# Device Reliability Analysis of NNBI Beam Source System Based on Fault Tree
## 2 NNBI Beam Source System  
The NNBI beam source system primarily provides stable and reliable negative ion beams for NNBI, preparing them for subsequent injection into the beamline. The NNBI beam source system mainly consists of two major components: the plasma generator and the negative ion accelerator. Its structure is illustrated in Figure 1. The plasma generator is primarily composed of an RF exciter and a large-area plasma expansion chamber. The alternating electric field generated by the RF coil excites the working gas to produce plasma [9]. The plasma expansion chamber is used to generate large-area, uniform plasma while slowing down the velocity of electrons in the plasma, thereby reducing the loss of negative ions [10]. The negative ion accelerator consists of an extraction system and an acceleration system, designed to extract negative ions from the RF-driven negative ion source and accelerate them to the required energy while controlling their trajectory to form a negative ion beam with specific optical properties. The accelerator includes four layers of electrodes: the plasma grid (PG), the extraction grid (EG), the acceleration grid (AG), and the ground grid (GG) [3]. The extraction system comprises the plasma grid and the extraction grid, where an electric field is applied between the plasma grid and the extraction grid to extract negative ions. The acceleration system involves a two-stage acceleration process: the electric fields applied between the extraction grid-acceleration grid and the acceleration grid-ground grid accelerate the negative ions to the target energy [11]. Additionally, the front section of the plasma grid is equipped with a bias plate featuring multiple \"window-frame\" apertures, which serves to reduce the number of electrons in the extraction region and improve plasma uniformity [12]. The operational sequence of the beam source system during a single-shot experiment is as follows: Under the centralized control system's coordinated scheduling, the power supply system delivers electrical energy meeting stringent technical specifications to the beam source apparatus according to predefined temporal parameters. Utilizing radio-frequency (RF) discharge principles, the beam source device excites and sustains source plasma. Through specially designed extraction structures, negative ions are efficiently extracted from the plasma and accelerated by multi-stage electric fields to form a high-energy negative ion beam. This beam propagates directionally through vacuum transmission ducts, sequentially traversing a neutralization conversion unit and an electromagnetic deflection system before ultimately impinging upon the power measurement target for energy deposition. Throughout this process, an integrated diagnostic system equipped with multiple sensors acquires beam characteristic parameters in real time, providing critical data for operational status monitoring. Concurrently, ancillary subsystems continuously supply precisely regulated coolants, instrumentation gases, and high-purity working gases to ensure optimal operating conditions are maintained during the entire beam generation and transmission process. Following discharge completion, the equipment undergoes a prescribed dwell period until the vacuum chamber reaches ideal conditions for subsequent discharge scheduling. All acquired data from each discharge cycle are systematically recorded in Excel spreadsheets for analysis [11; 13; 14]. The specific process is shown in Figure 2.  
Figure 1: Structure of the beam source system. During discharge experiments, the NNBI system operates under extreme conditions characterized by exceptionally high injection energies and voltages (typically reaching hundreds of kilovolts to megavolts) [15,16], combined with the requirement for certain subsystems, such as the cesium injection system, to function at elevated temperatures [17]. These demanding operational parameters frequently result in varying degrees of damage to critical components of the equipment.