Application of optical emission spectroscopy for the SNS H− ion source plasma studies

Abstract

Plasma emission spectroscopy for operating and developing the Spallation Neutron Source Abstract. The SNS H -ion source is a dual-frequency RF-driven (13.56-MHz low power continuous RF superimposed by 2-MHz high power pulsed RF with ~1.0 ms pulse length at 60 Hz), Cs-enhanced ion source. This paper discusses the applications of optical emission spectroscopy for the ion source plasma conditioning, cesiation, failure diagnostics, and studies of plasma build-up and outage issues.

Record performance of and extraction studies with the Spallation Neutron Source H− injector

“…However using lower-carbon stainless steel did not clearly reduce the blackness of the antennas. Plasma emission spectra contain sodium lines during the first few days when there is excess Cs in the plasma [23]. This suggests negligible sputtering after an initial period with heavy plasma impurities.…”

“…This was a serious issue in the early years when the subtle differences between RF system failures and antenna failures led to prolonged tuning in an effort to restore the RF system. This problem was reduced by a more robust RF installation on ground potential [6] and by routine analysis of the plasma light emission [23]. Likely, hidden excess porosity in strongly exposed locations, such as the leg bends, caused the porcelain to overheat and eventually to melt after prolonged exposure.…”

“…Likely, hidden excess porosity in strongly exposed locations, such as the leg bends, caused the porcelain to overheat and eventually to melt after prolonged exposure. Apparently only when the porcelain melts are there enough impurities in the bright hydrogen plasma to indicate an antenna failure, thus making it impossible to predict antenna failures [23].…”

H- ion source operational performance and latest development at the Spallation Neutron Source

“…The service duration of an ion source has recently been extended to ~4 months covering a whole run-cycle of the SNS neutron production operation without changing ion source. A newly installed ion source undergoes a startup process that includes outgassing and sputter-cleaning the plasma chamber, ion source cesiation and beam tuning [6]. Typically, a single dose of cesiation with <30 mg cesium covers several months of ion source operation with high beam current (>50 mA) [4].…”

Recent performance and ignition tests of the pulsed SNS H− source for 1-MW neutron production

“…This will continue at least until a safe method is demonstrated to ignite the plasma with the 2 MHz only. Additional efforts focus on understanding the nature of the plasma outages, which will include light emission studies [10], so that alternate mitigations can be developed.…”