Abstract
The Blumlein pulse forming line (BPFL) consisting of an inner coaxial pulse forming line (PFL) and an outer coaxial PFL is widely used in the field of pulsed power, especially for intense electron-beam accelerators (IEBA). The output voltage waveform determines the quality and characteristics of the output beam current of the IEBA. Comparing with the conventional BPFL, an IEBA based on a helical type BPFL can increase the duration of the output voltage in the same geometrical volume. However, for the helical type BPFL, the voltage waveform on a matched load may be distorted which influences the electron-beam quality. In this paper, an IEBA based on helical type BPFL is studied theoretically. Based on telegrapher equations of the BPFL, a formula for the output voltage of IEBA is obtained when the transition section is taken into account, where the transition section is between the middle cylinder of BPFL and the load. From the theoretical analysis, it is found that the wave impedance and transit time of the transition section influence considerably the main pulse voltage waveform at the load, a step is formed in front of the main pulse, and a sharp spike is also formed at the end of the main pulse. In order to get a well-shaped square waveform at the load and to improve the electron-beam quality of such an accelerator, the wave impedance of the transition section should be equal to that of the inner PFL of helical type BPFL and the transit time of the transition section should be designed as short as possible. Experiments performed on an IEBA with the helical type BPFL show reasonable agreement with theoretical analysis.
Highlights
The Blumlein pulse forming line (BPFL) [1] provides flat voltage pulses with an amplitude equal to the charging voltage when the load resistance is twice the impedance of the individual pulse forming lines (PFL)
The construction of the intense electron-beam accelerators (IEBA) based on helical type BPFL is illustrated in Fig. 1 which consists of the primary storage capacitor, field distortion switch [19], pulsed transformer, main switch, helical type BPFL, transition section, the load, and resistive divider
The equivalent circuit of the helical type BPFL discharging into the load is modeled where the transition section between the BPFL and the load is taken into account
Summary
The Blumlein pulse forming line (BPFL) [1] provides flat (rectangular) voltage pulses with an amplitude equal to the charging voltage when the load resistance is twice the impedance of the individual pulse forming lines (PFL). An electron-beam accelerator based on the helical type BPFL with water dielectric was investigated [10], the length of the PFL was only 1.35 m, but the duration of output pulse was about 200 ns. For the helical type BPFL, the transit time of inner PFL may be not equal to that of the outer PFL, causing the plateau region of the output voltage waveform of IEBA on the matched load to become distorted. The helical middle cylinder of BPFL results in unequal transit times for the inner and outer PFLs affecting the output voltage waveform. A formula for calculating the main pulse of the helical type BPFL is obtained, and the effect of the transition section between the helical type BPFL and the matched load on the output voltage of IEBA is analyzed. The experimental results are in agreement with the theoretical results, confirming that the output voltage waveform of an IEBA is improved significantly by optimizing the transition section, improving the flatness of the plateau region of the output waveform
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