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Chair experimental physics V "atom and laser physics" at the faculty of physics and astronomy at Ruhr University Bochum

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Nanosecond resolved ro-vibrational CO2 excitation measurement

The results in this dataset have been published in J. Phys. D: Appl. Phys. 54 (2021) 34LT02.
We report first ns-resolved absorption measurements of the ro-vibrational excitation of CO2.The high temporal resolution of 8 ns is made possible by a fast detector (rise-time 5 ns), sensitive in the mid-infrared region. The resolution is achieved by a slow temperature scan of a quantum cascade laser and a segmented data capturing scheme. A repetitively pulsed ns discharge in 10% CO2 + 90% He at 150 mbar and a repetition rate of 2 kHz is investigated. The evolution of the population densities of the different vibration modes as well as the associated vibrational and rotational temperatures within the discharge pulse of only 150 ns length are simultaneously determined and provide valuable insight into the CO2 excitation dynamics. A preferential excitation in the asymmetric vibrational mode is observed in the discharge phase shortly after the breakdown.

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Plasma Source Specification
Plasma Source Properties
The applied voltage by the DC high voltage generator is 2.25 kV with a voltage pulse length of 150 ns and a repetition rate of 2 kHz. The measured voltage at the powered electrode and current waveforms are supplied in the dataset in Fig 4. The voltage rise time from 10% to 90% amplitude is 15 ns. The measured peak voltage and current are 1.7 kV and 5.2 A, respectively.
Plasma Source Procedure
The discharge consists of two molybdenum electrodes with a cross-section of 20 × 1 mm placed vertically with a distance of 1 mm. Two glass plates are pressed tightly to the electrodes at the front and the back, yielding a well-confined discharge volume of 20 mm × 1 mm × 1 mm. Nanosecond voltage pulse generated by combining a DC power supply (Heinzinger, LNC 6000-10 neg) with a fast high-voltage switch (Behlke HTS-81) is applied to one electrode with the other one grounded.To protect the switch and increase the discharge stability, the current is limited by a 255 Ohm series resistor between the switch and cathode of the plasma reactor. A delay generator (Stanford Research Systems DG535) with internal triggering is used to trigger the switch with a repetition rate of f p = 2 kHz and an on-time of 150 ns. To allow easy discharge operation, the value of the repetition frequency is chosen within a range determined by the energy input to the system (to limit gas heating) and remaining seed electrons from the previous pulse (to ease discharge ignition). The incoming gas consists of 10% CO2 + He by mixing pure CO2 and He gas flow using two mass flow controllers (MKS instruments) with a total flow rate of 30 sccm. The pressure in the discharge chamber is monitored by a pressure gauge (Pfeiffer vacuum) and is kept constant at 145 mbar by fine adjusting the mechanical needle valve at the gas outflow.
Plasma Medium Name
Plasma Medium Properties
10% CO2+He gas mixture, total flow rate: 30 sccm, pressure: 145 mbar
Contact Name
Yanjun Du
Contact Email
Plasma Diagnostic Properties
A CW quantum cascade laser (Alpes) is used to scan the CO2 transitions in the wavelength range between 2288.4 and 2289.6 cm−1, enabling a simultaneous determination of the rotational temperature and the vibrational temperatures for both the symmetric and the asymmetric modes. After attenuation by a combination of a polarizer and a quarter-waveplate, the collimated light from the quantum cascade laser is guided into the discharge and finally focused by an off-axis parabolic mirror onto a fast IR detector (Vigo PVI-4TE-5, bandwidth 65 MHz). The signal from the detector is recorded by the PC-based oscilloscope. For the characterization of the laser wavelength, a silicon etalon (LightMachinery) with a free spectral range of 0.0176 cm−1 is introduced into the beam path by a flip mount.
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Yanjun Du