# I Questions about plasma physics

1. May 12, 2017

### mody mody

i study the electrostatic waves at plasma (as a beginner) and i come to some questions and hope to find a help

1) how could we ignore magnetic field (taking B =0) although we have oscillating charges (what particularly electrostatic wave means )

2) at studying ion acoustic they always assume that electron behave acrobatically ( w/k << vth) what does this mean ... and is the case such that we have both the waves (electron and ion waves) at the same time ??

my main reference is chen book for plasma physics ...

2. May 12, 2017

### Khashishi

1. Well, in general, there are many kinds of plasma waves which interact with the magnetic field. But there is one kind of wave that doesn't interact, and this is called an electrostatic wave or plasma oscillation. This wave has an oscillating electric field which is parallel to the magnetic field. Charged particles are free to move parallel to the magnetic field without any magnetic force, so the oscillatory motion in this direction is the same with and without magnetic field.

2. The word is adiabatically, not acrobatically. As far as I know, it means that the electrons are assumed to react instantly to the motion of the wave, so the electrons are always in a steady state distribution with respect to the fields. The electrons themselves are moving, but they are in orbits that cycle back and forth more quickly than the timescale of the wave. The ions, on the other hand, take some time to respond to the changing fields. What this means is that you can decouple the activity of the electrons and ions. The electron orbits are still changing over time, but a particular quantity that describes the electron orbits does not change.

The word adiabatic comes from the adiabatic invariant.
$J = \oint v dx = \oint \sqrt{\frac{2}{m}(W+e\phi)}$
Since the plasma is changing slowly compared to the motion of the electron, $J$ is approximately conserved.

If you have access, take a look at http://iopscience.iop.org/article/10.1088/0032-1028/12/12/003/meta

3. May 12, 2017

### mody mody

thank you .. things become more clearer

but i still have a question

at the same plasma .. could i have both electron and ion waves (although we didn't treat the electrons adiabatically at studying the electron waves ) ??!