# Device Reliability Analysis of NNBI Beam Source System Based on Fault Tree
###### Abstract  
Negative Ion Source Neutral beam Injection (NNBI), as a critical auxiliary heating system for magnetic confinement fusion devices, directly affects the plasma heating efficiency of tokamak devices through the reliability of its beam source system. The single-shot experiment constitutes a significant experimental program for NNBI. This study addresses the frequent equipment failures encountered by the NNBI beam source system during a cycle of experiments, employing fault tree analysis (FTA) to conduct a systematic reliability assessment. Utilizing the AutoFTA 3.9 software platform, a fault tree model of the beam source system was established. Minimal cut set analysis was performed to identify the system's weak points. The research employed AutoFTA 3.9 for both qualitative analysis and quantitative calculations, obtaining the failure probabilities of critical components. Furthermore, the F-V importance measure and mean time between failures (MTBF) were applied to analyze the system. This provides a theoretical basis and practical engineering guidance for enhancing the operational reliability of the NNBI system. The evaluation methodology developed in this study can be extended and applied to the reliability analysis of other high-power particle acceleration systems. beam source system; FTA; F-V importance measure; failure rate; reliability analysis; negative ion source 2020 June 2028 July 28 July 2028 July 28 July  
Tokamaks initially rely on ohmic heating; however, their efficiency drastically diminishes once temperatures reach approximately 3 keV. To overcome this limitation, auxiliary heating technologies have been developed, including Electron Cyclotron Resonance Heating (ECRH), Ion Cyclotron Resonance Heating (ICRH), Lower Hybrid Current Drive (LHCD), and Neutral Beam Injection (NBI) [1,2]. The synergistic interaction of these multiple mechanisms enables the breakthrough of temperature barriers, thereby creating the essential conditions for achieving controlled nuclear fusion [3,4]. Negative Ion Source Neutral Beam Injection (NNBI) is a critical technology in magnetic confinement fusion for heating plasma and driving current, playing a vital role in achieving fusion reactor ignition and burn control [5]. As fusion devices scale up, the energy requirements for neutral beams have increased from the range of tens of keV to the MeV level. Traditional positive ion sources suffer from reduced efficiency under high voltage. When beam energy exceeds 50 keV per atomic mass unit (keV/amu), NNBI systems based on negative ion sources become the preferred solution due to their high neutralization efficiency and cost-effectiveness. Among NNBI experiments, the single-shot experiment is particularly crucial. Given the extremely high energy involved in the experimental discharges, equipment malfunctions are unavoidable. Such failures directly compromise the integrity of experimental data, leading to inaccuracies and instability in the results. Consequently, regular maintenance of the experimental apparatus is imperative. The reliability of the NNBI system is of great significance to the stable operation of magnetic confinement fusion devices. However, existing studies have limitations: most of them focus on qualitative inspection of low-energy NBI systems or rely on empirical maintenance; a quantitative reliability model for the megavolt-level Negative Ion Source Neutral Beam Injection (NBI) beam source system has not been established; in addition, there is a lack of analysis on the correlation mechanism of failure rates under extreme operating conditions, and the reliability improvement measures lack theoretical support. Early fault analysis of the NNBI system was characterized by singularity and fragmentation: fault information arising during experiments was mostly limited to simple descriptions, and no systematic equipment reliability analysis framework was established.