Development and first operation of a Cavity Ring Down Spectroscopy diagnostic in the negative ion source SPIDER

TL;DR

This paper introduces a Cavity Ring Down Spectroscopy diagnostic for measuring negative ion density in the SPIDER negative ion source, providing first measurements and insights crucial for optimizing ion extraction in fusion applications.

Contribution

The paper presents the development and initial operation of a CRDS diagnostic for negative ion density measurement in the SPIDER ion source, a novel application in this context.

Findings

First negative ion density measurements in SPIDER using CRDS

Correlation between ion density and source parameters established

Diagnostic setup demonstrated effective for plasma diagnostics

Abstract

The Neutral Beam Injectors of the ITER experiment will rely on negative ion sources to produce 16.7 MW beams of H/D particles accelerated at 1 MeV. The prototype of these sources was built and is currently operated in the SPIDER experiment (Source for the Production of Ions o Deuterium Extracted from an RF plasma), part of the Neutral Beam Test Facility of Consorzio RFX, Padua. In SPIDER, the H-/D- ion source is coupled to a three grids 100 kV acceleration system. One of the main targets of the experimentation in SPIDER is to uniformly maximize the extracted current density; to achieve this it is important to study the density of negative ions available in proximity of the ion acceleration system. In SPIDER, line-integrated measurements of negative ion density are performed by a Cavity Ring Down Spectroscopy (CRDS) diagnostic. Its principle of operation is based on the absorption of the photons of a laser beam pulse by H-/D- photo-detachment; the absorption detection is enhanced by trapping the laser pulse in an optical cavity, containing the absorbing medium (i.e. negative ions). The paper presents and discusses the CRDS diagnostic setup in SPIDER, including the first measurements of negative ion density, correlated to the main source parameters.