Spectroscopic study of the neutral gas temperature of silicon based DC MHCD in various gases close to atmospheric pressure

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Spectroscopic study of the neutral gas temperature of silicon based DC MHCD in various gases close to

atmospheric pressure

Sylvain Iséni, Ronan Michaud, Philippe Lefaucheux, Volker Schulz-von der Gathen, Goran Sretenovic, R. Dussart

To cite this version:

Sylvain Iséni, Ronan Michaud, Philippe Lefaucheux, Volker Schulz-von der Gathen, Goran Sretenovic, et al.. Spectroscopic study of the neutral gas temperature of silicon based DC MHCD in various gases close to atmospheric pressure. 13th Frontiers in Low-Temperature Plasma Diagnostics, May 2019, Baf Honnef, Germany. 2019. �hal-02131666�

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Spectroscopic study of the neutral gas temperature of silicon based DC MHCD in various gases close to atmospheric pressure

S. Iseni

¹

, R. Michaud

¹

, P. Lefaucheux

¹

, G. B. Sretenovi´c

²

, V. Schulz-von der Gathen

³

et R. Dussart

¹

1

GREMI–UMR 7344, CNRS/Universit´e d’Orl´eans, FRANCE.

2

Faculty of Physics, University of Belgrade, SERBIE.

3

Department of Experimental Physics II, Ruhr-Universit¨ at Bochum, ALLEMAGNE.

Motivation

Micro hollow gas discharges (MHCD) have been on high interest to produce highly ionized gas while keeping the gas temperature close to room

temperature. This study focuses on the accurate measurement of the gas temperature in and out the cavity, operated in different regimes, by means of space resolved optical emission spectroscopy. Two approaches are

applied depending on the gas mixture (He, Ar, N₂):

I analysis of the profile of resonant atomic lines taking in to account the Van der Waals broadening,

I analysis of the relative population distribution of ro-vibrational N₂(C-B) bands.

Limitations of the latter approach will be discussed specifically. Heat transfer and temperature gradient will be discussed with regard to the geometry and the material properties of the present MHCD design.

Source plasma – micro-cavity plasma (MHCD)

Si (500µm)

Ni (0.4µm) SiO₂ (8µm) Ni (0.8µm) ballast

resistor

38µm

50µm +-

V_s

rotational symmetry axis plasma

Experimental conditions:

I Powered in DC,

I Polarity: positive ou negative, . 0.2 µ^{A `}^{a 600} µ^A,

. 2 mW `a 150 mW.

I Gas pressure 0.26 atm `a 1.2 atm, I Gas: He / Ar / N₂,

I MHCD arrays, e.g. 32 × 32 micro-cavities.

Imaging optical emission spectroscopy

bar

+

-

DC power supply X-Y-Z

gas valve

pressure gauge

oscilloscope

power control system spectrometer

Horiba - JY TRIAX 550

imaging system

CCD MHCD

chamber

window

voltage

probes computer

R_ballast ϑ

0 0.5 1

dimension / μm

−150

−100 0 50 100 150

dimension / μm

−150 −100 −50 0 50 100 150

Front image of a single MHCD (100 µm diameter). Integration of

the whole spectrum.

Resonance broadening and rotational temperature of N₂(C-B) The pressure broadening contributes to the Lorentzian full width at half maximum (FWHM) of the line, w_L. At atmospheric pressure, w_L is

dominated by the resonance broadening with a minor contribution due to the van der Waals interactions. Thus, w_L ≈ w_R + w_vdw, with,

w_R = K(0, 1)r_e π

rg_G

g_Rλ²₀λ_Rf_RN, (1)

w_vdw = 8.18 · 10¹²λ²₀ α¯R¯²^2/5

T_g µ

^3/10

· N. (2)

A detailed analysis of the line profile is used to determine T_g.

With reasonable assumptions, T_g is often considered to be equal to the

rotational temperature (T_rot) of diatomic molecules, e.g. N₂. The relative spectral intensity (I_J^J00⁰ ) of transitions from two different vibrational states (J ⁰ → J⁰⁰) reads,

I_J^J00⁰ ∝ N_v⁰S_J^J00⁰

ν_v^v00⁰^JJ⁰⁰⁰

⁴

exp

− hc

k_BT_rotF_v⁰(J⁰)

. (3)

I Spectral simulation based on molecular constants of N₂(C³Π_u) determined for J / 25.

I Fitting routine using only four parameters and two optimization passes;

computed confidence interval of 95 %, e.g. uncertainties.

Temperatures inside/outside the MHCD – T_g vs T_rot

He667nm - res.+vdw.

He667nm - res. Pipa 2015 He_667nm - res.

Ar750nm - res.+vdw.

Ar750nm - res. Pipa 2015 Ar_750nm - res.

T g / K

200 300 400 500 600 700 800 900

I_MHCD / μA

0 50 100 150 200 250 300 350 400

N₂(C-B)(0-2) - front N₂(C-B)(1-3) - front N₂(C-B)(0-2) - side N₂(C-B)(1-3) - side

T rot / K

300 320 340 360 380 400 420 440 460 480 500

I_MHCD / μA

0 50 100 150 200 250 300 350 400

T_g measured with line profile analysis.

(0.66 atm).

Temperatures inside/outside of the MHCD in He gas at 0.66 atm.

I Significant thermal differences between He and Ar,

I Discrepancies between T_g and T_rot ⇒ is N₂(C-B) in equilibrium in He. . . ? Focus on the temperature gradient. . .

He_667nm - front He_667nm - side He_667nm - side T g / K

300 320 340 360 380 400 420 440

I_MHCD / μA

0 50 100 150 200 250 300 350 400

Ar_750nm - front Ar_750nm - side T g / K

300 400 500 600 700 800 900

I_MHCD / μA

0 50 100 150 200 250 300 350 400

T_g in He (0.66 atm). T_g in Ar (0.66 atm).

I T_g front corresponds to the light from the negative glow (the brightest ionized volume) above the cathode sheath,

I Si substrate acts as a heat sink while the SiO₂ plays the role of a thermal isolation with the anode surface.

Comparison T_rot N₂(C-B) in MHCD and in guided ionization waves

933 mbar 666 mbar

267 mbar

N2(C-B)(0-2) N2(C-B)(1-3) N2(C-B)(2-4)

T rot / K

250 300 350 450 500 550

I_MHCD / μA

0 50 100 150 200 250 300 350

0-0 0-1 0-2

1-0 1-2 1-3

T rotN 2(C-B) \ K

300 325 400 425 450

pixel \ #

0 10 20 30 40

T_rot in pure N₂ MHCD. T_rot in He guided ionization waves at atmospheric pressure (APPJ).

I T_rot in pure DC MHCD and in guided ionization waves. . . T_rot ≈ T_g? Concluding remarks

I Higher T_g and T_rot outside the cavity than inside ⇒ heat flux disturbed by SiO₂ layer,

I Closer to atmospheric pressure, is T_rot of N₂(C-B) in equilibrium in He discharge. . . ?

I Iseni S, et. al., 2019 On the validity of neutral gas temperature by emission spectroscopy in micro-discharges close to

atmospheric pressure, PSST, 2019, in press.

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