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 Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi-110067, India b 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  z and  z provide the information of phase space occupied by the charged particles. The  =0,  =1, represents the upright ellipse or waist location, and large value of  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  ∼ 0 indicates well time bunched beam at RFQ entrance. In Fig. 2(b) as the Twiss parameter  z 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 ∗ Corresponding author at: Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi-110067, India. E-mail address: skkedia87@gmail.com (S.K. Kedia). 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 ( =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 -axis (beam axis). The spiral width is 50 mm 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.