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DESIGN, DEVELOPMENT AND COMMISSIONING OF 350 MHZ FOUR-VANE RADIO FREQUENCY QUADRUPOLE (RFQ) AND

ITS HIGH POWER RF SYSTEM

P. Singh Ion Accelerator Development Division

Dr. Pitamber Singh, Physics Group and his team Dr. Pitamber Singh, Physics Group and his team Dr. Pitamber Singh, Physics Group and his team Dr. Pitamber Singh, Physics Group and his team received the received the received the received the

DAE Group Achievement Award for the year 2013DAE Group Achievement Award for the year 2013DAE Group Achievement Award for the year 2013DAE Group Achievement Award for the year 2013

Abstract A 400 keV deuteron Radio-frequency quadrupole (RFQ) has been designed, developed and commissioned at BARC, Mumbai. In order to ease the fabrication a conventional (LANL) design procedure was adopted, where the vane voltage and cross section of RFQ are kept constant. The complete beam dynamics, 2D and 3D cavity dynamics were studied using the computer codes LIDOS, SUPERFISH and CST Microwave studio. The 1.1 m long RFQ operating at a frequency of 350 MHz required RF power of 60 kW to accelerate the deuteron beam to a final energy of 400 keV with beam transmission of ~ 95%. Introduction

Radio frequency quadrupole (RFQ) accelerators are extensively used as injectors in the high current linacs because of their remarkable capability of simultaneously focusing, bunching and accelerating the low-energy ion beams with high transmission and minimum emittance growth [1]. In a conventional accelerator such as the drift tube linac (DTL), bunching is accomplished prior to injection into the linac using RF bunching cavities. In buncher cavities, RF electric fields are applied to the DC input beam to produce a velocity modulation in which early particles are decelerated and late particles are accelerated. For high-current beams, the bunching is not very efficient because of the high space-charge forces at low energies. In high intensity beams, the bunching process causes an increase in the beam density, which increases the space-charge forces further resulting in an increase of the transverse beam emittance. The conventional DTL linacs, in order to make the efficient use of DTL structures, the extraction voltage of the ion source was increased to about 0.70 MV, generated using the Cockcroft Walton generators. The fundamental limitations of the conventional linacs was not only in terms of beam current, but also the reliable operation of HV columns, which was responsible for the large fraction of the machine downtime. Beams were also lost during their focusing and bunching before injection into the linac.A RFQ uses the velocity independent transverse electric focusing at low energies which gives a significant strong focusing as compared to conventional linacs that normally use velocity dependent magnetic lenses.This allows one to extend the practical range of operation of RFQs to



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low velocities, thus eliminating the need for large, high voltage dc accelerators. The RFQ accelerators are well suited for acceleration of beams with low velocities in the range of about 0.01 to 0.1 times the speed of light. Since RFQs bunch the beam adiabatically, by proper choice of parameters one can achieve a high beam transmission (> 90%). Brief description of the work

The front-end of the proton linac driver for an Accelerator-Driven System (ADS) has been conceptualized as a 20 MeV, 30 mA, CW proton linac, and has been implemented as the Low Energy High Intensity Proton Accelerator (LEHIPA) project at BARC [2]. This will consist of a 50 keV ion source, a 3 MeV, 4 m long, radio-frequency quadrupole (RFQ) and a 3-20 MeV, 12 m long, drift-tube linac (DTL).Since RFQ technology is challenging, it was decided to design and build, as a precursor, a 400 keV, 1.1 m long deuteron RFQ, which can also be used for 14 MeV neutron generation [3,4] using D-T reaction. After considerable design efforts, and after ascending a steep technological curve, this RFQ was developed and successfully commissioned [5,6]. The system consists of: (i) an ion-source that can deliver deuteron beams at energies up to 50 keV, (ii) a Low Energy Beam Transport (LEBT) line consisting of two solenoids [7], two steerers, and various diagnostic elements, (iii) the 400 keV RFQ [6], (iv) a 900-analyzing magnet (v) a 60 kW RF system at 350 MHz [8], and (vi) a 50 kW power-coupler [9] that couples the power into the RFQ with better than 99.5% efficiency. Based on the simulation studies, the RFQ made of OFE copper was machined and vacuum brazed at BATL, Trivandrum. For an RFQ to work efficiently, it must be made with high level of geometric precision. A series of measures were taken to achieve the desired level of accuracies. While brazing we adopted a two stage brazing process, in the first stage RFQ vanes were brazed, using Palcusil 5(68% Ag+27% Cu+5% Pd), in a horizontal vacuum furnace. In the second stage the ports and end flanges are brazed using Cusil (28% Cu+ 72% Ag) as a filler material. The RFQ after 1st and 2nd brazing is shown in Figs. 1 & 2. The various tolerances achieved during fabrication were: (1) vane position within ± 30 µm, (2) vane modulations within ± 20 µm, (3) vane thickness within ± 10 µm, (4) vane flatness within ± 20 µm.

Fig. 2: RFQ after 2nd Brazing

Fig. 1: RFQ after 1st Brazing



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After brazing, the RF characterization was done, this mainly involved frequency measurements, field tuning and quality factor measurements. Field distribution studies were done using the bead pull measurement setup developed in the lab [Fig. 3].

Initially, the magnetic measurements were done using a metallic bead made of brass of Φ6 mm for the flush tuner conditions. In order to achieve the required field distribution the perturbative analysis of the RFQ has been studied and an algorithm was written to tune the cavity. With the help of this algorthm, the quadrupole field levels were tuned within 5% (dQ/Q0) and dipole contribution < 4% (d1/Q0, d2/Q0) [Fig.4] Fig. 4: Final field distribution and dipole and quadrupole contributions

Fig. 3: Bead-pull measurement setup



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The tuners were adjusted to get the required field distribution as well as resonant frequency of 350 MHz. After the low power testing and cleaning the quality factor (Q0) was measured to be 6384 shown in Fig. 5, which is 73 % of the simulated value.

After the successful completion of cold testing, the high power RF conditioning of the RFQ was done. The RFQ required around 60 kW of RF power to compensate for beam acceleration, vane voltage generation and power dissipation across the RFQ cavity. Therefore, a 60 kW tetrode based RF system at 350 MHz fulfilling all the critical parameters required for the accelerator, like high reliability, ruggedness, good wall plug efficiency, high gain and maximum voltage standing wave ratio (VSWR) withstanding capability, RF radiation limit below IEEE standard, low harmonic contents, was designed, developed and tested [Fig.6]. The RF system comprises of many subsystems such as tetrode based high power amplifier of 60 kW, its driver and pre-driver amplifiers, directional couplers, rigid transmission lines, crowbar based fast protection circuit, 80 kW RF load, a junction circulator, programmable logic controller (PLC) based interlock and protection system, low conductivity cooling subsystems and many other bias supplies of low and high voltage ratings.

Fig. 5: Quality factor (Q0)



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The power was fed into the RFQ through 60 kW RF coupler [Fig.7].

The RFQ was conditioned to a peak power of 60 kW with a duty cycle of 0.1 % (1 ms pulse width and 1 Hz rep rate). Once the RFQ was conditioned successfully at a peak power of 60 kW the beam was injected. In order to accelerate the beam through the RFQ a beam line was set up shown in Fig.8. It mainly consists of a RF ion source (Alphatros), two solenoids, two steering magnets, one bending magnet and two Faraday cups. The ion source along with the accelerating

Fig. 7: 60 kW RF loop coupler

High Power Amplifier

Driver Power Amplifier

DC bias HV and LV

PLC based interlock and protection

Fig. 6: 60 kW tetrode based RF Amplifier



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column used to accelerate the beam to 50 keV is installed in high voltage deck. This high voltage deck was tested up to a voltage of 75 kV, so the input beam energy can be varied from 0-75 keV.

The emittance measurements of the injected beam is important for obtaining the emittance of the input beam within the accepted limit of the RFQ and also in optimizing the LEBT line that matches the beam from the ion source to the RFQ. A slit-wire scanner based emittance measurement setup was developed [Fig.9] for the transverse emittance measurements of H+ and D+ beams [Fig. 10]. The emittance measurement setup consists of movable slits of 0.3 mm width and movable tungsten wire of 0.05 mm diameter as shown in Figure 1. The spatial beam distribution is scanned by the slit while the angular distribution is scanned by the wire scanner located at a distance of 140 mm downstream the beam line. A 1 µm precision linear motion mechanism is used for slit and wire holder movement. The beam emittance can be measured simultaneously in both transverse directions in this setup. The beam dynamics simulation of the RFQ with injector line was performed using TRACEWIN with the measured input beam emittance parameters. The measured Twiss parameters for 100 µA, 50 keV, D+ beam are α = -0.29, β = 1.28 mm/mrad with an rms emittance, εyrms = 2.5 π.mm.mrad [10, 11]. Figure 2 shows the variation of normalized rms emittance of H+ and D+ beams as a function of beam current. The emittance of the input beam is well within the acceptance value of RFQ.

Fig.8: Beam line of 400 keV RFQ



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Results and conclusions We accelerated both the proton (H+) and deuteron (D+) beams through this RFQ. In order to accelerate the H+ beam the RF power needed was ~ 15 kW and input beam energy should be 25 keV. The H+ beam was accelerated to 200 keV with a transmission of 70%. The output beam energy and beam current are measured using the 900 bending magnet and a faraday cup. The variation of transmission with input beam energy and with the RF power was also measured. After successful acceleration of H+ beam the D+ beam at an input energy of 50 keV was accelerated to 400 keV at a peak power of 60 kW. The transmission of 95 % was measured. The output D+ beam energy and its energy spread were measured using 900 bending magnet and the results are shown in Fig.11. The results are in good agreement with the simulations.

Fig.11: Output energy of D+ beam

Acknowledgement Following members of the RFQ team ( P. Singh, Shrikrishna Gupta, S. Krishnagopal, Manjiri Pande, A. Agarwal, Rajesh Kumar, S.K. Singh, B.M. Kukreti, S.V.L.S. Rao, Rajni Pande, A. Basu, Shweta Roy, Piyush Jain, S.D. Shrotriya, Sapna P., Sonal Sharma, Jose V. Mathew, N.R. Patel, N.B.V. Subrahmanyam, J.P. Bhatt, S.V. Ware, Shiju A., M.R. Mishra, G.N. Singh, Pravin R. Parate, Laxman D. Tayade, Prashant P. Gorule, H.K. Sane, N.J. Koli, N.R. Dalvi, Mukesh Thapad, Gopal Joshi, N.K. Prasad) made excellent contributions towards design, development and commissioning of the 350 MHz four vane Radio Frequency Quadrupole (RFQ) and its high power RF system and we thank them all. Publications

[1] Design studies of a high current radio frequency quadrupole for accelerator driven systems prohramme, S.V.L.S. Rao and P. Singh, Pramana –J Physics 74 247 (2010), I.M Kapchinskii and V A Tepliakov, Prib. Tekh. Eksp 2, 19 (1970).

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[2] Accelerator Development in India for ADS programme, P. Singh, S. V. L. S. Rao, Rajni Pande, T. Basak, Shweta Roy, M. Aslam, P. Jain, S. C. L. Srivastava, Rajesh Kumar, P. K. Nema, S. Kailas and V. C. Sahni, Pramana- J Phys. 68, 331 (2007), Status of the LEHIPA Project at BARC, InPAC-2006, Nov 1-4, 2006, BARC, Mumbai, p50.

[3] Physics Design of a 400 keV D+ RFQ for PURNIMA Sub-Critical Facility, Shweta Roy, T. Basak, Rajni Pande, S. V. L. S. Rao, M. Aslam, P. Jain, P. Singh, P.K. Nema, S. Kailas and V.C. Sahni, BARC Report-BARC/2005/I/005

[4] Design of a RFQ based 14 MeV Neutron Generator for Studying Neutron Multiplication in a Sub-Critical Assembly, R. P. Anand, T. K. Basu, B.K. Godwal, V.K. Handu, S. Kailas, P.K. Nema, Manjiri Pande, B. V. Rama Rao, M. K.V. Rao, V.C. Sahni and P. Singh, Proc. DAE Symp. Nucl. Phys. (India) 46B, 574 (2003).

[5] Design, Development and Acceleration Experiments of a Four-Vane RFQ at BARC, S. V. L. S. Rao, Piyush Jain, Jose V. Mathew, Rajni Pande, Shweta Roy, S. Shrotriya, N. Patel, A. Shiju, , M. R. Mishra, Gireesh N. Singh,L. Tayade, Praveen Parate, B M Kukreti, Rajesh Kumar, Manjiri Pande, S. Krishnagopal, S. K. Gupta, P. Singh, InPAC 2013 at VECC, Kolkata, Nov.19-22, 2013

[6] Design, development and acceleration trials of Radio-frequency quadrupole, S V L S Rao, Piyush Jain, Rajni Pande, Shweta Roy, Jose Mathew, Rajesh kumar, Manjiri Pande, Srinivas Krishnagopal, S K Gupta and P Singh, Review of Scientific Instruments 85, 043304 (2014).

[7] A LEBT for the 400 keV RFQ Based Neutron Generator at BARC, Rajni Pande, S. V. L. S. Rao, Shweta Roy, Manas Mishra, L.D Tayade, P.R Parate, Sapna P., Shailaja Ware, S. K Singh, A. Agarwal, S. K Gupta, P. Singh, Proc. Indian Particle Accelerator Conference (InPAC-2011), Feb 15-18, 2011, IUAC, New Delhi.

[8] Design and development of 60 kW, 350 MHz RF system in CW and Pulse mode for 400 keV RFQ accelerator, Manjiri Pande, Sandip Shrotriya, Niranjan Patel, Shiju A, Sonal Sharma and P.Singh, IEEE-MTTS-IMARC 2013, Dec 14-16, 2013, New Delhi.

[9] Development of a 50 kW, 350 MHz Pulsed Power Coupler for RFQ conditioning, Rajesh Kumar, P. Singh, L.M. Joshi, Proc. Indian Particle Accelerator Conf.(InPAC 2011), Feb 15-18, 2011, IUAC, New Delhi.

[10] Transverse Emittance Measurement Studies of a Deuteron Ion Source Beam, Jose V. Mathew, S.V.L S. Rao, Rajni Pande, M.R. Mishra, L.D. Tayade, P.R. Parate, P. Singh, InPAC 2013 at VECC, Kolkata, Nov.19-22, 2013.

[11] Beam emittance measurements and simulations of injector line of RFQ, Jose Mathew, S V L S Rao, Rajni Pande and P. Singh, Review Scientific Instruments 86, 073306 (2015).

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BARC NEWSLBARC NEWSLETTERBARC NEWSLETTER ETTER FOUNDER…barc.gov.in/publications/nl/2015/spl2015/pdf/paper28.pdf · BARC NEWSLBARC NEWSLETTERBARC NEWSLETTER ETTER FOUNDER’S DAY