Tube diameter and wave frequency limitations when using the electromagnetic surface wave in the m=1 (dipolar) mode to sustain a plasma column

Summary Form only given, as follows. 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 f/sub m/ below which the discharge cannot be achieved; conversely, for a given operation frequency f, the tube radius must exceed some minimum value a/sub m/ for the plasma to be sustained. These minimum conditions required to obtain the discharge are observed to obey a scaling law of the form (af)/sub m/ approximately= constant, 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 that recovers the observed scaling laws determining the minimum tube radius and wave frequency values is proposed. It is further shown that using a surface wave to sustain a discharge in large-diameter tubes (>50 mm) at 2.45 GHz is bound to yield plasma columns with azimuthal inhomogeneities because the wave then propagates in the m=1 or higher order modes.<>