Research On 2.6MW Magnetron For Medical Linear Accelerator

Medical linear accelerator is an important device for radiotherapy. Magnetron is a key part of medical linear accelerator, as its microwave power source. Most magnetrons are symmetrical around the angular at present, whether the resonant cavities of magnetron are same or rising-sun system. However, the resonant system of 2.6MW magnetron studied in this paper is an asymmetric structure. The asymmetry is made up of two types. One is that the straps of the resonant system is breaking at some position,another is that resonant cavities located on both sides of the output port is added end slot, which make the cavities is different from others. This paper makes research on the resonant characteristics, the interaction between electron and high frequency field,and the energy coupling components.The effect of the straps and end slot of resonant system is analyzed, via researching the resonant mode, r-f electric field distribution, and mode separation. The results of simulation show that the straps make contribution to mode separation. When the straps are breaking, the degenerate mode is split so that the resonant frequency is not same but has difference. It also makes the r-f electric field distribution of ?-mode asymmetric,because the breaking leads the r-f electric field decrease around it. When the resonant system adds end slot base on the straps breaking, the asymmetry of ?-mode field distribution is enhanced. The field on the gap of resonant cavity added end slot is stronger, and field nearby the straps breaking is weaker. The asymmetry of r-f field form two regions, one is the stronger field another is the weaker field. The degenerate mode is hard to encourage because of frequency difference, which make the oscillation of magnetron is more stably. The weaker r-f electric field is beneficial to the interaction between electrons and fields, so the electric field will get more energy to improve the electronic efficiency of magnetron. However, the stronger electric field can improve the energy coupling ability, which can increase the circuit efficiency. What's more, the strong coupling between the magnetron and the load will reduce the r-f voltage in the resonant system, it can obtain a higher electron efficiency too. So the efficiency of magnetron can be improved greatly.So the high-frequency system of asymmetric structure can effectively improve the efficiency of magnetron. Under the same working conditions, the particle simulation shows that the electron efficiency and circuit efficiency of the symmetrical high frequency system are 0.4309 and 0.922 respectively, and the total efficiency is 39.73%.After the straps break, the electronic efficiency and the circuit efficiency are improved slightly. And the total efficiency is 40.44 %. When the resonant system added two end slots (position relationship was 60°), the electronic efficiency is 0.4781, circuit efficiency increased to 0.958, and the total efficiency of 45.80%. When the system adding 4 end slots on the both sides of the output resonant cavity, the electron efficiency is increased to 0.5354,the circuit efficiency is 0.938, and the total efficiency is increased to 50.22%. The output power of this structure is 2.599MW and the anode current is 115A.