The electric field in the He:N_2 nanosecond Atmospheric Pressure Plasma Jet (ns-APPJ) is studied using Electric-Field Induced Second Harmonic generation (E-FISH) technique. It is shown that the calibration obtained with a DC voltage applied to the discharge cell may lead to incorrect results of the electric field measurements. It is proposed to use nanosecond high voltage pulses at low repetition rates for the calibration instead of a DC voltage. The temporal development of the electric field in the discharge at different distances from the cathode is measured with high temporal (100~ps) and spatial (50~µm) resolution. An electric field profile structure similar to the one in streamers or ionization fronts is observed. The velocity of the propagation of the falling edge of the ionization front is determined as 0.85x10^6 m/s. The validity of the local field approximation, important for modeling of these kind of discharges, is confirmed for the present conditions based on time and space derivatives of the measured electric field. The temporal evolution of the electron density is obtained by the measured electrical current and the time resolved electric field measurement combined with the electron mobility calculated with BOLSIG+.
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DC voltage of 4kV is used to feed HV pulser. 1~kOhm resistor is installed parallel to the discharge cell. The voltage rise time from 10% to 90% amplitude is 4~ns, maximum amplitude is 3650~V. The rise time of the electric current is 2~ns and the maximum value is 33~A. The discharge is generated between two 1-mm flat Mo electrodes with a length of 20 mm mounted between two glass plates at 1 mm distance High voltage pulses of negative polarity are generated at a repotiion rate of 1 kHz by a hme made pulser based on a fast high voltage switch with on-time of 10 ns (Behlke HTS 80-12-UF) fed by DC voltage
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A He:N_2=5:1 gas mixture is supplied through an orifice in one of the glass plates at the center of the inter-electrode gap. A constant gas mixture content and a total flow rate of 60~sccm are maintained by two mass-flow controllers (MKS Instruments). The discharge cell is installed in a vacuum chamber to control the atmosphere and keep a constant pressure of 900~mbar during the measurements. The gas mixture is supplied at room temperature of 295~K.
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He:N_2=5:1 gas mixture, total flow rate: 60~sccm, pressure: 900~mbar
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The emission of a Nd-YAG laser (EKSPLA SL234) at 1064~nm with pulse duration of 100~ps is used to provide sub-ns time resolution. Two polarizers and a half-wave plate for 1064~nm are used to tune laser pulse energy. The second half-wave plate is installed in order to rotate the polarization of the laser beam relative to the electrodes. A red colored glass filter is used to suppress light at the second harmonic generated in the optical elements due to high laser intensity. the beam is directed along the electrodes. After the light is focused to the middle of the discharge gap along the electrodes in the plane located at half of the electrode thickness. The beam is collimated and the second harmonic is separated from the fundamental one by a dichroic mirror. A blue colored glass filter eliminates the residual light at 1064~nm, and, finally, a laser line filter with center wavelength at 532~nm and FWHM of 1~nm (Thorlabs FL532-1) is installed at the entrance of a photomultiplier tube (PMT; Hamamatsu H11901-210). Fast photodiode (Thorlabs FDS015) with rise time of about 200~ps is used to record timestamp of the laser pulse relative to the discharge. The time of light propagation from the diode to the discharge cell and the delay of the signal propagation in the BNC cable are taken into account to determine the time shift between E-FISH signal and signal from high voltage and current probes. Then a precise synchronization is performed by measurement of a Laplacian field at conditions without ignition of the discharge. The energy of the laser pulse is monitored by another photodiode (Thorlabs DET10A), which has a bigger area and, thus, is less sensitive to pulse-to-pulse laser profile variation. A glass diffuser plate is installed in front of the diode to additionally reduce the influence of possible laser profile instabilities.
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