A new inorganic material, C12A7 electride [1,2] has been experimentally studied as a candidate material for Cs-free hydrogen negative ion (H-) sources. A high production rate of H- was observed from a C12A7 electride surface immersed in hydrogen/deuterium low-pressure plasmas . In our previous work using a compact ECR ion source , it was found that the H- current extracted from the source with an electride Plasma Electrode (PE) is higher that with a clean molybdenum, by a factor of 80-100, as expected from the experimental results at Marseille . However, the absolute values of beam intensity were too low for an actual use and the dependence of H- current on the microwave power, was not observed clearly.
In the present study, key issues to realize a compact H- source for the actual use in accelerator injectors are investigated. Firstly, the ECR ion source was modified so that a dense plasma from the ECR region could be injected directly towards the PE. Preliminary results showed a clear dependence on the microwave power and the H- current increased by more than a factor 10.
In H- production on the cesiated metal surface, the work function (WF) of the surface plays the essential role because the H- electron might transfer back to empty states of the conduction band during it is moving away from the surface. In case of C12A7 electride, the connected cages form a new conduction band called “cage conduction band” (CCB). The WF from CCB is as low as 2.4 eV, but there is a wide gap to valence band maximum (~5.5 eV) from CCB. Moreover, it is reported that some of the encaged electron are replaced by the H- ions when the electride is exposed to hydrogen circumstance. Considering these aspects, the effect of the extraction hole shape, and the correlation to the surface work function of the electride PE will be studied, and prospects to a H- source for the actual use will be discussed.
 Y. Toda, H. Yanagi, E. Ikenaga, Jung Jin Kim, M. Kobata, S.Ueda, T. Kamiya, M. Hirano, K. Kobayashi, and H. Hosono, Advanced Materials 19, 3564 (2007).
 H. Hosono, Japanese Journal of Applied Physics 52 (2013) 090001.
 M. Sasao et al., Applied Physics Express 11, 066201 (2018).
 M. Kobayashi et al, AIP Conference Proceedings 2052, 020003 (2018).
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