Study On The Coaxial Spiral Blumlein Line Output Waveform Optimization

Development of high power microwave technology make an urgent need for pulsed power system's long pulse output and the compact construction.Coaxial spiral Blumlein line can achieve long pulse outputs without increasing the volume of pulse forming line,and has been widely used for long pulsed power source.On the issue of middle-cylinder spiral Blumlein line's prolonged rise time as well as plateau jitter,output waveform optimization research is carried out.In this dissertation,theoretical analysis and field circuit co-simulation on coaxial spiral Blumlein line are carried out,verification experiments of different forms of spiral Blumlein lines are conducted,and pulse forming line structure is improved to acquire high quality outputs based on the theoretical results.The detailed contents include the followings:1.Wave process of coaxial Blumlein line and relationship between load output voltage and the inner & outer line impedance are derived;parasitic transmission line is the cause for middle-cylinder spiral Blumlein line's output waveform deterioration.Impedance inequality of the inner & outer line is the main reason for the Blumlein line output waveform difference in different switch positions,.2.By CST field circuit co-simulation,output characteristics of Blumlein lines with different spiral structure are studied.For Blumlein line with spiral middle cylinder,parasitic transmission line prolong the rise time of the output pulse.It takes non-spiral transmission time for the voltage wave to propagate through the parasitic line.Through rational design,applying the inner & middle cylinder spiral in the same direction,a better load pulse output is obtained in certain turns ratio.When the inner & outer cylinder spiral Blumlein line discharge,in order to shield electromagnetic radiation,shielding cylinder will cause output waveform distortion when the switch in the outer line.Distortion can be decreased if low dielectric constant materials are filled between the shielding cylinder and outer cylinder.For Blumlein line with equal inner & outer line impedance,the output waveforms are identical by different switch positions.3.Verification experiments of different switch positions and different structure spiral Blumlein lines are conducted.When the switch is in the outer line,quasi-square wave pulse with rise time 45 ns,plateau time 94 ns,FWHM 139 ns is obtained from 5.5-turn inner cylinder & 20-turn inner cylinder spiral Blumlein line structure.Waveform quality improved significantly compared with 5.5-turn middle-cylinder spiral Blumlein line output.Shielding cylinder of the inner & outer cylinder spiral Blumlein line will cause output pulse width shortened and waveform distortion.Since the inner line impedance is greater than that of the outer line,there exists reflected pulse after main pulse when the switch is in the outer line;while voltage trailing appears after the main pulse when switch in the inner line.4.According to the theoretical results,laboratory's intense electron beam accelerator is transformed,by replacing the original non-helical inner cylinder into a 5.1 turn-spiral cylinder,quasi-square wave pulse with rise time 55 ns,plateau time 130 ns,FWHM 205 ns is obtained,the quality of the output pulse waveform significantly improved.