P. Leprince

Axial electron density and wave power distributions along a plasma column sustained by the propagation of a surface microwave

The propagation of electron surface waves can be used to sustain long plasma columns. The relation between the axial distribution of the electron density observed along the plasma column and the corresponding power distribution of the surface wave that produces it is investigated. It is found that the electron density decreases almost linearly along the plasma column in the direction of the wave propagation. This is explained by assuming that the number of electrons produced over a given axial length is proportional to the wave power absorbed over that same length.

Generation and confinement of microwave gas-plasma in photonic dielectric microstructure.

We report on a self-guided microwave surface-wave induced generation of ~60 μm diameter and 6 cm-long column of argon-plasma confined in the core of a hollow-core photonic crystal fiber. At gas pressure of 1 mbar, the micro-confined plasma exhibits a stable transverse profile with a maximum gas-temperature as high as 1300 ± 200 K, and a wall-temperature as low as 500 K, and an electron density level of 10¹⁴ cm⁻³. The fiber guided fluorescence emission presents strong Ar⁺ spectral lines in the visible and near UV. Theory shows that the observed combination of relatively low wall-temperature and high ionisation rate in this strongly confined configuration is due to an unprecedentedly wide electrostatic space-charge field and the subsequent ion acceleration dominance in the plasma-to-gas power transfer.

Microwave discharge in H2: influence of H-atom density on the power balance

We investigate a source of H atoms generated by a low-pressure surface wave discharge (2.45 GHz). We study the influence of microwave power both on the discharge characteristics on the H atom density, which has been measured by actinometry. Dissociation levels of H2 are much higher (75%) at low microwave power than at high power (10%). Unlike what has been found in oxygen surface wave plasmas, discharge characteristics depend strongly on microwave power, due to an important coupling between discharge equilibrium and kinetics of the atomic hydrogen. These results are explained taking into account the effect of discharge tube wall temperature on atomic recombination. The wall recombination probability gamma is estimated as a function of the microwave power: it ranges from 6*10-3 to 6*10-2, which is very high in comparison with values determined previously under post-discharge conditions.

Validity of actinometry to monitor oxygen atom concentration in microwave discharges created by surface wave in O2‐N2 mixtures

The validity of actinometry to monitor oxygen atom concentration in O2‐N2 microwave discharges created by surface wave is investigated. The plasma is created with a gas flow in a quartz tube of inner diameter 16 mm at pressures in the Torr range. First, it is shown that the reliability of actinometry can be deduced from the longitudinal profile of the actinometry signal. Second, absolute concentrations of oxygen atoms are estimated from the experimental actinometry signal and agree satisfactorily with concentrations simultaneously measured by vacuum ultraviolet (VUV) absorption downstream from the plasma. Moreover, upon varying the nitrogen percentage (0%–100%), it is evidenced that the actinometry signal is proportional to the concentration measured by VUV absorption. Furthermore, it is evidenced that the oxygen dissociation rate is only 2% in pure oxygen plasmas, while it reaches 15% (433 MHz) or 30% (2450 MHz) for mixtures containing more than 20% of nitrogen. This drastic increase in [O] upon the addi...

Microwave discharges produced by surface waves in argon gas

Reports on an experiment in argon discharges produced by surface waves at 2.45 GHz. A self-consistent description of these discharges at low pressure (p<10 Torr) is presented. Characteristics such as the electron density, the maintaining electric field (E), the collision frequency for momentum transfer v and the mean power required for maintaining the electron-ion pair theta are determined as functions of the product of the pressure and the plasma diameter. In particular scaling laws are verified for v and theta . It is concluded that experimental surface-wave discharges behave similarly to positive-column discharges.

Characterisation of a low-pressure oxygen discharge created by surface waves

Low-pressure oxygen discharges created by surface waves are characterised. Three discharge characteristics, the electron-neutral collision frequency nu , the maintaining electric field Eeff and the power required to maintain one electron theta , are deduced from the measurement of the electron density. They are independent of the microwave power and are studied as functions of the tube diameter (8-52 mm), the pressure (0.05-2 Torr) and the excitation frequency (390 or 210 MHz). The concentrations of ground state and singlet molecular oxygen are measured by VUV absorption and the concentration ratio of singlet to ground state molecular oxygen is about 10%, whatever the microwave power and the pressure. The ground state atomic oxygen concentration is measured downstream from the discharge by VUV absorption. The dissociation rate increases with the microwave power but remains less than 5%. A power balance model in the discharge, i.e. calculation of theta , is performed and yields a relation between theta /N and Eeff/N (n being the total neutral density), which is in good agreement with the experimental results. Finally, similarity laws are shown in the form theta /N and Eeff/N (Nr), where r denotes the tube radius.

Wave propagation and diagnostics in argon surface-wave discharges up to 100 Torr

Deals with the study of argon surface-wave produced discharges as the effective collision frequency nu becomes larger than the excitation pulsation omega (either at low frequency or at high pressure). Both conditions have been studied in the experiment reported. Discharges have been produced in the 1-100 Torr range in capillary tubes with 210 MHz and 2450 MHz excitation frequency. First, the set of Maxwell equations and boundary conditions is solved taking into account the attenuation due to collisions. Then the nu / omega influence is discussed. Secondly a diagnostic method providing the axial density profile ne(z) and the collision frequency nu is reported. Finally the authors find scaling laws for these discharges.

Spectroscopic temperature measurements in a microwave discharge

We report temperature measurements in a low-pressure hydrogen microwave plasma. Translational temperatures both of H and of (using Doppler broadening), as well as the rotational temperature of , are simultaneously determined. It is first shown that the rotational temperature of the excited state is not in equilibrium with the translational temperatures of the neutral particles. Then, using a high-resolution Fourier transform spectrometer, we show that the H atom kinetic temperature is higher than the one. This result is interpreted in considering the mechanisms of relaxation of the hot H atoms, produced by electron impact dissociation of , in the molecules and on the tube walls.

Influence of the radial electron density profile on the determination of the characteristics of surface-wave-produced discharges

Until recently, surface wave discharges have been studied, both experimentally and theoretically, using the wave propagation constant calculated in the assumption of a flat electron density profile. This paper deals with determination of wave characteristics (wavenumber, attenuation and fields) taking into account a radial profile of the electron density. The consequences for the experimental diagnostics are presented. In the range of low electron densities, the authors show that some results deduced from experimental measurements are spoilt if the radial electron density profile is not taken into account. They present an argon surface wave discharge (390 MHz) at low pressure (about 1 Torr) in a 76 mm diameter tube. The influence of the radial electron density profile is studied on the following experimental results: electron density, effective electron-neutral collision frequency and the mean power needed to maintain an electron in the discharge.

Spatial investigation of a large diameter microwave plasma

A large diameter plasma (120 mm) is sustained in argon/oxygen mixtures, at low pressure ( approximately=1 Torr) by surface waves at 2.45 GHz. We spatially describe the electromagnetic field distribution as well as the species distribution from diagnostics such as electric antenna probing and emission spectroscopy in the three spatial directions (z, r, phi ). According to azimuthal measures ( phi ), we show that the discharge is mainly maintained by an hexapolar hybrid mode (m=3). Longitudinal measures (z) exhibit modulations which could be explained by a multimode propagation whereas the radial spectroscopic measures (r) strongly depend on the electric field of the surface wave. However, atomic oxygen densities, nearly 1%, exhibit a quite homogeneous profile in the three spatial directions.