Spatially resolved diagnostics for optimization of large ion beam sources

Abstract

Giant negative ion sources for neutral beam injectors deliver huge negative ion currents, thanks to their multi-beamlet configuration. As the single-beamlet optics defines the transmission losses along the beamline, the extraction of a similar current for all beamlets is extremely desirable, in order to facilitate the beam source operation (i.e., around perveance match). This Review investigates the correlation between the vertical profile of beam intensity and the vertical profiles of plasma properties at the extraction region of the source, focusing on the influence of increasing cesium injection. Only by the combined use of all available source diagnostics, described in this Review, can beam features on the scale of the non-uniformities be investigated with a sufficient space resolution. At RF power of 50 kW/driver, with intermediate bias currents and a filter field of 2.4 mT, it is found that the central part of the four vertical beam segments exhibits comparable plasma density and beamlet currents; at the edges of the central segments, both the beam and electron density appear to decrease (probably maintaining fixed electron-to-ion ratio); at the bottom of the source, an increase of cesium injection can compensate for the vertical drifts that cause a much higher presence of electrons and a lower amount of negative ions.

Measure of negative ion density in a large negative ion source using Langmuir probes

“…A vertical arrangement of four probes in the middle of the plasma grid, one per segment, was used to characterize the vertical plasma uniformity, confirming what was observed also with other diagnostics [17]. Figure 7 compares the fit results obtained from these probes for two plasma blips with 𝑃 𝑅𝐹 = 50 kW/driver, 𝐼 𝐵𝐼 = 190 A, 𝐼 𝐵𝑃 = 80 A, same 𝐼 𝑃𝐺 = 1.5 kA but opposite direction of the filter field current.…”

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

“…During cesium injection, the expected increase of the negative ion current and simultaneous decrease of the co-extracted electron current were obtained and are in line with the results of the other large RF-based negative ion source, ELISE. The experimentation showed a tight relationship between the beam features and the plasma parameters measured in the vicinity of the PG [37]. It was possible to carry out a characterization of the beam features at least at an RF power of 30 kW/driver.…”

Characterization of plasmas in negative ion sources using a Cs-H Collisional Radiative model

“…is not yet equipped with any diagnostic capable of measuring the H − density. Serianni et al [20] showed that, in SPIDER, a good agreement could be obtained between measurements of 𝑛 𝐻 − and estimates from the beam current using the following approximation: Combining these hypotheses with the OES measurements and the CR model, estimates of the electron density can be obtained and are shown in table 1. These numbers must be considered cautiously as strong hypotheses on several other parameters were required, but the order of magnitude is consistent with the previous Langmuir probe campaigns in NIO1.…”