High Power Collinear Load Research Of S-band Linear Accelerator

Nowadays, electron linear accelerators are more and more employed in industry, agriculture, medical care and so on, while the miniaturization and mobility of LINAC is expected to advance all these applications. Aimed at substituting the output coupler and remnant power absorbing load of the LINAC, collinear load coated with high loss materials in its inner walls is expected to make it a reality. It will make the accelerating structure more compact, the size of the whole system decreased and the assembly procedure more simplified.In addition, the symmetry of the load will contribute to better beam quality. Foregoing researches on collinear load were restricted in lower remnant power levels for their rule-of-thumb means. In order to develop high-power S-band collinear load, systematic research on numerical design is performed and presented in this thesis. Several testing cavities following the design are processed out. The size, the operating frequency and quality factor of the cavities are measured. All they have reached the expected value. Then, A six-cavity FeSiAl collinear loads are designed and fabricated for absorbing 15 kW remnant power and -30db attenuation coefficient, The experimental results and the theoretical simulation predicted results are in agreement.In this thesis, firstly, the software CST is choosed as the main simulation tool, and in contrast, with the software Poisson SuperFish, verified the CST solving accuracy is confirmed for deformed 3D cavities. Then, the design parameters closely related to the performance of the load cavity, like operation frequency f, attenuation factor alpha,the group velocity Vg quality factor q, etc is discussed. A key problem to plan the location of the coating material in the inner wall of the cavities is solved Different from the traditional methods such as uniform power or same attenuation for every cavity, an optimized strategy based on uniform power principle is adopted, to make the actual load cavities with uniform temperature distribution when they are in operation.The electromagnetic parameters of the microwave absorbing material affect the performance of cavities seriously. In measuring the electromagnetic parameters of FeSiAl alloy, some uncertainty appeared. Simulation with CST is utilized to analyze the test results and find the uncertainty is caused by the uncertain gap between the test sample and test fixture. The simulation is very deliberate with several samples of the hollow test fixture, PTFE, crystals of Al₂O₃ and ZrO₂. The experiment simulations of the FeSiAl demonstrate that minor error of the sample size makes the experiment results great deviation from the true values.In the case of load cavities with FeSiAl coating, the accuracy control of the cavity simulation is studied. The effect of the coating volume upon the cavity frequency and Q factor is analyzed and the dimension compensations of the cavities are suggested for tuning the load cavities at 2856 MHz. The Orthogonal experimental method is utilized to investigate the sensitivity of the material permittivity (both real part and imaginary part) and permeability (both real part and imaginary part) to cavity characteristics. Five cavities with different coating dimensions are manufactured and their operating frequencies and Q values are measured. Meanwhile, the results are compared with the simulations of the cavities with actual dimensions. It is shown that the Q factor, which is characterization of the actual attenuation of the FeSiAl, agrees very well with the theoretical value. the Q factor of the resonant cavity is measured with the probe method. The relationship between Q factor and the length of the test probe is deduced and eventually the individual Q value of a load cavity is extracted. Simulation shows the FeSiAl load can support average power over 15 kW and the one-way attenuation is about 30 dB.As for Kanthal load cavities, the influence of coating position and area to the cavity properties is studied and the cavity dimensions under 2856 MHz are gained. Four cavities are produced and corresponding tests are executed. The measured operating frequencies and Q values are compared with CST simulations. The Q values indicate that the loss of the Kanthal coating is approximately 50% in contrast with theory.On the basis of the simulation analysis and experiment tests of the load cavities, in accordance with the targets of 15 kW average remnant power and 30 dB one-way attenuation, the structure design of six-cavity collinear load is accomplished. Firstly the design flow of the collinear load is summarized and generalized, Two collinear load are designed and both satisfy with the attenuation requirement. With the limitation of absorbing capacity, Kanthal load can only to support average power of 10 kW and the one-way attenuation is about 16 dB. The power distribution calculations point that the power of FeSiAl coating is uniform, the power of Kanthal coating on the cavity ring is also uniform, and the power density of Kanthal coating on the disk surface appears parabola-like.This work is supported by the NSFC (NO. 10775128).