The purpose of this work is property investigation of quasi-stationary plasma formations with high energy content for practical applications in high thermal physics and diagnostic of materials under extreme conditions. Investigated plasma formations are the result of two plasma counter-flows interaction process which is based on high-current discharges of plasma accelerators of erosion type in vacuum. Shadowgraphs of colliding plasma flows were made using knife and slit method. As a light source a specially made argon flash lamp was used. Averaged electron concentration in the interaction area was calculated from intensity distribution of shadowgraphs. In order to perform a correct shadow display a light filter system with transmission peak at 547 nm was mounted in front of the CCD-camera. At this wavelength the relative intensity in plasma spectrum was low while in argon lamp spectrum it was near-maximum. (C) 2012 Elsevier Ltd. All rights reserved.