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

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÷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.

First operations with caesium of the negative ion source SPIDER

“…At present, the beam source is operated in pulsed mode and it is equipped with a number of beam diagnostics and source diagnostics (see [21] and references therein). The diagnostic calorimeter STRIKE [22], beam emission spectroscopy (BES) [23], optical cameras, Allison emittance scanner [24], single-beamlet current monitors (new diagnostics based on LEM sensors) provide independent measurements of the beam properties, while Langmuir probes [25], optical emission spectroscopy [26], cavity ring down spectroscopy [27], laser absorption spectroscopy [28], Langmuir-Tayor detectors [29] can provide insight on the plasma and surface processes inside the plasma source.…”

Beam extraction under Cs-free conditions in HUST negative ion source

“…Some auxiliary experimental results on the axial plasma density distribution under various PG currents in the HUST source also demonstrated this phenomenon [29]. If the field strength continues to increase to a larger value (our setup cannot be reached yet due to the power source limitation), instead, it can penetrate deeper into the driver, depressing the production of ions, neutrals, and excited molecules, which will result in a decrease in electron density of the plasma in the extraction region [30,31]. The positive ion density, shown in figure 8(b), demonstrated a similarity to the electron density in both value and trend, indicating that the plasma at this position remained quasi-neutral.…”

SPIDER, the Negative Ion Source Prototype for ITER: Overview of Operations and Cesium Injection

“…Indeed, a vertical nonhomogeneity of the beam current was identified; particularly, it was also found that the beam is peaked in each beamlet group [11]. The ratio between the co-extracted electron and the negative ion currents was found to be at least in the range of several tens [13].…”