Investigation Of High Impedance Magnetically Insulated Line Oscillator

Magnetically insulated transmission line oscillator (MILO) is an excellent crossedfield device designed specifically to generate microwave power at the gigawatt level，which is a major hotspot in the field of high-power microwave (HPM) research at thepresent time. At present, the impedance of MILO is relatively low（~10）. As a result,the load current is quite large, which limits the further development and application ofMILO. On one hand, the anode plasma formation in the load region could result in thesevere pulse shortening and electrode erosion. On the other hand, intensive space chargeeffect could result in the relatively lower power conversion efficiency. Based on thisbackground, increasing the impedance of MILO can help to overcome these weaknesses.These research results also found a firm foundation for the design of the long-pulseMILO with repetitive operation. By the use of theoretical analysis, particle simulation,and experimental measurement, the high impedance MILO has been investigatedsystematically. A series of valuable results have been obtained.Firstly, according to the parapotential model, the magnetically insulated currentwas obtained. In order to increase the power conversion efficiency of MILO, the ratio ofload current-to-anode current should be decreased. Several methods could be used:increasing the side area of the cathode, decreasing the area of the cathode end,decreasing the distance between the side face of the cathode to the slow wave structure,increasing the distance between the cathode end and the electron collector, andincreasing the diode voltage. Based on the theory of parapotential flow, the influencelaw of both the ratio of cathode radius-to-anode radius and the diode voltage on theMILO impedance were studied, which provides efficient guidance for increasing theMILO impedance.Secondly, the tapered MILO was chosen as the basic structure, which could obtainhigher power conversion efficiency by comparison with the load-limited MILO. Thefield distribution of the resonance mode, the resonance frequency, and the quality factorare acquired. By employing a2.5-dimensional PIC code, the relation of MILOimpedance to both the ratio of cathode radius-to-anode radius and the diode voltagewere obtained numerically. It was found that for a given structure, within the range of2.1-3.1, the increase of the ratio of cathode radius-to-anode radius could increase theimpedance of MILO within the range of23.5-32.4; It was also found that theimpedance of MILO increased slowly with the range of28.2-30.7when the diodevoltage increased within the range of560-940kV. It was found that the lower thefrequency of MILO, the greater the upper limit of the MILO impedance. And further,the higher the impedance of MILO, the higher the diode voltage corresponding to themaximal power conversion efficiency. All associated factors were considered to increase the power conversion efficiency of the MILO during the optimizing prose, such as thechoice of impedance and frequency, the adjustment of magnetically insulated state, thechoice of the position of the electron emission area, and the optimization of theextraction and feedback of the microwave. The representative numerical results were asfollows. For the20MILO, the microwave output power was4.2GW with the initialelectron energy of675keV and the electron beam current of23.3kA. The microwavefrequency was2.5GHz. The beam-to-microwave power conversion efficiency was18.0%. For the30MILO, the microwave output power was5.5GW with the initialelectron energy of810keV and the electron beam current of26.9kA. The microwavefrequency was1.7GHz. The beam-to-microwave power conversion efficiency was25.3%. For the44MILO, the microwave output power was6.3GW with the initialelectron energy of1.1MeV and the electron beam current of25.0kA. The microwavefrequency was630MHz. The beam-to-microwave power conversion efficiency was22.7%.Finally, correlative experiments were carried out to verify the feasibility of the highimpedance MILO. The experimental study has been performed on the optimized20MILO designed with the help of simulation. The representative experimental resultswere as follows. When the voltage was665kV and the current was32.3kA, amicrowave was generated with power of1GW, mode of TM01and frequency of2.60GHz. Furthermore, the reason for the frequency drift, cathode erosion and lowpower conversion efficiency were investigated experimentally. Based on this, feasiblesolutions are given.