Experiments on plasma heating in open magnetic traps require a powerful electron beam with pulse length of 0.1–1 ms. Such a beam is expected to obtain in the source with a plasma cathode and high perveance multiaperture electron optical system. An appropriate technology is being developed at Budker Institute of Nuclear Physics (BINP), Novosibirsk. Here we introduce a prototype electron beam injector with the following design parameters: energy of electrons up to 150 keV, pulse duration of >0.1 ms and beam current up to a few hundred amperes. The injector is intended to operate in an external axial magnetic field of ~0.1 T. In this paper, the design of injector prototype is described and the first test experiments are presented.