Speaker
Description
The Neutral Beam Injection (NNBI) for ITER is based on large-scale RF plasma sources for negative hydrogen ions, which need to deliver high currents of H$^-$ and D$^-$ (up to 66 A) for up to one hour while maintaining the co-extracted current of electrons below the ion current. Cesium is used to enable the surface production of negative ions; however, the plasma-induced removal of Cs from the conversion surfaces leads to a steady decrease of the source performance during long pulses, particularly in deuterium operation.
The NNBI test facilities BATMAN Upgrade and ELISE (1/8 and 1/2 size of the ITER NBI source, respectively) contribute to the development program of the ITER NBI. Conditioning recipes and further measures (e.g. biased surfaces close to the extraction system) have been developed to stabilize and/or reduce the current of co-extracted electrons. These optimizations resulted in the reproducible achievement of almost 90% of the targeted extracted negative ion current (30 A) during 600 s hydrogen pulses at ELISE. Deuterium operation remains more challenging, since the co-extracted electron current increases much stronger during long pulses. An alternative Cs evaporation concept (“Cs shower”) has been tested at BATMAN Upgrade, allowing for direct control of the neutral Cs flux onto the converter surface during pulses. After further optimizations, 90% of the required scaled D$^-$ current was sustained for 1000 s for the very first time.
This contribution reports on these breakthroughs achieved at the NNBI ion sources towards long pulse operation and strategies to steady state operation are discussed. Results from plasma and beam diagnostics are presented.
| Email address | christian.wimmer@ipp.mpg.de |
|---|---|
| Classification/Track | Invited |
| Visitor's Visa | no |
| Classification | H- and D- sources for fusion, accelerators and other applications |