E. H. A. Granneman

The scattering of hydrogen from a cesiated tungsten surface

Abstract The reflection of protons from a partially cesiated tungsten surface is studied in the energy domain between 100 and 2000 eV and in the angular domain between 75° and 85° with respect to the surface normal. The study is performed by measuring the angular and energy distribution of the scattered negative ions. The reflection can take place along two paths. One path is reflection from the cesium surface layer, the other one is reflection from the tungsten substrate. A dependence of the final charge state on the path is observed. It is inferred that this phenomenon is due to incomplete neutralization of the protons scattered from the cesium layer. The energy loss of the reflected ions cannot be accounted for by using only the binary collision model. Also the electronic stopping of the atoms by the metal electrons is shown to be an important energy loss mechanism. Total conversion measurements of H + to H - combined with the measurements of the negatively charged fraction of the scattered particles, as reported in the proceeding paper, yield the particle reflection coefficient as a function of the angle of incidence. These reflection coefficients show that for angles of incidence less than 75° already more than 50% of the particles do not reflect from the surface. Total conversion efficiency measurements with H - ions as primary ions show that the influence of the initial charge state on the total conversion is very small.

A model for the stationary cesium coverage on a converter surface in a cesium seeded hydrogen discharge

A model is presented for the mechanism which establishes the dynamic equilibrium cesium coverage on a W(110) converter surface in the discharge chamber of a negative hydrogen ion source. The charge state of cesium particles arriving at the converter surface is found to be a crucial parameter. A coverage greater than 0.26 monolayers cannot be maintained if the cesium component is highly ionized. The corresponding negative hydrogen ion formation probability is far from optimum. This situation is probably present in all high‐density surface conversion negative ion sources employed nowadays.

Formation of H− by scattering H+ on a cesiated polycrystalline tungsten surface

We present measurements on the charge transfer and reflection of H+ ions which are scattered from a cesiated polycrystalline tungsten surface. The particle energy ranges from 400 to 2 keV, the angle of incidence with respect to the surface normal is varied between 65 and 90°. The measured values are compared with data obtained earlier for cesiated monocrystalline tungsten (110). The maximum differential H− fraction of scattered particles in the case of cesiated polycrystalline tungsten is 25%. This value is roughly a factor of 2 lower than that of cesiated monocrystalline tungsten (110). The maximum total conversion efficiency, that is the reflected H− current divided by the incident positive ion current, is 12%. This value is about a factor of 3 lower than that obtained for monocrystalline tungsten (110). The different behavior of the polycrystalline with respect to monocrystalline material cannot be explained theoretically by the difference in work function. Calculated values are a factor of 1.7 higher ...

Formation of negative hydrogen ions on a coadsorbed layer of cesium and hydrogen on W(110)

Negative ion formation on a W(110) surface which is covered with a coadsorbed layer of cesium and hydrogen is studied by scattering a proton beam from such a surface. The primary energy is 400 eV. The angle of incidence is 70° with respect to the surface normal. The hydrogen exposure ranges from 0 to 3000 L. The negative ion formation probability on a surface with 0.6 times the saturation cesium coverage is reduced by a factor of 4 by a hydrogen exposure of 3000 L. At small coverage the reduction is found to be proportional to the number of adsorbed hydrogen atoms. The formation probability on a surface which is covered with a thick cesium layer is hardly affected by a similar exposure. These phenomena are attributed to resonant electron transfer between a negative ion and an adsorbed hydrogen atom.

Formation of negative hydrogen ions on a cesiated W(110) surface; the influence of hydrogen implantation

The negative hydrogen ion formation process on a cesiated W(110) surface is studied by scattering a proton beam from such a surface. The primary energy ranges from 50 to 3000 eV. The angle of incidence with respect to the surface normal is 45° or 70°. A maximum negative‐ion formation probability of 0.67 is measured. This quantity does not depend on the angle of incidence. However, it is strongly influenced by the time the surface has been exposed to the beam. This effect is attributed to hydrogen implantation.

Matching of a cylindrical ion beam to a periodic quadrupole channel

Abstract A procedure to match a cylindrically symmetric ion beam to a periodic electrostatic quadrupole channel is described. He + beams are extracted from a plasma source with a voltage of 40 kV. Behind the ion source the beam current and emittance are measured. The extracted beam current is variable from 1 to 20 mA, while the measured emittance is 10–20 π mm mrad. For a chosen zero-current phase advance per cell μ 0 in the focusing channel the so-called periodic solution is calculated for the measured current and emittance. μ 0 is varied between 43° and 114°. The model, which is used for the calculations, is based on the Kapchinsky-Vladimir-sky equations. The quadrupole lens forces are calculated separately by solving the Laplace equation in 2 dimensions. Furthermore, the voltages on the first five quadrupole singlet lenses, which adapt the extracted beam to the periodic solution, are determined. The beam current and emittance are again measured after the sixth quadrupole singlet lens, which is the first lens of the periodic channel. These measured values are compared with the calculated beam parameters. A reasonable agreement is found.

Charge and energy transfer in collisions of Cs+ ions with a cesiated W(110) surface

A beam of Cs+ ions with an energy of 500, 1000, or 2000 eV is scattered from a cesiated W(110) target. The angle of incidence is 45° or 75° with respect to the surface normal. The charge state and energy of the scattered particles are measured. The influence of hydrogen coadsorption on the final charge state is investigated. All scattered cesium particles are neutrals when the surface work function is smaller than 2.6 eV. The scattered particles have suffered a pronounced energy loss. From the measurements an extrapolation is made to conditions relevant for surface conversion negative ion sources.

Multichannel acceleration of intense, low-energy ion beams

We present measurements on multichannel acceleration of intense, low‐energy ion beams with a multiple electrostatic quadrupole array linear accelerator. The beam properties are investigated for different transverse and longitudinal focusing strengths, which can be adjusted independently in the accelerator. A maximum He+ ion current of four times 2 mA has been accelerated from 40 to 115 keV with an acceleration efficiency of 50%. Measurements and computer simulations suggest that the current is limited by transverse fields and by mismatch and misalignment of the beams, whereas longitudinal fields are of minor importance. The energy spread in the bunches is roughly three times the gap voltage.

The role of cesium‐ion bombardment in the formation of negative hydrogen ions on a converter surface

A fundamental study of the formation of negative hydrogen ions via surface conversion is presented. Employed is a novel type of converter, namely a porous tungsten button with liquid cesium flowing through it towards the side which is in contact with the plasma. A high cesium coverage, i.e., a small work function, can easily be maintained with this approach. This is related to the high flux of neutral cesium atoms to which the converter is exposed and to the small cesium density in the discharge. Despite the small work function, we obtain negative‐ion yields which are an order of magnitude smaller than is usually found in more conventional experiments, in which the converter is cesiated via injection of cesium vapor into the discharge. Furthermore, our energy distributions show that no negative ions are formed via desorption by cesium‐ion impact. This gives a strong indication that the extracted negative hydrogen ions are primarily formed via this process in cesium seeded discharges. Our view is confirmed...

The formation of a negative hydrogen ion beam by surface conversion of a positive ion beam on cesiated tungsten surfaces

Presented are measurements on the generation of negative hydrogen ion beams via the conversion of positive ion beams on low work function surfaces placed outside the positive ion source. The converter surfaces are positioned parallel to the beam axis and the positive ion beam is directed onto them by the increase of the beam divergence caused by beam retardation. Results include a negative hydrogen ion beam current of 15 mA at a current density of 6.2 mA/cm2. The converter work function is lowered by the deposition of a Cs monolayer. The Cs coverage could be maintained during the bombardment by the intense H+ beam through surface migration of the Cs from a reservoir.