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The initial stage of prebreakdown cathode processes is studied with the help of the shadow method and electrooptical Kerr effect with laser light exposure of ≈ 3 ns. Experiments are performed by using triangular voltage pulses with time rise of ≈ 1 μs, time decay of ≈ 0.1 μs and total width of < 2 μs. The distance between electrodes 50 mm in diameter is 3 - 5 mm. Field strength varies from 275 kV/cm up to 500 kV/cm. Initial separate optical heterogeneities of size of 50-100 μm are registered at E ≥ 380 kV/cm. Spherical density disturbances with epicenter placed over the cathode are registered when the cathode "bush" reaches the size of ≥ 100 μm. In front of the "bush" the enhancement of electric field exceeding the average field several times in the gap is registered. However, fast discharge is developed from anode. Experiments performed without restriction of voltage pulse duration at E ≈ 300 kV/cm show that within 6 - 8 μs cathode disturbances increase up to the size of 2 - 4 mm and initiate the breakdown. The EHD model of subsonic cathode streamer development is considered. [ABSTRACT FROM AUTHOR]