Negative ion density in the ion source SPIDER in Cs free conditions

TL;DR

This study investigates how magnetic filter fields, bias currents, and electric fields influence negative ion density in the SPIDER ion source operating without cesium, aiming to optimize ion extraction for fusion applications.

Contribution

It provides new insights into the effects of magnetic filter strength and biasing conditions on negative ion density in Cs-free conditions, guiding operational optimization.

Findings

Optimal magnetic filter field between 1.6 mT and 3.2 mT for maximum negative ion density.

Positive biasing of the plasma grid increases negative ion density.

Electric fields in the acceleration system significantly affect negative ion production.

Abstract

The SPIDER experiment, operated at the Neutral Beam Test Facility of Consorzio RFX, Padua, hosts the prototype of the H-/D- ion source for the ITER neutral beam injectors. The maximization of the ion current extracted from the source and the minimization of the amount of co-extracted electrons are among the most relevant targets to accomplish. The Cavity Ring-Down Spectroscopy diagnostic measures the negative ion density in the source close to the plasma grid (the plasma-facing grid of the ion acceleration system), so to identify the source operational parameters that maximize the amount of negative ions which can be extracted. In this study SPIDER was operated in hydrogen and deuterium in Cs-free conditions, therefore negative ions were mostly produced by reactions in the plasma volume. This work shows how the magnetic filter field and the bias currents, present in SPIDER to limit the amount of co-extracted electrons, affect the density of negative ions available for extraction. The results indicate that the magnetic filter field in front of the acceleration system should be set between about 1.6 mT, condition that maximizes the density of available negative ions, and about 3.2 mT, condition that minimizes the ratio of electron current to ion current. The negative ion density also resulted to be maximized when the plasma grid and its surrounding bias plate was positively biased against the source body with a total current in the range 0 A{\div}100 A. The paper shows also how much, in Cs-free conditions, the electric fields in the acceleration system can affect the density of negative ions in the source, close to the plasma grid apertures.