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RNB post-accelerator for ISAC at TRIUMF— present and future
Nuclear Physics A 701 (2002) 647c–650c www.elsevier.com/locate/npe
RNB post-accelerator for ISAC at TRIUMF— present and future Robert E. Laxdal ∗ , Richard A. Baartman, Pierre Bricault, Gerardo Dutto, Roger Poirier, Paul Schmor, Guy Stanford 4004 Wesbrook Mall, Vancouver, BC, Canada V6T2A3
Abstract The post-accelerator for ISAC includes a 35 MHz RF Quadrupole (RFQ) to accelerate beams of A/q 30 from 2 keV/u to 150 keV/u and a post-stripper, 105 MHz variable energy drift tube linac (DTL) to accelerate ions of 3 A/q 6 to a final energy from 0.15 to 1.5 MeV/u. Recently funds have been allocated for an extension to the ISAC facility, ISAC-II, to permit acceleration of radioactive ion beams up to energies of at least 6.5 MeV/u for masses up to 150. The design concept and present status of both the ISAC-I project and the future ISAC-II project will be presented. 2002 Elsevier Science B.V. All rights reserved. Keywords: Linac; Heavy ion; Radioactive beams; Post-accelerator
1. ISAC-I 1.1. Concept A radioactive ion beam facility, ISAC, with on-line source and linear post-accelerator is being built at TRIUMF [1]. Beams of E 60 keV and A 238 are being delivered to the low-energy experimental area. A post-accelerator is being installed to supply radioactive beams to the high-energy experimental area. The accelerator chain includes a 35 MHz RFQ to accelerate beams of A/q 30 from 2 keV/u to 150 keV/u and a post-stripper, 105 MHz variable energy drift tube linac (DTL) to accelerate ions of 3 A/q 6 to a final energy between 0.15 to 1.5 MeV/u. Both linacs are required to operate cw to preserve beam intensity. A layout of the ISAC-I accelerator chain is shown in Fig. 1. * Corresponding author.
E-mail address: [email protected] (R.E. Laxdal). 0375-9474/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 5 - 9 4 7 4 ( 0 1 ) 0 1 6 6 0 - 8
648c
R.E. Laxdal et al. / Nuclear Physics A 701 (2002) 647c–650c
Fig. 1. The ISAC-I linear accelerator.
The ISAC RFQ is a split ring four-rod structure [2]. A total of 19 split rings, each feeding a 40 cm length of modulated electrodes, are housed in a square 1 m × 1 m tank with a total length of almost 8 m. The gross specifications include a bore radius of r0 = 7.4 mm, and a maximum intervane voltage of 74 kV corresponding to a power of 85 kW. Rings and electrodes are water cooled. A unique feature of the design is the constant synchronous phase of −25◦ [3]. For acceleration of radioactive beams space-charge is not a concern. In the ISAC RFQ, the buncher and shaper sections have been eliminated in favour of a four-harmonic sawtooth prebuncher located ∼ 5 m upstream of the RFQ in LEBT. This shortens the RFQ but in addition, injecting a prebunched beam yields a smaller longitudinal emittance at the expense of a slightly lower beam capture. The prebuncher operates at a fundamental frequency of 11.7 MHz, the third subharmonic of the RFQ. This introduces an 86 ns bunch spacing that is useful for some physics experiments. Beam simulations predict a capture of 81% of the beam accelerated in the 11.7 MHz bunches, ∼ 4% accelerated in the two neighbouring 35 MHz buckets, with 15% of the beam unaccelerated. The variable energy DTL [5] is based on a unique separated function approach with five independent interdigital H-mode (IH) structures, each with 0◦ synchronous phase, providing the acceleration and quadrupole triplets and three-gap bunching cavities between
R.E. Laxdal et al. / Nuclear Physics A 701 (2002) 647c–650c
649c
tanks providing transverse and longitudinal focussing, respectively. The DTL is designed to efficiently accelerate low-β heavy ions over a large operating range while maintaining high beam quality. The IH tanks consume only 60 kW of rf power to produce a total accelerating voltage of 8.1 MV over the 5.6 m length. To achieve a reduced final energy the higherenergy IH tanks are turned off and the voltage and phase in the last operating tank are varied. The three-gap split ring cavities between tanks are adjusted to maintain longitudinal bunching. In this way the whole energy range can be covered with 100% transmission and no longitudinal emittance growth. Final beam emittances of x,y = 0.2π µm and x,y = 0.73π keV/(u ns) are expected from the DTL. 1.2. Status Occupancy of the accelerator floor of the new ISAC building began in July 1997. To date the Off-Line Ion Source (OLIS), the Low-Energy Beam Transport (LEBT), the RFQ and two-thirds of the Medium-Energy Beam Transport (MEBT) have been commissioned with beam. Presently the last section of MEBT and the first section of the DTL, consisting of the first IH-tank, a triplet and a buncher, are being installed for a beam test in May 2000. The ISAC-I post-accelerator and high-energy beam transport (HEBT) are scheduled to be completed for beam delivery to two experimental stations by December 2000. The 35 MHz RFQ and MEBT rebuncher and the 105 MHz first DTL IH-tank and buncher have all been successfully tested at high power in cw mode. Stable beams of both N+ and N+ 2 from OLIS have been accelerated in the RFQ to 53 keV/u in an initial 7-ring configuration [4] and most recently to 150 keV/u in the final 19-ring configuration. Beam test results including beam capture, transverse emittance growth, transverse and longitudinal acceptance and beam energy spread all correspond well to beam simulations.
2. ISAC-II With ISAC-II the final energy of the post-accelerator is increased to 6.5 MeV/u and the mass range extended up to 150. The concept utilizes the existing ISAC-I RFQ for low-energy acceleration and therefore requires that the ion charge from the source obeys A/q 30. A charge-state booster (CSB) will be added after the mass separator to increase the charge state of ions with A > 30. The higher final energy imposes a higher optimum stripping energy of ∼ 400 keV/u with stripping efficiencies varying from 50% for the lightest ions to 15% for A = 150. An effective configuration to reach 400 keV/u is to continue the acceleration straight north of the RFQ–MEBT line. A DTL will be placed downstream of the existing MEBT bender after a short matching section. A room-temperature IH structure operating in cw mode will be used to accelerate the ions of A 30 from 0.15–0.4 MeV/u. The beam then is stripped and the ion charge selected in a new MEBT-2 that bends the beam through 90◦ to a line parallel to the ISAC-I–DTL line. Ions of mass to charge ratio 3 A/q 7 are matched into a superconducting DTL on this line and accelerated to at least 6.5 MeV/u and then transported to the experimental stations. The short independently phased cavities that make up the post-stripper linac allow a wide velocity acceptance and so the final energy
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R.E. Laxdal et al. / Nuclear Physics A 701 (2002) 647c–650c
Fig. 2. The ISAC-II linear accelerator complex.
for each ion can be optimized. In this case the final energy/nucleon for light ions could be more than double that for the high-mass particles. A schematic of the proposed ISAC-II linear accelerator complex is shown in Fig. 2. The project has just received funding. A 105 MHz superconducting quarter wave bulk niobium cavity has been designed; a prototype cavity will soon begin fabrication. A first stage consisting of ∼ 20 MV of accelerating voltage is planned for the end of 2003 with the complete installation expected by the end of 2005.
References [1] P. Schmor et al., The high intensity radioactive beam facility at TRIUMF, in: Proc. 6th European Part. Acc. Conf., EPAC98, Stockholm, 1998. [2] R. Poirier et al., RF systems of the TRIUMF ISAC facility, in: Proc. 1999 Part. Acc. Conf., PAC99, New York, 1999. [3] S. Koscielniak et al., Beam dynamics studies on the ISAC RFQ at TRIUMF, in: Proc. 1997 Part. Acc. Conf., PAC97, Vancouver, 1997. [4] R.E. Laxdal et al., Beam test results with the ISAC RFQ, in: Proc. 1999 Part. Acc. Conf., PAC99, New York, 1999. [5] R.E. Laxdal et al., A separated function drift tube linac for the ISAC project at TRIUMF, in: Proc. 1997 Part. Acc. Conf., Vancouver, 1997.
RNB post-accelerator for ISAC at TRIUMF— present and future Robert E. Laxdal ∗ , Richard A. Baartman, Pierre Bricault, Gerardo Dutto, Roger Poirier, Paul Schmor, Guy Stanford 4004 Wesbrook Mall, Vancouver, BC, Canada V6T2A3
Abstract The post-accelerator for ISAC includes a 35 MHz RF Quadrupole (RFQ) to accelerate beams of A/q 30 from 2 keV/u to 150 keV/u and a post-stripper, 105 MHz variable energy drift tube linac (DTL) to accelerate ions of 3 A/q 6 to a final energy from 0.15 to 1.5 MeV/u. Recently funds have been allocated for an extension to the ISAC facility, ISAC-II, to permit acceleration of radioactive ion beams up to energies of at least 6.5 MeV/u for masses up to 150. The design concept and present status of both the ISAC-I project and the future ISAC-II project will be presented. 2002 Elsevier Science B.V. All rights reserved. Keywords: Linac; Heavy ion; Radioactive beams; Post-accelerator
1. ISAC-I 1.1. Concept A radioactive ion beam facility, ISAC, with on-line source and linear post-accelerator is being built at TRIUMF [1]. Beams of E 60 keV and A 238 are being delivered to the low-energy experimental area. A post-accelerator is being installed to supply radioactive beams to the high-energy experimental area. The accelerator chain includes a 35 MHz RFQ to accelerate beams of A/q 30 from 2 keV/u to 150 keV/u and a post-stripper, 105 MHz variable energy drift tube linac (DTL) to accelerate ions of 3 A/q 6 to a final energy between 0.15 to 1.5 MeV/u. Both linacs are required to operate cw to preserve beam intensity. A layout of the ISAC-I accelerator chain is shown in Fig. 1. * Corresponding author.
E-mail address: [email protected] (R.E. Laxdal). 0375-9474/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 5 - 9 4 7 4 ( 0 1 ) 0 1 6 6 0 - 8
648c
R.E. Laxdal et al. / Nuclear Physics A 701 (2002) 647c–650c
Fig. 1. The ISAC-I linear accelerator.
The ISAC RFQ is a split ring four-rod structure [2]. A total of 19 split rings, each feeding a 40 cm length of modulated electrodes, are housed in a square 1 m × 1 m tank with a total length of almost 8 m. The gross specifications include a bore radius of r0 = 7.4 mm, and a maximum intervane voltage of 74 kV corresponding to a power of 85 kW. Rings and electrodes are water cooled. A unique feature of the design is the constant synchronous phase of −25◦ [3]. For acceleration of radioactive beams space-charge is not a concern. In the ISAC RFQ, the buncher and shaper sections have been eliminated in favour of a four-harmonic sawtooth prebuncher located ∼ 5 m upstream of the RFQ in LEBT. This shortens the RFQ but in addition, injecting a prebunched beam yields a smaller longitudinal emittance at the expense of a slightly lower beam capture. The prebuncher operates at a fundamental frequency of 11.7 MHz, the third subharmonic of the RFQ. This introduces an 86 ns bunch spacing that is useful for some physics experiments. Beam simulations predict a capture of 81% of the beam accelerated in the 11.7 MHz bunches, ∼ 4% accelerated in the two neighbouring 35 MHz buckets, with 15% of the beam unaccelerated. The variable energy DTL [5] is based on a unique separated function approach with five independent interdigital H-mode (IH) structures, each with 0◦ synchronous phase, providing the acceleration and quadrupole triplets and three-gap bunching cavities between
R.E. Laxdal et al. / Nuclear Physics A 701 (2002) 647c–650c
649c
tanks providing transverse and longitudinal focussing, respectively. The DTL is designed to efficiently accelerate low-β heavy ions over a large operating range while maintaining high beam quality. The IH tanks consume only 60 kW of rf power to produce a total accelerating voltage of 8.1 MV over the 5.6 m length. To achieve a reduced final energy the higherenergy IH tanks are turned off and the voltage and phase in the last operating tank are varied. The three-gap split ring cavities between tanks are adjusted to maintain longitudinal bunching. In this way the whole energy range can be covered with 100% transmission and no longitudinal emittance growth. Final beam emittances of x,y = 0.2π µm and x,y = 0.73π keV/(u ns) are expected from the DTL. 1.2. Status Occupancy of the accelerator floor of the new ISAC building began in July 1997. To date the Off-Line Ion Source (OLIS), the Low-Energy Beam Transport (LEBT), the RFQ and two-thirds of the Medium-Energy Beam Transport (MEBT) have been commissioned with beam. Presently the last section of MEBT and the first section of the DTL, consisting of the first IH-tank, a triplet and a buncher, are being installed for a beam test in May 2000. The ISAC-I post-accelerator and high-energy beam transport (HEBT) are scheduled to be completed for beam delivery to two experimental stations by December 2000. The 35 MHz RFQ and MEBT rebuncher and the 105 MHz first DTL IH-tank and buncher have all been successfully tested at high power in cw mode. Stable beams of both N+ and N+ 2 from OLIS have been accelerated in the RFQ to 53 keV/u in an initial 7-ring configuration [4] and most recently to 150 keV/u in the final 19-ring configuration. Beam test results including beam capture, transverse emittance growth, transverse and longitudinal acceptance and beam energy spread all correspond well to beam simulations.
2. ISAC-II With ISAC-II the final energy of the post-accelerator is increased to 6.5 MeV/u and the mass range extended up to 150. The concept utilizes the existing ISAC-I RFQ for low-energy acceleration and therefore requires that the ion charge from the source obeys A/q 30. A charge-state booster (CSB) will be added after the mass separator to increase the charge state of ions with A > 30. The higher final energy imposes a higher optimum stripping energy of ∼ 400 keV/u with stripping efficiencies varying from 50% for the lightest ions to 15% for A = 150. An effective configuration to reach 400 keV/u is to continue the acceleration straight north of the RFQ–MEBT line. A DTL will be placed downstream of the existing MEBT bender after a short matching section. A room-temperature IH structure operating in cw mode will be used to accelerate the ions of A 30 from 0.15–0.4 MeV/u. The beam then is stripped and the ion charge selected in a new MEBT-2 that bends the beam through 90◦ to a line parallel to the ISAC-I–DTL line. Ions of mass to charge ratio 3 A/q 7 are matched into a superconducting DTL on this line and accelerated to at least 6.5 MeV/u and then transported to the experimental stations. The short independently phased cavities that make up the post-stripper linac allow a wide velocity acceptance and so the final energy
650c
R.E. Laxdal et al. / Nuclear Physics A 701 (2002) 647c–650c
Fig. 2. The ISAC-II linear accelerator complex.
for each ion can be optimized. In this case the final energy/nucleon for light ions could be more than double that for the high-mass particles. A schematic of the proposed ISAC-II linear accelerator complex is shown in Fig. 2. The project has just received funding. A 105 MHz superconducting quarter wave bulk niobium cavity has been designed; a prototype cavity will soon begin fabrication. A first stage consisting of ∼ 20 MV of accelerating voltage is planned for the end of 2003 with the complete installation expected by the end of 2005.
References [1] P. Schmor et al., The high intensity radioactive beam facility at TRIUMF, in: Proc. 6th European Part. Acc. Conf., EPAC98, Stockholm, 1998. [2] R. Poirier et al., RF systems of the TRIUMF ISAC facility, in: Proc. 1999 Part. Acc. Conf., PAC99, New York, 1999. [3] S. Koscielniak et al., Beam dynamics studies on the ISAC RFQ at TRIUMF, in: Proc. 1997 Part. Acc. Conf., PAC97, Vancouver, 1997. [4] R.E. Laxdal et al., Beam test results with the ISAC RFQ, in: Proc. 1999 Part. Acc. Conf., PAC99, New York, 1999. [5] R.E. Laxdal et al., A separated function drift tube linac for the ISAC project at TRIUMF, in: Proc. 1997 Part. Acc. Conf., Vancouver, 1997.