Computer Physics Communications 177 (2007) 72–75
Low-pressure plasma generation inside slender tubes
F. Iza, J.K. Lee∗
Department of Electronics and Electrical Engineering, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea
Available online 28 February 2007
Low pressure (<50 mTorr) 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.
© 2007 Elsevier B.V. All rights reserved.
Keywords: ECR plasma; Coaxial magnetron; Magnetized discharge; Inner wall; Slender tube
Being able to plasma-treat the inner wall of slender tubes is
desirable to improve the performance of tubes and other instru-
ments in biomedical and industrial applications [1,2]. For ex-
ample,treatmentand coating of catheters can improvetheir bio-
compatibility and fouling properties. Similarly, a coating can
improve the lifetime of pipes that will carry corrosive materials.
Various techniques/configurations have been used in the past
to treat the inner wall of tubes. Electro-explosion , ion beams
and movable targets , external discharges , plasma im-
mersion ion implantation , and magnetized discharges 
provide means of treating the inner wall of slender tubes with
different performance in terms of repeatability, uniformity and
This paper studies low-pressure (<50 mTorr) argon dis-
charges inside tubes of inner radii on the order of millimeters.
The discharges are sustained between the tubes and auxiliary
wires introduced inside the tubes, i.e. coaxial configuration.
A detailed description of experimental set-ups can be found in
[8,9]. One- and two-dimensional particle-in-cell Monte Carlo
(PIC-MCC) collision simulations have been performed to study
the plasma characteristics under various magnetic field intensi-
E-mail address: firstname.lastname@example.org (J.K. Lee).
ties and driving schemes. PIC-MCC simulations capture parti-
cle kinetics without the assumptions required in fluid models
and have been successfully used in studies of fundamental and
applied plasma physics . The simulation results provide
a new insight into the underlying physics and are in good agree-
ment with experimental data.
2. Simulation model and device description
The discharges are sustained in a coaxial configuration. The
tube being treated acts as the outer electrode and a thin wire
introduced inside the tube as the inner electrode. For a sputter-
coating application, the thin wire is made of the coating ma-
terial and it is negatively biased so that the ion bombardment
sputters it. DC, pulsed-DC and microwave discharges can be
used to coat metallic and dielectric tubes [8,9]. In order to
effectively accelerate ions against the inner electrode (target)
operation at low pressure is required. Typically, this pressure is
below 50 mTorr and therefore magnetic confinement is needed
to limit the particle loss to the tube walls.
Simulations results are obtained using XPDC1 , an
open-source one-dimensional (1d3v) PIC-MCC code, and
a two-dimensional (2d3v) axisymmetric PIC-MCC plasma
simulator (APPS) developed by the authors. APPS assumes
azimuthal symmetry and solves for the radial and axial pro-
files. A multigrid solver is used to determine the electrostatic
0010-4655/$ – see front matter © 2007 Elsevier B.V. All rights reserved.