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A non-perturbing technique to characterize the indigenously developed spiral buncher cavity at high-power level
Nuclear Inst. and Methods in Physics Research, A 948 (2019) 162820
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
A non-perturbing technique to characterize the indigenously developed spiral buncher cavity at high-power level Sanjay Kumar Kedia a,b ,∗, Rajeev Mehta a a b
Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India Department of Physics, Indian Institute of Technology, New Delhi 110016, India
ABSTRACT The High Current Injector (HCI) at Inter-University Accelerator Centre (IUAC) incorporates a spiral buncher cavity to provide the time bunching of charged particles at the entrance of Drift Tube Linac (DTL). The spiral buncher cavity is a two-gap RF structure with its remarkable properties such as high shunt impedance, compactness, and high-frequency stability. The cavity was designed, developed, assembled, and successfully tested at IUAC. The cavity has been designed for ∼27 kV/gap at ∼1 kW of input power. At the low power level, the bead-pull technique was used to characterize the cavity, and the X-ray energy spectroscopy method was used to characterize the cavity at a high-power level. The X-ray measurement technique was used to determine the gap voltage across the drift tubes without perturbing the field. A thallium doped sodium iodide-based scintillator detector was used to measure the emitted X-ray from a cavity. The cavity was powered at a variable power level, and the X-ray energy spectrum was recorded. The measured and simulated results match closely within the error of ∼5%. The details of cavity characterization, high-power testing, and X-ray energy measurement method will be discussed in the article.
1. Introduction The accelerator augmentation program at the Inter-University Centre, New Delhi includes a High Current Injector (HCI) [1,2] accelerator to provide the wide variety of ion beams having A/q ≤ 6 for various experiments in the field of Materials Science, Molecular Physics, Atomic Physics, Radiation Biology, and Nuclear Physics. The HCI was chosen for its capability to provide the high current and ion species of noble gases such as Ne, Kr, Xe, etc. which are not possible with the combination of an existing Pelletron [3]-linac [4] system. The high current injector consists of a high-temperature superconducting electron cyclotron resonance (HTS-ECR) ion source [5] followed by one radio frequency quadrupole [6] (RFQ), and six drift tube linac [7] (DTL) accelerating cavities. The HTS-ECR ion source is capable of providing the output energy 8 keV/amu, which is further accelerated to 1.8 MeV/amu using RFQ and DTL cavities [7,8]. The complete layout of low energy beam transport section (LEBT), medium energy beam transport section (MEBT) section and location of the spiral buncher has been depicted in Fig. 1. The Spiral buncher cavity (Fig. 2) has been designed, developed, and installed in the MEBT section to provide the time bunched beam at the entrance of the first DTL cavity. A fine tuner was designed and installed to correct the frequency shift during the operation stage. The low-level RF characterizations were carried out using a bead-pull technique [9,10]. The measured frequency, quality factor, stored energy, and accelerating voltage closely matches with the design value. The spiral buncher cavity has been designed to produce ∼27 kV/gap with an input power of ∼1 kW at a quality factor of 4000. This optimal voltage is required to provide the time bunched beam at
the entrance of the first DTL cavity. The knowledge of gap voltage is necessary to precisely time bunch the charged particles at the entrance of the first DTL cavity. Direct measurement of the cavity field across the accelerating gaps cannot be accomplished because any probe capable of measuring the RF voltage/field would significantly perturb the cavity field [11]. The measurement of emitted X-ray is the only indirect/non-invasive technique to measure the RF field level. Therefore, X-ray diagnostic technique was used to characterize the cavity at the high-power level as it does not perturb or modify the RF field. X-ray diagnostic technique is based on the energy spectroscopy system that measures the energy of individual X-ray outside the buncher cavity. The endpoint of the Xray bremsstrahlung energy spectrum provides the maximum peak gap voltage from which the electric field level can be extrapolated. The set of data provides the relation between input power and the voltage generated across the accelerating gaps. There have been few reports available in which RF cavity field measurements have been discussed using X-ray energy spectroscopy method. The article reported in Ref. [11] presents the high-power Xray measurement for the multi-cell cavity in the transverse direction. The reported measurement error is ∼15%. The article referred in the Ref. [12] also presents the characterization of the RF cavity at high power using X-ray measurement technique. The discrepancies between the shape of the measured and calculated spectra are discussed in the report [12]. A detailed X-ray measurement technique to characterize the cavity at high-power has been discussed thoroughly. The setup has been
∗ 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.2019.162820 Received 12 January 2019; Received in revised form 6 August 2019; Accepted 19 September 2019 Available online 23 September 2019 0168-9002/© 2019 Elsevier B.V. All rights reserved.
S.K. Kedia and R. Mehta
Nuclear Inst. and Methods in Physics Research, A 948 (2019) 162820
Fig. 1. The complete layout of low energy and medium energy beam transport section of the high current injector.
calibrated with the standard source of X-ray (241 Am). The sensitivity, accuracy, reliability, and reproducibility of the measurement set up were checked with the repetitive measurement for the same time period during calibration using 241 Am source. A comparison between the simulated voltage (using CST MWS) and measured voltage (using Xray) was carried out at the various power level of the cavity. The error between simulated and measured voltage is always less than 5%.
cavity to provide effective acceleration or bunching to the charged particles across the gaps. These RF gaps are the rich source of the X-ray emission (Fig. 3). The negatively charged surface injects the electron in the RF gap due to field emission, and these electrons get accelerated and attain the kinetic energy according to gap voltage. Accelerated electrons hit on the positive surface of the gap and produce the Xrays. The two types of X-rays are being produced in the given scenario, namely characteristic X-ray and bremsstrahlung X-ray. Characteristic X-rays are emitted when electrons of outer shell jump into a vacancy of an inner shell of an atom and releasing X-rays photon known as ‘‘characteristic’’ X-ray.
2. X-ray emission from the RF gaps High voltage gaps in a vacuum environment are the well-known source of X-ray [11]. In a cavity, the time-varying field generates a voltage across gaps and accelerates the charged particles under high vacuum. The RF wave oscillates with the resonant frequency of the
The field emitted or stray electrons get accelerated by the RF field and stroked the cavity walls, produces bremsstrahlung X-rays. The 2
S.K. Kedia and R. Mehta
Nuclear Inst. and Methods in Physics Research, A 948 (2019) 162820
a lead collimated scintillator detector, a preamplifier, an amplifier, a power supply, and a multichannel analyzer. The X-ray photons emit fluorescence while passing through scintillator material. The emitted fluorescence photons are captured by the photomultiplier tube (PMT). PMT tube generates photoelectrons, and electrons get converted into a voltage pulse. The voltage pulse is proportional to the energy deposited by the X-ray photons. The voltage output of the PMT was fed into an Ortec linear amplifier (model number 571) and gain of the amplifier was adjusted as per the requirement. The amplified voltage was fed into an indigenous multichannel analyzer (MCA) of 4096 channels; each channel is corresponding to a different but evenly separated voltage range. The FREEDOM [14,15] software was used on a LINUX based operating system for the collection of events. FREEDOM [14,15] is an event mode data acquisition software written in house. The upgraded version of FREEDOM is CANDLE [15,16] data acquisition software. This detection process is linear, making the voltage output proportionally intensify the signal so that the multichannel analyzer can easily understand it. The 4096 channels MCA is capable of distinguishing a continuum spectrum of energy. The MCA sends a signal to the computer for every input voltage pulse. Every count was recorded corresponds to a specific channel number by a computer-based program. The data file can be recalled once it gets allocated. The lead collimated, X-ray detectors were installed in both transverse and longitudinal direction of the spiral buncher cavity, as shown in Fig. 5. The X-ray counts were measured using a lead sheet of aperture ∼3 mm, as shown in Fig. 5.
Fig. 2. The X-ray measurement setup including cavity, stem, spiral, power coupler, RF gaps, and two X-ray detectors (One side endplate is open for the demonstration of the RF gaps, the schematic layout of electronic setup has been depicted in Fig. 5).
4. Cavity development and simulations
bremsstrahlung X-rays (deceleration radiations) are the electromagnetic radiations produced by the deceleration of charged particles. Bremsstrahlung has a continuous spectrum, and the endpoint of the spectrum provides information about the maximum voltage across gaps. The X-rays have a maximum distribution of the energy up to the full kinetic energy of the accelerated electrons [12]. The angular distribution of the X-ray is the strongly peaked in the transverse direction (perpendicular to the beam direction); however, a finite no of X-ray is emitted throughout the full solid angle [13]. The emitted bremsstrahlung X-rays outside the cavity wall carries the signature of the gap voltage. The measurement of X-rays, outside the vacuum vessel, explores a technique to characterize the cavity at high-power without perturbing the cavity.
A 48.5 MHz spiral buncher cavity has been designed in Computer Simulation Technology, and Micro-Wave Studio (CST MWS) software and bead-pull measurements were performed to validate its design parameters. The simulation of TRACK and TRACE 3D code indicates that ∼27 kV/gap is required for longitudinal phase matching at the entrance of the DTL cavity. The various design parameters of the cavity have been tabulated in Table 1. The cavity simulation was carried out in CST software to record the change in resonant frequency. The data has been extracted, analyzed, and plotted. Fig. 6 represents a uniform frequency shift along with both the accelerating gaps. The identical frequency shift provides the information of equal gaps. Fig. 7 represents the simulated electric field with respect to distance (at time t = to ). The various RF parameters, including design frequency, quality factor [17], stored energy [17], and accelerating electric field, were simulated in the CST MWS software.
3. Experimental setup An experimental setup has been established for the detection of bremsstrahlung X-ray, as shown in Fig. 4. The detection setup includes
Fig. 3. The bremsstrahlung X-ray emission from the RF gap located inside the cavity.
3
S.K. Kedia and R. Mehta
Nuclear Inst. and Methods in Physics Research, A 948 (2019) 162820 Table 1 Design parameters of the spiral buncher cavity. Design frequency Longitudinal space Input energy Accelerating gap voltage Quality factor Shunt impedance Power @ (∼27 kV/gap) R/Q Stored energy (Uo ) @ (1 MV/m)
48.50 MHz 190 mm 180 keV/amu ∼27 kV/gap 4000 ∼3.2 MΩ ∼911 W ∼800 mJ ∼ 48 ( MV )2 m
Fig. 4. Schematic of electronics setup for X-ray detection.
Fig. 6. The simulated change in frequency concerning the beam axis in CST MWS.
Fig. 7. Time snap of the simulated electric field profile along the beam direction in CST MWS.
parameters, including frequency, quality factor, impedance losses, and S-parameters. The bead-pull experiment was performed to validate the electric field pattern. The cavity has been powered using a 2 kW solidstate RF amplifier to check the power handling capacity of the cavity. The measured bead-pull profile validates the electric field pattern along the beam direction (Fig. 8). Fig. 8 demonstrate a comparison of field strength using bead pull and CST simulation. Fig. 9 shows the behavior of the spiral buncher cavity at highpower. On increasing the forward power, a vacuum was degraded, signal strength was increased, and reflected power was increased. The frequency has been decreased with increase in the forward power. The
Fig. 5. The pictorial schematic setup for X-ray detection, including lead shielded X-ray detector and cavity.
5. Results and discussion The cavity has been designed, fabricated, and assembled indigenously. The cavity has been characterized at both low and high-power level. A Vector Network Analyzer was used to measure various RF 4
S.K. Kedia and R. Mehta
Nuclear Inst. and Methods in Physics Research, A 948 (2019) 162820 Table 2 The comparison between simulated and experimentally measured RF parameters.
Design frequency Quality factor Stored energy (Uo ) @ (1 MV/m) Average accelerating voltage Power @ (∼27 kV/gap) Shunt impedance R/Q
CST MWS
Bead-Pull
48.50 MHz 4000 mJ 50 ( MV )2
48.50 3800 mJ ∼ 48 ( MV )2
m
0.50 MV/m 911 W 3.2 MΩ 800
VNA
S11
48.50 3800 −32 dB
m
∼0.48 MV/m ∼963 W ∼2.90 MΩ ∼805
S12 S21 S22 Impedance
−56 dB −56 dB No signal 50.35 Ω
Fig. 8. The comparison of electric field strength using CST simulation and bead-pull technique.
Fig. 10. The complete experimental setup for X-ray detection, including lead shielded X-ray detector and cavity.
The voltage generated across the accelerating gaps was calculated using the analytical calculation and various power level. 𝜔𝑈 𝑃 𝑈 = 𝑈0 𝐸02 ( ) 𝜔𝑈0 𝐸02 𝑄= 𝑃 √ 𝑉𝑔𝑎𝑝 = 0.870 × 𝑃𝑤𝑎𝑡𝑡 (kV)
(1)
𝑄=
Fig. 9. The cavity behavior at high-power.
(2) (3) (4)
where Q, P(watt) stands for quality factor and power dissipations, respectively. Ea presents the accelerating voltage in MV/m unit. U (J) stands for the stored energy and U0 ( ( J )2 ) is the stored energy at 1
change in capacitance and inductance was observed due to temperature drift. The cavity has been powered more than 48 h continuously to check its frequency stability. All the parameters became constant during continuously powering the cavity for more than half an hour at a particular power level. The various RF parameters namely, frequency, stored energy, average accelerating electric field, total voltage, and quality factor, were experimentally calculated using the bead-pull technique. The comparison between measured and simulated RF parameters have been tabulated below (Table 2). The simulated and measured parameters match closely, as shown in Table 2.
MV m
MV/m electric field. Vgap presents the voltage across the drift tubes in volts. The power required to generate ∼27 kV/gap is ∼963 W at 3800 quality factors. The theoretical voltage generated across each gap at various power level has been tabulated in Table 3. The experimental setup of X-ray measurement has been shown in Fig. 10. The X-ray setup has been calibrated using 241 Am sources. The 241 Am source emits a gamma-ray (or low energy gamma-ray) peak of 59.6 keV originating from nuclear decay of 237 Np. Generally, gammaray energy should be greater than 100 keV; therefore, we have denoted 5
S.K. Kedia and R. Mehta
Nuclear Inst. and Methods in Physics Research, A 948 (2019) 162820
Fig. 11. Demonstration of FWHM of the scintillator detector.
Fig. 12. The X-ray energy spectrum at 1 kW power (detector installed in the transverse direction).
it as an X-ray peak. Further, in various literature, the 241 Am spectrum is termed, as X-ray spectrum. The FWHM of the scintillator detector is ∼14%, as demonstrated in Fig. 11. The cavity has been powered at 1000 W, and X-ray spectrums were recorded in both transverse and longitudinal direction, as shown in Figs. 12 and 13, respectively. The pulse pileup was prevented from reducing the count rates to maximum ∼1000/s at the high energy (power) end. The spectrum was analyzed in Freedom software to determine the endpoint of the spectrum. The intensity of X-ray in the longitudinal direction is 15 times lower than transverse direction due to poor solid angle in the longitudinal direction. A time-weighted background spectrum has been subtracted from the X-ray data spectrum. The strength of the background is around <1% of the X-ray data at 1 kW of input power. The linear region of the bremsstrahlung spectrum was determined using a nonlinearasymmetric peak fitting in the Origin software, as shown in Fig. 12. The endpoint energy has been determined by the projection of straight lineleast square fit, in the portion of the spectrum where it is almost linear, near the end of the spectrum [11,18]. A fitting window was set across a linear region of the energetic X-ray spectrum to most accurately project the spectrum endpoint [11,18]. The two baselines have been drawn in the linear regime, near the end of the spectrum, as shown in Fig. 12. A point where the straight line passing through the baseline intersects the abscissa provides the information of endpoint energy of the bremsstrahlung spectrum [18]. The measured energy of an X-ray is 28.5 keV at 1000-W power, in both the direction as shown in Figs. 12 and 13.
Fig. 13. The X-ray energy spectrum at 1 kW power (detector installed in longitudinal direction). The endpoint energy has been deduced from the shape of the spectra.
Table 3 The comparison between numerically calculated and experimentally measured energy of X-ray.
The cavity power has been increased from 600 W to 1000 W in a step of 100 W, as shown in Fig. 14. The X-ray spectrums were recorded for the various power level and analyzed, as shown in Fig. 14. No X-ray counts were detected below 600 W.
Power (W)
Numerically calculated (kV)
Experimentally measured (kV)
Absolute error (%)
1000 900 800 700 600
27.5 26.1 24.6 23.0 21.3
28.5 27 25.5 24 22
3.6 3.4 3.7 4.3 3.3
6. Conclusion
Fig. 15 reveals that both energy and the intensity of the spectrum are getting increased with increase in the forward power. The voltage across the drift tubes has been increased on increasing the forward power of the cavity, as shown in Fig. 15. The comparison between analytically calculated and measured energy of X-ray has been graphically presented in Fig. 16.
A spiral buncher cavity has been designed, developed, and tested indigenously. The cavity was characterized thoroughly using a beadpull, Vector Network Analyzer, and X-ray energy spectroscopy method. The voltage across RF gaps was measured using the X-ray energy spectroscopy method at high power. The measured data were analyzed, and the bremsstrahlung spectrum fitting was done. The bremsstrahlung fitting reveals that the measured and calculated results match reasonably well within the error of ∼5%. The offline testing of the cavity has been completed and installed in the HCI beam hall. The cavity characterization with the ion beam will be presented separately.
Table 3 represents the numerically calculated and experimentally measured value of X-ray in the range of 600 W to 1000 W. Table 3 provides valuable information to bunch the charged particles of different energy. The error in the numerically calculated and the measured value is less than 5% as shown in Table 3. 6
S.K. Kedia and R. Mehta
Nuclear Inst. and Methods in Physics Research, A 948 (2019) 162820
Fig. 16. The comparison between simulated and measured X-ray voltage.
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, pp. 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. Ghosh, R. Mehta, G.K. Chaudhary, A. Rai, P. Patra, B.K. Sahu, A. Pandey, D.S. Mathuria, J. Chacko, A. Chaudhary, S. Kar, S. Babu, M. Kumar, S.S.K. Sonti, K.K. Mistry, J. Zacharias, P.N. Prakash, T.S. Datta, A. Mandal, D. Kanjilal, A. Roy, Superconducting linac at Inter-University Accelerator Centre: Operational challenge and solutions, Phys. Rev. ST Accel. Beams 12 (2009) 040101. [5] 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. [6] R. Ahuja, A. Kothari, C.P. Safvan, Sugam Kumar and P Ram Sankar, Design & fabrication of Radio Frequency Quadrupole (RFQ) accelerator at IUAC, New Delhi, in: Proc. InPAC2009, RRCAT Indore. [7] B.P. Ajith Kumar, 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. [8] 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. [9] 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, pp. 280. [10] John Byrd, RF Cavity bead pull measurements, Lawrence Berkeley Laboratory, USPAS and CCAST Beijing China, 1998. [11] G.O. Bolme, G.P. Boincourt, K.F. Johnson, R.A. Lohsen, O.R. Sander, L.S. Walling, Measurement of RF accelerator cavity field level at high power from X-ray emission, in: Proc LINAC Mexico, 1990, pp. 219. [12] J.P. Duke, A.P. Letchford, D.J.S. Findlay, Measurement of RF cavity voltages by X-ray spectrum measurements, in: Proc. XX International Linac Conference, California, 2000, pp. 164. [13] J.D. Jackson, Classical Electrodynamics, John Wiley and Sons, Inc., New York, 1975, pp. 701–715. [14] B.P. Ajith Kumar, E.T. Subramaniam, K. Singh, R.K. Bhomik, Freedom-High Speed DAS, in: Proc. SANAI 1997, Trombay, India. [15] http://www.iuac.res.in/NIAS/. [16] B.P. Ajith Kumar, E.T. Subramaniam, R.K. Bhomik, CANDLE-Collection and analysis of nuclear data using linux network, in: Proc. DAE SNP 2001, Kolkata, India. [17] Thorsten Hellert, Martin Dohlus, Detuning related coupler kick variation of a superconducting nine-cell 1.3 GHz cavity, Phys. Rev. ST Accel. Beams 21 (2018) 042001. [18] J. Marcia C. Silva, Silvio B. Herdade, Patrcia Lammoglia, P.R. Costa, R.A. Terini, Determination of voltage applied on X-ray tubes from the bremsstrahlung spectrum obtained with a silicon PIN photodiode, Med. Phys. 27 (2000) 11.
Fig. 14. The X-ray energy spectrum at a variable power level (detector installed in the transverse direction). The endpoint energies have been deduced from the shape of the spectra.
Fig. 15. Intensity versus the energy of the X-ray spectra on the same scale.
Acknowledgments We would like to thank Vandana Singh (Teacher, Kendriya Vidyalaya) for proofreading and valuable comments and Dr Prashant Sharma (PhD, IUAC), for his help in the endpoint energy determination. We would like to acknowledge Aneeqa Basheer (Project Student, IUAC) for her participation in data analysis and discussion. We wish to extend our particular thanks appreciation to R Ruby Santhi, VVV Satyanarayana, Saneesh N for their contribution in data acquisition setup. 7
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
A non-perturbing technique to characterize the indigenously developed spiral buncher cavity at high-power level Sanjay Kumar Kedia a,b ,∗, Rajeev Mehta a a b
Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India Department of Physics, Indian Institute of Technology, New Delhi 110016, India
ABSTRACT The High Current Injector (HCI) at Inter-University Accelerator Centre (IUAC) incorporates a spiral buncher cavity to provide the time bunching of charged particles at the entrance of Drift Tube Linac (DTL). The spiral buncher cavity is a two-gap RF structure with its remarkable properties such as high shunt impedance, compactness, and high-frequency stability. The cavity was designed, developed, assembled, and successfully tested at IUAC. The cavity has been designed for ∼27 kV/gap at ∼1 kW of input power. At the low power level, the bead-pull technique was used to characterize the cavity, and the X-ray energy spectroscopy method was used to characterize the cavity at a high-power level. The X-ray measurement technique was used to determine the gap voltage across the drift tubes without perturbing the field. A thallium doped sodium iodide-based scintillator detector was used to measure the emitted X-ray from a cavity. The cavity was powered at a variable power level, and the X-ray energy spectrum was recorded. The measured and simulated results match closely within the error of ∼5%. The details of cavity characterization, high-power testing, and X-ray energy measurement method will be discussed in the article.
1. Introduction The accelerator augmentation program at the Inter-University Centre, New Delhi includes a High Current Injector (HCI) [1,2] accelerator to provide the wide variety of ion beams having A/q ≤ 6 for various experiments in the field of Materials Science, Molecular Physics, Atomic Physics, Radiation Biology, and Nuclear Physics. The HCI was chosen for its capability to provide the high current and ion species of noble gases such as Ne, Kr, Xe, etc. which are not possible with the combination of an existing Pelletron [3]-linac [4] system. The high current injector consists of a high-temperature superconducting electron cyclotron resonance (HTS-ECR) ion source [5] followed by one radio frequency quadrupole [6] (RFQ), and six drift tube linac [7] (DTL) accelerating cavities. The HTS-ECR ion source is capable of providing the output energy 8 keV/amu, which is further accelerated to 1.8 MeV/amu using RFQ and DTL cavities [7,8]. The complete layout of low energy beam transport section (LEBT), medium energy beam transport section (MEBT) section and location of the spiral buncher has been depicted in Fig. 1. The Spiral buncher cavity (Fig. 2) has been designed, developed, and installed in the MEBT section to provide the time bunched beam at the entrance of the first DTL cavity. A fine tuner was designed and installed to correct the frequency shift during the operation stage. The low-level RF characterizations were carried out using a bead-pull technique [9,10]. The measured frequency, quality factor, stored energy, and accelerating voltage closely matches with the design value. The spiral buncher cavity has been designed to produce ∼27 kV/gap with an input power of ∼1 kW at a quality factor of 4000. This optimal voltage is required to provide the time bunched beam at
the entrance of the first DTL cavity. The knowledge of gap voltage is necessary to precisely time bunch the charged particles at the entrance of the first DTL cavity. Direct measurement of the cavity field across the accelerating gaps cannot be accomplished because any probe capable of measuring the RF voltage/field would significantly perturb the cavity field [11]. The measurement of emitted X-ray is the only indirect/non-invasive technique to measure the RF field level. Therefore, X-ray diagnostic technique was used to characterize the cavity at the high-power level as it does not perturb or modify the RF field. X-ray diagnostic technique is based on the energy spectroscopy system that measures the energy of individual X-ray outside the buncher cavity. The endpoint of the Xray bremsstrahlung energy spectrum provides the maximum peak gap voltage from which the electric field level can be extrapolated. The set of data provides the relation between input power and the voltage generated across the accelerating gaps. There have been few reports available in which RF cavity field measurements have been discussed using X-ray energy spectroscopy method. The article reported in Ref. [11] presents the high-power Xray measurement for the multi-cell cavity in the transverse direction. The reported measurement error is ∼15%. The article referred in the Ref. [12] also presents the characterization of the RF cavity at high power using X-ray measurement technique. The discrepancies between the shape of the measured and calculated spectra are discussed in the report [12]. A detailed X-ray measurement technique to characterize the cavity at high-power has been discussed thoroughly. The setup has been
∗ 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.2019.162820 Received 12 January 2019; Received in revised form 6 August 2019; Accepted 19 September 2019 Available online 23 September 2019 0168-9002/© 2019 Elsevier B.V. All rights reserved.
S.K. Kedia and R. Mehta
Nuclear Inst. and Methods in Physics Research, A 948 (2019) 162820
Fig. 1. The complete layout of low energy and medium energy beam transport section of the high current injector.
calibrated with the standard source of X-ray (241 Am). The sensitivity, accuracy, reliability, and reproducibility of the measurement set up were checked with the repetitive measurement for the same time period during calibration using 241 Am source. A comparison between the simulated voltage (using CST MWS) and measured voltage (using Xray) was carried out at the various power level of the cavity. The error between simulated and measured voltage is always less than 5%.
cavity to provide effective acceleration or bunching to the charged particles across the gaps. These RF gaps are the rich source of the X-ray emission (Fig. 3). The negatively charged surface injects the electron in the RF gap due to field emission, and these electrons get accelerated and attain the kinetic energy according to gap voltage. Accelerated electrons hit on the positive surface of the gap and produce the Xrays. The two types of X-rays are being produced in the given scenario, namely characteristic X-ray and bremsstrahlung X-ray. Characteristic X-rays are emitted when electrons of outer shell jump into a vacancy of an inner shell of an atom and releasing X-rays photon known as ‘‘characteristic’’ X-ray.
2. X-ray emission from the RF gaps High voltage gaps in a vacuum environment are the well-known source of X-ray [11]. In a cavity, the time-varying field generates a voltage across gaps and accelerates the charged particles under high vacuum. The RF wave oscillates with the resonant frequency of the
The field emitted or stray electrons get accelerated by the RF field and stroked the cavity walls, produces bremsstrahlung X-rays. The 2
S.K. Kedia and R. Mehta
Nuclear Inst. and Methods in Physics Research, A 948 (2019) 162820
a lead collimated scintillator detector, a preamplifier, an amplifier, a power supply, and a multichannel analyzer. The X-ray photons emit fluorescence while passing through scintillator material. The emitted fluorescence photons are captured by the photomultiplier tube (PMT). PMT tube generates photoelectrons, and electrons get converted into a voltage pulse. The voltage pulse is proportional to the energy deposited by the X-ray photons. The voltage output of the PMT was fed into an Ortec linear amplifier (model number 571) and gain of the amplifier was adjusted as per the requirement. The amplified voltage was fed into an indigenous multichannel analyzer (MCA) of 4096 channels; each channel is corresponding to a different but evenly separated voltage range. The FREEDOM [14,15] software was used on a LINUX based operating system for the collection of events. FREEDOM [14,15] is an event mode data acquisition software written in house. The upgraded version of FREEDOM is CANDLE [15,16] data acquisition software. This detection process is linear, making the voltage output proportionally intensify the signal so that the multichannel analyzer can easily understand it. The 4096 channels MCA is capable of distinguishing a continuum spectrum of energy. The MCA sends a signal to the computer for every input voltage pulse. Every count was recorded corresponds to a specific channel number by a computer-based program. The data file can be recalled once it gets allocated. The lead collimated, X-ray detectors were installed in both transverse and longitudinal direction of the spiral buncher cavity, as shown in Fig. 5. The X-ray counts were measured using a lead sheet of aperture ∼3 mm, as shown in Fig. 5.
Fig. 2. The X-ray measurement setup including cavity, stem, spiral, power coupler, RF gaps, and two X-ray detectors (One side endplate is open for the demonstration of the RF gaps, the schematic layout of electronic setup has been depicted in Fig. 5).
4. Cavity development and simulations
bremsstrahlung X-rays (deceleration radiations) are the electromagnetic radiations produced by the deceleration of charged particles. Bremsstrahlung has a continuous spectrum, and the endpoint of the spectrum provides information about the maximum voltage across gaps. The X-rays have a maximum distribution of the energy up to the full kinetic energy of the accelerated electrons [12]. The angular distribution of the X-ray is the strongly peaked in the transverse direction (perpendicular to the beam direction); however, a finite no of X-ray is emitted throughout the full solid angle [13]. The emitted bremsstrahlung X-rays outside the cavity wall carries the signature of the gap voltage. The measurement of X-rays, outside the vacuum vessel, explores a technique to characterize the cavity at high-power without perturbing the cavity.
A 48.5 MHz spiral buncher cavity has been designed in Computer Simulation Technology, and Micro-Wave Studio (CST MWS) software and bead-pull measurements were performed to validate its design parameters. The simulation of TRACK and TRACE 3D code indicates that ∼27 kV/gap is required for longitudinal phase matching at the entrance of the DTL cavity. The various design parameters of the cavity have been tabulated in Table 1. The cavity simulation was carried out in CST software to record the change in resonant frequency. The data has been extracted, analyzed, and plotted. Fig. 6 represents a uniform frequency shift along with both the accelerating gaps. The identical frequency shift provides the information of equal gaps. Fig. 7 represents the simulated electric field with respect to distance (at time t = to ). The various RF parameters, including design frequency, quality factor [17], stored energy [17], and accelerating electric field, were simulated in the CST MWS software.
3. Experimental setup An experimental setup has been established for the detection of bremsstrahlung X-ray, as shown in Fig. 4. The detection setup includes
Fig. 3. The bremsstrahlung X-ray emission from the RF gap located inside the cavity.
3
S.K. Kedia and R. Mehta
Nuclear Inst. and Methods in Physics Research, A 948 (2019) 162820 Table 1 Design parameters of the spiral buncher cavity. Design frequency Longitudinal space Input energy Accelerating gap voltage Quality factor Shunt impedance Power @ (∼27 kV/gap) R/Q Stored energy (Uo ) @ (1 MV/m)
48.50 MHz 190 mm 180 keV/amu ∼27 kV/gap 4000 ∼3.2 MΩ ∼911 W ∼800 mJ ∼ 48 ( MV )2 m
Fig. 4. Schematic of electronics setup for X-ray detection.
Fig. 6. The simulated change in frequency concerning the beam axis in CST MWS.
Fig. 7. Time snap of the simulated electric field profile along the beam direction in CST MWS.
parameters, including frequency, quality factor, impedance losses, and S-parameters. The bead-pull experiment was performed to validate the electric field pattern. The cavity has been powered using a 2 kW solidstate RF amplifier to check the power handling capacity of the cavity. The measured bead-pull profile validates the electric field pattern along the beam direction (Fig. 8). Fig. 8 demonstrate a comparison of field strength using bead pull and CST simulation. Fig. 9 shows the behavior of the spiral buncher cavity at highpower. On increasing the forward power, a vacuum was degraded, signal strength was increased, and reflected power was increased. The frequency has been decreased with increase in the forward power. The
Fig. 5. The pictorial schematic setup for X-ray detection, including lead shielded X-ray detector and cavity.
5. Results and discussion The cavity has been designed, fabricated, and assembled indigenously. The cavity has been characterized at both low and high-power level. A Vector Network Analyzer was used to measure various RF 4
S.K. Kedia and R. Mehta
Nuclear Inst. and Methods in Physics Research, A 948 (2019) 162820 Table 2 The comparison between simulated and experimentally measured RF parameters.
Design frequency Quality factor Stored energy (Uo ) @ (1 MV/m) Average accelerating voltage Power @ (∼27 kV/gap) Shunt impedance R/Q
CST MWS
Bead-Pull
48.50 MHz 4000 mJ 50 ( MV )2
48.50 3800 mJ ∼ 48 ( MV )2
m
0.50 MV/m 911 W 3.2 MΩ 800
VNA
S11
48.50 3800 −32 dB
m
∼0.48 MV/m ∼963 W ∼2.90 MΩ ∼805
S12 S21 S22 Impedance
−56 dB −56 dB No signal 50.35 Ω
Fig. 8. The comparison of electric field strength using CST simulation and bead-pull technique.
Fig. 10. The complete experimental setup for X-ray detection, including lead shielded X-ray detector and cavity.
The voltage generated across the accelerating gaps was calculated using the analytical calculation and various power level. 𝜔𝑈 𝑃 𝑈 = 𝑈0 𝐸02 ( ) 𝜔𝑈0 𝐸02 𝑄= 𝑃 √ 𝑉𝑔𝑎𝑝 = 0.870 × 𝑃𝑤𝑎𝑡𝑡 (kV)
(1)
𝑄=
Fig. 9. The cavity behavior at high-power.
(2) (3) (4)
where Q, P(watt) stands for quality factor and power dissipations, respectively. Ea presents the accelerating voltage in MV/m unit. U (J) stands for the stored energy and U0 ( ( J )2 ) is the stored energy at 1
change in capacitance and inductance was observed due to temperature drift. The cavity has been powered more than 48 h continuously to check its frequency stability. All the parameters became constant during continuously powering the cavity for more than half an hour at a particular power level. The various RF parameters namely, frequency, stored energy, average accelerating electric field, total voltage, and quality factor, were experimentally calculated using the bead-pull technique. The comparison between measured and simulated RF parameters have been tabulated below (Table 2). The simulated and measured parameters match closely, as shown in Table 2.
MV m
MV/m electric field. Vgap presents the voltage across the drift tubes in volts. The power required to generate ∼27 kV/gap is ∼963 W at 3800 quality factors. The theoretical voltage generated across each gap at various power level has been tabulated in Table 3. The experimental setup of X-ray measurement has been shown in Fig. 10. The X-ray setup has been calibrated using 241 Am sources. The 241 Am source emits a gamma-ray (or low energy gamma-ray) peak of 59.6 keV originating from nuclear decay of 237 Np. Generally, gammaray energy should be greater than 100 keV; therefore, we have denoted 5
S.K. Kedia and R. Mehta
Nuclear Inst. and Methods in Physics Research, A 948 (2019) 162820
Fig. 11. Demonstration of FWHM of the scintillator detector.
Fig. 12. The X-ray energy spectrum at 1 kW power (detector installed in the transverse direction).
it as an X-ray peak. Further, in various literature, the 241 Am spectrum is termed, as X-ray spectrum. The FWHM of the scintillator detector is ∼14%, as demonstrated in Fig. 11. The cavity has been powered at 1000 W, and X-ray spectrums were recorded in both transverse and longitudinal direction, as shown in Figs. 12 and 13, respectively. The pulse pileup was prevented from reducing the count rates to maximum ∼1000/s at the high energy (power) end. The spectrum was analyzed in Freedom software to determine the endpoint of the spectrum. The intensity of X-ray in the longitudinal direction is 15 times lower than transverse direction due to poor solid angle in the longitudinal direction. A time-weighted background spectrum has been subtracted from the X-ray data spectrum. The strength of the background is around <1% of the X-ray data at 1 kW of input power. The linear region of the bremsstrahlung spectrum was determined using a nonlinearasymmetric peak fitting in the Origin software, as shown in Fig. 12. The endpoint energy has been determined by the projection of straight lineleast square fit, in the portion of the spectrum where it is almost linear, near the end of the spectrum [11,18]. A fitting window was set across a linear region of the energetic X-ray spectrum to most accurately project the spectrum endpoint [11,18]. The two baselines have been drawn in the linear regime, near the end of the spectrum, as shown in Fig. 12. A point where the straight line passing through the baseline intersects the abscissa provides the information of endpoint energy of the bremsstrahlung spectrum [18]. The measured energy of an X-ray is 28.5 keV at 1000-W power, in both the direction as shown in Figs. 12 and 13.
Fig. 13. The X-ray energy spectrum at 1 kW power (detector installed in longitudinal direction). The endpoint energy has been deduced from the shape of the spectra.
Table 3 The comparison between numerically calculated and experimentally measured energy of X-ray.
The cavity power has been increased from 600 W to 1000 W in a step of 100 W, as shown in Fig. 14. The X-ray spectrums were recorded for the various power level and analyzed, as shown in Fig. 14. No X-ray counts were detected below 600 W.
Power (W)
Numerically calculated (kV)
Experimentally measured (kV)
Absolute error (%)
1000 900 800 700 600
27.5 26.1 24.6 23.0 21.3
28.5 27 25.5 24 22
3.6 3.4 3.7 4.3 3.3
6. Conclusion
Fig. 15 reveals that both energy and the intensity of the spectrum are getting increased with increase in the forward power. The voltage across the drift tubes has been increased on increasing the forward power of the cavity, as shown in Fig. 15. The comparison between analytically calculated and measured energy of X-ray has been graphically presented in Fig. 16.
A spiral buncher cavity has been designed, developed, and tested indigenously. The cavity was characterized thoroughly using a beadpull, Vector Network Analyzer, and X-ray energy spectroscopy method. The voltage across RF gaps was measured using the X-ray energy spectroscopy method at high power. The measured data were analyzed, and the bremsstrahlung spectrum fitting was done. The bremsstrahlung fitting reveals that the measured and calculated results match reasonably well within the error of ∼5%. The offline testing of the cavity has been completed and installed in the HCI beam hall. The cavity characterization with the ion beam will be presented separately.
Table 3 represents the numerically calculated and experimentally measured value of X-ray in the range of 600 W to 1000 W. Table 3 provides valuable information to bunch the charged particles of different energy. The error in the numerically calculated and the measured value is less than 5% as shown in Table 3. 6
S.K. Kedia and R. Mehta
Nuclear Inst. and Methods in Physics Research, A 948 (2019) 162820
Fig. 16. The comparison between simulated and measured X-ray voltage.
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, pp. 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. Ghosh, R. Mehta, G.K. Chaudhary, A. Rai, P. Patra, B.K. Sahu, A. Pandey, D.S. Mathuria, J. Chacko, A. Chaudhary, S. Kar, S. Babu, M. Kumar, S.S.K. Sonti, K.K. Mistry, J. Zacharias, P.N. Prakash, T.S. Datta, A. Mandal, D. Kanjilal, A. Roy, Superconducting linac at Inter-University Accelerator Centre: Operational challenge and solutions, Phys. Rev. ST Accel. Beams 12 (2009) 040101. [5] 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. [6] R. Ahuja, A. Kothari, C.P. Safvan, Sugam Kumar and P Ram Sankar, Design & fabrication of Radio Frequency Quadrupole (RFQ) accelerator at IUAC, New Delhi, in: Proc. InPAC2009, RRCAT Indore. [7] B.P. Ajith Kumar, 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. [8] 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. [9] 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, pp. 280. [10] John Byrd, RF Cavity bead pull measurements, Lawrence Berkeley Laboratory, USPAS and CCAST Beijing China, 1998. [11] G.O. Bolme, G.P. Boincourt, K.F. Johnson, R.A. Lohsen, O.R. Sander, L.S. Walling, Measurement of RF accelerator cavity field level at high power from X-ray emission, in: Proc LINAC Mexico, 1990, pp. 219. [12] J.P. Duke, A.P. Letchford, D.J.S. Findlay, Measurement of RF cavity voltages by X-ray spectrum measurements, in: Proc. XX International Linac Conference, California, 2000, pp. 164. [13] J.D. Jackson, Classical Electrodynamics, John Wiley and Sons, Inc., New York, 1975, pp. 701–715. [14] B.P. Ajith Kumar, E.T. Subramaniam, K. Singh, R.K. Bhomik, Freedom-High Speed DAS, in: Proc. SANAI 1997, Trombay, India. [15] http://www.iuac.res.in/NIAS/. [16] B.P. Ajith Kumar, E.T. Subramaniam, R.K. Bhomik, CANDLE-Collection and analysis of nuclear data using linux network, in: Proc. DAE SNP 2001, Kolkata, India. [17] Thorsten Hellert, Martin Dohlus, Detuning related coupler kick variation of a superconducting nine-cell 1.3 GHz cavity, Phys. Rev. ST Accel. Beams 21 (2018) 042001. [18] J. Marcia C. Silva, Silvio B. Herdade, Patrcia Lammoglia, P.R. Costa, R.A. Terini, Determination of voltage applied on X-ray tubes from the bremsstrahlung spectrum obtained with a silicon PIN photodiode, Med. Phys. 27 (2000) 11.
Fig. 14. The X-ray energy spectrum at a variable power level (detector installed in the transverse direction). The endpoint energies have been deduced from the shape of the spectra.
Fig. 15. Intensity versus the energy of the X-ray spectra on the same scale.
Acknowledgments We would like to thank Vandana Singh (Teacher, Kendriya Vidyalaya) for proofreading and valuable comments and Dr Prashant Sharma (PhD, IUAC), for his help in the endpoint energy determination. We would like to acknowledge Aneeqa Basheer (Project Student, IUAC) for her participation in data analysis and discussion. We wish to extend our particular thanks appreciation to R Ruby Santhi, VVV Satyanarayana, Saneesh N for their contribution in data acquisition setup. 7