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Waveguide-coupled cavities for energy recovery linacs
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 528 (2004) 220–224
Waveguide-coupled cavities for energy recovery linacs S.S. Kurennoy*, D.C. Nguyen, L.M. Young Los Alamos National Laboratory, Los Alamos, NM 87545, USA
Abstract A novel scheme for energy recovery linacs used as FEL drivers is proposed. It consists of two parallel beam lines, one for electron beam acceleration and the other for the used beam that is bent after passing through a wiggler. The used beam is decelerated by the structure and feeds the cavity fields. The main feature of the scheme is that RF cavities are coupled with waveguides between these two linacs. The waveguide cut through the two beam pipes provides an efficient mechanism for energy transfer. The superconducting RF cavities in the two accelerators can be shaped differently, with an operating mode at the same frequency. This provides HOM detuning and therefore reduces the beam break-up effects. Another advantage of the proposed two-beam scheme is easy tuning of the cavity coupling by changing the waveguide length. r 2004 Elsevier B.V. All rights reserved. PACS: 41.60.Cr; 29.17.+w; 29.27.Bd; 41.75.Fr Keywords: FEL; Energy recovery; CW; High current; High power
1. Introduction Energy recovery (ER) will be an important feature of many, if not all, future high-power FELs. This is due to the fact that in FELs an electron beam passing through a wiggler radiates a rather small fraction of its energy, typically 1–2% in oscillator FELs and up to 5–7% in amplifier FELs. For an FEL system to be efficient, the remaining significant beam power must be recovered. Moreover, if this power is not recycled in the system, disposing of the spent beam could be a *Corresponding author. Tel.: +1-505-665-1459; fax: +1505-665-2904. E-mail address: [email protected] (S.S. Kurennoy).
difficult problem: the high-power beam dumping becomes a serious challenge. There are different options for energy recovery. One is reusing the electron beam after it has passed through the wiggler. After conditioning and acceleration, the beam is returned to the wiggler again and again. This scheme is more appropriately called ‘‘beam recovery.’’ It is better suited for oscillator-type FELs, where the beam quality degrades less after passing through the wiggler. In a sense, storage-ring FELs are the extreme example of this approach. Another ER option is to recover only the energy contained in the spent beam, not the beam itself. This can be achieved by two different methods. The beam, after it has passed through the wiggler,
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.04.051
ARTICLE IN PRESS S.S. Kurennoy et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 220–224
(1) is sent again through the same accelerating structure as the initial beam, but in a decelerating phase, or (2) is directed through a separate decelerating structure. The last structure should be strongly coupled to the accelerating structure to transfer the recovered energy efficiently. These two methods are classified as the ‘‘same-structure (or same-cell) ER’’ and ‘‘different-structure ER.’’ The idea of energy recovery in accelerators has a long history. Starting from a first proposal by Tigner [1], this idea led to the Reflexotron [2], and then to experiments on the different-structure ER in two copper linacs with bridge couplers at LANL [3], and the same-cell ER in a superconducting (SC) linac at SLAC [4]. The development of ER culminated recently in a record high average power of 1.7 kW CW FEL (2.3 kW by now) achieved in the SC IR Demo FEL at JLab [5] with the same-cell ER. Obviously, all ER schemes have the common advantage of making an FEL system more effective overall. An important consideration of the FEL system accelerator efficiency, given fixed beam energy at injection and at the wiggler, is related to the ratio Pb/Pw, where Pb is the beam power and Pw is the power dissipated in accelerator structures. The higher this ratio, the more efficient the ER can be. SC systems certainly have a big advantage here, both for ‘‘same-cell’’ and ‘‘different-structure’’ ER. Disadvantages of ER schemes include generally more complex systems than those without ER, except for the beam dump. Additional problems in beam dynamics include possible instabilities at high currents, such as beam break-up (BBU). The transverse BBU appears to be the most dangerous in recirculating ER linacs [6]. From this viewpoint, normal-conducting structures, where damping of high-order modes (HOM) is easier than in SC structures and Qfactors are lower, can be better. For the same reason, SC ‘‘different-structure’’ ER has an advantage over SC ‘‘same-cell’’ ER with respect to the beam stability: one can detune HOM in its decelerating structure. The ‘‘same-cell’’ ER seems to be a favorite choice today, in large part because of economical reasons. We would like to propose here a simple option for coupling RF cavities in two linacs of the
221
‘‘different-structure’’ ER that can be useful and important for future really high-power FELs.
2. Waveguide-coupled cavities for ER 2.1. Waveguide coupling Three-cell bridge couplers were used for energy transfer between two copper linacs at LANL [3]. Their central cell served also for the RF power input. Four tuning posts per bridge allowed changing the field in the decelerator from 15% to 120% of that in the accelerator. Unfortunately, such bridges are too complicated to be implemented in SC linacs. It would be good to have couplers that are simpler. We have explored an option with a coaxial coupler and found that getting the coupling coefficient on the order of a few percent this way was very difficult. On the other hand, a coupler consisting of a waveguide (WG) section that cut through two beam pipes was found to give a strong coupling; it is easily tunable as well. The model coupler layout is shown in Fig. 1. 2.2. Model of two waveguide-coupled cavities To demonstrate the idea, we intentionally chose a simple model with two differently shaped
Fig. 1. Model coupler layout. Two beam paths are shown by the lines.
ARTICLE IN PRESS 222
S.S. Kurennoy et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 220–224
cavities—one in an accelerating linac and the other in a decelerator. The first cavity is a cylindrical pillbox (C), while the second one has a squareshaped cross section (S), but they both have the same fundamental frequency at 700 MHz. The WG coupler here is a section of a standard WR1150, but it can be some other WG. The coupling is regulated by the size of irises connecting the WG with the cavities. We studied our simple model with the MicroWave Studio (MWS) [7] to calculate the mode frequencies and coupling. The working mode
Fig. 2. Electric field of the fundamental p/2 mode at 700 MHz.
frequency can easily be tuned to 700 MHz by small changes of the transverse dimensions of the two cavities. The electric field of this mode is shown in Fig. 2. If we consider a three-cell system consisting of two linac cells, C and S with the coupling cell WG in between, the mode can be classified as a p/2-mode. Obviously, the coupling cell is not excited in this mode. Small changes of the WG length make two neighbor modes (0- and p-type, Fig. 3) separated from the working mode by the same shift, but in opposite directions: at f0 ¼ 694 MHz and fp ¼ 706 MHz. Hence, the coupling is k ¼ ðfp f0 Þ=fp=2 ¼ 12=700 ¼ 0:017 for the model in Fig. 2, where the radii of the coupling apertures are equal to that of the beam pipe, 65 mm. For the model in Fig. 1, where the cavity–WG coupling irises are larger (r ¼ 80 mm), the coupling is 3.5%. The coupling also depends on the separation between the cavity and WG, i.e., on the iris thickness. The HOMs in the model are uncoupled because the cavities in two linacs have different shapes. This is clearly illustrated in Fig. 4, where two dipole modes are plotted. The mode fields are contained mostly in their ‘native’ cavities. The WG length Lwg in our model was chosen to be slightly larger than one full WG wavelength at 700 MHz, lwg ¼ 63:02 cm.The WG ends are at about Lwg =4 from the cavity axes, and the axis separation is close to Lwg =2: This makes a compact structure that can fit, for example, in a single
Fig. 3. Electric fields of 0- and p-modes at 694 and 706 MHz.
ARTICLE IN PRESS S.S. Kurennoy et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 220–224
223
Fig. 4. Electric fields of two HOMs at 804 and 822 MHz.
cryomodule. However, the WG length can be increased by an integer number of lwg/2, if needed. Slightly changing the WG section length gives us an effective tuning option. The flat ends of the WG section can be made thin enough to implement mechanical push–pull tuners. 2.3. Coupled multi-cell cavities in linacs The same approach can be applied for multi-cell cavities in linacs. Fig. 5 illustrates the working mode at 700 MHz for a model with 2-cell cavities. One can see that again the WG coupler is not excited in this mode, while the cavities have field patterns typical for 2-cell p-modes. From the viewpoint of mode classification in a 5-cell cavity consisting of cells C1, C2, WG coupler, S1, and S2, this working mode has the structure (1,1,0,1,1). It can be called 3p/4-mode, as the fourth mode in the 5-mode pass band. This pass band has its lowest, 0-mode at 687.5 MHz and the highest, pmode at 704 MHz. The cell coupling coefficient here is 2.1%.
3. Discussion A simple model with simplified cavities was presented to illustrate the idea of WG-coupled linacs for energy recovery in FELs. The WG
Fig. 5. Electric field of the ER working mode at 700 MHz.
coupling between cavities in the accelerator and decelerator structures appears to be simple, effective, and easily tunable. It is important to note that this scheme can be implemented in SC linacs as well. Of course, for SC linacs the cavities should be, most likely, of the multi-cell elliptical type, but the WG coupling will work for them too, as one can see from Section 2.3. A rectangular WG was used in our simple model. Possible multipacting effects in such a WG can be mitigated by choosing a different WG shape, e.g., a WG with an elliptical crosssection.
ARTICLE IN PRESS 224
S.S. Kurennoy et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 220–224
4. Conclusions
Acknowledgements
A simple scheme for coupling RF cavities in the ‘‘different-structure’’ energy recovery linacs used as high-power FEL drivers is proposed. The RF cavities are coupled between the two linacs—an accelerator and decelerator—with waveguides that cut through two beam pipes, providing an efficient mechanism for energy transfer. The RF cavities in the two linacs can be shaped differently to detune HOMs and increase BBU thresholds. Using a simple model, we have shown that the WG coupling is simple, tunable, and can be implemented for multi-cell cavities in SC RF linacs. It can be a useful option on a path to high-power ER FELs.
The authors would like to acknowledge useful discussions with F.L. Krawczyk, D.L. Schrage, and R.L. Wood (LANL).
References [1] M. Tigner, Nuovo Cimento 37 (1965) 1228. [2] S.O. Schriber, E.A. Heighway, IEEE NS-22 (1975) 1060. [3] D.W. Feldman, et al., Nucl. Instr. and Meth. A 259 (1987) 26. [4] T.I. Smith, et al., Nucl. Instr. and Meth. A 259 (1987) 1. [5] G.R. Neil, et al., Phys. Rev. Lett. 84 (2000) 662. [6] L. Merminga, Nucl. Instr. and Meth. A 483 (2002) 107. [7] MicroWave Studio, CST, Darmstadt, Germany.
Nuclear Instruments and Methods in Physics Research A 528 (2004) 220–224
Waveguide-coupled cavities for energy recovery linacs S.S. Kurennoy*, D.C. Nguyen, L.M. Young Los Alamos National Laboratory, Los Alamos, NM 87545, USA
Abstract A novel scheme for energy recovery linacs used as FEL drivers is proposed. It consists of two parallel beam lines, one for electron beam acceleration and the other for the used beam that is bent after passing through a wiggler. The used beam is decelerated by the structure and feeds the cavity fields. The main feature of the scheme is that RF cavities are coupled with waveguides between these two linacs. The waveguide cut through the two beam pipes provides an efficient mechanism for energy transfer. The superconducting RF cavities in the two accelerators can be shaped differently, with an operating mode at the same frequency. This provides HOM detuning and therefore reduces the beam break-up effects. Another advantage of the proposed two-beam scheme is easy tuning of the cavity coupling by changing the waveguide length. r 2004 Elsevier B.V. All rights reserved. PACS: 41.60.Cr; 29.17.+w; 29.27.Bd; 41.75.Fr Keywords: FEL; Energy recovery; CW; High current; High power
1. Introduction Energy recovery (ER) will be an important feature of many, if not all, future high-power FELs. This is due to the fact that in FELs an electron beam passing through a wiggler radiates a rather small fraction of its energy, typically 1–2% in oscillator FELs and up to 5–7% in amplifier FELs. For an FEL system to be efficient, the remaining significant beam power must be recovered. Moreover, if this power is not recycled in the system, disposing of the spent beam could be a *Corresponding author. Tel.: +1-505-665-1459; fax: +1505-665-2904. E-mail address: [email protected] (S.S. Kurennoy).
difficult problem: the high-power beam dumping becomes a serious challenge. There are different options for energy recovery. One is reusing the electron beam after it has passed through the wiggler. After conditioning and acceleration, the beam is returned to the wiggler again and again. This scheme is more appropriately called ‘‘beam recovery.’’ It is better suited for oscillator-type FELs, where the beam quality degrades less after passing through the wiggler. In a sense, storage-ring FELs are the extreme example of this approach. Another ER option is to recover only the energy contained in the spent beam, not the beam itself. This can be achieved by two different methods. The beam, after it has passed through the wiggler,
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.04.051
ARTICLE IN PRESS S.S. Kurennoy et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 220–224
(1) is sent again through the same accelerating structure as the initial beam, but in a decelerating phase, or (2) is directed through a separate decelerating structure. The last structure should be strongly coupled to the accelerating structure to transfer the recovered energy efficiently. These two methods are classified as the ‘‘same-structure (or same-cell) ER’’ and ‘‘different-structure ER.’’ The idea of energy recovery in accelerators has a long history. Starting from a first proposal by Tigner [1], this idea led to the Reflexotron [2], and then to experiments on the different-structure ER in two copper linacs with bridge couplers at LANL [3], and the same-cell ER in a superconducting (SC) linac at SLAC [4]. The development of ER culminated recently in a record high average power of 1.7 kW CW FEL (2.3 kW by now) achieved in the SC IR Demo FEL at JLab [5] with the same-cell ER. Obviously, all ER schemes have the common advantage of making an FEL system more effective overall. An important consideration of the FEL system accelerator efficiency, given fixed beam energy at injection and at the wiggler, is related to the ratio Pb/Pw, where Pb is the beam power and Pw is the power dissipated in accelerator structures. The higher this ratio, the more efficient the ER can be. SC systems certainly have a big advantage here, both for ‘‘same-cell’’ and ‘‘different-structure’’ ER. Disadvantages of ER schemes include generally more complex systems than those without ER, except for the beam dump. Additional problems in beam dynamics include possible instabilities at high currents, such as beam break-up (BBU). The transverse BBU appears to be the most dangerous in recirculating ER linacs [6]. From this viewpoint, normal-conducting structures, where damping of high-order modes (HOM) is easier than in SC structures and Qfactors are lower, can be better. For the same reason, SC ‘‘different-structure’’ ER has an advantage over SC ‘‘same-cell’’ ER with respect to the beam stability: one can detune HOM in its decelerating structure. The ‘‘same-cell’’ ER seems to be a favorite choice today, in large part because of economical reasons. We would like to propose here a simple option for coupling RF cavities in two linacs of the
221
‘‘different-structure’’ ER that can be useful and important for future really high-power FELs.
2. Waveguide-coupled cavities for ER 2.1. Waveguide coupling Three-cell bridge couplers were used for energy transfer between two copper linacs at LANL [3]. Their central cell served also for the RF power input. Four tuning posts per bridge allowed changing the field in the decelerator from 15% to 120% of that in the accelerator. Unfortunately, such bridges are too complicated to be implemented in SC linacs. It would be good to have couplers that are simpler. We have explored an option with a coaxial coupler and found that getting the coupling coefficient on the order of a few percent this way was very difficult. On the other hand, a coupler consisting of a waveguide (WG) section that cut through two beam pipes was found to give a strong coupling; it is easily tunable as well. The model coupler layout is shown in Fig. 1. 2.2. Model of two waveguide-coupled cavities To demonstrate the idea, we intentionally chose a simple model with two differently shaped
Fig. 1. Model coupler layout. Two beam paths are shown by the lines.
ARTICLE IN PRESS 222
S.S. Kurennoy et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 220–224
cavities—one in an accelerating linac and the other in a decelerator. The first cavity is a cylindrical pillbox (C), while the second one has a squareshaped cross section (S), but they both have the same fundamental frequency at 700 MHz. The WG coupler here is a section of a standard WR1150, but it can be some other WG. The coupling is regulated by the size of irises connecting the WG with the cavities. We studied our simple model with the MicroWave Studio (MWS) [7] to calculate the mode frequencies and coupling. The working mode
Fig. 2. Electric field of the fundamental p/2 mode at 700 MHz.
frequency can easily be tuned to 700 MHz by small changes of the transverse dimensions of the two cavities. The electric field of this mode is shown in Fig. 2. If we consider a three-cell system consisting of two linac cells, C and S with the coupling cell WG in between, the mode can be classified as a p/2-mode. Obviously, the coupling cell is not excited in this mode. Small changes of the WG length make two neighbor modes (0- and p-type, Fig. 3) separated from the working mode by the same shift, but in opposite directions: at f0 ¼ 694 MHz and fp ¼ 706 MHz. Hence, the coupling is k ¼ ðfp f0 Þ=fp=2 ¼ 12=700 ¼ 0:017 for the model in Fig. 2, where the radii of the coupling apertures are equal to that of the beam pipe, 65 mm. For the model in Fig. 1, where the cavity–WG coupling irises are larger (r ¼ 80 mm), the coupling is 3.5%. The coupling also depends on the separation between the cavity and WG, i.e., on the iris thickness. The HOMs in the model are uncoupled because the cavities in two linacs have different shapes. This is clearly illustrated in Fig. 4, where two dipole modes are plotted. The mode fields are contained mostly in their ‘native’ cavities. The WG length Lwg in our model was chosen to be slightly larger than one full WG wavelength at 700 MHz, lwg ¼ 63:02 cm.The WG ends are at about Lwg =4 from the cavity axes, and the axis separation is close to Lwg =2: This makes a compact structure that can fit, for example, in a single
Fig. 3. Electric fields of 0- and p-modes at 694 and 706 MHz.
ARTICLE IN PRESS S.S. Kurennoy et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 220–224
223
Fig. 4. Electric fields of two HOMs at 804 and 822 MHz.
cryomodule. However, the WG length can be increased by an integer number of lwg/2, if needed. Slightly changing the WG section length gives us an effective tuning option. The flat ends of the WG section can be made thin enough to implement mechanical push–pull tuners. 2.3. Coupled multi-cell cavities in linacs The same approach can be applied for multi-cell cavities in linacs. Fig. 5 illustrates the working mode at 700 MHz for a model with 2-cell cavities. One can see that again the WG coupler is not excited in this mode, while the cavities have field patterns typical for 2-cell p-modes. From the viewpoint of mode classification in a 5-cell cavity consisting of cells C1, C2, WG coupler, S1, and S2, this working mode has the structure (1,1,0,1,1). It can be called 3p/4-mode, as the fourth mode in the 5-mode pass band. This pass band has its lowest, 0-mode at 687.5 MHz and the highest, pmode at 704 MHz. The cell coupling coefficient here is 2.1%.
3. Discussion A simple model with simplified cavities was presented to illustrate the idea of WG-coupled linacs for energy recovery in FELs. The WG
Fig. 5. Electric field of the ER working mode at 700 MHz.
coupling between cavities in the accelerator and decelerator structures appears to be simple, effective, and easily tunable. It is important to note that this scheme can be implemented in SC linacs as well. Of course, for SC linacs the cavities should be, most likely, of the multi-cell elliptical type, but the WG coupling will work for them too, as one can see from Section 2.3. A rectangular WG was used in our simple model. Possible multipacting effects in such a WG can be mitigated by choosing a different WG shape, e.g., a WG with an elliptical crosssection.
ARTICLE IN PRESS 224
S.S. Kurennoy et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 220–224
4. Conclusions
Acknowledgements
A simple scheme for coupling RF cavities in the ‘‘different-structure’’ energy recovery linacs used as high-power FEL drivers is proposed. The RF cavities are coupled between the two linacs—an accelerator and decelerator—with waveguides that cut through two beam pipes, providing an efficient mechanism for energy transfer. The RF cavities in the two linacs can be shaped differently to detune HOMs and increase BBU thresholds. Using a simple model, we have shown that the WG coupling is simple, tunable, and can be implemented for multi-cell cavities in SC RF linacs. It can be a useful option on a path to high-power ER FELs.
The authors would like to acknowledge useful discussions with F.L. Krawczyk, D.L. Schrage, and R.L. Wood (LANL).
References [1] M. Tigner, Nuovo Cimento 37 (1965) 1228. [2] S.O. Schriber, E.A. Heighway, IEEE NS-22 (1975) 1060. [3] D.W. Feldman, et al., Nucl. Instr. and Meth. A 259 (1987) 26. [4] T.I. Smith, et al., Nucl. Instr. and Meth. A 259 (1987) 1. [5] G.R. Neil, et al., Phys. Rev. Lett. 84 (2000) 662. [6] L. Merminga, Nucl. Instr. and Meth. A 483 (2002) 107. [7] MicroWave Studio, CST, Darmstadt, Germany.