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The role of the plasma electrode (PE) bias in the extraction process in a large-volume hybrid multicusp negative-ion source, which is driven by 2.45-GHz microwaves, is studied. Spatially resolved negative-ion and electron density measurements close to the extractor aperture were performed under various pressures (1-4 mtorr) by means of the electrostatic probe and photodetachment technique. As the low positive voltage applied to the PE is slightly increased (from 4 to 7 V), the electron temperature passes through a minimum (0.2-0.6 eV), while simultaneously, both the negative-ion density and the [H.sup.-] extracted current reach a maximum (~1-2 x [10.sup.9] [cm.sup.-3] and ~0.5 mA/c[m.sup.2], respectively). Optimum pressure values for the extracted negative-ion current and the negative-ion density are found between ~1.5 and 3 mtorr. It is deduced that the negative-ion density measured in the center of the source cannot be directly correlated with the ion extracted current. The electron density and the associated extracted electron current linearly decrease as a function of the PE bias. The physical mechanisms explaining the experimental results are discussed. Index Terms--Electrostatic probe, hydrogen, microwaves, negative ions, photodetachment, plasma electrode (PE) bias.

We review the evolution of volume production negative hydrogen ion sources since the discovery in 1977 of the new phenomenon, designated as volume production and attributed to dissociative electron attachment of low energy electrons to rovibrationally excited molecules. The experimental verification in 2005 of the reality of this mechanism is reported. The magnetically filtered tandem sources, using hot filaments or inductively coupled radio frequency discharges, proposed in order to make use of the volume production mechanism, are used as continuous wave sources for cyclotrons and short pulse sources for synchrotrons. The extraction physics, required to correlate the negative ion and electron densities near the extraction opening with the extracted currents, is discussed taking into account the recently measured [H.sup.-]/[D.sup.-] ion temperatures. It is also shown that the extracted negative ion current can be predicted from the directed flow velocity (measured by two laser photodetachment) and the negative ion density measured in the extraction region plasma. Progress in modeling the volume production negative hydrogen ion sources is briefly summarized. Main attention has been paid to some recent topics, such as negative ion temperature and specific for two negative species plasmas transport in a weak transverse magnetic field. A new view on the potential of volume production making use of the vibrationally excited molecules produced on surfaces (plasma electrode, walls) by recombinative desorption is presented. Index Terms--Ion source, magnetic filter, negative ion, recombinative desorption, volume production.

The role of the plasma electrode (PE) bias in the extraction process in a large-volume hybrid multicusp negative- ion source, which is driven by 2.45-GHz microwaves, is studied. Spatially resolved negative-ion and electron density measurements close to the extractor aperture were performed under various pressures (1-4 mtorr) by means of the electrostatic probe and photodetachment technique. As the low positive voltage applied to the PE is slightly increased (from 4 to 7 V), the electron temperature passes through a minimum (0.2-0.6 eV), while simultaneously, both the negative-ion density and the H- extracted current reach a maximum (~1-2 × 109 cm-3 and ~0.5 mA/cm2, respectively). Optimum pressure values for the extracted negative-ion current and the negative-ion density are found between ~1.5 and 3 mtorr. It is deduced that the negative-ion density measured in the center of the source cannot be directly correlated with the ion extracted current. The electron density and the associated extracted electron current linearly decrease as a function of the PE bias. The physical mechanisms explaining the experimental results are discussed. [ABSTRACT FROM AUTHOR]

This paper discusses the possibility that there is an enhancement of volume production due to gettering of atomic hydrogen by the caesium deposited on the walls, which reduces the negative ion destruction by associative and non-associative detachment. Such an enhancement would take place for all plasma grid bias voltages. Furthermore at plasma grid bias voltages higher than the plasma potential there is flow of the negative ions from the bulk plasma. [ABSTRACT FROM AUTHOR]

The article presents information on an increase in research and development activities in the field of ion sources. Research in this field is divided into various fields namely--accelerator injection, fusion, ion implantation, ion beam lithography, and basic research. Many review papers have been published and presented in a wide part of the ion source panorama, outlining progress around the world in these selected areas. The major areas of research in the field of ion source include--electron cyclotron resonance ion sources, electron beam ion sources and traps, laser ion sources, its applications in accelerator and fusion and ion sources for industrial applications.

When considering the state-of-the-art on H- ion sources, ions can be produced either by plasma-surface interaction and/or inside the plasma volume. For the production of negative ions by surface ionization, a low work function material is required. For this purpose, cesium has been used in many cases at LBNL, JAEA, KEK, and in other facilities [M. Bacal, Nucl. Fusion 46, 250 (2006)]. Despite an enhancement in the negative ion production (by a factor of 2.5 in JAEA source), the use of cesium could lead to many drawbacks in the plasma functioning of ITER, for example. An alternative material to cesium could lead to an important improvement for negative ion source.For this purpose, both theoretical and experimental studies must be undertaken. Surface mechanisms have to be taken into account both for creation and loss mechanisms: (i)By recycling the atomic hydrogen into highly vibrationally excited molecular hydrogen via recombinative desorption on specific surfaces (fresh tantalum on surface increases the negative ion density [M. Bacal, A. A. Ivanov, Jr. et al., Rev. Sci. Instrum. 75, 1699 (2004)] by more than 60%). It has been shown for a rigid substrate model that both the recombination cross section and the degree of vibrational excitation are highly sensitive to the nature of the surface [B. Jackson and D. Lemoine, J. Chem. Phys. 114, 474 (2001)].(ii)By surface passivation, which could lead to a substantial decrease in H2 (X,v″) wall losses.In order to understand the fundamental mechanisms of surface production and losses, “Camembert III” experimental setup, recently settled in the LPSC laboratory (Grenoble, France) is used. In this experimental structure, hydrogen multidipolar plasma sustained by microwaves (2.45 GHz) presents the potential advantage to operate either in a metallic or a conductive chamber. The inner walls could be then frequently coated, by sputtering or chemical vapor deposition techniques, with no opening of the chamber with various materials. First experiments of H- surface production will be performed on target material. Hence, perfectly knowing the target material (in terms of composition and structural and physical properties), H- ion density production near the target surface will be monitored by laser photodetachment [M. Bacal, Rev. Sci. Instrum. 71, 3981 (2000)]. Even if this diagnostic gives no direct information on high rovibrationally excited level of the H2 molecule, its implementation and use are far less complicated than vuv LIF. [ABSTRACT FROM AUTHOR]

This paper discusses the possibility that there is an enhancement of volume production due to gettering of atomic hydrogen by the caesium deposited on the walls, which reduces the negative ion destruction by associative and non-associative detachment. Such an enhancement would take place for all plasma grid bias voltages. Furthermore at plasma grid bias voltages higher than the plasma potential there is flow of the negative ions from the bulk plasma.