New Developments in Trapped Electron Mode Turbulence

The onset of TEM turbulence is believed to limit density gradients in Alcator C-Mod internal transport barriers.$^{1 }$ We have recently modified GS2 to make direct comparisons with phase contrast imaging measurements of density fluctuations in the ITB.$^{2}$ Further, the TEM critical density gradient is nonlinearly upshifted,$^{1}$ analogous to the Dimits shift for ITG turbulence.$^{3}$ In the ITG case, ion-ion collisions reduced the upshift by damping zonal flows.$^{4}$ In contrast, this new TEM nonlinear upshift persists in the presence of realistic ion-ion and electron-ion collisions, and increases with collisionality.$^{5}$ Quasi-periodic bursts arise near threshold, with a period dependent on the relative primary growth and zonal flow damping rates. Nonlinear simulations of this regime confirm the role of zonal flows. $^{1}$D. R. Ernst et al., Phys. Plasmas 11 (2004) 2637. $^{2}$D. R. Ernst \textit{et al.,} 2004 IAEA Fusion Energy Conference, paper IAEA-CN116/TH/4-1, http://www-naweb.iaea.org/napc/physics/fec/fec2004/datasets/TH 4-1.html, and A. Long \textit{et al}., this conference. $^{3}$A. M. Dimits \textit{et al.,} Phys. Plasmas 7(3)(2000) 969. $^{4}$Z. Lin \textit{et al}., Phys. Rev. Lett. 7(5) (2000) 1857. $^{5}$D.R. Ernst, K. Zeller, and W. Dorland, 2005 Sherwood Int'l Fusion Theory Conference, P3-33.