Bispectral experimental estimation of the nonlinear energy transfer in two-dimensional plasma turbulence

Experimental density and potential fluctuation data from a 2D probe array have been analysed to study the turbulent cascade in a toroidally confined magnetized plasma. The bispectral analysis technique used is from Ritz et al ( 1989 Phys. Fluids B 1 153) and Kim et al ( 1996 Phys. Plasmas 3 3998). It has been tested on Hasegawa-Wakatani turbulence simulation data and then applied to experimental data from the TJ-K torsatron. The analysis has been done in 2D wavenumber space. Analysis of simulated and experimental data recover the analytically known growth rate and dispersion relation for drift-wave turbulence. The density fluctuations, which at the realistic collisionality are advected as a passive scalar with the vorticity, show power transfer from large to small scales, while the spectral power in potential fluctuations, which represents the energy, is transferred as an inverse cascade to larger scales. This is the first experimental evidence for the dual turbulent cascade in a magnetized plasma.