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Study On The Vacuum Surface Flashover Of High-power Microwave Window

Posted on:2014-12-27Degree:DoctorType:Dissertation
Country:ChinaCandidate:G X ChengFull Text:PDF
GTID:1108330479979667Subject:Electronic Science and Technology
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Due to recent advances in peak output power(~GW) and pulse width(~100 ns) of high-power microwave(HPM) devices, the ability to transmit and radiate this power into atmosphere has been an area of intense interest. One major factor of limiting the transmission and radiation of HPM has been the dielectric window breakdown, which is frequently observed to occur at vacuum-dielectric interface. This phenomenon, known to cause a pulse shortening of the transmitted power, can consequently constrain the achievement of higher output power, wider pulse duration of HPM device. Therefore, understanding dielectric window breakdown is of great importance. Methods to suppress dielectric window breakdown are also of great interest. In this dissertation, the vacuum-dielectric interface breakdown is studied. The possibilities for polycarbonate(PC) and polyetherimide(PEI) to be applied as new dielectric windows are investigated, and the effects of grooved dielectric on enhancing power handing capacity of dielectric window are analyzed. Besides, methods to characterize dielectric properties of HPM windows are developed. The main contents of this dissertation are listed as follows:1. A 1D3 V electro-magnetic PIC-MCC code, which involves electron simulated outgassing, is developed to study the HPM driven dielectric window breakdown. With this code, the transition of window breakdown from vacuum multipactor to rf plasma is obtained. It is found that, even there is no dc electric field on the dielectric surface, rf magnetic field can itself drive electron back to the dielectric surface, and function as to initiate the breakdown. The electron deposited power could be as high as 50% of the incident rf power during the occurence of breakdown. Effects of external electric field and magnetic field on suppressing the dielectric window breakdown are studied. It is found that external electric field with direction pointing to dielectric surface suppresses effectively the multipactor effect. While for external magnetic field with its direction perpendicular to rf electric field and parallel to dielectric surface suppresses effectively the breakdown if its gyrofrequency is equal to the angular frequency of rf electric field.2. Different dielectric windows are tested with ~100 ns pulsed high voltage and ~100 ns pulsed HPM in order to characterize the power handling capacity of dielectric windows. The possibilities for PC and PEI to be applied as new dielectric windows are studied. Meanwhile, the effects of grooved dielectrics on enhancing the power handling capacity of dielectric window are analyzed.Experiment with ~100 ns pulsed high voltage found that PC performs better than HDPE and PTFE. Grooves on dielectric surface increase promisingly the flashover time lags. With diode voltage of 265 k V, electrode gap spacing of 100 mm, and grooves 2 mm in width, 2 mm in depth, and 4 mm in period, the flashover time lag of PTFE is increased from 45±3 ns to 78±3 ns, while for HDPE, its flashover time lag is increased from 67±9 ns to 98±9 ns, and for PC, its flashover time lag is increased from 93±5 ns to 177±9 ns. Using a high-speed framing camera, the temporal evolutions of flashover channels of flat and grooved dielectrics are obtained. It is seen that flashover channel of flat sample forms a straight line across the two electrodes. While for grooved sample, its channel is almost along the circular edge of the sample. This consequently results in a considerably bended trajectory, which is about π/2 times the distance of that of flat one.Experiment with ~100 ns pulsed HPM found that flat PEI performs better than PC and HDPE. While with grooved structure, the power handling capacity of PEI and PC are almost the same. HDPE has the lowest insulation strength, no matter with flat or grooved structure. Grooved window has much higher power handling capacity than flat one. With electric field of 63 k V/cm, and microwave frequency of 3.73 GHz, the far field pulse widths of flat HDPE, PC, and PEI windows are 62±4 ns, 72±4 ns, and 78±5 ns, respectively. While with grooves 1.5 mm in width, 1.0 mm in depth, and 2.5 mm in period, the far field pulse widths of HDPE, PC, and PEI windows are 85±5 ns, 105±5 ns, and 105±8 ns, respectively. Effect of surface roughness on the power handling capacity of dielectric window is examined. It is found that dielectric window with certain surface roughness performs better than optical grade one. After many operations of flashovers, the damage of HDPE dielectric window is much milder than those of PC and PEI. Few tree-like tracks can be found on flat PC and PEI, but with grooved structure, obvious tree-like tracks can be found, especially for grooved PEI, Its lifetime is limited.3. Three different resonators, including a TM011 and TE011 dual-mode dielectric resonator, a modified TE01 n cylindrical cavity, and a dual-band measurement system, are developed in order to characterize the dielectric properties of HPM windows. Using these three resonators, PTFE, HDPE, PC, and PEI, together with two alumina ceramics and a glass fiber reinforced plastic are measured. It is demonstrated that the proposed dual-mode dielectric resonator can be applied to machinable dielectric. The modified TE01 n cavity can be used for non-destructive measurement of plane dielectric with thickness larger than 5 mm. While for the dual-band measurement system, it can be used to thin layer dielectric with thickness less than 5 mm. The measurement uncertainties of dielectric constant by these three resonators are within 2%, while the uncertainties of dielectric loss would be different depending on the measurement structure. For the proposed dual-mode dielectric resonator and the modified TE01 n cavity, the measurement uncertainties of dielectric loss are within 10 %. While for the dualband measurement system, when it is in the form of a split dielectric resonator, the uncertainty of dielectric loss would exceed 10 %. This is due to the fact that the energy filling factor is too small in the dielectric.
Keywords/Search Tags:vacuum surface flashover, high-power microwave, dielectric window, grooved dielectric, power handling capacity, dielectric properties
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