It is well known that for a collisionless plasma, we have: $$ \Lambda = n_e \lambda_{De}^3 \gg 1 $$
which means that the Debye sphere is densely populated. Whereas it is the opposite for a collisional plasma, where the Debye sphere is sparsely populated. Isn't this quite counter - intuitive? Since we expect collisions to play a bigger role for a more densely populated region?
Also, why would collective behavior be more dominant in the case of a densely populated Debye sphere?
What is the Debye length from physical point of view? It modifies the Coulomb interaction in plasma, producing Yukawa-like potential, $$V(r)\sim \frac{e^{-r/\lambda_D}}{r}.$$ The case of large $\lambda_D$ corresponds to weak long-range interaction of electrons in plasma, whereas in case of small Debye length electrons are interacting on long-range scale. Then, consider the plasma parameter, $$\Lambda = n\lambda_D^3.$$ With fixed $n$ and large $\lambda_D$ we deal with wekly-interacting electrons. It means that it is possible to use mean-field description: you can approximate electron-electron interactions as interaction of electron with external field. Roughly speaking, you simply can write $$\frac{e^2}{r}\sim eE_{\text{mean-field}}.$$ In such set-up, we deal with collective behavior: a response on external field is the simply sum of response of individual electron. In addition, the hydrodynamic description is valid.
