- Email: [email protected]
FFAG proton driver for muon source
Nuclear Instruments and Methods in Physics Research A 451 (2000) 300}303
FFAG proton driver for muon source Y. Mori KEK-Tanashi, 3-2-1 Midori-cho, Tanashi-shi, Tokyo 188-0002, Japan
Abstract Fixed "eld alternating gradient (FFAG) focusing synchrotron is attractive as a proton driver for the generation of intense secondary particles such as muons. The magnetic "eld of the FFAG synchrotron is static, therefore, the repetition rate of acceleration could be increased more than ten times larger, such as 1 kHz, than that of an ordinary rapid cycling synchrotron (RCS) if an e$cient high-voltage rf accelerating system becomes available. Recently, a new type of high gradient and broad-band rf cavity using a high-permeability magnetic alloy (MA) has been developed and the FFAG focusing becomes very promising. In this paper, a design of 1.5 GeV and 10 MW beam power of proton driver is presented. In order to clarify the feasibility of rapid cycling FFAG synchrotron experimentally, a proof-of-principle (POP) machine, which accelerates protons up to 1 MeV with 1 kHz repetition, is under development. 2000 Elsevier Science B.V. All rights reserved.
1. Introduction High-intensity medium-energy (1 GeV}10 GeV) proton beams are required for many applications such as spallation of neutron source, acceleratordriven system for nuclear energy production or proton-driver for muon collider. In these applications, large beam power of more than 1 MW is required. In order to realize such large beam power with an ordinary proton synchrotron, rapid cycling of beam acceleration is inevitable. The magnetic "eld is time varying according to beam acceleration in an ordinary synchrotron, the eddy-current power loss in the magnets becomes serious when the repetition rate of the accelerating cycle is increased and the magnetic "eld ramping exceeds more than 200 T/s. On the other hand, the acceler-
E-mail address: [email protected] (Y. Mori).
ated particle number per pulse is limited by the space-charge e!ect. Practically, the maximum repetition of the rapid cycling synchrotron is limited to be less than 50 Hz or so. Therefore, the maximum available beam power would be at most about 1 MW [1]. However, the beam in the synchrotron is stable, because it is strongly focused in the transverse and longitudinal directions, and the instantaneous beam current in the ring becomes very large. Fixed-"eld alternating gradient (FFAG) synchrotron, thus, becomes attractive for this purpose, because the repetition rate of the accelerating cycle could be raised ten times or more compared to that of an ordinary synchrotron. The idea of a FFAG synchrotron was proposed independently by Ohkawa [2] and Symon et al. [3] in the early 1950s, and electron-beam machines demonstrating this principle have been successfully built up in the MURA project [3].
0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 5 5 5 - 6
Y. Mori / Nuclear Instruments and Methods in Physics Research A 451 (2000) 300}303
2. Pros and cons of FFAG In FFAG synchrotron, where the magnetic "eld is constant in time, the shape of the magnetic "eld should be such that the betatron tunes for both the horizontal and vertical planes should be constant for all closed orbits and far from all of the dangerous resonance lines. The above condition is called &zero chromaticity'.
* K "0, *p K F where h is the azimuth, K the focusing strength about the orbit of momentum p and K , the refer ence focusing strength for p"p . A magnetic "eld satisfying the scaling conditions described above must generally have the form B(r,h)"B
rI L r F h!1 ln , G r r G
where 1 is the spiral angle. If 1 is zero, the magnetic "eld does not depend on h, and the corresponding orbit points are distributed on a radial vector. This type of magnetic shape is called &radial sector' (see Fig. 1a). On the other hand, if h behaves in a logarithmic manner, such as r h!1 ln "const., r G
301
the orbits remain geometrically similar, but move around the beam center towards larger radii. This type is called &spiral sector' (see Fig. 1b). The FFAG synchrotron is very attractive for the acceleration of intense proton beams as described above and several proposals have been submitted [4,5]. However, no practical proton-beam machine has been built so far. One of the most di$cult technical issues in a high-repetition FFAG synchrotron is the rf acceleration. The required accelerating rf voltage per turn is
*<"2p(1#n)
dr p. dt
Here, dr/dt is the orbit excursion rate. In the case of a 1 GeV FFAG proton synchrotron with the repetition rate of 1 kHz, the requested rf voltage becomes almost 1 MV. This is a rather di$cult number if an ordinary ferrite-loaded rf cavity is used as in the case of the proton synchrotron so far. In the ordinary ferrite-loaded rf cavity, the maximum accelerating "eld gradient is at most 10 kV/m or so. Therefore, more than 100 m long straight sections are necessary for the rf cavities in the ring, although the total circumference of the 1 GeV FFAG proton synchrotron would be less than 150 m. Recently, a new type of high-gradient rf cavity using a high-permeability magnetic alloy has been developed at KEK for the JHF project, and a "eld
Fig. 1. Magnet con"guration of FFAG focusing. Left is a radial sector(a) and right a spiral sector(b) [3].
ACCELERATORS
302
Y. Mori / Nuclear Instruments and Methods in Physics Research A 451 (2000) 300}303
gradient of 100 kV/m has been successfully achieved [4]. Using this high-gradient cavity, the most di$cult technical issue in realizing a highrepetition RF voltage can be solved. The beam parameters of a preliminary design of a 1.5 GeV and 10 MW beam power FFAG proton synchrotron are listed in Table 1. A schematic layout of this machine is shown in Fig. 2. The b- and dispersion functions are presented in Fig. 3.
3. Proof-of-principle (POP) machine In order to clarify the feasibility of a very rapid cycling in FFAG proton synchrotron, we have been developing a small POP (proof-of-principle) machine. In this POP machine, the maximum energy is limited to 1 MeV and the repetition acceleration rate is 1 kHz. A schematic layout of the POP machine is presented in Ref. [5]. The magnet con"guration is of a radial sector type and an eightfold symmetry was choosen. Each sector consists of three dipole magnets which form a DFD triplet. The "eld index of each dipole magnet is 2.5. The
Table 1 Parameters of a 1.5 GeV, 10 MW FFAG proton synchrotron Injection energy Extraction energy Beam intensity Harmonic number Max. repetition rate Average beam current Numbers of sector Max. average beam radius Injection Extraction Betatron tunes RF frequency Injection Extraction Max. rf voltage Long. acceptance Transv. acceptance Dipole magnetic "eld Injection Extraction
200 MeV 1.5 GeV 5.5;10 ppp 1 750 Hz 10 mA 16 12.31 m 14.15 m h:3.25, v:3.25 2.20 MHz 3.11 MHz 750 kV 4 eVs h:350 p mm mrad v:350 p mm mrad 0.437 T 1.325 T
Fig. 2. Schematic layout of 1.5 GeV, 10 MW FFAG proton synchrotron.
maximum magnetic "elds of the focusing and defocusing dipole magnets are 0.5 and 0.2 T, respectively. The 3D-"eld maps are calculated with OPERA-3D and their results used for beam tracking simulation. The average beam radius changes from 0.81 to 1.13 m when the beam energy varies from 100 keV to 1.1 MeV. The half-gap heights of the magnet at the radius of 0.75 and 1.15 m are 73 and 25 mm, respectively. The betatron tunes for horizontal and vertical directions are functions of the "led index and of the integrated magnetic "eld. The design values of betatron tunes for horizontal and vertical directions are 2.25 and 1.345, respectively. They were calculated using the SAD code and by particle tracking in the "eld generated by OPERA 3D; slight discrepancies were observed and the tunes change gradually during acceleration. For a constant radial displacement as a function of time (dr/dt"const.), the rf voltage has to be increased from 1.1 to 3.1 kV. This rf voltage can be easily obtained by a magnetic alloy (MA) loaded rf cavity. The rf frequency changes from 0.85 to 2.05 MHz.
4. Summary A 1.5 GeV and 10 MW multi-orbit proton synchrotron (MOS) with "xed "eld alternating
Y. Mori / Nuclear Instruments and Methods in Physics Research A 451 (2000) 300}303
303
Fig. 3. Beam parameters of the 1.5 GeV FFAG proton synchrotron.
gradient (FFAG) focusing has been designed. Although the repetition rate for accelerating cycle is rather high i.e. 750 Hz, the required rf voltage is relatively small, only 580 kV, because of small ring size. A 1 MeV proof of principle FFAG proton synchrotron is under development. It is aimed at achieving a repetition frequency as high as 1 kHz.
References [1] JHF Accelerator Design Study Report, KEK Report 97-16. [2] C. Ohkawa, Proceedings of the annual meeting of JPS, 1953. [3] K.R. Symon et al., Phys. Rev. 103 (1956) 1837. [4] Y. Mori, Proceedings of EPAC98, Stockholm, 1998, p. 299. [5] A. Ueno et al., Proceedings of PAC99, New York, p. 2271.
ACCELERATORS
FFAG proton driver for muon source Y. Mori KEK-Tanashi, 3-2-1 Midori-cho, Tanashi-shi, Tokyo 188-0002, Japan
Abstract Fixed "eld alternating gradient (FFAG) focusing synchrotron is attractive as a proton driver for the generation of intense secondary particles such as muons. The magnetic "eld of the FFAG synchrotron is static, therefore, the repetition rate of acceleration could be increased more than ten times larger, such as 1 kHz, than that of an ordinary rapid cycling synchrotron (RCS) if an e$cient high-voltage rf accelerating system becomes available. Recently, a new type of high gradient and broad-band rf cavity using a high-permeability magnetic alloy (MA) has been developed and the FFAG focusing becomes very promising. In this paper, a design of 1.5 GeV and 10 MW beam power of proton driver is presented. In order to clarify the feasibility of rapid cycling FFAG synchrotron experimentally, a proof-of-principle (POP) machine, which accelerates protons up to 1 MeV with 1 kHz repetition, is under development. 2000 Elsevier Science B.V. All rights reserved.
1. Introduction High-intensity medium-energy (1 GeV}10 GeV) proton beams are required for many applications such as spallation of neutron source, acceleratordriven system for nuclear energy production or proton-driver for muon collider. In these applications, large beam power of more than 1 MW is required. In order to realize such large beam power with an ordinary proton synchrotron, rapid cycling of beam acceleration is inevitable. The magnetic "eld is time varying according to beam acceleration in an ordinary synchrotron, the eddy-current power loss in the magnets becomes serious when the repetition rate of the accelerating cycle is increased and the magnetic "eld ramping exceeds more than 200 T/s. On the other hand, the acceler-
E-mail address: [email protected] (Y. Mori).
ated particle number per pulse is limited by the space-charge e!ect. Practically, the maximum repetition of the rapid cycling synchrotron is limited to be less than 50 Hz or so. Therefore, the maximum available beam power would be at most about 1 MW [1]. However, the beam in the synchrotron is stable, because it is strongly focused in the transverse and longitudinal directions, and the instantaneous beam current in the ring becomes very large. Fixed-"eld alternating gradient (FFAG) synchrotron, thus, becomes attractive for this purpose, because the repetition rate of the accelerating cycle could be raised ten times or more compared to that of an ordinary synchrotron. The idea of a FFAG synchrotron was proposed independently by Ohkawa [2] and Symon et al. [3] in the early 1950s, and electron-beam machines demonstrating this principle have been successfully built up in the MURA project [3].
0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 5 5 5 - 6
Y. Mori / Nuclear Instruments and Methods in Physics Research A 451 (2000) 300}303
2. Pros and cons of FFAG In FFAG synchrotron, where the magnetic "eld is constant in time, the shape of the magnetic "eld should be such that the betatron tunes for both the horizontal and vertical planes should be constant for all closed orbits and far from all of the dangerous resonance lines. The above condition is called &zero chromaticity'.
* K "0, *p K F where h is the azimuth, K the focusing strength about the orbit of momentum p and K , the refer ence focusing strength for p"p . A magnetic "eld satisfying the scaling conditions described above must generally have the form B(r,h)"B
rI L r F h!1 ln , G r r G
where 1 is the spiral angle. If 1 is zero, the magnetic "eld does not depend on h, and the corresponding orbit points are distributed on a radial vector. This type of magnetic shape is called &radial sector' (see Fig. 1a). On the other hand, if h behaves in a logarithmic manner, such as r h!1 ln "const., r G
301
the orbits remain geometrically similar, but move around the beam center towards larger radii. This type is called &spiral sector' (see Fig. 1b). The FFAG synchrotron is very attractive for the acceleration of intense proton beams as described above and several proposals have been submitted [4,5]. However, no practical proton-beam machine has been built so far. One of the most di$cult technical issues in a high-repetition FFAG synchrotron is the rf acceleration. The required accelerating rf voltage per turn is
*<"2p(1#n)
dr p. dt
Here, dr/dt is the orbit excursion rate. In the case of a 1 GeV FFAG proton synchrotron with the repetition rate of 1 kHz, the requested rf voltage becomes almost 1 MV. This is a rather di$cult number if an ordinary ferrite-loaded rf cavity is used as in the case of the proton synchrotron so far. In the ordinary ferrite-loaded rf cavity, the maximum accelerating "eld gradient is at most 10 kV/m or so. Therefore, more than 100 m long straight sections are necessary for the rf cavities in the ring, although the total circumference of the 1 GeV FFAG proton synchrotron would be less than 150 m. Recently, a new type of high-gradient rf cavity using a high-permeability magnetic alloy has been developed at KEK for the JHF project, and a "eld
Fig. 1. Magnet con"guration of FFAG focusing. Left is a radial sector(a) and right a spiral sector(b) [3].
ACCELERATORS
302
Y. Mori / Nuclear Instruments and Methods in Physics Research A 451 (2000) 300}303
gradient of 100 kV/m has been successfully achieved [4]. Using this high-gradient cavity, the most di$cult technical issue in realizing a highrepetition RF voltage can be solved. The beam parameters of a preliminary design of a 1.5 GeV and 10 MW beam power FFAG proton synchrotron are listed in Table 1. A schematic layout of this machine is shown in Fig. 2. The b- and dispersion functions are presented in Fig. 3.
3. Proof-of-principle (POP) machine In order to clarify the feasibility of a very rapid cycling in FFAG proton synchrotron, we have been developing a small POP (proof-of-principle) machine. In this POP machine, the maximum energy is limited to 1 MeV and the repetition acceleration rate is 1 kHz. A schematic layout of the POP machine is presented in Ref. [5]. The magnet con"guration is of a radial sector type and an eightfold symmetry was choosen. Each sector consists of three dipole magnets which form a DFD triplet. The "eld index of each dipole magnet is 2.5. The
Table 1 Parameters of a 1.5 GeV, 10 MW FFAG proton synchrotron Injection energy Extraction energy Beam intensity Harmonic number Max. repetition rate Average beam current Numbers of sector Max. average beam radius Injection Extraction Betatron tunes RF frequency Injection Extraction Max. rf voltage Long. acceptance Transv. acceptance Dipole magnetic "eld Injection Extraction
200 MeV 1.5 GeV 5.5;10 ppp 1 750 Hz 10 mA 16 12.31 m 14.15 m h:3.25, v:3.25 2.20 MHz 3.11 MHz 750 kV 4 eVs h:350 p mm mrad v:350 p mm mrad 0.437 T 1.325 T
Fig. 2. Schematic layout of 1.5 GeV, 10 MW FFAG proton synchrotron.
maximum magnetic "elds of the focusing and defocusing dipole magnets are 0.5 and 0.2 T, respectively. The 3D-"eld maps are calculated with OPERA-3D and their results used for beam tracking simulation. The average beam radius changes from 0.81 to 1.13 m when the beam energy varies from 100 keV to 1.1 MeV. The half-gap heights of the magnet at the radius of 0.75 and 1.15 m are 73 and 25 mm, respectively. The betatron tunes for horizontal and vertical directions are functions of the "led index and of the integrated magnetic "eld. The design values of betatron tunes for horizontal and vertical directions are 2.25 and 1.345, respectively. They were calculated using the SAD code and by particle tracking in the "eld generated by OPERA 3D; slight discrepancies were observed and the tunes change gradually during acceleration. For a constant radial displacement as a function of time (dr/dt"const.), the rf voltage has to be increased from 1.1 to 3.1 kV. This rf voltage can be easily obtained by a magnetic alloy (MA) loaded rf cavity. The rf frequency changes from 0.85 to 2.05 MHz.
4. Summary A 1.5 GeV and 10 MW multi-orbit proton synchrotron (MOS) with "xed "eld alternating
Y. Mori / Nuclear Instruments and Methods in Physics Research A 451 (2000) 300}303
303
Fig. 3. Beam parameters of the 1.5 GeV FFAG proton synchrotron.
gradient (FFAG) focusing has been designed. Although the repetition rate for accelerating cycle is rather high i.e. 750 Hz, the required rf voltage is relatively small, only 580 kV, because of small ring size. A 1 MeV proof of principle FFAG proton synchrotron is under development. It is aimed at achieving a repetition frequency as high as 1 kHz.
References [1] JHF Accelerator Design Study Report, KEK Report 97-16. [2] C. Ohkawa, Proceedings of the annual meeting of JPS, 1953. [3] K.R. Symon et al., Phys. Rev. 103 (1956) 1837. [4] Y. Mori, Proceedings of EPAC98, Stockholm, 1998, p. 299. [5] A. Ueno et al., Proceedings of PAC99, New York, p. 2271.
ACCELERATORS