Yu. I. Belchenko

Surface negative ion production in ion sources

Negative ion sources and the mechanisms for negative ion production are reviewed. Several classes of sources with surface origin of negative ions are examined in detail: surface‐plasma sources where ion production occurs on the electrode in contact with the plasma, and ‘‘pure surface’’ sources where ion production occurs due to conversion or desorption processes. Negative ion production by backscattering, impact desorption, and electron‐ and photo‐stimulated desorption are discussed. The experimental efficiencies of intense surface negative ion production realized on electrodes contacted with hydrogen‐cesium or pure hydrogen gas‐discharge plasma are compared. Recent modifications of surface‐plasma sources developed for accelerator and fusion applications are reviewed in detail.

Ion sources at the Novosibirsk Institute of Nuclear Physics (invited)

A review of investigations in the physics and technology of ion sources, developed in the Institute of Nuclear Physics in Novosibirsk is presented. Distinctive features of physical processes and technical characteristics of plasma sources of gaseous ions, negative ion surface‐plasma sources, electrohydrodynamic (liquid metal) ion sources are considered. In original design plasma sources, ion beams with a current of up to 90 A and energies 1–30 keV are formed by four‐electrode multislit extraction systems from highly ionized, high brightness plasma flux, generated by an high‐current arc discharge with a cold cathode in a small cross‐section diaphragmed channel, and directed with a magnetic field of a special configuration. Plasma jet expansion for a very low ion temperature (0.1 eV) production is used. In surface plasma sources, the fluxes of negative ions are produced when electrons are captured from the electrode surface at the electron affinity level of sputtered and reflected particles. A discharge of ...

Direct current H− source for the medicine accelerator (invited)

A compact cw hydrogen negative ion source having reliable operation and a simplified maintenance is developed at Budker Institute of Nuclear Physics for a tandem accelerator of boron capture neutron therapy installation. The source uses a Penning discharge with a hydrogen and cesium feed through the hollows in the cathodes. Discharge voltage is about 60–80 V, current 9 A, hydrogen pressure 4–5 Pa, magnetic field 0.05–0.1 T, and cesium seed <1 mg/h. Negative ions are mainly produced on the cesiated anode surface due to conversion of hydrogen atoms. An optimal anode temperature is 250–350 °C. Negative ion beam current is directly proportional to the discharge current and to the emission hole area. A triode system for the beam extraction and acceleration system is used. The flux of accompanying extracted electrons was decreased by filtering in the transverse magnetic field. This electron flux was intercepted to the special electrode, biased at 4 kV potential with respect to the anode. Source stable cw operat...

Optimization of Cs deposition in the 1/3 scale hydrogen negative ion source for the large helical device-neutral beam injection system

A compact cesium deposition system was used for direct deposition of cesium atoms and ions onto the inner surface of the 1/3 scale hydrogen negative ion source for the large helical device-neutral beam injection (LHD-NBI), system. A small, well defined amount of cesium deposition in the range of 3–200 mg was tested. Negative ion extraction and acceleration were carried out both in the pure hydrogen operation mode and in the cesium mode. Single Cs deposition of 3–30 mg to the plasma chamber has produced temporary 2–5 times increases of H− yield, but the yield was decreased within several discharge pulses to the previous steady-state value. Two consecutive 30 mg depositions done within a 3–5 h/60 shot interval, produced a similar temporary increase of H− beam, but reached a large H− yield steady-state value. Deposition of larger 0.1–0.2 g Cs portions with a 20–120 h/150–270 shot interval improved the H− yield for a long (2–5 days) period of operation. Directed depositions of Cs to the various walls of the p...

Inductively driven surface-plasma negative ion source for N-NBI use (invited)

The long-pulse surface-plasma source prototype is developed at Budker Institute of Nuclear Physics for negative-ion based neutral beam injector use. The essential source features are (1) an active temperature control of the ion-optical system electrodes by circulation of hot thermal fluid through the channels, drilled in the electrode bodies, (2) the concaved transverse magnetic field in the extraction and acceleration gaps, preventing the electrons trapping and avalanching, and (3) the directed cesium deposition via distribution tubes adjacent to the plasma grid periphery. The long term effect of cesium was obtained just with the single cesium deposition. The high voltage strength of ion-optical system electrodes was improved with actively heated electrodes. A stable H(-) beam with a current ∼1 A and energy 90 keV was routinely extracted and accelerated.

Development of a negative ion-based neutral beam injector in Novosibirska)

A 1000 keV, 5 MW, 1000 s neutral beam injector based on negative ions is being developed in the Budker Institute of Nuclear Physics, Novosibirsk in collaboration with Tri Alpha Energy, Inc. The innovative design of the injector features the spatially separated ion source and an electrostatic accelerator. Plasma or photon neutralizer and energy recuperation of the remaining ion species is employed in the injector to provide an overall energy efficiency of the system as high as 80%. A test stand for the beam acceleration is now under construction. A prototype of the negative ion beam source has been fabricated and installed at the test stand. The prototype ion source is designed to produce 120 keV, 1.5 A beam.

Directed cesium deposition into a large volume negative‐ion source

A compact, safe system for directed cesium deposition into large volume sources is described. It consists of a small oven mounted on a feedthrough, which permits one to rotate and move the oven across the discharge chamber length or to remove it from the vacuum box to reload the oven. Special industrial pellets containing the cesium compound (30% of cesium chromate +70% titanium) were used for a pure cesium release. The pellets are insensitive to pollution by air and can provide the cesium release during several cycles of heating and cooling in long‐term experiments. There was no ‘‘poisoning’’ impurities degassing (oxygen, water) during the standard oven operation. The Cs system was reliably operated for cesium deposition in multiampere negative‐ion source experiments.

Efficient cesiation in RF driven surface plasma negative ion source

Experiments on hydrogen negative ions production in the large radio-frequency negative ion source with cesium seed are described. The system of directed cesium deposition to the plasma grid periphery was used. The small cesium seed (∼0.5 G) provides an enhanced H(-) production during a 2 month long experimental cycle. The gradual increase of negative ion yield during the long-term source runs was observed after cesium addition to the source. The degraded H(-) production was recorded after air filling to the source or after the cesium washing away from the driver and plasma chamber walls. The following source conditioning by beam shots produces the gradual recovery of H(-) yield to the high value. The effect of H(-) yield recovery after cesium coverage passivation by air fill was studied. The concept of cesium coverage replenishment and of H(-) yield recovery due to sputtering of cesium from the deteriorated layers is discussed.

Recovery of cesium in the hydrogen negative ion sources

Cesium recovery from the polluted layers in the 1/3 scale hydrogen negative ion source for LHD-NBI system has tested. It was found that the cesium recovery can be produced by additional discharges as from the cesium layer, aged by tungsten and residual gas, so as from the cesium layers, polluted by an occasional water leak. The highest cesium recovery to negative ion production was produced by a xenon arc, while glow discharge and arcing in hydrogen were less effective. The mechanism of recovery is the ejection of cesium from the underlying enriched layer by the arc and its transport to the surface.

Negative hydrogen ion production in the hollow cathode Penning surface‐plasma source

A small hollow cathode Penning surface‐plasma source (SPS) was developed and studied. The H− yield was proportional to the emission apertures area and increased over a wide range of discharge current. The H− yield, with an intensity of up to 0.95 A and an emission current density of up to 3.6 A/cm2, was obtained in a pulsed mode. With the discharge current of 20 A and a pulse duration of 60 s, an H− yield with current of 0.1 A was obtained. The H− emission current density had approximately the same value for various diameters (0.5–7 mm) and thicknesses (0.3–4.0 mm) of cylindrical emission holes, if the thickness of hole walls did not exceed the hole diameter. The H− yield extracted through the thick conical emission holes had a value 25% higher than that for a thin cylindrical hole with the same permeable diameter. Dependencies of the H− yield versus magnetic field and hydrogen feed were different from that of the standard Penning SPS. The optimal cesium coverage of the electrodes was stable for both high...