Laser diagnostics for negative ion source optimization: insights from SPIDER at the ITER Neutral Beam Test Facility

TL;DR

This work addresses the challenge of optimizing large, high-energy H/D beams for ITER Heating Neutral Beams by employing laser diagnostics on the SPIDER negative ion source, a full-scale NB prototype at NBTF. It leverages CRDS to monitor $H^-$/$D^-$ densities and LAS to track caesium neutral density, enabling correlation with other diagnostics and operational parameters. The paper details upgrades to CRDS (including a second LOS and a decoupled frame) and analyzes Cs dynamics under vacuum and plasma conditions, quantifying how Cs density scales with Cs evaporation rate and RF power, and demonstrating a Cs density drop during plasma pulses supported by optical emission trends. Collectively, these diagnostics provide critical, calibration-free insights for ITER-relevant source optimization and guide future modelling efforts for Cs transport and negative ion production in challenging fusion environments.

Abstract

The ITER Heating Neutral Beams (HNBs) require large, high-energy H/D atom beams (285/330 A/m^2 extracted current density, and 1/0.87 MeV acceleration energy, respectively for H and D). To address the associated challenges, the SPIDER negative ion RF beam source at the Neutral Beam Test Facility (NBTF) in Padova (Italy) serves as a full-scale source prototype with a 100 kV triode accelerator, for design validation and performance verification. SPIDER is equipped with two advanced laser diagnostics to monitor key plasma parameters; Cavity Ring-Down Spectroscopy (CRDS) is used to measure H$^-$\slash D$^-$ ion densities, while Laser Absorption Spectroscopy (LAS) tracks caesium neutral density in the source. These measurements are essential for optimizing negative ion production and meeting ITER source targets. We present diagnostic upgrade details, recent experimental results, and correlations with other machine parameters. Since CRDS relies on a single 4.637-meter-long optical cavity, the longest used in such sources, it has demonstrated sensitivity to alignment. Based on recent experimental experience, structural improvements are being implemented to enhance both stability and measurement reliability. LAS has mainly been employed as a tool to monitor the caesium conditioning status of SPIDER. Additionally, due to a distributed measurement over four lines of sight, LAS has proven effective in monitoring the caesium distribution within the source. This work demonstrates the essential role of laser diagnostics in developing ITER-relevant plasma sources and informs ongoing efforts to improve measurement accuracy in challenging environments.

Figures (8)

• Figure 1: View of the laser beam path for CRDS and LAS in the context of the complete source and chamber.
• Figure 2: (a) Single Ring-Down signal showing an exponential decay; (b) Decay time as a function of time during a plasma discharge with flat RF power; (c) Calculated H- density waveform. Source discharge parameters were RF power 100 kW, filter field 1.1 kA, source pressure 0.35 Pa.
• Figure 3: Top: $\mathrm{H^{-}}$ density and ratio of emission lines intensity $H_{\alpha}/H_{\beta}$, the shaded areas correspond to the beam extraction phases; center: RF power and filter field current; bottom: extraction voltage
• Figure 4: Trend of the CRDS cavity decay time throughout the experimental campaign before and after each plasma discharge.
• Figure 5: Development of a new structural design for the CRDS diagnostic aimed at enhancing robustness and incorporating a second line-of-sight positioned above the second beam segment.