Low-pressure plasma generation inside slender tubes

Low pressure (<50mTorr) argon discharges inside tubes of a few millimeters in diameter have been studied by means of one- and two-dimensional particle-in-cell Monte Carlo collision simulations. Magnetically confined DC and microwave discharges have been sustained in a coaxial configuration. For DC discharges, the magnetic field needs to be strong enough to confine secondary electrons emitted from the cathode, i.e. the amplitude of the cycloidal motion described by secondary electrons has to be smaller than the discharge gap. For microwave excited discharges, the power absorption profile depends on the magnitude of the magnetic field. Power absorption beyond the sheath boundary in the plasma bulk and better confinement for magnetic fields below the ECR condition lead to maximum densities at ωc/ωrf∼0.5.