Diagram of the experimental setup.
Measured peak circuit current ( ) and discharge initiation delay ( ). The initiation delay was measured as the delay of the initial rise of the current pulse from the time of peak displacement current. Error bars represent the standard deviation from three measurements.
Streamer velocity measured as a linear fit to the region of quasi-steady-state propagation outside the glass capillary. The raw data were acquired from short-lived atomic helium transitions at 706.5 nm and 587.6 nm marking the streamer head in 0.5 mm steps. Error bars represent the standard deviation from three measurements.
Temporal profiles of (a) He( ) and (b) Ar(1s 5) densities (solid line). The He( ) profile is shown in a helium plasma jet and the Ar(1s 5) profile is shown with a 0.6% Ar admixture. The measured circuit current (dotted line) and indicated atomic emission (dashed line) line are superimposed. The emission profiles are shown with arbitrary units. The signal delays were all measured relative to the time of peak displacement current marking the rise of the voltage pulse after the respective propagation delays were subtracted from each signal. Note the difference in timescales.
Peak number density of He( ) ( ) and Ar(1s 5) ( ) as a function of Ar admixture. The densities were corrected for quenching using the measured logarithmic decay rates. These are lower bound estimates of the absolute density per the discussion in Sec. II .
Quenching rates for the He( ) metastable as a function of Ar concentration. The fitted rate coefficient is valid up to 2% Ar admixture.
Contributions to the total Ar(1s 5) metastable density from the active discharge ( ) and afterglow ( ) components as a function of Ar admixture. The densities were corrected for quenching using the measured logarithmic decay rates.
Energy level diagram representing important atomic and molecular species excited in a He-Ar discharge. Solid arrows indicate conversion reactions that can ultimately populate the Ar(4s) sublevel which contains two metastable states 1s 3 and 1s 5. Dotted arrows indicate loss channels thatdepopulate the Ar(4s) sublevel, including radiative loss from resonant Ar(4s, 1s 2) and Ar(4s, 1s 4) transitions.
Relevant data for transitions used in laser absorption spectroscopy. is the center wavelength of the primary transition. For the He( ) transition, the subscripts a and b , respectively, refer to the J = 2 and J = 1 sublevels. is the coefficient for spontaneous emission, and are ratios of statistical weights, and is the separation between overlapping components, if present.
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