# Device Reliability Analysis of NNBI Beam Source System Based on Fault Tree
## 6 Conclusions  
This study established a fault tree model for beam source system arcing events in NNBI and conducted both qualitative analysis and quantitative calculations using AutoFTA 3.9 software. The following conclusions can be drawn regarding this system:  
1. Currently, the NNBI system's overall performance parameters and design specifications are as follows: beam power of 2 MW, pulse duration \(\geq\) 1000 s, and particle species (H\({}^{0}\) or H\({}^{-}\) + H\({}^{0}\) + H\({}^{+}\)) [30]. Continuous beam source debugging is required to achieve these  
Figure 12: Histogram of F-V importance analysis for each fault bottom event in the beam source system. Figure 13: Histogram of F-V importance analysis for each second-order fault event in the NNBI beam source system. design goals. Currently, the fault-free operation time is only within the range of [42.6; 68] hours. To meet the operation requirements, the mean time between failures (MTBF) must be significantly improved [26]. 2. Analysis of the failure distribution in Figure 12 reveals that the most frequent failure modes in the beam source system include the following: lower crucible pipeline valve malfunctions, insufficient surface flatness of the Plasma Grid (PG), clogging of PG apertures, and impurity accumulation on PG electrodes. Crucible valve failures are mostly caused by excessive valve temperature. Since this problem is induced by the necessary experimental environment, the impact on experimental stability can be minimized through high-frequency maintenance and stockpiling of replacement components. The frequent failures of plasma electrodes are mainly caused by the intrusion of impurities such as Cs in the expansion chamber due to the high temperature and high pressure of the experimental environment, and the installation of purification devices can be considered in the subsequent stage. Solving the above core problems can significantly improve the reliability of the entire system during the experiment. 3. As shown in Figure 13, Plasma Grid (PG) failures and Plasma Expansion Chamber (EC) failures constitute the most prevalent failure modes in the beam source system. The main cause of failures lies in the high temperature and high pressure of the experimental environment. In response to this environmental characteristic, it is necessary to strengthen the inspection and renewal of experimental materials; optimizing these subsystems will significantly improve the overall stability of the beam source system. 4. The International Thermonuclear Experimental Reactor (ITER) has also conducted similar reliability analysis work. In ITER's reliability analysis, a more diverse set of technical approaches has been adopted: on the one hand, a comprehensive evaluation framework is built through the multi-dimensional integration of RAMI (Reliability, Availability, Maintainability, Inspectability) and PSA (Probabilistic Safety Assessment) [6], while FTA is applied to the reliability analysis of multiple systems; on the other hand, the Cognitive Reliability and Error Analysis Method (CREAM) is introduced to conduct an in-depth exploration of potential risks from three dimensions: personnel operation, technical processes, and organizational management [7]. Based on these methods, ITER has completed a number of practical engineering applications--for instance, it has formulated and implemented maintenance optimization schemes for rectifier cabinets and bypass thyristors [8], which has effectively improved equipment performance; meanwhile, it has completed the risk assessment of the vacuum vessel pressure suppression system, providing a scientific basis for the safe operation of the system [6].