Investigation Of Water-dielectric Spiral Strip Blumlein Line

A high power accelerator capable of producing long voltage pulse is of great interest in the pulse power region. One of the most widely employed devices to achieve long pulse is to use spiral pulse forming line(PFL) to modify the coaxial one. Two different types of spiral PFL, spiral wire PFL and spiral strip PFL, which differ from each other primarily in the configuration of the spiral conductor, are both effective method to achieve long pulse at a given length. Compared with the spiral wire PFL, the spiral strip PFL has a desirable attribute for including simplicity. So, more attention should be paid to it, especially the water-dielectric spiral strip PFL. For most of the accelerator, the transformer oil was employed as dielectric of the spiral PFL, which resulted in the accelerator with large dimension and huge weight. This thesis presents preliminary investigation on the water-dielectric spiral strip Blumlein line. The theoretical analysis, the simulations and experiments of this type of accelerator are carried out. A series of valuable results are obtained from this work.Firstly, the current in the helical line, the electric field between conductors and the breakdown field are theoretically analysed and simulated. The caculated and simulated results are in good agreement and it is demonstrated that the water-dielectric spiral strip Blumlein line satisfies the thin current layer model and quasi-stationary parameter assumption. The accelerator can work stable without breakdown occurred under the operating voltage of 540kV.Secondly, according to the thin current layer model and quasi-stationary parameter assumption, the inductance, capacitance, transmission time and characteristic impedance of spiral PFL are theoretically caculated, and KARAT code is used for simulation to verify the results obtained from the theoretiacal caculation.Thirdly, the transmission process of electromagnetic wave in the PFL and the influence of the connected inductance between conductors of PFL on the diode voltage is analysed experimentally and theoretically. The process of the tansformer charging spiral PFL and the prepulse in the diode are originally investigated. The theoretical results agree with simulated results.Based on the above research, a long-pulse accelerator with spiral PFL is established. It consists of a primary storage capacitor system, an air core spiral strip transformer, a water-dielectric spiral strip Blumlein line and a field-emission diode. The experimental results show that the diode voltage is more than 500kV, the electron beam current of diode is about 22kA, and the pulse duration is about 200ns. Although diode voltage measured in the experiment is a little lower than simulated one. The main reason is loss of high power spark gap switch and errors of circuit parameters.