An exhaustive experimental investigation of the conditions required to sustain a plasma column through the propagation of the m=1 mode surface wave has been conducted. It reveals that, given a discharge tube radius a, there corresponds a minimum frequency value fm below which the discharge cannot be achieved; conversely, for a given operating frequency f, the tube radius must exceed some minimum value am for the plasma to be sustained. These minimum conditions required to obtain the discharge are observed to obey a scaling law of the form (fa)m≂const., where the constant is independent of the gas nature and pressure. Theoretically, the dispersion equation of the m=1 mode wave shows no low-frequency cutoff. However, it is found that the specific dependence of the wave attenuation coefficient on the frequency and on the tube diameter can ultimately account for the observed limitations when the wave is used to sustain a plasma. A discharge stability criterion is proposed that recovers the observed scaling law determining the minimum tube radius and wave frequency values.
Determining the wavelength of a wave propagating in a medium nonuniform along the wave path implies using a measurement technique that provides enough spatial resolution to permit following up the wavelength variation resulting from such a nonuniformity. This paper presents a theoretical analysis of the respective merits of two phase sensitive methods used for that purpose, under both weak and strong wave attenuation conditions. Modifications to these methods are recommended which assure both accuracy and spatial resolution of the wavelength measurements. These conclusions are substantiated by numerical simulations of phase measurements along surface wave‐sustained plasma columns used as an example of axially nonuniform and lossy media.
Radial density distributions of excited atoms in plasma columns of helium, neon, and argon, sustained by a travelling electromagnetic surface wave, are examined as a function of frequency over the range 200 kHz – 2450 MHz. This investigation is conducted using an end-on measurement method. At low frequencies (<50 MHz), these radial distributions show a maximum at the axis (J0 Bessel-like behavior), whereas as frequency is increased beyond 50 MHz up to 2450 MHz, the radial distributions flatten and finally exhibit a minimum at the axis with a maximum close to the tube wall. Comparison with a DC positive column plasma, working under the same gas-pressure and tube-diameter conditions, is made as a function of cross-section average electron density. The surface-wave discharge operated in the microwave frequency range (>300 MHz) yields larger cross-section average densities for atoms in a metastable or resonant state, typically a factor of 2–3 at 1011 electrons∙cm−3. This result arises because the two types of discharges have different radial-density distributions for excited atoms.
The plasma column sustained by an electromagnetic surface wave under the free-fall regime, a still unexplored pressure domain for such a plasma, has been experimentally investigated. Because of the relatively large values of the frequency (2.45 GHz) and plasma diameter (56 mm), the wave propagates either in the dipolar (m=1) or quadrupolar (m=2) mode rather than in the azimuthally symmetric mode (m=0). The radial distributions of the electron density and temperature, obtained by means of electrostatic probes, show the existence of a resonant absorption mechanism of the wave electric field by the electrons. The measured value of the power loss per electron, theta , is found to be four to ten times larger than the value extrapolated from a diffusion model for the m=0 mode. Such a discharge is intended to supply plasma in a reactor operating with a multipolar magnetic confinement used for surface processing.
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