VI Int. Workshop on Microwave Discharges: Fundamentals and Applications

September 11-15, 2006, Zvenigorod, RUSSIA

Hot anIsotropic Electron population in A Low-Pressure COAXIAL Microwave Discharge

Simon Letout, Philippe Leprince, Lionel Teulé-Gay, Caroline Boisse-Laporte

Laboratoire de Physique des Gaz et Plasmas, Univ. Paris-Sud XI, Orsay, France.

Surface wave (SW) discharges are used to sustain overdense plasmas. Local plasma resonances can occur wherever the electron density induces the condition pe (plasma frequency) = hf (wave frequency), leading to a significant enhancement of the electric field component, parallel to the local density gradient. Their presence can play a major role in the maintenance of low-pressure SW discharges by generating hot electron fluxes, through a non-collisional (transit time) heating mechanism in the narrow resonance peaks. The latter eventually constitute a mean of resonance detection, otherwise too narrow for direct observation. The existence of these resonances has been discussed in the case of large-area SW discharges [1] and the observation of high-energy electrons was reported in [2].

This paper investigates the development of electron-plasma resonances within a coaxial SW (2.45 GHz) discharge. The coaxial structure consists of a copper tube (=10mm) surrounded by a quartz tube (=30mm), which acts as a propagation surface for a TM00 SW mode. A low-pressure (<100 mTorr) argon plasma is created outside the quartz tube, and is delimited by a metallic cylinder (=150mm). In addition to the expected end-of-column axial resonance [1], 1D self-consistent calculations [3] report the existence of resonances associated to radial density gradients at the plasma lateral boundaries (near both the quartz tube and the metallic shield). Directional planar probes were used to investigate the electron population, considering the anisotropy induced by the resonant SW field. Probe characteristics exhibit a significant increase in the electronic current over a wide range of probe potentials, depending on the radial position and the observation direction. Such behaviour reveals the presence of anisotropic high-energy electron populations. Extra probe currents were simulated by additional electron populations, assuming 1D-drifting Maxwellian velocity distribution functions [4]. Experimental results yield drift velocities up to 25 eV and density ratios of only a few 10-2, which is coherent with the presence of an energetic electron beam.

References

  1. I. P. Ganachev, et al. Plasma Sources Sci. Technol., 2002, 11, A178.
  2. J. Kudela, et al. Appl. Phys. Lett., 2000, 76, 1249.
  3. L. L. Alves, et al. Proc. MD5 Ed. A. Ohled INP Greifswald: 2003, 90.
  4. W. R. Hoegy, et al. Rev. Sci. Instrum., 1999, 70, 3015.