Design And Experiment Of ClIC Choke-mode Accelerating Structure

X-band normal-conducting high gradient accelerating structures are being studiedfor the Multi-TeV linear collider or the compact X-ray FEL facility. However, the strongwakefield excited by beam in X-band structures will cause emittance dilution or beaminstability. Several kinds of structures with higher-order-mode (HOM) suppression werethence developed in order to reduce the wakefield effect on the beam.The choke-mode accelerating structure with HOM damping was first proposed byT.Shintake in1992. Take the advantage of easier fabrication and lower pulsedtemperature rise, the choke-mode structure is studied as an alternative design for themain linac of Compact Linear Collider (CLIC). Theoretical design for X-bandchoke-mode structure will be investigated in the dissertation, other issues aboutexperiments and the manufacture will also be preliminarily discussed.Based on electromagnetical field distribution in the choke-mode cavity, aequivalent circuit model is developed to determine its HOM suppression. Tuningformula for the accelerating mode in the choke-mode cavity is derived from this circuitmodel and can be used to assist designing the structures and estimate parameters.Using the equivalent circuit model, the new choke-mode geometry is developed toimprove its wakefield suppression. The matched load for this choke-mode cavity is alsodesigned. The new choke-mode design meets the beam dynamics requirements and isverified by GDFIDL simulations.Optimization of tapered choke-mode structure is carried out and aims onimproving the RF-to-beam efficiency and reducing the surface electric or magnetic field.A genetic algorithm for tapered accelerating structures is discussed and is used to searchthe optimum solution. A new tapered24-cell choke-mode structure is designed and hasa higher RF efficiency and lower surface field.Refer to the circuit model for the choke-mode structure, HOMs are damped in theradial line and the choke parts. A radial line experiment is carried out to measure theabsorption of HOMs in these parts, which will verify HOM suppression of choke-modecavity. Multi-short calibration method is developed for the radial line experiment due tolack of standard calibration kit for radial line. Required calibration accuracy has beenachieved in the measurements using multi-short calibration method. Several choke structures has been tested in the radial line experiments and verifies the HOMsuppression of the choke-mode cavities. The matched load for choke-mode structure isalso tested in the radial line experiments and meets the design target of the matchedload.