## 1 Introduction  
Agriculture productivities used to be important drivers of Thailand's economic growth. There are many exported agricultural products, including rice and a variety of fruits and vegetables. Recently, many countries have imposed strict regulations on imported fruits as a result of pathogenic microorganisms. To ensure the safety of fresh fruits and vegetables and prevent the introduction of unwanted diseases, irradiation with accelerator machines is widely used to improve the physical, chemical or biological properties of the products. Synchrotron Light Research Institute (SLRI), Thailand, has researched, developed and designed the prototype of a Linear Accelerator (LINAC) for X-ray sterilization with an electron energy of 6 MeV. The LINAC system produces the X-ray by accelerating the 6-MeV electron beam to hit a target with high Z. The target used to convert the electron beam to bremsstrahlung is tungsten. The acceleration of the electron beam requires three main components, consisting of an electron gun, an accelerator structure and a radio frequency generator or magnetron that can oscillate and produce an electromagnetic field as a standing wave within each cavity of the accelerator tubes. A magnetron is a high-power oscillator generating an electromagnetic field and used in a wide range of applications including military radar, aerospace, navigation and medical use [1, 2, 3, 4, 5, 6]. Among the various high-power oscillators, a magnetron can be manufacturedwith compact size, high efficiency, light weight and simple structure. E2V, located in Chelmsford, has been manufacturing magnetrons for civil marine applications since 1947 based on an S-band design, and they have developed an X-band magnetron. In the 1950s, a magnetron adapted for use in photon beam radiotherapy was developed which requires a pulsed RF source to accelerate electrons in the cavities of LINAC [7]. The different physical structures of the anode block in a magnetron are sector, hole-slot, rising sun, ellipse and vane-type [8; 9; 10]. The applied voltage between the anode and cathode in the magnetron-producing DC field and the magnetic field is applied perpendicular to the electric field in the interaction space and resonant cavities in the anode block. If the values of the electric and magnetic field are suitable, a resonance condition occurs and the desired microwave frequency is generated by the cavities. The dimensions and shape of the cavities and the interaction space between the anode and cathode determine the operation frequency of magnetrons [10]. There are several methods to determine a magnetron's resonant frequency, such as theoretical equivalent circuit of geometry analysis, field theory, computer simulation and experimental measurement. Each method has different advantages. The theoretical equivalent circuit method can obtain a resonant frequency formula as a function of the structure dimensions of a magnetron according to the relation between resonant frequency and structure dimensions. Thereby, it can analyze the effect of the structures on the resonant frequency of the magnetron [11]. The field theory is very sophisticated and it requires advanced mathematics to solve the electromagnetic equation. Computer simulation is usually used for design and optimization, but it requires long periods of time. Particle-in-cell (PIC) simulation is the electromagnetic solver in CST Particle Studio (CST-PS), which is widely used for magnetron design. It is based on finite integration for the simulation of electromagnetic fields in the desired model and is usually used to estimate the fundamental mode frequency of a relativistic magnetron and tune the resonant frequency. The last method is the frequency measurement, which is not suitable for magnetron design, but it is well suited to the practical application of Hole-Slot-Type magnetrons. In this paper, we focus on the effects of disturbances such as temperature and vacuum pressure on the twelve-hole-slot-type magnetron which is used for the 6-MeV LINAC.