24P121-R
Basic plasma experiment in DC plasma device- Landau damping of ion acoustic waves
DCプラズマ装置におけるイオン音波の基礎実験
Y.Nishimura (西村泰太郎)
Institute of Space, Astrophysical, and Plasma Sciences, National Cheng Kung University
1 University Road, Tainan 70101, Taiwan
Landau damping of ion acoustic wave (IAW) in a DC plasma device is studied. The ion acoustic wave isgenerated by an excitation grid and the wave amplitude is observed in space by spatially distributed probes.
Plasma parameters are in the range of 109 cm-3 for the density and a few electron volts for the electrontemperature. When the hydrogen impurities are added to argon plasmas the ion acoustic wave damps due tothe increase of the particles with velocities close to the phase velocity of IAW.

1. Basic experiments at educational environment
Wave-particle interaction [1,2] plays animportant role in regulating microscopic andmacroscopic instabilities in collision-less plasmas.
In this work, Landau damping of ion acoustic wave
in a DC plasma device is studied. We aim at
education and training of the graduate students [3].
In Ref.[3], the ion acoustic wave is generated by anexcitation grid (with a real frequency “1”) and theLandau damping is observed in space by spatiallydistributed probes. The background argon pressure
is set to order of m torr. Plasma parameters are in
the range of ~10 9 cm-3 for the density and a few
electron volts for the electron temperature.
Damping of ion acoustic waves with and withouthydrogen impurities are investigated. When thehydrogen impurities are added to argon plasmas, thewave damps significantly (since the number of theparticles with velocities close to the phase velocityof IAW increase, see Fig.1).
2.Discussions
The DC plasma device consists of a metal
vacuum vessel of a cylindrical shape. Relatively
inexpensive experimental setting are considered(see e.g. Ref.[4], the Soup-pot experiment uses
permanent magnets to create cusp fields in
improving confinement) at our educational
environment. Landau damping in itself is areversible process in the absence of collisions. Now,
we consider whether we can generate plasma echoes(which is feasible in the DC plasma setting) byexciting the “2” wave at the second excitation grid.
Possible effects of Coulomb collisions on plasmaechoes [6,7] (both by increasing the ion densities
and the neutrals), which can significantly alter thedistribution function, and thus the damping process,are also discussed.

Fig.1. A conceptual diagram of the velocity
distribution of argon plasmas and hydrogen plasmas.
Due to a large population of hydrogen plasmas at“v=vthA ’’, damping can be enhanced significantly.
Acknowledgments
This presentation is supported by National Cheng Kung
University and Taiwan’s (R.O.C.) Top University
Projects, D100-32007 and D100-32001. The author
thanks Professor Yasushi Nishida and Professor Eichiro
Kawamori for discussions.
References[1] L. D. Landau: J. Phys. (U.S.S.R.) 10 (1946) 25.
[2] J. Dawson: Phys. Fluids 4 (1961) 869.
[3] Based on the author’s experience at the University of
Wisconsin-Madison: “Ion acoustic wave damping in
A DC Plasma Device” Y. Nishimura et al.: (1996)
unpublished.
[4] R. Limpaecher and K.R. Mackenzie: Rev. Sci.
Instrum. 44 (1973) 726.
[5] Relatively large plasma chambers are existent aswell at NCKU.
[6] D.R. Baker, N.R. Ahern, and A.Y. Wong: Phys. Rev.
Lett. 20 (1968) 318.
[7] J.H. Malmberg, C.B. Wharton, R.W. Gould, and T.M.
O’Neil: Phys. Rev. Lett. 20 (1968) 95.