HIGH BRIGHTNESS ELECTRON LINAC WITH RF GUN AND ACCELERATING STRUCTURE ON BACKWARD WAVE M.I. Ayzatsky, E.Z. Biller, A.N. Dovbnya, V.A. Kushnir, V.V. Mitrochenko, L.V. Reprintzev, D.L. Stepin, NSC KIPT 1, Academicheskaya Str., Kharkov, 310108 Ukraine 1 INTRODUCTION LIC ( Laser Injector Complex ) facility was developed and constructed in view of shaping and acceleration of high - brightness electron beams. This R&D was to be followed by beam research in the following arias: - ultra - short wave generation; - wake-field generation in plasmas and other systems; - relativistic electron beam focusing in plasmas; - testing of the diagnostic equipment developed within the framework of the VLEPP program. The electron energy at the accelerator output was to be 15 to 20 MeV which is sufficient to carry out the above programs. One of the major factors taken into account during development of the facility was limitation of the RF - power (20 - 25 MW) obtainable using the available klystron KIU-12. For creation of the multipurpose applications facility it was necessary to solve the following problems: - the injector choice and development in order to form intense electron beams with a small emittance and a broad variation of pulse width; - development of an accelerating section allowing to efficiently accelerate intense beams both in the steady-state regime and in the stored energy one; - design of the accelerated beam diagnostic facility; - design of the technological support and other control facilities. LIC accelerator complex was commissioned during 2 years ( 1991 - 1992 ). In 1994, the facility was shut down in order to reconstruct the injector and assemble experimental devices in the area of plasma physics. The work was renewed in 1995. This paper described LIC facility, as well as results of beam performance obtained during testes held in other locations. 2 ACCELERATOR DESCRIPTION LIC layout is given in Fig. 1. Its basic components are an RF-gun, the accelerating section, beam steering and its focusing elements, beam diagnostics, cooling water system and control elements. The RF system includes a klystron with the maximum power operation up to 25 MW, a set of waveguides, controllable phase shifter and attenuator. 2.1 The injector system It is well known that RF-gun is a injector element, which allows to create a multipurpose accelerator facility with a high-brightness beam. It is this version that has been chosen by us while designing LIC accelerator. Developed in 1991, the injector system comprises an RF-gun, focusing elements, beam steering elements and a laser-driver. The latter is a Nd:YAG laser, with a double-pass amplifier and a frequency-converter providing for operation at the 2-nd; 3-d and 4-th harmonics of the fundamental frequency (λ=1064 nm). The pulse energy at 7 ns duration is respectively equal to 60, 10 and 6 mJ. The driver-laser is equipped with a focusing and remote-control systems. The RF-gun, consisting of a single cavity E 010 resonator, was, basically, designed for operation in the nano-second current pulse regime within VLEPP program. In the meantime, the gun allowed operation with thermionic-emission cathodes and to perform studies at pulse duration of several microseconds. The gun resonator was equipped with channel to transport laser irradiation on to the cathode and frequency tuner. As photo-cathode, we used our tested BaNi pressed oxide [1], whose diameter was substantially lager, 14 mm. The gun experimental studies were done at the special stand. The cathode irradiated at λ=355 nm (pulse energy in 7 ns being 5 mJ) produced current of 11 A at the gun exit. Gun testing shoved that BaNi cathode with a larger area decreases electric reliability of the cavity. Besides, a decrease in the loaded cavity quality factor was 20%. All considered particle energy at the gun exit did not exceed 0.5 MeV which did not provide for the necessary capturing of electrons in the acceleration process at the accelerating structure with β =1. Considering this circumstance and in order to widen the scientific research performed on the accelerator, its injector part was upgraded in 1994. While designing the new injector, we had in mind the basic goal set ahead during work on the accelerator: creation of a multipurpose universal facility. While doing this, we took into account the fact that the optimum conditions of electron bunch - forming in photo - emission and thermionic RF-gun are different, and to create them into one the same cavity is a very tough proposition. On the other hand, the beam