LOCAL TRANSIENT PHENOMENA INDUCED IN AN INERT GAS PLASMA BY A SHORT PULSE V. KREJC:f Institute of Physics, CzechosL Acad. Sci., Prague*) G. POPA University A. I. Cuza, lasi, Romania A microsecond periodic pulse was applied on thin probes in a positive column of a neon or helium glow discharge. The local transient phenomena provoked by this pulse were detected by a photomultiplier and presented in a time-distance display. They are qualitatively described and explained. The results are in good agreement with previous theories. 1. INTRODUCTION In theories dealing with the wave of stratification and other phenomena, which can be provoked in a glow discharge plasma by a short local change of plasma para- meters, it is usually supposed that the initial disturbance has the form of a Dirac pulse [1, 4]. However, as it is very difficult to realize a very narrow and very short disturbance of the positive column experimentally, the verification of such theories is based mostly on experiments, in which the region of the initial disturbance has the length of the order of the tube diameter. In such experiments the initial stages of the transient phenomena predicted by the theories cannot be observed, since they are usually narrower than the tube diameter and are wiped off by the broad pulse. In the present paper experiments are described in which the initial pulse was lead on a pair of thin probes reaching to the axis of the tube. The presence of the probes caused a minor permanent disturbance of the positive column. However, the short voltage pulse applied on the probes changed the plasma parameters in a very small region only and the following transient processes exceeded in amplitude the permanent pattern caused in the plasma by the presence of the probes. Our experiments cor- respond therefore better to the theoretical initial conditions, than experiments with an external electrode and reveal some phenomena hitherto unobserved. 2. EXPERIMENTAL ARRANGEMENT AND RESULTS The tube used in most of our experiments was about 190 cm long and 8 cm thick. It had cylindrical nickel electrodes and was filled with spectrally pure neon or helium. The platinum probes 0.1 mm thick situated both about 40 cm from the cathode were *) Na Slovance 2, Praha 8, Czechoslovakia. Czech. J. Phys. B 23 (1973) 161