Influence of the excitation frequency on the density of helium metastable atoms in an atmospheric pressure dielectric barrier discharge

Abstract

Diffuse dielectric barrier discharges in atmospheric-pressure helium can be sustained over a wide range of excitation frequencies (from, but not restricted, 25 kHz to 15 MHz). The aim of the present paper is to identify the specific characteristics of the discharge modes that can be sustained in this frequency range, namely, the atmospheric-pressure Townsend-like discharge (APTD-L) mode, the atmospheric-pressure glow discharge (APGD) mode, the Ω mode, the hybrid mode, and the RF-α mode. This is achieved experimentally, by measuring the density of helium metastable atoms, which are known to play a driving role on the discharge kinetics. This density is measured by means of two absorption spectroscopy methods, one using a spectral lamp and the other one using a diode laser as a light source. The first one provides the time-averaged atom densities in the singlet He(21S) and triplet He(23S) metastable states, while with the second one we access the time-resolved density of He(23S) atoms. Time-averaged measurements indicate that the He(23S) density is relatively low in the APTD-L, the Ω and the RF-α modes (<4×1016 m−3) slightly higher in the APGD mode (2–7×1016 m−3), and still higher (>1×1017 m−3) in the hybrid mode. The hybrid mode is exclusively observed for frequencies from 0.2 to 3 MHz. However, time-resolved density measurement shows that at 1 MHz and below, the hybrid mode is not continuously sustained. Instead, the discharge oscillates between the Ω and the hybrid mode with a switching frequency about the kilohertz. This explains the significantly lower power required to sustain the plasma as compared to above 1 MHz.

Time and space-resolved experimental investigation of the electron energy distribution function of a helium capacitive discharge at atmospheric pressure

“…Using the rate constant k 2 = 2 × 10 −46 m 3 s −1 [26] and integrating the cross section for the electron impact excitation of the metastable from the ground state [27], T hot can be found for a given set of n g , n e and n He * . Using values of n e obtained from electrical measurements (similar measurement method as in [18]) and n He * from absorption measurement (similar to those reported in [19]) we are able to sample the high energy portion of the EEDF ( 19.8 eV).…”

“…For this purpose, the generalized distribution is estimated with the help of the space and time-resolved T warm from figures 2 and 3 together with space and time-resolved T hot . As our measurement methods for n He * and n e (described in [19] and [18] respectively) are not space and time-resolved, T hot is obtained from extrapolation of our data according to the time and space variations of n He * and n e reported in similar (helium atmospheric-pressure CCRF) discharges [14,31]. This was done by, first estimating the difference between the reported maximum (and minimum) value and the reported average value, then proportionally applying the difference to our measured averaged values of n He * and n e .…”

“…Quenching reactions and other radiative decay reactions are also taken into account [16]. The number density of helium metastable atoms used for detailed analysis of OES, measured with the help of laser absorption spectroscopy (not reported here), are the same as those reported in a previous work [19].…”

Systematic investigation of the effect of N2 admixture ratio on barrier discharge in helium

“…The presence of a layer of insulation in front of the electrode reduces avalanche multiplication, prevents arc formation, and thus stabilizes the discharge. APNP generated by DBD in a parallel electrode configuration using helium [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17], nitrogen [18][19][20][21][22], and He-N 2 mixtures [23][24][25][26][27][28][29][30] as the working gas has been extensively studied. Discharge is stable for a limited set of input parameters [17].…”

Ultra-high-resolution optical absorption spectroscopy of DC plasmas at low pressure using a supercontinuum laser combined with a laser line tunable filter and a HyperFine spectrometer

“…While offering generally excellent signal-to-noise ratios as well as spatial and temporal resolution, this technique provides only relative concentration unless a complex calibration is performed [18][19][20][21][22]. Similarly, wavelength-resolved optical absorption measurements using a tunable diode laser, also known as tunable diode laser absorption spectroscopy (TDLAS), allows for the determination of the line-of-sight averaged number density of absorbing species [2,8,[23][24][25][26][27][28][29][30][31]. In this case, the loss of light intensity across a plasma volume due to absorption from select states is directly measured.…”