Investigation Of A Long-pulse Accelerator With Water-insulated Helical Line

Pulsed power technology (PPT) is widely used in national defense and economy areas. In recent years, increasing the average power is one of the most important directions of PPT developments and more attention has been given to long-pulse technology. In the dissertation, based on the further investigation of helical transmission line characteristic, a set of water-dielectric helical pulse forming line (PFL) accelerator, as the long-pulse experiments platform, is designed and built.Firstly, the electromagnetic field in the helical line is theoretically analyzed. For the helical PFL applied in pulsed power apparatus, the radial electric field between inner and outer conductor is greater than other transverse electric fields by two orders of magnitude. The most energy flows along the helical electric conduction path, and the velocity of the electromagnetic wave has an inverse ratio to the helical angle.Secondly, the characteristic impedance and wave velocity are calculated by combined methods of electromagnetic fields and circuits. Considering that both of them are related to frequency, the concept of average characteristic impedance and average wave velocity are introduced and the calculation formulae are defined. From the fundamental test, the wave velocities of the helical line and the dispersion character as well as its effect on pulse waveform are obtained. Investigations show that the defined formulae have considerable precision to design and calculate the helical PFL.Based on the study on PFL charge process, the pre-pulse problems of the diode load are analyzed and solved. The method of connecting a parallel resistance to transition section is proposed. Calculation results show that as long as the paralleled resistance is greater than the load by about two orders, the pre-pulse voltage can be well controlled. According to the study on PFL discharge process, the spectrum and waveform functions for the pulse formed by helical PFL are also deduced. Obtaining the data of the switch risetime and the structure parameters of the PFL, the dispersion effect can be easy evaluated based on these pulse waveforms calculated by the functions.Then, to provide insight into the charge and discharge process of the helical PFL, the effect of structure parameters on charge voltage is simulated by PSPICE codes. Results show that compared with the coaxial PFL, the helical PFL needs more charge time because of the biggish inductance. The static and dynamic electromagnetic field distribution in helical PFL, especially the axis electric field value, is also simulated by KARAT codes. The simulation results give intuitive pictures that during the pass-through period of risetime of voltage wave, if the helical angle increases, or the rise time of main switch decreases , or the load resistance decreases, the axis electrical value will become larger. Based on the above research, a set of long-pulse accelerator with helical PFL is built. According to the operation law of the apparatus, the relevant measure system is setup and the pre-pulse problem is effectively solved by the resistance voltage divider of the load measurement. The accelerator has the 0.41 m diameter and 2.2 m total length. The characteristic impedance of the PFL is about 20Ω, and a pulse of 296 ns (FWHM) with a risetime about 12 ns, a fall time about 150 ns is obtained on a diode with impedance of about 20Ω. The voltage is bigger than 400 kV, but the pulse has aovershot and a long delay .The problem of overshot and long delay in output waveform is also discussed inthis paper. Simulations give the results that the unmatched impedance part has the influence on output waveforms of the accelerator. Except for the theory and code simulation analyseis, the technical methods to improve the waveform are put forward: by means of the annulus made of macromolecule material to make the impedance of the joint section close to that of the helical line and decrease electric length, the overshot is avoided. By adjusting the impedance and electric length of the transition part, the risetime is decreased. The experimental results on machining load are as below: the pulse voltage is bigger than 400 kV, the rise time is about 18 ns, the fall time is about 40 ns, and the pulse top duration about 200 ns. Experiments are also carried out to drive a microwave source with slow-wave structure and low magnetic field, and the X-band high power microwave (HPM) with pulse width of larger than 100 ns and power of hundreds mega-watt level is successfully generated.At last, by using glycol or glycol solution as insulation medium, the characteristic impedance of the PFL can be varied in the range of 20～27Ω, which can make the accelerator drive more different microwave sources and match exactly with different loads.