Abstract
A retarding field analyzer (RFA) that consists of three grids and a collector was developed, and the measurement of an ion beam that passes through plasma was demonstrated. First, a suitable grid potential structure to allow the measurement of an ion beam in plasma was investigated. After this investigation, a helium ion beam was measured without the production of plasma. It was found that the helium ion beam current was significantly overestimated when an unoptimized potential structure was utilized. One probable reason for the overestimation is secondary electron emission. Next, ion beam measurement in low density helium ionizing plasma was conducted. Accompanying the onset of the beam extraction, the collector current clearly increased, which implies that the beam ions penetrated through the plasma and reached the RFA. Subsequently, similar measurements were conducted after the electron density of the helium plasma was changed. Since a nearly identical beam extraction condition was retained, the ion beam current obtained after plasma production was almost constant. However, the ion beam current obtained during plasma production increased as the electron density increased. A calculation of the ion beam envelope indicated that space charge neutralization by bulk electrons could account for the increase in the ion beam current.
Highlights
In magnetic confinement fusion research, control of heat fluxes flowing onto divertor plates has been one of the most crucial problems yet to be solved
When the inner wall (Al2O3 insulator) of the retarding field analyzer (RFA) is bombarded by the diverged ions, secondary electrons are emitted from the surface
A retarding field analyzer consisting of three grids and a collector was introduced into the radio-frequency plasma source DTALPHA
Summary
In magnetic confinement fusion research, control of heat fluxes flowing onto divertor plates has been one of the most crucial problems yet to be solved. Plasma volumetric recombination has an important role in detached plasma formation.. Electron energy removal is important for forming and maintaining the detached plasma. Especially in tokamak devices, edge localized modes (ELMs) associated with improved plasma confinement at the peripheral region periodically transport energetic plasma particles into the divertor region. There is a concern that pulsed heat loads could exceed the heat load limit of the divertor plates as the energy of the plasma particles, exhausted by ELM events, potentially rises to several keV. A comprehensive understanding of the plasma volumetric recombination that coexists with energetic plasma inflow has been an important research topic
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