* Corresponding author. Present address: Radiophysical Fac- ulty, Kiev Shevchenko University, Vladimirskaya St. 64, Kiev 252033, Ukraine. Tel.: #380-44-4581832. E-mail address: viv@vhome.kiev.ua (V.I. Vysotskii). Nuclear Instruments and Methods in Physics Research A 455 (2000) 123}127 Creating and using of superdense micro-beams of relativistic electrons V.I. Vysotskii*, S.V. Adamenko, V.A. Stratienko, N.G. Tolmachev Laboratory of Electrodynamical Investigation, Concern **ENRAN++, Kiev, Ukraine Abstract The problem of creation and the results of experimental investigation of superdense subrelativistic electrons micro- beams with current density 10 A/cm)j)10 A/cm, duration t"1}10 ns and energy about 0.01}1 MeV are discussed. The interaction of these micro-beams with condensed target and ions beams are investigated. The problems of isomers excitation and active medium for both X-ray laser and gamma-laser creation by use of super-dense electrons micro-beams are also studied.  2000 Elsevier Science B.V. All rights reserved. PACS: 29.17.#w; 29.25.Bx; 29.27.Bd; 42.55.Vc Keywords: Superdense micro-beams; Relativistic electrons; Plasma; Nuclei excitation; X-laser; Gamma-laser 1. Introduction The problem of generation of relativistic and subrelativistic electrons is among most challenging ones. Using such beams, it would be possible to develop both X-ray and gamma lasers and to excite short-lived nuclear isomers. As shown by estimates, electron beams of a density not less than j" 10}10 A/cm are needed for this purpose. Accordingly, a current density j"10} 10 A/cm is needed for excitation of long-lived isomers. A critical current density j"10}10 A/cm is required for an X-ray laser based on the phenomenon of stimulated emission of radiation at channeling of relativistic positrons in single crystals. Obtaining such current densities with the use of traditional linear and ring-type accelerators is im- possible. Some models of microaccelerators and optimized macro accelerators, providing for achievement of required current densities will be discussed below. Some "elds of their application will also be considered. 2. Creation of traveling inversion for an X-ray laser in plasma environment with the aid of an annular pulse of femtosecond laser The problem of generation of a superdense elec- tron beam for exiting a traveling pumping in an X-ray laser can be solved by the use of an annular 0168-9002/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 7 1 8 - X LASER BEAM INTERACTIONS