Browsing by Author "Varushkin S."

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    Current-driven ion-acoustic and potential-relaxation instabilities excited in plasma plume during electron beam welding
    (2014-01-01) Trushnikov D.; Mladenov G.; Belenkiy V.; Koleva E.; Varushkin S.
    Many papers have sought correlations between the parameters of secondary particles generated above the beam/work piece interaction zone, dynamics of processes in the keyhole, and technological processes. Low- and high-frequency oscillations of the current, collected by plasma have been observed above the welding zone during electron beam welding. Low-frequency oscillations of secondary signals are related to capillary instabilities of the keyhole, however; the physical mechanisms responsible for the high-frequency oscillations (>10 kHz) of the collected current are not fully understood. This paper shows that peak frequencies in the spectra of the collected high-frequency signal are dependent on the reciprocal distance between the welding zone and collector electrode. From the relationship between current harmonics frequency and distance of the collector/welding zone, it can be estimated that the draft velocity of electrons or phase velocity of excited waves is about 1600 m/s. The dispersion relation with the properties of ion-acoustic waves is related to electron temperature 10 000 K, ion temperature 2 400 K and plasma density 1016 m-3, which is analogues to the parameters of potential-relaxation instabilities, observed in similar conditions. The estimated critical density of the transported current for creating the anomalous resistance state of plasma is of the order of 3 A·m-2, i.e. 8 mA for a 3-10 cm2 collector electrode. Thus, it is assumed that the observed high-frequency oscillations of the current collected by the positive collector electrode are caused by relaxation processes in the plasma plume above the welding zone, and not a direct demonstration of oscillations in the keyhole. © 2014 Author(s).
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    Heat distribution simulation in electron-beam surface modification of 316L stainless steel samples
    (2020-06-02) Koleva E.; Tsonevska T.; Koleva L.; Vutova K.; Mladenov G.; Naplatanova M.; Trushnikov D.; Varushkin S.
    A time-dependent heat model is implemented for the simulation of the heat distribution in 316L stainless steel samples during electron-beam irradiation. The influence is studied of the electro-beam power and processing time on the temperature field produced in the samples. The simulation results are compared with real experimental data of electron-beam surface modification of 0.5-mm-thick 316L stainless steel samples using the ELIT-60 installation in the Institute of Electronics BAS. The simulation and the experimental results show good agreement.
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    Using bremsstrahlung X-Ray for positioning of the filler wire during electron beam surfacing
    (2018-10-19) Varushkin S.; Trushnikov D.; Koleva E.; Shamov D.
    This work aims to assess the bremsstrahlung X-Ray signal from the processing zone obtained by the use of a solid wire during electron beam surfacing. Applications of the bremsstrahlung X-Ray signal in the control systems allow positioning of the filler wire. During the experiments, a scintillation detector based on monocrystalline iodine cesium and silicone photomultiplier was used for registration of the X-Ray signal. Circle oscillations of the electron beam create the informative component. Synchronous storage method was applied for mathematical treatment of the recorded signal.