| Intense electron beam accelerators (IEBA) have been widely used in the field ofnational denfense and industrial applications, such as nuclear explosion simulators,inertial confinement fusion, high power microwave generation, electron beam pumpedhigh power laser, Z-Pinch, X-ray generation, ion sources et al. At present, increasingpower, repetitive rate and the duration of the output pulse are the importantdevelopment trend for an accelerator. However, improving quality of the output voltagewaveform and the electron-beam is also very important for the requirement ofapplications. Therefore, to improve the quality of the output pulse of IEBA, theoperating characteristic of IEBA based on spiral Blumlein line (SBL) and correlativetechnologies are investigated. The main contributions of this dissertation are listed asfollows.1. Investigation of output characteristics of IEBA based on SBL.The analytic expression of output voltage, describing exactly the transienttransmission process of the SBL with a purely resistive load, has been derived bysolving the telegraph equations with Laplace transform, and the output characteristics ofIEBA based on SBL are analyzed. Meanwhile, the effects of the switch transitionsection (STS) between main switch and middle cylinder of the SBL on the outputvoltage are discussed by wave propagation method, and the effects of the outputtransition section (OTS) between SBL and the load on the output voltage are analyzedby solving the telegraph equations. It is obtained that the wave traveling time of STSinfluences considerably the rise-time, plateau-time, and fall-time of the voltagewaveform at the matching load. The wave impedance and transmission time of OTSinfluence considerably the fluctuation of the main pulse voltage waveform at the load.So to improve the quality of the output pulse of IEBA, the transition sections should bedesigned properly.2. Investigation of operating characteristics of IEBA at Load anomalyThe characteristics of output voltage of IEBA and voltage at the output terminal ofhigh voltage pulsed transformer (HVPT) are analyzed at four kinds of load anomaly.They are: load short, load open, surface flashover on the load surface and electronemission of cathode holder. It is found that load short, load open, and surface flashoveron the load surface will affect the voltage at the output terminal of HVPT, and anoscillation wave will be formed, which affect the electrical insulation of HVPT anddecrease the lifetime of HVPT. Meanwhile, the waveform of the load voltage is alsomodified, especially at the conditions of load open and surface flashover on the loadsurface. When the load is open, the amplitude of the output main pulse of IEBA is twicethe charging voltage of SBL. However, the amplitude of the output pulse of IEBA is modified by the voltage at the output terminal of HVPT, and the resistance of mainswitch channel has a great effect on the amplitude of the load voltage. When surfaceflashover occurs on the load surface, the pulse duration of the output voltage will bedecreased. So, during the operation of IEBA, load short, load open and surfaceflashover in vacuum chamber should be avoided. Also, the electron emission of cathodeholde is one of the main factors which affect the flatness of the output voltage of IEBA.3. Long gap surface flashover using180nanosecond quasi-square pulsesInsulator is one of the absolutely necessary devices in the output terminal of IEBA.The surface flashover characteristics with long gap spacing length of Polymethylmethacrylate (PMMA), High density polyethylene plastics (HDPE) and Polyamide (PA)under180ns quasi-square wave with voltage of several hundred kilovolts areinvestigated by using an IEBA. It is found that PA had better capability to hold-offvoltage than HDPE and PMMA. If the flashover occurs at the flat part of the main pulse,the value of the peak flashover current is very high, this is disadvantageous to the pulsedpower device. Meanwhile, if the flashover occurr after the main pulse, there is littleeffect on the pulsed power device. So, it must be avoided that the flashover occurs at theflat part of the main pulse. According to the impedance of the flashover channel, thedevelopment of the flashover channel would be divided into three stages. Firstly,electron emits from CTJ and transmits to the anode, the impedance is very high.Secondly, flashover channel starts to form, and the impedance is changed from veryhigh to dozens of ohms. At last, the flashover channel is formed, and the impedance ofthe flashover channel is decreased to less than1. However, after formation of theflashover channel, the impedance of the flashover channel is inductive and thewaveform of the applied voltage is determined by the inductance of the flashoverchannel directly. Meanwhile, a pretreatment method with different shots is used topre-treat the HDPE sample, it is obtained that the best way to improve the hold-offvoltage of the insulator is to increase the voltage step by step during the pretreatmentprocess.4. Synchronization of high speed framing camera and IEBAHigh speed framing camera (HSFC) is one of the important devices to visualize theevolution of discharging and plasma generation. However, to image the physics processexactly, the problem of the synchronism between the IEBA and HSFC should be takeninto account. A novel trigger program is proposed to realize the synchronization ofHSFC and IEBA. The trigger program which includes light signal acquisition radiatedfrom main switch of IEBA and signal processing circuit could provide a trigger signalwith rise time of17ns and amplitude of about5V, the delay time between the highlevel voltage (3V) of trigger signal and the start of the flat top of load voltage is about60ns, which is bigger than the intrinsic delay time of HSFC. So the trigger programcould realize the synchronization of HSFC and IEBA. At the same time, using the trigger module, HSFC and IEBA, the images of the developmental process of surfaceflashover channel of insulating material with gap spacing of170mm in vacuum undernanosecond quasi-square pulses is obtained in detail, and the change of luminosity isanalyzed during the surface flashover process. The emission images of a stainless steelcathode at short anode–cathode gap (5mm) are also obtained. |
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