Xiaowei Wu

High-Gradient Performance of X-Band Choke-Mode Structures

The choke-mode accelerating structure is one of the higher-order-mode (HOM) damping structures. It has the advantage of relatively simple fabrication and low surface magnetic field. C-band choke-mode accelerating structures have been successfully applied in multibunch XFEL. However, the X-band choke-mode study remains in the theoretical design stage. The high-gradient performance of the choke is still unknown. Five different single-cell choke-mode accelerating structures were designed, fabricated and high-gradient tested to study the related RF breakdown characteristics. It was observed that high electric field and small choke dimension caused serious breakdowns in the choke which was the main limitation of the high-gradient performance. The Choke-mode accelerating structures reached 130 MV/m by decreasing the electric field and increasing the choke gap. A new quantity was proposed to give the high-gradient performance limit of choke-mode accelerating structures due to RF breakdown. The new quantity was obtained from the summary of the high-gradient experiments and could be used to guide high-gradient choke-mode accelerating structure design.

Development of a C-Band 4/8 Mev Dual-Energy Accelerator for Cargo Inspection System

Modern cargo inspection system applies dual-energy Xray for material discrimination. Based on the compact Cband 6 MeV standing-wave accelerating structures developed at Tsinghua University, a compact C-band 4/8 MeV dual-energy accelerator has been proposed, fabricated and tested. Compared with that of the conventional S-band 3/6 MeV dual-energy accelerator at Tsinghua University, the volume and the weight of the C-band one has been reduced by ~40% and ~30%, respectively. Detailed review of this C-band dual-energy accelerator is present in the paper. INTRODUCTION Nowadays, cargo inspection system is essential for homeland security and customs control. Dual-energy Xray is necessary for effectively discrimination of materials with similar atomic numbers [1]. Over the last decade, dual-energy accelerators within the energy range of 1-10 MeV have been developed by various research groups [16]. Most of these accelerators are operated at S-band (2856 or 2998 MHz). Accelerators operated at higher frequency such as C-band (5712 MHz) and X-band (9300 MHz) are preferable when space is limited and accurate positioning is required [7]. Compared with X-band ones, C-band accelerators require less production precision and are less likely to detune after brazing [7]. Recently, the research of C-band accelerators for medical and industrial applications is of high interest and many prototypes have been developed [7-12]. Two prototypes of C-band 6 MeV standing-wave biperiodic on-axis coupled linear accelerating structure have been developed at Tsinghua University [7]. A compact 4/8 MeV dual-energy accelerator has been developed based on the prototypes by adjusting the input power as well as the beam loading. ACCELERATING STRUCTURE The prototypes of C-band 6 MeV standing-wave linear accelerating structure have a compact size with a narrow spectrum and a small spot size that can be used for medical and industrial applications. The two prototypes share nearly the same design for the bunching cells and normal cell. Compared with the prototype I, the prototype II has one more normal cell to lower the required input power to achieve the same acceleration. The main parameters of the accelerating structure prototype II are listed in Table 1. In Table 1, the length of the accelerating structure excludes the thermal-cathode gun and the flange at the exit for test. Pm is the required input pulsed power to accelerate 130 mA beam to the nominal 6 MeV. Ea is the corresponding accelerating gradient of the normal cell. Table 1: Parameters of the C-band 6 MeV Accelerating Structure Prototype II Parameter Value Frequency (MHz) 5712 Number of bunching cells 3 Number of normal cells 10 Length (cm) ~31 Quality factor 10000 External coupling factor 1.6 Pm (MW) 1.95 Ea (MV/m) 24 The accelerating structure is cooled by a water jacket. The prototype II after fabrication is shown in Fig. 1. Figure 1: The C-band 6 MeV accelerating structure prototype II. DUAL-ENERGY ACCELERATOR The compact C-band 4/8 MeV dual-energy standingwave accelerator has been developed based on the accelerating structure prototype II, as shown in Fig. 2. The power source of the accelerator is the VMC3109 magnetron from CPI and it’s fed by a domestic solid state modulator. The maximum pulsed and average power are 2.5 MW and 2.5 kW, respectively. Taking the attenuation of the waveguide system between the magnetron and accelerator to be 0.5 dB, the maximum pulsed input power available for the accelerator is ~2.23 MW. The energy variation has been achieved by adjusting the input power as well as the beam loading. Proceedings of IPAC2016, Busan, Korea TUPOW016 08 Applications of Accelerators U04 Security ISBN 978-3-95450-147-2 1775 C op yr ig ht