Speaker
Description
Diagnostics at the region between magnetic filter to plasma grid of negative ion sources are important to understand the mechanism of the process through production to extraction of negative ions in source plasmas seeded caesium (Cs). Millimeter microwave interferometry and cavity ringdown (CRD) measurement are reliable diagnostic methods to measure line-averaged densities of electrons and negative ions, respectively.
The plasma close to the plasma grid, however, changes due to the vector field of magnetic field changing three dimensionally, and a “local” measurement at any point is required in such cases. Langmuir probe and photo-detachment probe are suitable for the purpose, while those methods have ambiguities caused by magnetic field and electrostatic field resulted from bias potential at plasma grid to obtain accurate physics values. In addition, influence of photoelectrons to a Langmuir probe was observed at the measurement of Cs-seeded plasma formed in the extraction region of LHD ion source.
In this presentation, we report the problems to apply Langmuir and photo-detachment probes and discuss the conditions to correct the magnetic field and to reduce the influences of photoelectron to the measured values.
