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

Abstract

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 gamma approximate to -root pi/2k(parallel to) lambda(De)omega(2)(pi)/omega(pe); and (3) the collisional damping dominates for the large parallel wavenumber, and the decay constant is given as gamma approximate to -nu(in)/16, where nu(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) = (2-9) x 10(11) cm(-3), T-e = 3.7 - 3.8 eV, T-i = 0.2 - 0.3 eV and collision frequency nu(in) = 23 - 127 kHz. Although the wavelength should be specified with the given parameters of DiPS-2, the collisional damping is found to be gamma = (-0.9 to - 5) x 10(4) rad s(-1) for k(parallel to) lambda(De) = 10, while the Landau damping is found to be gamma = (-4 to - 9) x 10(4) rad s(-1) for k(parallel to) lambda(De) = 0.1.