Characterization and Optimization of Cesium Delivery Tube for Stable Flux in Negative Ion Source (CeWNIS)

Negative hydrogen ion ($H^{-}$) sources are essential for high-energy neutral beam injection (NBI) systems in fusion reactors, supporting efficient plasma heating and current drive. At the Center of Plasma Physics, Institute for Plasma Research (CPP-IPR), a filamentary discharge-based$H^{-}$source, termed as cesiated tungsten dust-driven negative hydrogen ion source (CeWNIS), has been developed. This source uses cesium (Cs)-coated tungsten (W) micrometer-sized dust as an alternative to conventional Cs vapor deposition on the plasma grid. Stable and repeatable Cs delivery, which is crucial for maintaining a low work-function surface, is provided by two independently controlled cesium ovens, while the resulting flux is monitored using a surface ionization detector (SID). The observed flux density of Cs is on the order of$10^{19}~\mathrm {atoms~m^{-2} s^{-1}}$, with$175~^{\circ } \mathrm {C}$–$200~^{\circ } \mathrm {C}$identified as the optimal oven temperature to achieve uniform and sustained Cs emission. Subsequently, W dust was introduced via an ultrasonic dispenser, enabling Cs coating and the formation of low work function particulates. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses confirmed the crystalline BCC structure and$2.5~\mathrm {\mu m}$average grain size of the W dust. Under steady-state plasma conditions, Cs-coated W dust facilitated effective$H^{-}$ion generation, with Langmuir probe diagnostics indicating an$H^{-}$ion density on the order of$10^{16}~\mathrm {m^{-3}}$. These findings demonstrate the viability of the Cs-coated dust approach and highlight the need for a valve-controlled Cs delivery system, along with uniform heating of the delivery tube. A mechanical upgrade to achieve this will be considered in future iterations of the system.