Commissioning of C-band Standing-wave Accelerator

A C-band standing-wave electron accelerator for a compact X-ray source is being commissioned at ACEP (Advanced Center for Electron-beam Processing in Cheorwon, Korea). It is capable of producing 4-MeV electron beam with pulsed 50-mA. The RF power is supplied by the 5-GHz magnetron with pulsed 1.5 MW and average 1.2 kW. The accelerating column is a biperiodic and on-axis-coupled structure operated with π/2mode standing-waves. It consists of 3 bunching cells, 6 normal cells and a coupling cell. As a result of cold tests, the resonant frequency of the accelerating column is 4999.17 MHz at the π/2-mode and the coupling coefficient is 0.92. The field flatness was tuned to be less than 2%. In this paper, we present commissioning status with design details of the accelerator system. INTRODUCTION The electron accelerator is widely used for industrial applications, for example, a medical diagnosis and therapy, sterilization of products, material surface processing, and a contraband detection [1]. Sterilization processing requires an average beam power of several tens of kilowatts which depends on the processing speed. The contraband detection requires 5-10 MeV with the pulsed beam current of about 150 mA [2, 3]. We are developing an electron accelerator for an X-ray source, for industrial application. It is producing 4-MeV electron beam with pulsed 50-mA, using pulsed 1.5-MW C-band RF. In this paper, we present design of the overall accelerator system and measurement results of the prototype cavity. Also we present fabrication and RF conditioning of the actual accelerating structure. ACCELERATOR OVERVIEW An RF source for the accelerator is a 5-GHz CPI magnetron. It is capable of producing 1.5 MW with 4-μs pulse length and 200-Hz repetition rates. The RF power is transmitted to the accelerating column through the WR187 waveguide network, which is filled with atmospheric pressure SF6 gas. A pulse modulator supplies the 40-kV and 90-A pulsed power to the magnetron with 4-μs pulse length [4]. It also supplies 20kV pulsed voltage to an E-gun. The E-gun is a diode-type thermionic DC gun, capable of injecting pulsed 150-mA beam. Table 1: The Design Parameters for the Accelerator Operating Frequency 5 GHz Input Pulsed RF Power 1.5 MW Pulse Length 4 μs Repetition Rate 200 Hz E-gun Voltage 20 kV Input Pulsed Beam Current 150 mA Output Beam Energy 4 MeV Output Pulsed Beam Current 50 mA Output Average Beam Power 160 W Type of Structure Bi-periodic, On-axis coupled Operating Mode SW π/2 mode Beam Aperture Diameter 10 mm Average Accelerating Gradient 13.3 MV/m Number of Cells 10 Inter-cell Coupling 6% Quality Factor 11000 Effective Shunt Impedance 90 MΩ/m Transit-time Factor 0.81 Values for normal cells. For a compact system, the accelerating structure is attached to the E-gun directly and a pre-buncher with a drift tube is omitted, as shown in Figure 1. Furthermore, no solenoids magnet is used since the beam current is low enough to be focused by the intrinsic focusing effect of the standing-wave electric field [5]. A bi-periodic and on-axis coupled structure is adopted for the π/2-mode standing-wave structure [6]. To increase the inter-cell coupling up to 6%, the magnetic coupling slot is bored on the wall between the accelerating cavity and the coupling cavity. The first three cells, in the Figure 2, are bunching cells with βph = 0.7, and later six cells are _____________________________________________________ *Work supported by MKE, Korea and POSTECH Physics BK21 Program. highlong@postech.ac. kr Proceedings of IPAC’10, Kyoto, Japan THPEA033 07 Accelerator Technology T06 Room Temperature RF 3753 normal cells with βph = 1. The last one is a coupler cell attached to the tapered C-band waveguide. Figure 1: The schematic diagram of accelerator system. Figure 2: The cross-sectional view of the accelerating structure. The inter-cell magnetic coupling slots are skewed to this plane. PROTO-TYPE TEST The aluminium proto-type cavities are fabricated for confirming the cavity design [7]. With measurements of the cells, the resonant frequency of the bunching cells is 5000.000±0.661 MHz and the one of the normal cells is 4999.600±0.163 MHz. A proto-type test of a coupler cell is conducted to decide the dimension of the RF input coupler. If the beam power Pb is 0.2 MW and the cavity dissipated power Pc is 1.3 MW, the critical coupling condition, where accelerating efficiency becomes maximized, is calculated as follows: