L-H transition physics and energetics of zonal flows in the H-1 heliac

Finite frequency zonal flows, or geodesic acoustic modes observed in the H-1 heliac [1], play an important role in plasma dynamics during spontaneous L-H transitions [2]. Here we describe detailed dynamics of the turbulence spectra as plasma approaches the bifurcation from L to H mode. The inverse energy cascade [3] leads to the accumulation of the turbulence energy in the largest vortex seen as the mean ExB flow, similar to spectral condensation in 2D fluid turbulence [4], until geodesic effects due to the compressibility of the flow lead to the generation of the side-band modes, as shown in numerical simulation [5]. The number of these modes gradually increases towards the L-H transitions. It is speculated that the shear decorrelation mechanism eventually forbids the geodesic conversion of the Er energy into the energy of the side-band modes which reinforces spectral condensation and leads to L-H transition.\newline [1] M. G. Shats and W. M. Solomon, Phys. Rev. Lett.\textbf{ 88, }045001 (2002).\newline [2] H. Punzmann, M. G. Shats, Phys. Rev. Lett. \textbf{93}, 125003 (2004).\newline [4] H. Xia and M. G. Shats, Phys. Plasmas \textbf{11,} 561 (2004).\newline [3] M. Shats et al, Phys. Rev. E \textbf{71}, 046409 (2005) \newline [5] B.D. Scott, New J. Physics \textbf{7}, 92 (2005)