How can a plasma exhibit both quasineutrality and collective motion? Since, over a Debye Length $\lambda$, very small compared to the characteristic length $L$ of a plasma, a potential due to a source charge is essentially screened, how can plasma particles communicate over long distances and establish a collective behavior?
Let me cite the following statements by F.F. Chen in "Introduction to Plasma Physics and Controlled Fusion", Springer 2016:
"If the dimensions $L$ of a system are much larger than $\lambda$, then whenever local concentrations of charge arise or external potentials are introduced into the system, these are shielded out in a distance short compared with $L$, leaving the bulk of the plasma free of large electric potentials or fields. (...) It takes only a small charge imbalance to give rise to potentials of the order of $KT/e$. The plasma is “quasineutral”; that is, neutral enough so that one can take $n_i\approx n_e\approx n$, where $n$ is a common density called the plasma density, but not so neutral that all the interesting electromagnetic forces vanish."
So, is collective motion due to these electromagnetic forces originated from the "small charge imbalance"? Or is it a different mechanism, for example the result of many short-range interactions?
 A: 
So, is collective motion due to these electromagnetic forces originated from the "small charge imbalance"? Or is it a different mechanism, for example the result of many short-range interactions?

The electrostatic shielding within a Debye sphere results in the plasma behaving like a fluid, i.e., the equivalent of a fluid parcel would be something like a Debye sphere.  It is collective because an external electromagnetic force acts on all of the charged particles.  Yes, you can apply an electric field externally with a scale length larger than the Debye length.  The Debye length is the characteristic scale over which positive and negative charges are balanced.  The Debye length does not represent the longest scale length of any electric fields within the plasma.  For instance, all electromagnetic waves (of which I am aware) have wavelengths longer than one Debye length (e.g., Langmuir waves have wavelengths on the order of the electron skin depth, which is often much much larger than the Debye length in an over dense plasma).
