- Email: [email protected]
Design, development and power test of a spiral buncher RF cavity for the high current injector at IUAC
Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162776
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Design, development and power test of a spiral buncher RF cavity for the high current injector at IUAC Rajeev Mehta a , Sanjay Kumar Kedia a,b ,∗, Rajeev Ahuja a , R.V. Hariwal a a b
Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi-110067, India Department of Physics, Indian Institute of Technology Delhi, New Delhi-110016, India
ABSTRACT A 48.5 MHz spiral buncher cavity in the Medium Energy Beam Transport section (MEBT) of the High Current Injector (HCI) provides longitudinal beam matching at the entrance of the Drift Tube Linac (DTL). The spiral type cavity has been chosen for its high shunt impedance and mechanical stability. The cylindrical chamber was fabricated of copper plated stainless-steel, while OFHC copper was used for the inner components, leading to better results than previous similar cavities. The experimentally measured quality factor, shunt impedance, and resonance frequency fit very well with the design values. The bead-pull measurement technique has been used to validate the electric field profile. A fine tuner has been developed and installed to correct the frequency shift while the cavity is phase and amplitude locked. An air-cooled power coupler has been designed, developed, and installed. The cavity has been tested up to the full power of 1 kW to produce 27 kV across each gap. The X-ray energy spectroscopy technique has been used to measure the gap voltages across the drift tubes. The designed and measured value of the quality factor is 4000 and 3600, respectively. The details of the design, development, and power testing are discussed in this article.
1. Introduction The High Current Injector [1,2], when operational, will overcome the low current limitation of the existing 15UD Pelletron [3] accelerator at Inter-University Accelerator Centre (IUAC). HCI will also provide ion species like inert gases which are currently not possible with the existing Pelletron Accelerator [4] facility due to use of negative ion source. A 12.125 MHz Multi-Harmonic Buncher [4] installed in the Low Energy Beam Transport (LEBT) section (Fig. 1) will bunch the DC beam to ∼1 ns time bunched beam at the entrance of RFQ as shown in Fig. 2(a). Trace 3D ion optics code has been used to match the longitudinal beam parameters between RFQ [5] and DTL [6]. Figs. 2 and 3 are simulated results of TRACE 3D code. The Twiss parameters 𝛼z and 𝛽z provide the information of phase space occupied by the charged particles. The 𝛼 = 0, 𝛽 = 1, represents the upright ellipse or waist location, and large value of 𝛼 represents the large time width of the beam. The X and Y axes present the ion bunch width and energy spread respectively concerning the synchronous particle. In Fig. 2a, the value of Twiss parameter 𝛼 ∼ 0 indicates well time bunched beam at RFQ entrance. In Fig. 2(b) as the Twiss parameter 𝛼z increases from ∼0 to ∼4.6, indicating increases in the bunch time width at the entrance of DTL. Since the longitudinal acceptance of the DTL cavity is ∼1 ns, a buncher is required in the MEBT section to provide the longitudinal matching between RFQ and DTL. 2. Design and simulations The literature review [7–11] was conducted while designing the buncher. The spiral type cavity was chosen over a quarter-wave type
cavity since they are characterized by high shunt impedance [7], compact structure [8], and high efficiency in a frequency range of 27 MHz to 200 MHz [9]. The cavity design reported in Ref. [9] generates a bunching voltage of ∼27 kV/gap at 3.2 kW power level. This design includes a carburized steel for outer housing and aluminum for the central conductor. The designed and measured values of the quality factor are 4068 and 702, respectively. The low conductivity of an aluminum inner conductor and a carburized steel result in low-quality factor (Q) and shunt impedance. Reports also exist wherein the design, development, and power test of the spiral buncher cavity has been discussed [12]. The cavity reported in Ref. [12] has been designed and developed at 35.36 MHz frequency to provide the longitudinal bunching of 150 keV/amu ions. The measured and simulated values of the quality factor are 2740 and 5520, respectively. The measured and simulated values of shunt impedance at the resonance frequency are 370 kΩ and 686 kΩ, respectively. An improved spiral buncher cavity as compared to the reference cited above has been designed to provide longitudinal matching at the input of DTL. Since the ion velocity is low (𝛽 = 0.0196) in the MEBT section, we have selected operating frequency as 48.5 MHz, though the physical dimensions are comparatively larger than at 97 MHz which is the operating frequency of the DTL. The CST Microwave Studio [13] (CST MWS) software has been used to simulate the desired resonant frequency and to maximize the shunt impedance by optimizing the length, width, and pitch of the spiral. The central conductor has been developed with a non-uniform cross-section, and the depth of the spiral is kept constant along the 𝑧-axis (beam axis). The spiral width is 50 mm
∗ Corresponding author at: Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi-110067, India. E-mail address: [email protected] (S.K. Kedia).
https://doi.org/10.1016/j.nima.2019.162776 Received 5 November 2018; Received in revised form 16 September 2019; Accepted 16 September 2019 Available online 21 September 2019 0168-9002/© 2019 Elsevier B.V. All rights reserved.
R. Mehta, S.K. Kedia, R. Ahuja et al.
Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162776
Fig. 1. The complete layout of LEBT and MEBT section of the HCI.
Table 1 Design parameters of the spiral buncher cavity.
at the stem and tapered to 25 mm at the drift tube. Though a higher ratio of tank length to spiral depth yield higher shunt impedance [14], depth of 30 mm has been chosen for the mechanical stability of the structure. The shunt impedance of the cavity has been optimized by simulating the spacing between spiral turns in MWS. The gaps were optimized for a charged particle having a normalized velocity (𝛽0 ) of 0.0196. The design provides enough margin to correct the manufacturing and alignment related errors causing drift in resonant frequency by altering dimensions of inner components. The power dissipated in various sections has been calculated. Since 70% of the
Operating frequency Quality factor Optimum velocity (𝛽0 ) Shunt impedance Required voltage 𝐸maximum /𝐸kilpatrick
2
48.50 MHz 4000 0.0196 2.98 MΩ 27 kV/Gap 0.345
R. Mehta, S.K. Kedia, R. Ahuja et al.
Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162776
Fig. 2. Longitudinal Phase Space plot (a) At the entrance of RFQ, (b) At the entrance of DTL Tank-1 (after RFQ) without buncher.
Fig. 3. Longitudinal Phase Space at the entrance of DTL Tank-1 (when spiral buncher is ON).
Fig. 4. Electric field profile along the beam axis in the MWS model (Profile is symmetric to the center of the cavity).
Table 2 Mechanical parameters of the designed spiral buncher cavity in solid works and MWS. Tank diameter Tank length (z-axis) Maximum spiral width (x-y plane) Depth of spiral (z-axis) No of spiral turns/sections Drift tube ID Tank inner diameter
steel. The simulated RF parameters are listed in Table 2. The electric field profile along the beam axis has been simulated using MWS model (Fig. 4). While simulating electric field profile in MWS, we used close to 1 million mesh cells, any further increase in the number of meshes does not change the simulated value appreciably. The uncertainty in the simulation was close to 0.1% within the given conditions. Fig. 5(a) & (b) show the electric and magnetic field intensity, respectively along the spiral for the fundamental mode. The electric field intensity is maximum at the spiral end towards the drift tube for the effective acceleration of the charged particles and minimum near the stem area. Fig. 5(b) reveals that magnetic field intensity is maximum near the stem. An inductive coupler has been employed near the stem area to provide the maximum magnetic coupling. The surface
1377 mm 190 mm 50 mm 30 mm 2/5 20 mm 820 mm
power is dissipated in the spiral and stem, they have been fabricated of OFHC copper, and water-cooling channels have been provided inside the spiral section. Only 30% of power is dissipated in the end walls and tank (Fig. 5c); they are fabricated using copper plated stainless
Fig. 5. The CST simulations (a) Electric field intensity pattern along the spiral, (b) Magnetic field intensity pattern along the spiral, (c) Surface current intensity pattern along the spiral in MWS model.
3
R. Mehta, S.K. Kedia, R. Ahuja et al.
Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162776
Fig. 9. Frequency tuner position versus frequency shift. Fig. 6. The installation of various components of spiral buncher and mirror finish surface after buffing. Table 3 The measurement of accelerating voltage and stored energy at both as center and off-axis.
Center_fwd Center_revs Off-axis_down_4 mm Quad_2 mm
Ea (MV/m)
Stored energy (mJ) @ (1 MV/m)
4.5 4.52 4.47 4.55
49.3 48.9 50.0 48.3
The measured quality factor and shunt impedance fit very well with designed values. 3. Mechanical structure and measurements After the final optimization of design parameters, a mechanical model has been prepared in SolidWorks for the fabrication purpose. A stem is mounted at the bottom of the tank, and a two and a half turn spiral section has been fixed inside the jaw of the stem (Fig. 6). An aircooled power coupler has been designed and developed to provide the 50-Ω impedance matching between RF power source and cavity. Fig. 7 shows the measured frequency (48.6267 MHz) and impedance (49.921 Ω). The target frequency (48.50 MHz) was achieved by decreasing the radius of the stem by steps of 1 mm. Fig. 8 plots the simulated and measured frequency shift in response to change in the radius of the stem. A fine frequency tuner has been employed to fix the frequency shift during the operation stage. A movable capacitive tuning plate gives a frequency tuning range of 180 kHz with the linear movement of 45 mm. Fig. 9 shows the measured value of frequency and frequency shift concerning the tuner position. The measured and simulated value of frequency shift matches reasonably well within an error of 5%. Initially, the measured frequency and quality factor were within 99.9% and 65% respectively, under atmospheric pressure conditions. The buffing cleaned the copper surface, removed the dents and voids, and improved the quality factor by 40%. The measured quality factor of the spiral buncher after buffing is 3600.
Fig. 7. 50-Ω impedance matching between source and load using a vector network analyzer.
Fig. 8. Frequency tuning by varying the stem diameter.
4. Bead-pull measurements The standard bead-pull technique was used to map the electric field profile [15–18]. This technique has been used to validate the field profile and to measure the various RF parameters such as stored energy, effective accelerating electric field, frequency, and quality factor. A spherical Teflon bead of diameter 2.2 mm has been used for the bead-pull measurements. A fully automatized bead-pull setup has been established using LabVIEW codes, as shown in the schematic diagram, Fig. 10.
current density is maximum near the stem and minimum near the drift tube (Fig. 5c). Specifications of the spiral bunchers are presented in Table 1. The buncher cavity requires 27 kV/gap to provide longitudinal matching at the entrance of the first DTL cavity. This designed gap voltage has been achieved at ∼1 kW of input RF power. The designed and measured values of the quality factor are 4000 and 3600, respectively. 4
R. Mehta, S.K. Kedia, R. Ahuja et al.
Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162776
Fig. 10. The schematic diagram of the bead-pull measurement setup. Table 4 The deviation between simulated and measured RF parameters. RF parameters
Simulated (MWS)
Measured (bead-pull)
Deviation (%)
Resonant frequency Accelerating electric field (𝐸a ) Stored energy @ (1 MV/m) d (= 𝛽𝜆/2) Shunt impedance Quality factor
48.50 MHz 4.55 48 mJ 60 mm 2.98 MΩ 4000
48.50 MHz 4.51 MV/m 49.1 mJ 59 mm 2.9 3600
0.0 0.87 2.3 1.67 2.6 10
Fig. 12. Frequency shift w.r.t. the distance along the beam trajectory (bead radius is 1 mm).
Fig. 11. Change in frequency w.r.t. to the distance along the beam trajectory (bead radius is 1 mm).
The bead-pull measurement data (Fig. 11) was analyzed, and various RF parameters were calculated (Table 4). The off-axis measurements were also carried out to measure the uniformity of the electric field within the gaps. The bead-pull results are summarized in Table 3. Fig. 11 reveals that both the accelerating gaps are not the same. The design value for each RF gap is 21 ± 0.5 mm. The frequency shift is inversely proportional to the dimension of accelerating gaps; therefore, the frequency shift of 141 kHz is equivalent to ∼2 mm shorter gap as compared to the design value. The error might occur due to alignment or mishandling of the central conductor. Due to geometrical constraints, we could not measure both the gaps simultaneously during assembly.
Fig. 13. Calculated electric field profile w.r.t. the distance along with beam trajectory (time snap).
5
R. Mehta, S.K. Kedia, R. Ahuja et al.
Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162776
energy calculated from bead-pull data (∼49.1 mJ) matches closely with the designed value, as shown in Table 4. 5. RF power test A 48.50 MHz, 2-kW solid-state RF amplifier was used to power the spiral buncher cavity. The RF cavity has been baked for more than 48 h at ∼60 degrees centigrade to remove the outgassing. The cooling channels were connected to a water supply and checked for vibrations of spiral and water leakages. No frequency shift has been observed due to a flow of water, confirming the high stability of the structure. The schematic diagram of a power test is shown in Fig. 14. The multipacting has been observed below 250 W. Both pulsed and continuous high-power conditioning have been carried out to clean the RF surface. The input flow and temperature of the water have been maintained at 6 liters per minute (lpm) and 22 ◦ C, respectively. The reflected power, vacuum, pick up signal, and frequency have been measured at various forward power levels, as shown in Fig. 15. A frequency shift of −12 kHz was observed on increasing the forward power of the cavity. It can be explained by the change in the length of the spiral as a response to an increase in temperature. After continuous powering and conditioning for a couple of days, we were able to increase the input power to 1 kW, as shown in Figs. 15 and 16. The cavity response in open loop mode at 1 kW has been recorded for 12 h, as shown in Fig. 16. Initially, due to thermal instability shift in resonance frequency was observed and this was corrected using a frequency tuner. The reflected power, frequency, and cavity pick up were measured as a function of time. The reflected power was found to be constant for a time span of 12 h. The X-ray energy spectroscopy method has been used to characterize the cavity at high power. The simulated and measured value of X-ray energy is closely matching within an error of 5%. The measured X-ray energy validates the cavity design at high power.
Fig. 14. The schematic diagram of the power setup.
The cavity was reopened, and the accelerating gap was measured one by one using a slip gauge and found that the gap length was off by 1.9 mm, confirming the bead-pull result. The cavity was realigned, and bead-pull measurement has been carried out again, as shown in Fig. 12. The off-axis measurements are performed to check the uniformity of the field. The field is uniform up to 4 mm off-axis from the center (Fig. 12), and results are tabulated in Table 3. Labels: cent_fwd: Bead is moving in +z direction i.e. along the beam direction. Coordinates are (x = 0, y = 0, z = 0 to +L, where L is length of the spiral). cent_revs: Bead is moving in −z direction. Coordinates are (x = 0, y = 0, z = L to 0, where L is length of the spiral). down_4 mm: Bead is moving in +z direction. Coordinates are (x = 0, y = −4, z = 0 to +L, where L is length of the spiral). quad_2 mm: Bead is moving in +z direction. Coordinates are (x = −2, y = −2, z = 0 to +L, where L is length of the spiral). The measured and simulated electric field profile (Fig. 13) and the change in resonant frequency (Fig. 12) match satisfactorily. The stored
Fig. 15. The RF characterization at various power level (a) The reflected power follows the same trend as forwarding power, (b) The cavity pick-up is continuously increasing on increasing the forward power, (c) Frequency shows a negative trend on increasing the forward power, (d) Vacuum is getting degraded for few minutes on increasing the forward power.
6
R. Mehta, S.K. Kedia, R. Ahuja et al.
Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162776
nique matches well with the simulated value. The cavity is fully stable at the desired power level. The cavity has been tested and installed in the beamline. The cavity testing with the beam will be reported separately. References [1] D. Kanjilal, C.P. Safvan, G.O. Rodrigues, P. Kumar, U.K. Rao, A. Mandal, A. Roy, High current injector at Nuclear Science Centre, in: Proc. APAC Korea, 2004, p. 73. [2] R. Mehta, et al., The high current injector at IUAC-overview and status, in: Proc. InPAC2015, TIFR Mumbai, India. [3] D. Kanjilal, S. Chopra, M.M. Narayanan, Indira S. Iyer, Vandana Jha, R. Joshi, S.K. Datta, Testing and operation of the 15ud pelletron at NSC, Nucl. Instrum. Methods A 328 (1993) 97. [4] S.K. Kedia, R. Mehta, Design and development of a chopping and deflecting system for the high current injector at IUAC, Nucl. Instrum. Methods A 889 (2018) 22. [5] R. Ahuja, A. Kothari, C.P. Safvan, Sugam Kumar, P. Ram Sankar, Design & fabrication of Radio Frequency Quadrupole (RFQ) accelerator at IUAC, New Delhi, in: proc. InPAC’09, RRCAT Indore, February. [6] B.P. Ajith Kumar, J. Zacharias, R. Hariwal, R. Mehta, C.P. Safvan, R.E. Laxdal, Beam optics and resonator design for the 97 MHz DTL at IUAC, in: proc. InPAC2009, RRCAT Indore, India. [7] A. Schempp, H. Klein, Properties of spiral loaded cavities, Nucl. Instrum. Methods 135 (1976) 409. [8] A.K. Mitra, A. Chan, Design consideration and measurements of a 35 MHz, spiral rebuncher cavity, TRIUMF Design Note, 1998. [9] Sun Lie Peng, Zhao Hong Wei, Sun Zhou Ping, He Yuan, Shi Ai Min, XIAO Chen, Du Xiao Nan, Zhang Cong, Zhang Zhou Li, Design study of the SSC-LINAC re-buncher, Chin. Phys. C 37 (2013) 027002. [10] D. Kanjilal, G. Rodrigues, P. Kumar, A. Mandal, A. Roy, C. Bieth, S. Kantas, P. Sortais, Performance of first high temperature superconducting ECRIS, Rev. Sci. Instrum. 77 (2006) 03A317. [11] Amit Roy, Amit roy accelerator development at the nuclear science centre, Current Sci. 76 (1999) 149. [12] A.K. Mitra, R.L. Poirier, A 35 MHz re-buncher cavity for the TRIUMF ISAC facility, in: Proc. PAC, New York, 1999, p. 839. [13] https://www.cst.com/products/cstmws. [14] A.K. Mitra, R.L. Poirier, High power test of the 35 MHz re-buncher cavity for the TRIUMF ISAC facility, in: Proc. EPAC Austria, 2000, p. 1978. [15] E.L. Gintzton, ‘‘Microwave Electronics’’, Van Nostrand, Princetron, New Jersey, 2009, p. 81. [16] Sumit Som, Sudeshna Seth, Aditya Mandal, Surajit Ghosh, Bead-Pull measurement using phase shift technique in multi-cell elliptical cavity, in: Proc. IPAC, Spain, 2011, p. 280. [17] P.A. McIntosh, Perturbation Measurement on RF Cavities At Daresbury, SERC Daresbury Lab, Daresbury, UK EPAC, 1994. [18] John Byrd, RF cavities bead-pull measurements, USPAS and CCAST, Beijing China, 1998.
Fig. 16. The power characterization of the cavity at 1 kW power (a) The pickup becomes constant after some initial fluctuations, (b) The frequency remained constant after initial 2–3 h, (c), (d) The Reflected power and forward power remain constant with time at a constant power level.
Conclusion In conclusion, an improved spiral buncher cavity has been designed, developed and tested to provide the longitudinal matching between RFQ and DTL. The spiral buncher cavity was characterized at both low and high-power levels to validate its design.The designed and measured parameters closely match with each other. In addition, the measured gap voltage using an X-ray measurement spectroscopy tech-
7
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Design, development and power test of a spiral buncher RF cavity for the high current injector at IUAC Rajeev Mehta a , Sanjay Kumar Kedia a,b ,∗, Rajeev Ahuja a , R.V. Hariwal a a b
Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi-110067, India Department of Physics, Indian Institute of Technology Delhi, New Delhi-110016, India
ABSTRACT A 48.5 MHz spiral buncher cavity in the Medium Energy Beam Transport section (MEBT) of the High Current Injector (HCI) provides longitudinal beam matching at the entrance of the Drift Tube Linac (DTL). The spiral type cavity has been chosen for its high shunt impedance and mechanical stability. The cylindrical chamber was fabricated of copper plated stainless-steel, while OFHC copper was used for the inner components, leading to better results than previous similar cavities. The experimentally measured quality factor, shunt impedance, and resonance frequency fit very well with the design values. The bead-pull measurement technique has been used to validate the electric field profile. A fine tuner has been developed and installed to correct the frequency shift while the cavity is phase and amplitude locked. An air-cooled power coupler has been designed, developed, and installed. The cavity has been tested up to the full power of 1 kW to produce 27 kV across each gap. The X-ray energy spectroscopy technique has been used to measure the gap voltages across the drift tubes. The designed and measured value of the quality factor is 4000 and 3600, respectively. The details of the design, development, and power testing are discussed in this article.
1. Introduction The High Current Injector [1,2], when operational, will overcome the low current limitation of the existing 15UD Pelletron [3] accelerator at Inter-University Accelerator Centre (IUAC). HCI will also provide ion species like inert gases which are currently not possible with the existing Pelletron Accelerator [4] facility due to use of negative ion source. A 12.125 MHz Multi-Harmonic Buncher [4] installed in the Low Energy Beam Transport (LEBT) section (Fig. 1) will bunch the DC beam to ∼1 ns time bunched beam at the entrance of RFQ as shown in Fig. 2(a). Trace 3D ion optics code has been used to match the longitudinal beam parameters between RFQ [5] and DTL [6]. Figs. 2 and 3 are simulated results of TRACE 3D code. The Twiss parameters 𝛼z and 𝛽z provide the information of phase space occupied by the charged particles. The 𝛼 = 0, 𝛽 = 1, represents the upright ellipse or waist location, and large value of 𝛼 represents the large time width of the beam. The X and Y axes present the ion bunch width and energy spread respectively concerning the synchronous particle. In Fig. 2a, the value of Twiss parameter 𝛼 ∼ 0 indicates well time bunched beam at RFQ entrance. In Fig. 2(b) as the Twiss parameter 𝛼z increases from ∼0 to ∼4.6, indicating increases in the bunch time width at the entrance of DTL. Since the longitudinal acceptance of the DTL cavity is ∼1 ns, a buncher is required in the MEBT section to provide the longitudinal matching between RFQ and DTL. 2. Design and simulations The literature review [7–11] was conducted while designing the buncher. The spiral type cavity was chosen over a quarter-wave type
cavity since they are characterized by high shunt impedance [7], compact structure [8], and high efficiency in a frequency range of 27 MHz to 200 MHz [9]. The cavity design reported in Ref. [9] generates a bunching voltage of ∼27 kV/gap at 3.2 kW power level. This design includes a carburized steel for outer housing and aluminum for the central conductor. The designed and measured values of the quality factor are 4068 and 702, respectively. The low conductivity of an aluminum inner conductor and a carburized steel result in low-quality factor (Q) and shunt impedance. Reports also exist wherein the design, development, and power test of the spiral buncher cavity has been discussed [12]. The cavity reported in Ref. [12] has been designed and developed at 35.36 MHz frequency to provide the longitudinal bunching of 150 keV/amu ions. The measured and simulated values of the quality factor are 2740 and 5520, respectively. The measured and simulated values of shunt impedance at the resonance frequency are 370 kΩ and 686 kΩ, respectively. An improved spiral buncher cavity as compared to the reference cited above has been designed to provide longitudinal matching at the input of DTL. Since the ion velocity is low (𝛽 = 0.0196) in the MEBT section, we have selected operating frequency as 48.5 MHz, though the physical dimensions are comparatively larger than at 97 MHz which is the operating frequency of the DTL. The CST Microwave Studio [13] (CST MWS) software has been used to simulate the desired resonant frequency and to maximize the shunt impedance by optimizing the length, width, and pitch of the spiral. The central conductor has been developed with a non-uniform cross-section, and the depth of the spiral is kept constant along the 𝑧-axis (beam axis). The spiral width is 50 mm
∗ Corresponding author at: Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi-110067, India. E-mail address: [email protected] (S.K. Kedia).
https://doi.org/10.1016/j.nima.2019.162776 Received 5 November 2018; Received in revised form 16 September 2019; Accepted 16 September 2019 Available online 21 September 2019 0168-9002/© 2019 Elsevier B.V. All rights reserved.
R. Mehta, S.K. Kedia, R. Ahuja et al.
Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162776
Fig. 1. The complete layout of LEBT and MEBT section of the HCI.
Table 1 Design parameters of the spiral buncher cavity.
at the stem and tapered to 25 mm at the drift tube. Though a higher ratio of tank length to spiral depth yield higher shunt impedance [14], depth of 30 mm has been chosen for the mechanical stability of the structure. The shunt impedance of the cavity has been optimized by simulating the spacing between spiral turns in MWS. The gaps were optimized for a charged particle having a normalized velocity (𝛽0 ) of 0.0196. The design provides enough margin to correct the manufacturing and alignment related errors causing drift in resonant frequency by altering dimensions of inner components. The power dissipated in various sections has been calculated. Since 70% of the
Operating frequency Quality factor Optimum velocity (𝛽0 ) Shunt impedance Required voltage 𝐸maximum /𝐸kilpatrick
2
48.50 MHz 4000 0.0196 2.98 MΩ 27 kV/Gap 0.345
R. Mehta, S.K. Kedia, R. Ahuja et al.
Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162776
Fig. 2. Longitudinal Phase Space plot (a) At the entrance of RFQ, (b) At the entrance of DTL Tank-1 (after RFQ) without buncher.
Fig. 3. Longitudinal Phase Space at the entrance of DTL Tank-1 (when spiral buncher is ON).
Fig. 4. Electric field profile along the beam axis in the MWS model (Profile is symmetric to the center of the cavity).
Table 2 Mechanical parameters of the designed spiral buncher cavity in solid works and MWS. Tank diameter Tank length (z-axis) Maximum spiral width (x-y plane) Depth of spiral (z-axis) No of spiral turns/sections Drift tube ID Tank inner diameter
steel. The simulated RF parameters are listed in Table 2. The electric field profile along the beam axis has been simulated using MWS model (Fig. 4). While simulating electric field profile in MWS, we used close to 1 million mesh cells, any further increase in the number of meshes does not change the simulated value appreciably. The uncertainty in the simulation was close to 0.1% within the given conditions. Fig. 5(a) & (b) show the electric and magnetic field intensity, respectively along the spiral for the fundamental mode. The electric field intensity is maximum at the spiral end towards the drift tube for the effective acceleration of the charged particles and minimum near the stem area. Fig. 5(b) reveals that magnetic field intensity is maximum near the stem. An inductive coupler has been employed near the stem area to provide the maximum magnetic coupling. The surface
1377 mm 190 mm 50 mm 30 mm 2/5 20 mm 820 mm
power is dissipated in the spiral and stem, they have been fabricated of OFHC copper, and water-cooling channels have been provided inside the spiral section. Only 30% of power is dissipated in the end walls and tank (Fig. 5c); they are fabricated using copper plated stainless
Fig. 5. The CST simulations (a) Electric field intensity pattern along the spiral, (b) Magnetic field intensity pattern along the spiral, (c) Surface current intensity pattern along the spiral in MWS model.
3
R. Mehta, S.K. Kedia, R. Ahuja et al.
Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162776
Fig. 9. Frequency tuner position versus frequency shift. Fig. 6. The installation of various components of spiral buncher and mirror finish surface after buffing. Table 3 The measurement of accelerating voltage and stored energy at both as center and off-axis.
Center_fwd Center_revs Off-axis_down_4 mm Quad_2 mm
Ea (MV/m)
Stored energy (mJ) @ (1 MV/m)
4.5 4.52 4.47 4.55
49.3 48.9 50.0 48.3
The measured quality factor and shunt impedance fit very well with designed values. 3. Mechanical structure and measurements After the final optimization of design parameters, a mechanical model has been prepared in SolidWorks for the fabrication purpose. A stem is mounted at the bottom of the tank, and a two and a half turn spiral section has been fixed inside the jaw of the stem (Fig. 6). An aircooled power coupler has been designed and developed to provide the 50-Ω impedance matching between RF power source and cavity. Fig. 7 shows the measured frequency (48.6267 MHz) and impedance (49.921 Ω). The target frequency (48.50 MHz) was achieved by decreasing the radius of the stem by steps of 1 mm. Fig. 8 plots the simulated and measured frequency shift in response to change in the radius of the stem. A fine frequency tuner has been employed to fix the frequency shift during the operation stage. A movable capacitive tuning plate gives a frequency tuning range of 180 kHz with the linear movement of 45 mm. Fig. 9 shows the measured value of frequency and frequency shift concerning the tuner position. The measured and simulated value of frequency shift matches reasonably well within an error of 5%. Initially, the measured frequency and quality factor were within 99.9% and 65% respectively, under atmospheric pressure conditions. The buffing cleaned the copper surface, removed the dents and voids, and improved the quality factor by 40%. The measured quality factor of the spiral buncher after buffing is 3600.
Fig. 7. 50-Ω impedance matching between source and load using a vector network analyzer.
Fig. 8. Frequency tuning by varying the stem diameter.
4. Bead-pull measurements The standard bead-pull technique was used to map the electric field profile [15–18]. This technique has been used to validate the field profile and to measure the various RF parameters such as stored energy, effective accelerating electric field, frequency, and quality factor. A spherical Teflon bead of diameter 2.2 mm has been used for the bead-pull measurements. A fully automatized bead-pull setup has been established using LabVIEW codes, as shown in the schematic diagram, Fig. 10.
current density is maximum near the stem and minimum near the drift tube (Fig. 5c). Specifications of the spiral bunchers are presented in Table 1. The buncher cavity requires 27 kV/gap to provide longitudinal matching at the entrance of the first DTL cavity. This designed gap voltage has been achieved at ∼1 kW of input RF power. The designed and measured values of the quality factor are 4000 and 3600, respectively. 4
R. Mehta, S.K. Kedia, R. Ahuja et al.
Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162776
Fig. 10. The schematic diagram of the bead-pull measurement setup. Table 4 The deviation between simulated and measured RF parameters. RF parameters
Simulated (MWS)
Measured (bead-pull)
Deviation (%)
Resonant frequency Accelerating electric field (𝐸a ) Stored energy @ (1 MV/m) d (= 𝛽𝜆/2) Shunt impedance Quality factor
48.50 MHz 4.55 48 mJ 60 mm 2.98 MΩ 4000
48.50 MHz 4.51 MV/m 49.1 mJ 59 mm 2.9 3600
0.0 0.87 2.3 1.67 2.6 10
Fig. 12. Frequency shift w.r.t. the distance along the beam trajectory (bead radius is 1 mm).
Fig. 11. Change in frequency w.r.t. to the distance along the beam trajectory (bead radius is 1 mm).
The bead-pull measurement data (Fig. 11) was analyzed, and various RF parameters were calculated (Table 4). The off-axis measurements were also carried out to measure the uniformity of the electric field within the gaps. The bead-pull results are summarized in Table 3. Fig. 11 reveals that both the accelerating gaps are not the same. The design value for each RF gap is 21 ± 0.5 mm. The frequency shift is inversely proportional to the dimension of accelerating gaps; therefore, the frequency shift of 141 kHz is equivalent to ∼2 mm shorter gap as compared to the design value. The error might occur due to alignment or mishandling of the central conductor. Due to geometrical constraints, we could not measure both the gaps simultaneously during assembly.
Fig. 13. Calculated electric field profile w.r.t. the distance along with beam trajectory (time snap).
5
R. Mehta, S.K. Kedia, R. Ahuja et al.
Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162776
energy calculated from bead-pull data (∼49.1 mJ) matches closely with the designed value, as shown in Table 4. 5. RF power test A 48.50 MHz, 2-kW solid-state RF amplifier was used to power the spiral buncher cavity. The RF cavity has been baked for more than 48 h at ∼60 degrees centigrade to remove the outgassing. The cooling channels were connected to a water supply and checked for vibrations of spiral and water leakages. No frequency shift has been observed due to a flow of water, confirming the high stability of the structure. The schematic diagram of a power test is shown in Fig. 14. The multipacting has been observed below 250 W. Both pulsed and continuous high-power conditioning have been carried out to clean the RF surface. The input flow and temperature of the water have been maintained at 6 liters per minute (lpm) and 22 ◦ C, respectively. The reflected power, vacuum, pick up signal, and frequency have been measured at various forward power levels, as shown in Fig. 15. A frequency shift of −12 kHz was observed on increasing the forward power of the cavity. It can be explained by the change in the length of the spiral as a response to an increase in temperature. After continuous powering and conditioning for a couple of days, we were able to increase the input power to 1 kW, as shown in Figs. 15 and 16. The cavity response in open loop mode at 1 kW has been recorded for 12 h, as shown in Fig. 16. Initially, due to thermal instability shift in resonance frequency was observed and this was corrected using a frequency tuner. The reflected power, frequency, and cavity pick up were measured as a function of time. The reflected power was found to be constant for a time span of 12 h. The X-ray energy spectroscopy method has been used to characterize the cavity at high power. The simulated and measured value of X-ray energy is closely matching within an error of 5%. The measured X-ray energy validates the cavity design at high power.
Fig. 14. The schematic diagram of the power setup.
The cavity was reopened, and the accelerating gap was measured one by one using a slip gauge and found that the gap length was off by 1.9 mm, confirming the bead-pull result. The cavity was realigned, and bead-pull measurement has been carried out again, as shown in Fig. 12. The off-axis measurements are performed to check the uniformity of the field. The field is uniform up to 4 mm off-axis from the center (Fig. 12), and results are tabulated in Table 3. Labels: cent_fwd: Bead is moving in +z direction i.e. along the beam direction. Coordinates are (x = 0, y = 0, z = 0 to +L, where L is length of the spiral). cent_revs: Bead is moving in −z direction. Coordinates are (x = 0, y = 0, z = L to 0, where L is length of the spiral). down_4 mm: Bead is moving in +z direction. Coordinates are (x = 0, y = −4, z = 0 to +L, where L is length of the spiral). quad_2 mm: Bead is moving in +z direction. Coordinates are (x = −2, y = −2, z = 0 to +L, where L is length of the spiral). The measured and simulated electric field profile (Fig. 13) and the change in resonant frequency (Fig. 12) match satisfactorily. The stored
Fig. 15. The RF characterization at various power level (a) The reflected power follows the same trend as forwarding power, (b) The cavity pick-up is continuously increasing on increasing the forward power, (c) Frequency shows a negative trend on increasing the forward power, (d) Vacuum is getting degraded for few minutes on increasing the forward power.
6
R. Mehta, S.K. Kedia, R. Ahuja et al.
Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162776
nique matches well with the simulated value. The cavity is fully stable at the desired power level. The cavity has been tested and installed in the beamline. The cavity testing with the beam will be reported separately. References [1] D. Kanjilal, C.P. Safvan, G.O. Rodrigues, P. Kumar, U.K. Rao, A. Mandal, A. Roy, High current injector at Nuclear Science Centre, in: Proc. APAC Korea, 2004, p. 73. [2] R. Mehta, et al., The high current injector at IUAC-overview and status, in: Proc. InPAC2015, TIFR Mumbai, India. [3] D. Kanjilal, S. Chopra, M.M. Narayanan, Indira S. Iyer, Vandana Jha, R. Joshi, S.K. Datta, Testing and operation of the 15ud pelletron at NSC, Nucl. Instrum. Methods A 328 (1993) 97. [4] S.K. Kedia, R. Mehta, Design and development of a chopping and deflecting system for the high current injector at IUAC, Nucl. Instrum. Methods A 889 (2018) 22. [5] R. Ahuja, A. Kothari, C.P. Safvan, Sugam Kumar, P. Ram Sankar, Design & fabrication of Radio Frequency Quadrupole (RFQ) accelerator at IUAC, New Delhi, in: proc. InPAC’09, RRCAT Indore, February. [6] B.P. Ajith Kumar, J. Zacharias, R. Hariwal, R. Mehta, C.P. Safvan, R.E. Laxdal, Beam optics and resonator design for the 97 MHz DTL at IUAC, in: proc. InPAC2009, RRCAT Indore, India. [7] A. Schempp, H. Klein, Properties of spiral loaded cavities, Nucl. Instrum. Methods 135 (1976) 409. [8] A.K. Mitra, A. Chan, Design consideration and measurements of a 35 MHz, spiral rebuncher cavity, TRIUMF Design Note, 1998. [9] Sun Lie Peng, Zhao Hong Wei, Sun Zhou Ping, He Yuan, Shi Ai Min, XIAO Chen, Du Xiao Nan, Zhang Cong, Zhang Zhou Li, Design study of the SSC-LINAC re-buncher, Chin. Phys. C 37 (2013) 027002. [10] D. Kanjilal, G. Rodrigues, P. Kumar, A. Mandal, A. Roy, C. Bieth, S. Kantas, P. Sortais, Performance of first high temperature superconducting ECRIS, Rev. Sci. Instrum. 77 (2006) 03A317. [11] Amit Roy, Amit roy accelerator development at the nuclear science centre, Current Sci. 76 (1999) 149. [12] A.K. Mitra, R.L. Poirier, A 35 MHz re-buncher cavity for the TRIUMF ISAC facility, in: Proc. PAC, New York, 1999, p. 839. [13] https://www.cst.com/products/cstmws. [14] A.K. Mitra, R.L. Poirier, High power test of the 35 MHz re-buncher cavity for the TRIUMF ISAC facility, in: Proc. EPAC Austria, 2000, p. 1978. [15] E.L. Gintzton, ‘‘Microwave Electronics’’, Van Nostrand, Princetron, New Jersey, 2009, p. 81. [16] Sumit Som, Sudeshna Seth, Aditya Mandal, Surajit Ghosh, Bead-Pull measurement using phase shift technique in multi-cell elliptical cavity, in: Proc. IPAC, Spain, 2011, p. 280. [17] P.A. McIntosh, Perturbation Measurement on RF Cavities At Daresbury, SERC Daresbury Lab, Daresbury, UK EPAC, 1994. [18] John Byrd, RF cavities bead-pull measurements, USPAS and CCAST, Beijing China, 1998.
Fig. 16. The power characterization of the cavity at 1 kW power (a) The pickup becomes constant after some initial fluctuations, (b) The frequency remained constant after initial 2–3 h, (c), (d) The Reflected power and forward power remain constant with time at a constant power level.
Conclusion In conclusion, an improved spiral buncher cavity has been designed, developed and tested to provide the longitudinal matching between RFQ and DTL. The spiral buncher cavity was characterized at both low and high-power levels to validate its design.The designed and measured parameters closely match with each other. In addition, the measured gap voltage using an X-ray measurement spectroscopy tech-
7