The space-time-averaging procedure and modeling of the RF discharge

Abstract. An analysis is presented of the effect of the RF frequency on the active media of CO2 waveguide lasers. It is found that the characteristics are improved with increasing RF frequency because the space charge sheath width decreases with increasing excitation frequency. We also found that the sheath width decreases with the discharge current; this fact was never discussed before. The higher the exciting frequency the higher is the maximum input power of the discharge in the stable low current mode. It is attractive to extend the input power while keeping the discharge in this mode. Finally, a stabilizing excitation technique is described for the inherent unstable region of the discharge.It is well known that the best performance of CO 2 waveguide lasers is obtained with RF excitation [1][2][3][4][5][6]. The choice of the used excitation frequency was mainly determined by practical arguments like the available power supplies. Its impact on the laser performance has not been studied thoroughly. So far this problem received little attention. There are two main points of view. There is the opinion that a change of frequency influences only the properties of the quasi-neutral plasma of the active medium [-2,7]. The other one is that the frequency changes the thickness of the charge sheaths near the electrodes so that the energy deposition into the quasineutral plasma and consequently the laser output are influenced [8,9]. The theoretical background for these considerations was discussed either phenomenologically or qualitatively.In a previous paper we presented the laser characteristics of the CO2 waveguide system with RF discharge [5]. The present paper, based on the previous model, will analyse the mechanism of laser excitation as it depends on various parameters, especially on the RF field frequency. Initial assumptions and system descriptionThe system that will be analysed theoretically is our experimental device [4,5] having a waveguide discharge volume of 2.25 × 2.25 x 370 mm 3. The used gas mixture is CO2-Nz-He (1:1:8) at a pressure of 100 Tort. The RF voltage is by means of parallel shunt inductors homogeneously applied to the two parallel electrodes. The field distribution in the discharge is calculated by solving the Poisson equation with the appropriate boundary conditions determined by the oscillating voltage over the electrodes. Because of the homogeneity of the RF discharge the problem is treated one-dimensionally with the coordinate x perpendicular to the electrode surfaces. The Boltzmann equation for the electron energy distribution is then solved by using the relevant cross-sections for the electronic excitations, vibrational excitations, dissociation of the CO2 molecule and the ionisations of N2 and CO2. The discharge heat is removed through both electrodes and through the ceramic side walls of the cavity. The considered frequencies of the electric field are in the range from 62 to 233 MHz. The relaxation frequency of the electron drift velocity is assumed to be larger than the frequency of ...

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“…The criterion for a stable stationary current in the electrical circuit with constant voltage of the power supply is [16]:…”

Section: Influence Of Frequency On the Current Stability In The Electmentioning

confidence: 99%

Influence of the electric field frequency on the performance of a RF excited CO 2 waveguide laser

Ochkin¹,

Witteman²,

Ilukhin³

et al. 1996

Applied Physics B: Lasers and Optics

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“…An explanation of this phenomenon is given in Section IV. The analytical solution for the electron energy distribution function of electrons is presented in Section V. We performed self consistent calculations of RFC discharges over a wide range of pressures, and power densities according to the fast modeling techniques, developed in [5] and [6].…”

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confidence: 99%

“…The approximations have been discussed in detail in [5] and [6], and we reproduce them briefly below.…”

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confidence: 99%

“…The ion continuity equation averaged over the RF period can be used [5], [6] ( 4) where is time-averaged ion flux, and is time averaged ionization rate. Under the conditions that half of the discharge gap and the sheath thickness exceed the ion mean free paths, ion motion can be described by the drift-diffusion approximation is the ambipolar diffusion coefficient.…”

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confidence: 99%

“…The ionization by gamma electrons is governed by the flux of electrons , electrons move quickly through the sheath toward the plasma and their flux increases. Integrating the flux starting from the electrode results in exponential flux multiplication [6] ( 6) where is the coefficient of secondary ion-electron emission, the coefficient is a function of the electric field of the ionic space charge (3). Equation (6) utilizes a local approximation for electrons.…”

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confidence: 99%

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