STREAMER DISCHARGE MECHANISM SparkGap Townsendov vs. streamer mechanizmus Pressure (p), interelectrode distance (d) and overvoltage Kover = (U – Us)/Us , where Us je is the static onset voltage according to the Paschen curve. The p and Kover determine the discharge mechanism: glow discharge arc discharge Townsend Streamer mechanism mechanism → ← Ultrafast spark gap for sub-nanosecond HV switching for special radars Hydrogen spark gap pressurized up to 55 atm. : d/Ƭ = 0.45 mm /68 x 10-12 s = 7 x 108 cm/s Ion velocity ~ 105 cm/s Formation of narrow HV pulses using caoxial Blumlein forming line http://pulsedpower.de/pulsedpower_engineering.html Textové pole: (A speed of the wave propagation in coax lines is about two-thirds the light speed => 1m thin coax delays ~5ns) (A speed of the wave propagation in coax lines is about two-thirds the light speed => 1m thin coax delays ~5ns) (A speed of the wave propagation in coax lines is about two-thirds the light speed => 1m thin coax delays ~5ns) In the beginning of 20th century the visualization of electron avalanches was possible using the Wilson´s cloud chamber , a standard camera, and the HV pulse forming line:, PowerPoint Template PowerPoint Template Výsledok vyhľadávania obrázkov pre dopyt streamer field avalanche Guided ionization waves: Theory and experiments - ScienceDirect Calculation of streamer development in MPGDs in an axisymmetric ... Calculation of streamer development in MPGDs in an axisymmetric ... The anode-directed (negative) streamer propagates in the direction of the drift of fast electrons and usually is not important for the subsequet discharge. The positive streamer is very important for the subsequent discharge development since at its arrival to the cathode it creates the cathode spot similar to the cathode region of a glow discharge. Such cathode spot can be very quickly be transferred in a „hot“ cathode spot of an arc discharge The positive streamer head propagates in the direction opposite to the drift of fast electrons with the speed on the order of 108 cm/s. It can be understood as the phase movement of the point of the maximum el. el. field strenght. „Seed electrons“ initiating the avalanches in the streamer head vicinity are generated by photoionization. (The drift of positive ions can be neglected on the considered time scale !) Guided ionization waves: Theory and experiments - ScienceDirect Calculation of streamer development in MPGDs in an axisymmetric ... → → → „seed electrons“ Generated by the photoionization \begin{figure}\begin{center} \par\epsfig{file=eps/positive-streamer.eps, width=5in}\par\par\par\end{center}\end{figure} Šírenie čela adného streameru – pohyb oblasti max. intenzity poľa \begin{figure}\begin{center} \par\epsfig{file=eps/positive-streamer.eps, width=5in}\par\par\par\end{center}\end{figure} Obrázok, na ktorom je svetlo Automaticky generovaný popis Obrázok, na ktorom je snímka obrazovky Automaticky generovaný popis Propagation of the negative streamer by the drift of fast electrons can be enhanced by the photoionization Trigatron is the name for the additional trigger electrode to a spark gap Trigatron´s advantage: By a small discharge generated by a short on the order of 100V – 1 kV voltage pulse Vt we can ignite (i.e. switch) at well-defined time and Vg voltage value (on the order of 1 kV – 1 MV) SparkGap TRIGATRON: shutter-camera records Streak camera records : STREAK CAMERA \begin{figure}\begin{center} \par\epsfig{file=eps/positive-streamer.eps, width=5in}\par\par\par\end{center}\end{figure} poliach? The explanation: El. field: Light: \begin{figure}\begin{center} \par\epsfig{file=eps/positive-streamer.eps, width=5in}\par\par\par\end{center}\end{figure} Streak and shutter camera records: <= the secondary streamers Streak camera record in a 2 cm ambient air gap with an anode radius of 50 μm. The applied voltage and streak are 36.8kV and 20 ns, respectively. ICCD camera records made in a 1.3 cm ambient air gap with an anode radius of 80 μm, applied voltage of 35 kV at exposure times of 2 ns, part (b) . To generate the non-equllibrium plasma at near-atmospheric-pressures (important for many applications) it is necessary to avoid of „ (e) The filamentary glow to arc transition“ It can be done in the following types ot the non-stationary high-pressure discharges (or their cobinations) 1.Impulse discharges (impulse coronas, the discharges on TEA lasers) 2. 2.Corona discharges (DC or AC) 3.Dielectric barrier discharges ( one or several dielectric „barriers“ are situated on the lectrode surfaces, or in the gap) 1. Impulse corona discharge for plasmachemical applications (see the primary p and the secondary s streamers) https://www.intechopen.com/books/air-pollution-a-comprehensive-perspective/non-thermal-plasma-techn ic-for-air-pollution-control A rod electrode made of stainless steel, 0.5 mm in diameter and 10 mm in length was placed concentrically in a copper cylinder, 76 mm in diameter. Impulse corona discharg with a dielectric barrier on the anode surface Photograph of positive „impulse corona“ in needle – water gap 4 mm long. Typical voltage and current waveforms http://enviro.fmph.uniba.sk/index.php?link=research&topic=15 Obrázok, na ktorom je svetlo, sedenie, tmavé, osvetlený Automaticky generovaný popis Obrázok, na ktorom je mapa Automaticky generovaný popis STREAMER BREAKDOWN ALWAYS OCCURS IN THE FOLLOWING SEQUENCE OF EVENTS (a) The avalanche stage, wherein the streamer initiating charge in a localized region is formed by charges generated in a single avalanche or more often accumulated in a sequence of avalanches (b) The positive primary streamer initiation: after an initial delay, when the streamer initiating charge partially shields itself from the external field forming a ‘critical’ region of relatively dense plasma (1013–1015 cm−3 ) resulting in the primary positive streamer starts to propagate. (i) (c)The positive streamer propagation, where the primary streamer head propagates as a luminous spot of the diameter typically less than 1 mm with the velocity usually in the range 107 –108 cm s−1 followed by a less luminous streamer trail. (d) The streamer arrival to the cathode, forming an active glow-discharge type cathode spot, which is effectively producing the electrons by direct impact ionization in the cathode fall (i) (e) The filamentary glow to arc transition is often initiated by the growth of secondary streamers (i) (i) (i) (i) (i) (i) (i)