Investigation Of Related Techniques For A Vacuum Clean High-Current Diode

Vacuum diodes are among the most important parts of high power microwave (HPM) systems. To achieve maturity and practical applications, HPM need benefit from many aspects of conventional technology, and the most important of these is clean environments and improved vacuum techniques in the diodes. In this dissertation, to make reliable vacuum sealing, a ceramic vacuum interface was developed following the compact and transportable demand: a novel radial insulation structure was designed and the hydrodynamic loading of the ceramic component was considered. To maintain high vacuum condition during the repetition rate (rep-rate) operations, for MILO tube, the pulse desorption of the velvet cathode and its characteristics under high rep-rate were quantified. Meanwhile, a distributed pump method was proposed to improve the transient pumping ability of MILOs. For BWO tube, to avoid anode thermal desorption, cooling of beam collector in high average power was discussed. These efforts set a good foundation for the clean development of a high-current vacuum diode. The detailed contents and innovative work include the followings.1. The study of the correlative technologies of a ceramic-vacuum interface(1) A ceramic radial insulation structure for a compact diodeAccording to the theories of vacuum flashover and the rules for radial insulators, a"cone-column"anode outline and the cathode shielding rings were adopted. The electrostatic field along the insulator surface was obtained by FEA simulating. By adjusting the outline of the anode and reshaping the shielding rings, the electric fields were well distributed and the field around the cathode triple junction was effectively controlled. Statistical weighted method was applied to estimate the surface breakdown field. The high voltage test was carried out on a water dielectric spiral PFL accelerator with 200ns pulse duration, and results show that the diode can work stably with the hold-off field of 44 kV/cm. The experimental results agree with the theoretical and simulated results.(2) Outgassing properties of the ceramic insulated diodeA pumping model including the influence of flow conductance was set up to simulate the material outgassing rate and pump down time. Experimentally, the outgassing characteristics of materials between normal temperature and heated state are compared. Results show that heating can efficiently accelerate the outgassing rate and rise the vacuum sealing time.(3) Hydrodynamic loading of the ceramic component due to pulsed discharge in waterFirstly, the mechanic properties of the ceramic component including static, modal, and dynamic were analyzed. Further, by combining empirical formulae of plasma driven water shock theory with a self-consistent underwater explosive approach, a finite element model was introduced to investigate the shockwave behaviors. The pressure-time history and ceramic mechanical response to pressure waves were presented. In order to get the pressure profile and verify the calculation models, the arc pressure test including"point-plane"electrode system was carried out based on a 10-stage Marx generator. The variations of peak pressure, shock velocity and main pulse width under different breakdown condition were measured. The relationship between peak pressure and shock energy was also figured out and the experimental formula has a close approximation to the simulation results. These results may give a good reference for estimation of the water shock when designing ceramic interface in pulsed power machines.2. The study of related techniques for vacuum clean under rep-rate operations(1) Characteristics of a velvet cathode under high rep-rate pulse operationTo determine the feasibility of velvet cathodes under high rep-rate operation, a series of experiments are carried out on a high-power diode, driven by a ~300 kV, ~6 ns, ~100 ? and 1~ 300 Hz rep-rate pulser, Torch-02. Results of time-resolved pressure history, discharge waveforms, and velvet microstructure changes are presented. With the pulse repetition rate (rep-rate) increasing, it was found that the equilibrium pressure during the pulse series from 30 Hz to 300 Hz increased hyper-linearly that led to diode pressures of ~0.01 Pa to ~1 Pa. When the pressure exceeded 0.1 Pa, the beam current degradation was observed in terms of pulse-to-pulse instability increment and pulse width decline. Velvet curling and erosion also became apparent. The nature behind these results was discussed closely related to the gas ionization due to such high background pressures.(2) A distributed pumping model for a MILO tubeTo improve the pumping ability for a rep-rate MILO, a model was developed for the molecular movement and collision in the vacuum diode chamber and MILO tube on the basis of the Monte-Carlo method. According to the three dimension particle distribution, the idea of distributed pumping was proposed. In this way, another pump system which is close to the velvet desorption source was introduced and located between MILO tube and antenna. Simulation predicts were verified through the initial experimental test carried out on Torch-01 pulser. Results show that the distributed pumping can efficiently reduce the characteristic time of pressure drop to one-fifth of the single pumping. The distributed pumping method was also proved to be feasible to ameliorate the vacuum condition for MILO rep-rate operations.(3) Thermal management of a repetitive operated high-current beam collectorTo avoid anode thermal desorption, based on a high-current beam collector, the effecting area due to bombardment by energetic electrons was experimentally quantified, and the power per unit area as thermal loading for a 700 keV and 7 kA beam current was calculated. By using an ANSYS-APDL package, the 10-100 Hz results of temperature history and distribution of the collector were typically presented. The relationship between the equilibrium temperature and convective coefficient for different pulse repetition rate was also obtained. In addition, the hydraulic system necessary to achieve appropriate forced cooling fluid flow was given. These results supplied an efficient way to the optimizing design and reasonable use of a repetitive operated beam collector in HPM tubes.