Study and Simulations of Klystron Modulator Pulse Transformer

Abstract

Pulse transformer transients are among the most common problems faced, during pulse modulator design. In order to deal with these problems, Pulse transformer’s parasitic elements must be minimized during pulse transformer designing. This work contributes towards evaluating parasitic elements i-e distributed capacitance and leakage inductance of Pulse Transformer before construction for more accurate dimensioning of Pulse transformer to achieve desired pulse specifications. Two approaches are adapted to evaluate Pulse transformer parasitic elements. The first one is Analytical approach and the second one is FEMM simulations. In analytical method, pulse transformer cross section in 2D is divided into six regions. Electrical and magnetic energies expressions are derived assuming some approximations in shapes of regions. Electrical energies of different regions of pulse transformer found are multiplied by corresponding depths, depending on dimensions of pulse transformer. Distributed capacitance is evaluated by the electrical energies found. The second approach is 2D finite element method simulations, carried out on FEMM software. For electrical energies same regions as in analytical method, are simulated in FEMM. Modeling of secondary funnel winding in FEMM is done by split conductor. Number of conductors is equal to number of secondary winding turns. Voltage assigned to each conductor is gradually increased from zero to full scale output voltage. With the said modeling more precise electrical energy is calculated to have better estimate of distributed capacitance. For Leakage inductance evaluation, the whole pulse transformer cross section is simulated in magneto static mode. The magnetic energy associated with region in between secondary and primary winding is primarily responsible for leakage inductance. The post processing is done to evaluate distributed capacitance and leakage inductance from electric and magnetic energies respectively. Pulse transformer response to different values of parasitic elements is simulated and analyzed using IEEE standard equivalent circuit for Pulse Transformer. Klystron is a nonlinear load, whose impedance varies with the operating voltage. Klystron manufacturer provides pulse specification requirement for its stable operation. An excessive overshoot can cause arcing in Klystron gun making its life shorter. Variation in voltage at pulse flat top causes viii change in phase of microwave produced. In high energy physics experiments, change in microwave power phase results in outside detector collisions in collider applications and experiment results are lost. Similarly an excessive undershoot will damage klystron and measuring equipment on waveguide assembly as klystron is a unidirectional load. Klystron Modulators are designed to meet pulse parameters specifications like rise-time, flat top stability, voltage droop and tail for stable and efficient operation of Klystrons. Pulse transformer parasitic elements play a crucial role in meeting the desired pulse specifications. In this thesis analytical modeling of leakage inductance and parasitic capacitance for three winding topologies are compared considering space between primary and secondary windings. The best one topology having least product of leakage inductance and distributed capacitance is further explored for detail analysis, considering all the regions inside pulse transformer. Finite element method (FEM) simulations are carried out of Funnel winding pulse transformer in FEMM software for more accurate dimensioning of pulse transformer. Finally Pulse transformer core is studied and DC core reset is described with choke designing for efficient utilization of Core cross section area in pulsed operation