A review of diagnostic techniques for high-intensity negative ion sources

Abstract

Plasma and beam diagnostic methods for high-current-density negative ion sources are reviewed. The pulse laser–induced photodetachment method was widely used to measure negative ion density in the ion source plasma. The method was used to understand fundamental processes of negative hydrogen (H−) ion formation and destruction in hydrogen plasmas. Species composition and plasma parameters in H− ion sources can be estimated from optical emission spectroscopy data, while electrostatic probes are utilized to obtain local plasma parameters in the ion source. Plasma transport toward the extractor is studied by combining a laser beam for photodetachment and a structured electrostatic probe to visualize a flow pattern of H− ions. Spatial distributions of large-area H− ion beams are monitored by infrared thermography images, showing interactions among multiple beamlets. These diagnostic methods deepen the understanding of how H− ions are produced and extracted through beam-forming apertures. The acquired knowledge on ion source plasma and beam transport improves the ion source design of the next-generation H− ion sources for fusion experiments and accelerators. The essence of research and development for negative ion source study is summarized through briefly tracing the history.

Characterization of Cs-free negative ion production in the ion source SPIDER by cavity ring-down spectroscopy

“…The CRDS diagnostic was used to characterize and maximize the availability of negative ions at the PG, as a function of the source operation parameters. The volume production of negative ions grows with the plasma density, which in turn grows with the input power used to sustain the plasma [ [5], [6], [10]]. Figure 3a shows how the Hdensity grows as a function of total RF power, each RF generator providing 1/4 of the total.…”

“…This is possible thanks to the Cavity Ring-Down Spectroscopy (CRDS) [7] diagnostic installed in SPIDER [ [8], [9]], which provides Line-of-Sight (LoS) integrated measurements of H -/Ddensity in proximity of the ion extraction apertures. This technique has been successfully applied to other several negative ion sources for fusion applications [ [6], [10]- [17]]. The CRDS diagnostic in SPIDER can provide H -/Ddensity measurements at 10 Hz rate, with a minimum detection threshold of few 10 15 m -3 .…”

Beam instability in the vicinity of beam extraction region of negative ion source

“…4. The density fluctuation in the vicinity of the PG is measured by a Langmuir probe (LP) [7]. The location of the probe measurement is 12 mm from the PG, and at the almost center of the aperture, where the presheath is formed when the extraction voltage is applied to the second grid (extraction grid).…”

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

“…To counteract the co-extraction of electrons, magnets embedded in the EG produce alternated vertical magnetic fields that dump the co-extracted electrons onto the EG surface, while causing minimal deviations of the negative ion trajectories [1]. Furthermore, the amount of electrons in the proximity of the PG apertures is reduced in two ways; first, a current IPG flows vertically in the PG, generating a magnetic filter field (1.6 mT close to the PG per 1 kA of current) that locally reduces density and energy of electrons [ [1], [5], [10], [20]]. Second, the plasma potentials can be modified by making currents flow between different electric components of the source [ [5], [10], [20]- [22]].…”