Characteristics of collisional damping of surface ion-acoustic mode in Divertor Plasma Simulator-2 (DiPS-2)

The dissipation of ion-acoustic surface waves propagating in a semi-bounded and collisional plasma which has a boundary with vacuum is theoretically investigated and this result is used for the analysis of edge-relevant plasma simulated by Divertor Plasma Simulator-2 (DiPS-2). The collisional damping of the surface wave is investigated for weakly ionized plasmas by comparing the collisionless Landau damping with the collisional damping as follows: (1) the ratio of ion temperature $({T_i})$ to electron temperature $({T_e})$ should be very small for the weak collisionality $({T_i}/{T_e} ≪ 1)$; (2) the effect of collisionless Landau damping is dominant for the small parallel wavenumber, and the decay constant is given as $γ ≈{-} \sqrt {π /2} {k_\parallel }{λ _{De}}ω _{pi}^2/{ω _{pe}}$; and (3) the collisional damping dominates for the large parallel wavenumber, and the decay constant is given as $γ ≈{-} {ν _{in}}/16$, where ${ν _{in}}$ is the ion-neutral collisional frequency. An experimental simulation of the above theoretical prediction has been done in the argon plasma of DiPS-2, which has the following parameters: plasma density ${n_e} = (\textrm{2--9)} × \textrm{1}{\textrm{0}^{11}}\textrm{c}{\textrm{m}^{ - 3}}$, ${T_e} = 3.7- 3.8\textrm{eV}$, ${T_i} = 0.2- 0.3\textrm{eV}$ and collision frequency ${ν _{in}} = 23- 127\textrm{kHz}$. Although the wavelength should be specified with the given parameters of DiPS-2, the collisional damping is found to be $γ = ( - 0.9\textrm{to} - 5) × {10^4}\textrm{rad}{\textrm{s}^{ - 1}}$ for ${k_\parallel }{λ _{De}} = 10$, while the Landau damping is found to be $γ = ( - 4\textrm{to} - 9) × {10^4}\textrm{rad}{\textrm{s}^{ - 1}}$ for ${k_\parallel }{λ _{De}} = 0.1$.