Studies of surface-plasma negative ion sources at Novosibirsk

Abstract

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