Discharge chamber length is one of the factors in electron cyclotron resonance ion thruster performance optimization. It adjusts the distance between bulk plasma and grid system to change the plasma density upstream the screen grid, which will impact the ion beam current and focusing state to achieve optimization purpose. However, new evidence shows discharge chamber length play an important role in ionization during ion beam extraction, which means the effect of discharge chamber length on the performance of electron cyclotron resonance ion thruster should be renewed. After applying grid voltages, another high electron temperature region located upstream the screen grid is observed in the integrated simulation using particle-in-cell with Monte Carlo collision method, while traditional discharge chamber simulation has not seen that. The paper believed that the high electron temperature region exists objectively, because the Child-Langmuir sheath will repel electrons moving towards screen grid back to magnetic mirrors again. Those electrons will gain energy from microwave, and finally form a high electron temperature region along the Child-Langmuir sheath. This phenomenon implies that discharge chamber length can adjust the high electron temperature distribution upstream screen grid to impact the plasma generation. Therefore, the paper systematically studies the influence mechanism of discharge chamber length on discharge and ion beam performance of thruster by adopting the integrated simulation. In this paper, three ion thrusters with different discharge chamber length are simulated. Under the conditions of same magnetic field and operation parameters, the electron energy gain, plasma parameter distributions and ion beam current are compared. The results show that shorter discharge chamber length has higher electron energy gain, plasma density and voltage, but less ion beam current. This abnormal phenomenon also be seen in experiments. By analyzing the ionization rate inside the chamber, it can be seen that high-temperature electrons upstream screen grid make a significant contribution on ionization. And thus, a little bit longer discharge chamber length with lower plasma density inside the chamber has bigger ion beam current for having higher plasma density upstream the screen grid. According to this phenomenon, an electron heating mode is proposed:electrons gain energy by reciprocating through the electron cyclotron resonance layer between the Child-Langmuir sheath and magnetic mirrors. This heating mode can be used as a supplement to the electronic constraints outside the magnetic mirrors to improve the energy utilization efficiency of the thruster, which can provide a new idea for the electron cyclotron resonance ion thruster design in the future.
