Investigations on Cs-free alternatives for negative ion formation in a low pressure hydrogen discharge at ion source relevant parameters

Caesium (Cs) is applied in high power negative hydrogen ion sources to reduce a converter surface’s work function and thus enabling an efficient negative ion surface formation. Inherent drawbacks with the usage of this reactive alkali metal motivate the search for Cs-free alternative materials for neutral beam injection systems in fusion research. In view of a future DEMOnstration power plant, a suitable material should provide a high negative ion formation efficiency and comply with the RAMI issues of the system: reliability, availability, maintainability, inspectability. Promising candidates, like low work function materials (molybdenum doped with lanthanum (MoLa) and LaB6), as well as different non-doped and boron-doped diamond samples were investigated in this context at identical and ion source relevant parameters at the laboratory experiment HOMER. Negative ion densities were measured above the samples by means of laser photodetachment and compared with two reference cases: pure negative ion volume formation with negative ion densities of about 1 × 10 15 m − 3 and the effect of H− surface production using an in situ caesiated stainless steel sample which yields 2.5 times higher densities. Compared to pure volume production, none of the diamond samples did exhibit a measurable increase in H− densities, while showing clear indications of plasma-induced erosion. In contrast, both MoLa and LaB6 produced systematically higher densities (MoLa: ×1.60; LaB6: ×1.43). The difference to caesiation can be attributed to the higher work functions of MoLa and LaB6 which are expected to be about 3 eV for both compared to 2.1 eV of a caesiated surface.

[1]  U. Fantz,et al.  Efficiency of Cs-free materials for negative ion production in H2 and D2 plasmas , 2017 .

[2]  R. Friedl.  Enhancing the accuracy of the Fowler method for monitoring non-constant work functions. , 2016, The Review of scientific instruments.

[3]  Julien Hillairet,et al.  Technological and physics assessments on heating and current drive systems for DEMO , 2015 .

[4]  M. Bacal,et al.  Negative hydrogen ion production mechanisms , 2015 .

[5]  U. Fantz,et al.  Comparison of measured and modelled negative hydrogen ion densities at the ECR-discharge HOMER , 2015 .

[6]  U. Fantz,et al.  Temperature dependence of the work function of caesiated materials under ion source conditions , 2015 .

[7]  U. Fantz,et al.  Investigations on Caesium-free Alternatives for H- Formation at Ion Source Relevant Parameters , 2015 .

[8]  C. Wimmer,et al.  Dependence of the source performance on plasma parameters at the BATMAN test facility , 2015 .

[9]  Pravin Kumar,et al.  Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on H− yield , 2014 .

[10]  U. Fantz,et al.  Fundamental studies on the Cs dynamics under ion source conditions. , 2014, The Review of scientific instruments.

[11]  C. Wimmer.  Characteristics and Dynamics of the Boundary Layer in RF-driven Sources for Negative Hydrogen Ions , 2014 .

[12]  T. Minea,et al.  Negative-ion surface production in hydrogen plasmas: modeling of negative-ion energy distribution functions and comparison with experiments , 2013 .

[13]  U. Fantz,et al.  Influence of cesium on the plasma parameters in front of the plasma grid in sources for negative hydrogen ions , 2013 .

[14]  U. Fantz,et al.  Development of negative hydrogen ion sources for fusion: Experiments and modelling , 2012 .

[15]  A. Gicquel,et al.  Enhanced negative ion yields on diamond surfaces at elevated temperatures , 2011 .

[16]  C. Wimmer,et al.  Work function measurements during plasma exposition at conditions relevant in negative ion sources for the ITER neutral beam injection. , 2011, The Review of scientific instruments.

[17]  R. Gutser,et al.  PIC code for the plasma sheath in large caesiated RF sources for negative hydrogen ions , 2009 .

[18]  Francis F. Chen,et al.  Langmuir probes in RF plasma: surprising validity of OML theory , 2009 .

[19]  S. Dietrich,et al.  Application of a collisional radiative model to atomic hydrogen for diagnostic purposes , 2009 .

[20]  H. Kwon,et al.  Surface production of H− ions by backscattering of H+and H+2 ions in the 3–50 eV ion energy range , 2008 .

[21]  S. Prawer,et al.  Work function of hydrogen-terminated diamond surfaces under ion impact , 2007 .

[22]  G. D. de Wijs,et al.  Ab initio and work function and surface energy anisotropy of LaB6. , 2006, The journal of physical chemistry. B.

[23]  U. Fantz,et al.  Spectroscopy—a powerful diagnostic tool in source development , 2006 .

[24]  C. Martens,et al.  Overview of the RF source development programme at IPP Garching , 2006 .

[25]  M. Bacal,et al.  Physics aspects of negative ion sources , 2006 .

[26]  M. Sasao,et al.  Contribution of wall material to the vibrational excitation and negative ion formation in hydrogen negative ion sources (invited) , 2004 .

[27]  Yiman Wang,et al.  Emission ability of La–Sc–Mo cathode , 2004 .

[28]  S. Ustaze,et al.  Dissociative electron attachment and dipolar dissociation of H − electron stimulated desorption from hydrogenated diamond films , 2001 .

[29]  A. Borisov,et al.  REVIEW ARTICLE: Negative ion formation in the scattering of atoms and ions from dielectric surfaces , 2000 .

[30]  V A Esaulov,et al.  Negative ion formation in the scattering of atoms and ions from dielectric surfaces , 2000 .

[31]  G. Dollinger,et al.  Electron stimulated desorption on diamond (100) as a negative hydrogen source , 1999 .

[32]  U. Fantz,et al.  Spectroscopic diagnostics of the vibrational population in the ground state of and molecules , 1998 .

[33]  L. Schlapbach,et al.  Electron affinity and work function of differently oriented and doped diamond surfaces determined by photoelectron spectroscopy , 1998 .

[34]  P. Wurz,et al.  Hydrogen and oxygen negative ion production by surface ionization using diamond surfaces , 1997 .

[35]  M. Bacal,et al.  Basic processes of negative hydrogen ion production and destruction in sources and beams (invited) , 1996 .

[36]  Brian S. Lee,et al.  Surface production of H− ions by hyperthermal hydrogen atoms , 1992 .

[37]  T. Iwasaki,et al.  Development of a double plasma type negative ion source , 1992 .

[38]  M. Bacal,et al.  Effect of filament material and area on the extracted current from a volume H- ion source , 1992 .

[39]  T. Inoue,et al.  Effect of filament material and area on the extracted current from a volume H- ion source , 1992 .

[40]  A. Karo,et al.  Recombination and dissociation of H2+ and H3+ ions on surfaces to form H2(v‘): Negative‐ion formation on low‐work‐function surfaces , 1990 .

[41]  J. Los,et al.  Negative ion formation at a barium surface exposed to an intense positive-hydrogen ion beam , 1988 .

[42]  O. Fukumasa,et al.  Effect of wall material on negative ion production in a hydrogen plasma , 1987 .

[43]  K. W. Ehlers,et al.  Effect of wall material on H− production in a multicusp source , 1985 .

[44]  W. Graham.  Wall material and wall temperature effects on negative ion production in a hydrogen plasma , 1983 .

[45]  K. W. Ehlers,et al.  Extraction of volume‐produced H− ions from a multicusp source , 1983 .

[46]  J. Los,et al.  Theoretical models of the negative ionization of hydrogen on clean tungsten, cesiated tungsten and cesium surfaces at low energies , 1982 .

[47]  G. W. Hamilton,et al.  Measurement of H- density in plasma by photodetachment. , 1979, The Review of scientific instruments.

[48]  R. Janev.  Destruction of negative ions near solid state surfaces , 1974 .

[49]  V. Dudnikov,et al.  A powerful injector of neutrals with a surface-plasma source of negative ions , 1974 .