Surface-plasma Negative Ion Source Research Articles

Development of high-voltage negative ion based neutral beam injector for fusion devices

A prototype of a powerful high-voltage neutral beam injector, based on acceleration of negative hydrogen ions and their neutralization, is under development at Budker Institute of Nuclear Physics (BINP). The design of the BINP high-voltage injector includes several innovative components, important for injector operation stability and overall efficiency. It includes a multi-aperture long-pulse surface-plasma negative ion source with thermostabilized grid, ithe magnetic system with concaved field lines in the ion-optic system (IOS) and the distributed cesium deposition system. The injector scheme incorporates a wide-aperture low-energy beam transport (LEBT) section, plasma target for negative ionneutralization, and recuperators of non-neutralized ions. Several test stands were constructed at BINP for injector component studies. This paper describes the results of experiments on negative ion beam production, transport through LEBT, ion acceleration to energy up to 240 keV and transport through the high voltage beam transport (HEBT) section to the distance ∼10 m from the source. The parameters of the transported beam, which were measured at several points along the beam line and at the beam dump calorimeter, are presented. The beam transport efficiency as a function of various ion source, LEBT and HEBT parameters is measured and compared with the calculated values. The results dof plasma neutralization target study are presented as well.

Effect of plasma grid bias on extracted currents in the RF driven surface-plasma negative ion source.

Extraction of negative ions from the large inductively driven surface-plasma negative ion source was studied. The dependencies of the extracted currents vs plasma grid (PG) bias potential were measured for two modifications of radio-frequency driver with and without Faraday screen, for different hydrogen feeds and for different levels of cesium conditioning. The maximal PG current was independent of driver modification and it was lower in the case of inhibited cesium. The maximal extracted negative ion current depends on the potential difference between the near-PG plasma and the PG bias potentials, while the absolute value of plasma potential in the driver and in the PG area is less important for the negative ion production. The last conclusion confirms the main mechanism of negative ion production through the surface conversion of fast atoms.

High voltage holding in the negative ion sources with cesium deposition.

High voltage holding of the large surface-plasma negative ion source with cesium deposition was studied. It was found that heating of ion-optical system electrodes to temperature >100 °C facilitates the source conditioning by high voltage pulses in vacuum and by beam shots. The procedure of electrode conditioning and the data on high-voltage holding in the negative ion source with small cesium seed are described. The mechanism of high voltage holding improvement by depletion of cesium coverage is discussed.

Comparative analysis of continuous-wave surface-plasma negative ion sources with various discharge geometry

Negative ion extraction from continuous-wave (CW) magnetron and semiplanotron discharges was studied and it was compared with that for the source with Penning electrode geometry. The CW negative ion beam up current to 13 mA was extracted from the magnetron source with emission aperture of 3.5 mm in diameter, while the beam with current up to 8 mA was obtained from the semiplanotron source modification. Characteristics of CW magnetron and semiplanotron sources are presented and analyzed.

Cesium control and diagnostics in surface plasma negative ion sources

For efficient and reliable negative ion generation it is very important to improve a cesium control and diagnostics. Laser beam attenuation and resonance fluorescence can be used for measurement of cesium distribution and cesium control. Resonant laser excitation and two-photon excitation can be used for improved cesium ionization and cesium trapping in the discharge chamber. Simple and inexpensive diode lasers can be used for cesium diagnostics and control. Cesium migration along the surface is an important mechanism of cesium escaping. It is important to develop a suppression of cesium migration and cesium accumulation on the extraction system.

Studies of surface-plasma negative ion sources at Novosibirsk

STUDIES OF SURFACE-PLASMA NEGATIVE ION SOURCES AT NOVOSIBIRSK Yu. I. Belchenko, G. E. Derevyankin, G. I. Dimov, and V. G. Dudnikov UDC 621.385(088.8) The recharging method is often used to control particle flows in accelerator technology and magnetic controlled thermonuclear synthesis systems. This method permits use of a change in particle charge state to control motion in electric and magnetic fields, change the magnitude and direction of particle acceleration, "switch on" and "switch off" the action of macroscopic fields, affect particle distribution in phase space, etc. G. I. Budker produced a significant contribution to the development of the recharging method for control of accelerated particle flows. He prosposed the use of dissociation of rapid molecular ions upon collisions with residual gas and plasma to fill magnetic hot plasma traps in his first study of mirror traps in 1954. At his initiative the Nuclear Physics Institute of the Siberian Branch, Academy of Sciences of the USSR developed a recharging method for proton injection into cyclical accelerators and accumulators [i, 2], together with physical bases for producing intense atomic beams of high energy hydrogen isotopes for injec- tion into thermonuclear traps [3-6]. A convenient method for producing high energy hydrogen particles is stripping of accelerated negative hydrogen ions which easily lose their "excess" electron in collisions on the recharging target and have a high (0.6-0.9) coefficient of conversion into atoms over a wide range of particle energies. Studies of H- ion sources at the Nuclear Physics Institute of the Siberian Branch, Aca- demy of Sciences of the USSR were begun in connection with development of the recharging method for proton injection into accelerators. For this purpose plasma sources were developed at the Institute (Elers type sources) with pulsed current up to 8 mA, together with recharg- ing sources first at a current of 15 mA, later up to I00 mA. However the characteristics of these sources did not permit complete realization of the advantages of recharging injection so that the latter were not widely used in accelerators. The situation with respect to production of negative ion beams improved radically after observation and experimental study of a new surface-plasma mechanism for negative ion forma- tion in gas discharges [3]. Using this mechanism a number of surface-plasma H- ion sources were developed for accelerators. Moreover, the surface-plasma method proved so effective that it could be used for development of high power negative ion sources for controlled thermo- nuclear synthesis. Major contributions to development of surface-plasma sources were also made by labora- tories in the USA (Brookhaven, Berkeley, Los Alamos, Oak Ridge, Fermi Lab), a number of laboratories in the Soviet Union, Europe, and Japan. These results are reflected in the proceedings of the International Symposium on Production and Neutralization of Negative Ions [7-9], accelerator conferences, and many other publications. In the present study we will briefly consider the results obtained at the Nuclear Physics Institute of the Siberian Branch~ Academy of Sciences of the USSR. Some achievements of these studies were considered in [4, 5, i0, ii], which offer citations of the original literature involved. Studies of the Physical Basis of the Surface-Plasma Method for Producing Negative Ion Beams. The first major results which served as a base for development of the surface-plasma method for generating negative ion beams and surface-plasma sources were obtained at the Institute in 1970. At that time note was made of the possibility in principle of forming H- ions due to secondary emission of negative ions upon bombardment by ions of surfaces with a reduced work function. Experiments performed revealed that upon bombardment of metal Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 4, pp. 106-115, July-August, 1987. Original article submitted December 26, 1986. 568 0021-8944/87/2804-0568512.50 9 1988 Plenum Publishing Corporation