Lin Yuzheng

Theoretical Study of a Plasma-Filled Relativistic Cerenkov Generator With Coaxial Slow-Wave Structure

The linear and nonlinear theory of a plasma-filled relativistic Cherenkov generator with coaxial slow-wave structure (SWS) is developed. Linear analysis includes the space-charge limiting current, dispersion relations, dispersion curves, and coupling impedance. In nonlinear theory, the electron beam space-charge effect, nonsynchronous interaction, and complex reflection coefficients of the electromagnetic wave at the boundaries of the SWS are taken into account. With the assumption of quasi steady state, the self-consistent equations are numerically calculated using the fourth-order Runge-Kutta method. The effects of the relative phase between inner and outer corrugations, phase and amplitude of the complex reflection coefficient at the boundaries of the SWS, and plasma density on device efficiency are discussed. Efficiencies of up to 37% without plasma and about 50% with plasma density of np = 3-7 times 1011 cm-3 are obtained.

A portable X-band on-axis standing wave linac structure

A portable X-band on-axis standing wave electron linear accelerating structure has been developed that is suitable for portable radiation therapy and radiography. The phase-focusing technique is used. The design parameters of a 2 MeV, X-band, on-axis coupled, SW accelerating guide operated in the /spl pi//2 mode is described. A prototype 150 mm long structure has been machined, brazed and sealed, and the experimental data of the cold tests and beam tests are presented.

Simulation Studies of a Relativistic Klystron With Strong Input Power

As one of the potential high-power microwave devices on the level of gigawatts, relativistic klystron amplifier (RKA) can be used for power combination to further improve the radiation microwave power. Because of self-excited oscillation of the intense relativistic electron beam, the frequency and phase characteristics of general relativistic klystron devices are independent with the driven microwave pulse. In order to obtain frequency and phase locking, a method of improving the input power is put forward to inhibit these parasitic oscillations. This paper reports a particle-in-cell simulation study of the RKAs characteristics under the condition of strong power feeding. By making use of the 500-keV 6-kA electron beam, the simulation results show that strong input power can inhibit the parasitical oscillations in cavity and modulate the beam very well with only one cavity. About 5.4-kA modulation current and a microwave with power of 1.4 GW, bandwidth of 5%, and efficiency of 50% are obtained. The new amplifier driven by the strong input power proves to be a potential device to attain high amplitude stability, high efficiency, high spectral purity, wide bandwidth, and low level of phase and amplitude noise.

PRELIMINARY EXPERIMENT OF THE THOMSON SCATTERING X-RAY SOURCE AT TSINGHUA UNIVERSITY

The X-ray source based on Thomson scattering of ultrashort laser pulse with a relativistic electron beam is a means of generating a tunable, narrow bandwidth and ultrashort pulse of hard X-rays. Such a sub-picosecond hard X-ray source is proposed at Tsinghua University, and a preliminary experiment with a 16 MeV Backward Traveling electron linac and a 1.5 J, 6 ns Q-switched Nd:YAG laser is carried out first. A 6 ns pulse X-ray with a peak energy of 4.6 keV and an intensity of 1.7×104 per pulse is generated successfully in the experiment. The experimental setup, result and discussion are reported in this paper.

Theoretical investigation of an electron beam propagating through a wide gap cavity

This paper presents a self-consistent nonlinear theory of the current and energy modulations when an electron beam propagates through an inductively-loaded wide gap cavity. The integro-differential equations are obtained to describe the modulation of the beam current and kinetic energy. A relativistic klystron amplifier (RKA) model is introduced, which uses an inductively-loaded wide gap cavity as an input cavity. And a numerical code is developed for the extended model based on the equations, from which some relations about the modulated current and modulated energy are numerically given.

Development of Mini-LIA and Primary Experiments

Mini-LIA is a miniature of a linear induction accelerator developed by China Academy of Engineering Physics and Tsinghua University in 2007. It has been constructed with a thermionic cathode in an electron injector and a metglas core in the induction accelerator cavities. A double-pulsed electron beam was produced for the first time in China on the Mini-LIA with a thermionic cathode in the electron gun and a metglas core in the induction accelerator cavities. A double-pulsed beam current of more than 1.1 A was obtained on condition of 80 kV double-pulsed high voltage produced by pulsed power system supplying to the injector and accelerating modules. Some primary experiments for measuring the parameters of Mini-LIA has been performed, and some beam characterizations of Mini-LIA are presented. Further improvement is underway.

Multislit-Based Emittance Measurement of Electron Beam from a Photocathode Radio-Frequency Gun

Measurement of emittance for a space-charge dominated electron beam from a photocathode rf gun is performed by employing the multislit-based method at Accelerator Laboratory of Tsinghua University. We present the design considerations on the multislit system and the experimental results, with special attention to the study of space charge induced emittance growth. The experimental results are in reasonable agreement with the PARMELA simulations.

Experimental Characterization of Sub-picosecond Electron Bunch Length with Coherent Diffraction Radiation

Diffraction radiation is one of the most promising candidates for electron beam diagnostics for the International Linear Collider, x-ray free electron lasers and energy recovery linac due to its non-intercepting characteristics. We report the non-intercepting measurement of sub-ps electron bunch length with coherent diffraction radiation. The bunch length is measured with a Martin—Puplett interferometer and the detailed longitudinal bunch shape is reconstructed with the Kramers—Kronig relation. The rms bunch length is found to be about 0.73 ps, which confirms a successful commissioning of the bunch compressor and the interferometer.

Higher-order modes in periodic accelerating structures analyzed using the cavity mode method

The cavity mode method is a new method for computing the dispersion curves and fields of accelerating structures. The method assumes that the field of interest is expanded with open and short modes. These modes are constructed on the basis of the resonant fields in a unit cell. With the wave equation and the Floquet boundary conditions, the method is mathematically simplified into a general eigenvalue problem to calculate the dispersion curve and fields for any phase shift in the structures. This method has been used in analyzing the higher-order modes in the periodic structures. In further study, it is also expected to be useful for analyzing non-periodic structures in the next generation linear collider.

Genetic algorithm diagnosis of individual cell frequencies in a coupled cavity chain

Abstract Genetic algorithms (GAs) are used to diagnose individual cell frequencies in a coupled cavity chain without any perturbing objects. The cell frequencies and couplings between the cells are determined in terms of the measured pass-band performance. The analysis simplifies the tuning processes and enables the tuning of the sealed cavities possible. The problem is solved as an optimization problem with GAs used as a function optimizer. The convergence is improved using the nonlinear least-squares method as a modification of the GAs, which also improves the precision of the solution. The effectiveness of the new technique is demonstrated through some numerical examples and the results are compared with those of the conventional GAs.