers at the PF ring in near future.
References [1] T. Koseki, M. Izawa, Y. Kamiya, Rev. Sci. Instrum. 66 (1995) 1892. [2] M. Izawa et al., J. Synchrotron Rad. 5 (1998) 369. [3] A. Ando, Proceedings of the First Asian Conference on Particle Accelerator, Tsukuba, 1998, p. 646. [4] H. Takaki et al., The Lattice of 1.0–1.6 GeV VSX Storage Ring, Nucl. Instr. and Meth. A 467–468 (2001), in these proceedings. [5] T. Koseki et al., Proceedings of the Sixth European Conference on Particle Accelerator, Stockholm, 1998, p. 1776. [6] M. Izawa, T. Koseki, Y. Kamiya, T. Toyomasu, Rev. Sci. Instr. 66 (1995) 1910. [7] M. Izawa et al., Proceedings of the First Asian Conference on Particle Accelerator, Tsukuba, 1998, p. 758. [8] S. Sakanaka et al., Proceedings of the IEEE Conference on Particle Accelerator, Vancouver, 1997, p. 2983. [9] T. Takeuchi et al., Proceedings of the IEEE Conference on Particle Accelerator, Vancouver, 1997, p. 2986. [10] T. Takahashi et al., Proceedings of the IEEE Conference on Particle Accelerator, New York, 1999, p. 904. [11] T. Koseki et al., Proceedings of the IEEE Conference on Particle Accelerator, Vancouver, 1997, p. 3066. [12] Activity Report of SRL-ISSP 1997/1998, The University of Tokyo, 1999, p. 11.
Coaxial HOM coupler for the 500 MHz RF damped cavity T. Kosekia,*, M. Izawab, T. Takahashib, Y. Kamiyaa, K. Satohc, H. Ogatac a
Synchrotron Radiation Laboratory, The Institute for Solid State Physics (ISSP), University of Tokyo, 5-1-5 Kashiwanoha, Kashiwashi, Chiba 277-8581, Japan b Photon Factory (PF), High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan c Toshiba Corporation, Tsurumi, Yokohama 230-0045, Japan
Abstract We have developed a new higher-order modes (HOMs) coupler of coaxial waveguide type for the 500 MHz damped cavity. An SiC ceramics is adopted as microwave absorber. Two prototype models of the HOM coupler have been fabricated and tested. The detailed design of the coupler is described in this paper. The results of RF characteristics measurement and high power conditioning are also presented. # 2001 Elsevier Science B.V. All rights reserved. PACS: 29.20.D Keywords: RF cavity; Higher-order mode; Microwave absorber; SiC
1. Introduction We developed a 500 MHz damped cavity for high brilliance synchrotron radiation sources. The cavity has a large diameter (140 mm) beam-duct made of silicon–carbide (SiC) microwave absorber. The HOMs excited in the cavity are guided out to the beam duct and dissipated in the absorber [1]. The cavities of this type have been already adopted and operated well in the PF ring of KEK [2] and the NEW SUBARU ring of Himeji Institute of Technology [3]. The cavity also aims at being installed in the VSX storage ring, the ultra-low emittance synchrotron light source,
*Corresponding author. Tel.: +81-471-36-3565; fax: +81471-34-6041. E-mail address: [email protected] (T. Koseki).
which is being proposed by the University of Tokyo [4]. The HOMs with frequencies lower than the cutoff frequency of the beam-duct cannot propagate the beam duct, and some of them remain in the cavity with rather high Q-values. Such dangerous ‘‘trapped modes’’ are TM011, TM020, TM021, TE111, TM110 and TM111 modes. To suppress the instabilities due to them, the frequency-detuning method using fixed tuners is applied in the PF ring successfully [2]. By choosing suitable block lengths of the fixed tuners, frequencies of the trapped modes are well detuned so that the instability can be avoided. However, for a ring with a larger circumference such as the VSX ring, the method becomes less effective, because of low revolution frequency. In such a case, it becomes more preferable to reduce Q-values of the trapped modes.
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 2 4 9 - 2
92
T. Koseki et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 91–94
We have been developing a coaxial HOM coupler with an SiC microwave absorber. It can be attached to the horizontal and vertical fixedtuner ports of the cavity. Recently, two prototype models of the HOM coupler have been fabricated and tested.
2. Coaxial HOM coupler Fig. 1 shows the schematic view of the HOM coupler. It has a rod-shaped coupling antenna with a coaxial waveguide. A microwave absorber is attached to the end of the coaxial waveguide in order to absorb the extracted HOM power. The waveguide are terminated by the stainless flange to seal vacuum. The fixed-turner block mounted on the ICF-flange is used for detuning frequencies of a few trapped modes, unable to couple to the rod antenna. The fixed-tuner block and the rod antenna are made of OFHC copper and cooled by water. The dimensions of the rod-antenna and fixed-tuner block were determined by the measurement using cold models of the HOM coupler [5]. The length of the tuner block indicated as LB in Fig. 1, depends on the parameters of the ring in which the cavity is installed. For a beam test of the HOM coupler at the PF ring, LB was chosen as 79.9 mm, that is suitable for detuning the TM110 mode.
As the microwave absorber, we adopted the SiC ceramic, CERASIC-B (‘‘pressureless sintered SiC’’, Toshiba Ceramics Co. Ltd.), which is the same material as the absorber in the beam-duct. The SiC ceramic gave satisfactory results in various studies on the HOM absorber for damped cavities [6–9]. The SiC absorber is made in a taper shape to reduce the reflection coefficient of microwave. It is divided in the azimuthal direction into four pieces and brazed on the inner conductor. Heat load on the SiC absorber is removed via the inner conductor. A disk spacer made of macor is put in the waveguide to support the inner conductor. The shapes and positions of the SiC absorber and macor spacer were determined by the electromagnetic simulation code, HFSS. In this design, VSWR obtained is less than 1.7 in the frequency range between 0.7 and 1.4 GHz [10]. The simple structure and compact size are notable features of the HOM coupler. Specially, the size of waveguide on the outside of ICF flange are adequately small. It is advantageous to avoid interference with other components of the storage ring, such as synchrotron radiation beamlines.
3. Low power measurements We attached two HOM couplers to the cavity and measured their RF characteristics at low
Fig. 1. Schematic view of the HOM coupler.
T. Koseki et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 91–94
power levels. The S-parameters of the cavity were measured using an HP8510C network analyzer in both cases with and without HOM couplers. In the case without coupler, the fixed-tuner of 81 mm block length was attached to the port. All data were taken after adjusting the frequency of the accelerating mode to be 500.1 MHz by the tuner and taken at atmospheric pressure. The measured frequencies and Q-values of all trapped modes are summarized in Table 1. The required Q-values Qc , below which the coupled-bunch instabilities are not induced at the nominal operation mode for the VSX ring, are also given in the table. For the TM011, TM021, TE111 and TM111 modes, the reflection responses (S11 ) measured by a small rod antenna at monitor port are shown in Fig. 2. The Q-values of all trapped modes except a few modes are reduced drastically by the coupler whereas the Q-value of the accelerating mode is not affected. Only three modes, TM110H, TM110V and TM020, cannot couple to the rod antenna and still have high Q-values. For the TM110 modes, the resonant frequencies strongly depend on the length of fixed-tuner blocks [5]. It is expected that they can be detuned easily so as not to induce the instabilities. For the TM020 mode, however, the frequency shift by the fixed-tuner is not so large
93
that the detuning method is less effective [5]. If a strong instability due to TM020 occurs at the VSX ring, the other method such as bunch-by-bunch feedback and Landau cavity, should be applied to suppress the instability.
4. High power test We carried out a high power conditioning of the HOM couplers at the RF test bench of the Photon Factory. A detailed description of the bench is given in elsewhere [11]. The cavity was evacuated by a 300 1/s turbo molecular pump through the beam duct. The vacuum pressure before applying RF power was 4.8 10 6 Pa. Levels of input RF power was gradually increased so as to keep the vacuum pressure below 5 10 4 Pa and then it reached 60 kW after 4 h from the start. The temperatures of the cavity wall, cooling water channels and the HOM couplers were monitored by thermocouples. The measured temperatures of the HOM couplers were almost the same as those of the cavity wall through the conditioning. Thus it implies that the HOM coupler can be used at the input RF power much higher than 60 kW. The input RF power was, however, restricted to 60 kW
Table 1 The frequencies and Q-values of all trapped modes and the accelerating mode. Qc means the critical Q-value, i.e., target values of mode damping for the VSX ring Without couplers
With horizontal coupler
With vertical coupler
With horizontal and vertical couplers
Frequency (MHz)
Q
Frequency (MHz)
Frequency (MHz)
Frequency (MHz)
Longitudinal TM010 500.1 TM011 792.8 TM020 1312.1 TM021 1371.2
37000 22000 9000 10000
500.1 37000 789.7 190 1312.2 8600 Not measurable
500.1 37000 790.3 230 1312.3 8300 Not measurable
Transverse TE111H TE111V TM110V TM110H TM111H TM111V
36000 35000 8600 42000 11000 10000
Not measurable 706.9 35000 789.2 8600 793.1 40000 Not measurable 991.2 9000
702.7 34000 Not measurable 789.2 8600 793.1 40000 988.5 9500 Not measurable
701.0 706.5 788.9 792.8 985.2 990.3
Q
Q
500.1 786.9 1312.4
789.2 793.1
Q
Qc
37000 120 7600 Not measurable
150 520 520
Not measurable Not measurable 8600 45000 Not measurable Not measurable
3700 3700 100 100 60 60
94
T. Koseki et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 91–94
Fig. 2. Measured trapped modes without and with the HOM couplers: (a) the TM011 mode. A small peak on the ‘‘with coupler’’ line is the TM110H mode which does not couple to the rod antenna, (b) the TM021 mode, (c) the TE111 mode and (d) the TM111 mode.
at maximum in this test, as a trouble occurred at the input coupler of the cavity.
5. Conclusions and future plans The HOM coupler strongly couples to six trapped modes in the cavity and reduce their Qvalues drastically without affecting the accelerating mode. With two HOM couplers, input RF power up to 60 kW was applied to the cavity successfully, and unusual heating was not observed on the couplers. Although the input power of 60 kW is a sufficient level for low energy (1.0 GeV) operation of the VSX ring, more RF power is required for high energy (1.6 GeV) operation in order to obtain much longer Touschek lifetime [12]. Thus we will continue the high power test and try to increase the input power up to 100 kW. Furthermore, we will carry out actual beam test of the HOM couplers at the PF ring in near future.
References [1] T. Koseki, M. Izawa, Y. Kamiya, Rev. Sci. Instrum. 66 (1995) 1892. [2] M. Izawa et al., J. Synchrotron Rad. 5 (1998) 369. [3] A. Ando, Proceedings of the First Asian Conference on Particle Accelerator, Tsukuba, 1998, p. 646. [4] H. Takaki et al., The Lattice of 1.0–1.6 GeV VSX Storage Ring, Nucl. Instr. and Meth. A 467–468 (2001), in these proceedings. [5] T. Koseki et al., Proceedings of the Sixth European Conference on Particle Accelerator, Stockholm, 1998, p. 1776. [6] M. Izawa, T. Koseki, Y. Kamiya, T. Toyomasu, Rev. Sci. Instr. 66 (1995) 1910. [7] M. Izawa et al., Proceedings of the First Asian Conference on Particle Accelerator, Tsukuba, 1998, p. 758. [8] S. Sakanaka et al., Proceedings of the IEEE Conference on Particle Accelerator, Vancouver, 1997, p. 2983. [9] T. Takeuchi et al., Proceedings of the IEEE Conference on Particle Accelerator, Vancouver, 1997, p. 2986. [10] T. Takahashi et al., Proceedings of the IEEE Conference on Particle Accelerator, New York, 1999, p. 904. [11] T. Koseki et al., Proceedings of the IEEE Conference on Particle Accelerator, Vancouver, 1997, p. 3066. [12] Activity Report of SRL-ISSP 1997/1998, The University of Tokyo, 1999, p. 11.