Design and development of a focusing electromagnet for RF linac of a cargo scanner B. Nayak n , S. Acharya, D. Bhattacharjee, Shiv Chandan, N. Choudhury, V. Sharma, K.C. Mittal, L.M. Gantayet Accelerator and Pulsed Power Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India article info Article history: Received 22 March 2014 Received in revised form 2 August 2014 Accepted 3 August 2014 Available online 10 August 2014 Keywords: Focusing solenoid Beam optics Cargo scanning linac abstract Cargo scanning of contraband objects by high energetic X-rays requires compact and self-contained RF linacs. To achieve this goal, a 6 MeV, 700 W standing wave RF electron linac has been developed at Accelerator and Pulse Power Division, BARC. To obtain the image of scanned objects with good resolution requires a focused electron beam at the target. This paper presents the design, development and integration of a solenoid magnet with a compact RF linac and its performance with electron beam for cargo scanning applications. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Cargo inspection is one of the important components of the homeland security. At ports, linac based cargo scanning systems [1] are used to scan cargos such that the transport of contraband objects can be examined. A cargo inspection system mainly consists of an X-ray source, an array of detectors, a software to re-construct the image from the electronic signal of detectors, control systems and the gantry or other mechanic moving systems. Because high energy X-rays are needed to penetrate the cargo, normally an electron linac is served as the X-ray source [1–3] In these systems, the linac acts as the source of X-rays, which fall on the cargo and are then detected by a detector system [4]. Both permanent magnets [5–10] and electromagnets [11–14] are used to focus the beam from the linac to smaller spot sizes. Although the use of permanent magnet eliminates the require- ment for a power supply but it is very difficult to adjust the magnetic field to optimize the accelerator performance. So for a mobile system like RF linac cargo scanners, it is preferable to use solenoid for focusing purposes. An RF electron linac of energy 6 MeV designed and developed by Accelerator and Pulse Power Division, BARC is used to generate X-rays for imaging by impinging the electron beam on a Tungsten target [1]. In the present work, design of a post-acceleration solenoid magnet [15] and its performance with the accelerator is discussed. The initial beam size of 6 mm is focused to a lesser diameter which is necessary for image with good resolution to scan cargos. Analytical calculations and simulations with electro- magnetic software CST particle studio [16–20] are done to realize a compact magnet. The cargo is divided into several slices, each slice is pictured and then they are integrated by software to generate a good quality image of the hidden object. A primary collimator is located immediately after the target to generate a fanned X-ray beam and shield undesirable X-rays. To test the performance of magnet with the electron beam, it is integrated with the accel- erator and variation of the X-ray dose [21] at the Tungsten target with the magnet coil current is measured by an ionization chamber [4] placed at the collimator exit. This paper is organized into two parts. In the first part, we calculate the beam optics for the optimized design of the focusing electromagnet. We solve the RMS beam envelope equation and find the required magnetic field to reduce the beam size for X-ray generation. We carry out numerical computation using Electro- magnetic software CST particle studio in order to optimize the dimensions and parameters of the magnet. In the second part, the test results of the magnet are presented and its performance after integration with the accelerator is discussed. The accelerating module of the linac consists of an electron gun, Accelerating cavities, a solenoid magnet, collimator and X-ray target. A 85 keV thermionic Triode electron gun serves as the injector to the RF linac. The standing wave, bi-periodic, on-axis coupled cavity linac operates in π/2 mode at a frequency of 2856 MHz. The RF linac cavity is fed with a magnetron-based microwave power source [5 MW (peak)/3 kW (average)]. An electromagnet with eight pancake coils in series is used to focus the output beam of the linac. A water-cooled Tungsten X-ray target Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A http://dx.doi.org/10.1016/j.nima.2014.08.001 0168-9002/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ91 22 27524574. E-mail address: biswaranjan.nayak1@gmail.com (B. Nayak). Nuclear Instruments and Methods in Physics Research A 764 (2014) 340–346