A previous study of transport [1] and turbulences [2] in Tore Supra Ion Cyclotron Resonance (ICRH) hydrogen minority heating discharges has shown the existence of a critical temperature gradient in the effective electron themal conductivity. In this work, we try to better clarify in which conditions there exists an effective critical gradient for other electron heating schemes. To this end, we have analyzed a large database of about hundred discharges, using helium as working gas, heated by either Fast Wave (direct electron heating, FWEH) or Lower Hybrid waves (current drive, LHCD). These hot electron plasmas, in which more than 90% of the RF input power is coupled to the electrons, are characterized by neglgible electron-ion collisional coupling. The database is tested against the models for the turbulent electron thermal conductivity.

The results reported here show good correlation between the data and the electromagnetic revised version [3] of the Electron Temperature Gradient (ETG) turbulence model. This model has a theoretical critical temperature gradient (‹Tc) which is found to be unimportant, since is much lower than the experimental gradient. Comparison of experimental value of ‹Tc, determined from the database, with the theoretical formula is also presented in this paper.

[1] G.T. Hoang et al, Nucl. Fusion 38 (1998) 117
[2] L. Colas et al, Nucl. Fusion 38 (1998) 903
[3] W. Horton, Reviews of Mod. Physics 71 (1999) 735

A previous study of transport [1] and turbulence [2] in Tore Supra Ion Cyclotron Resonance (ICRH) hydrogen minority heating discharges has shown the existence of a critical temperature gradient in the effective electron thermal conductivity. In this work, we try to better clarify in which conditions an effective critical gradient exists for other electron heating schemes. To this end, we have analyzed a large database of about hundred discharges using helium as working gas. The plasmas were heated by either Fast Wave (direct electron heating, FWEH) or Lower Hybrid waves (current drive, LHCD). These hot electron plasmas, in which more than 90% of the RF input power is coupled to the electrons, are characterized by negligible electron-ion collisional coupling.

The database is tested against the revised model of electron thermal conductivity [3]. This model is based on electromagnetic turbulence driven by the electron temperature gradient (ETG) and collisionless electron skin depth. It has a theoretical critical temperature gradient (‹Tc). The results reported here show good correlation between the data and this updated model. Moreover, in our experiments, ‹Tc is found to be unimportant, since it is much lower than the experimental gradient. This work also compares the formulas of critical threshold for the ETG turbulence with the experimental values determined from the database.

[1] G.T. Hoang et al, Nucl. Fusion 38 (1998) 117
[2] L. Colas et al, Nucl. Fusion 38 (1998) 903
[3] W. Horton, Reviews of Mod. Physics 71 (1999) 735

Topic F

A six-dimensional nonlinear dynamics model is derived for the basic energy components of the night-side magnetotail coupled to the ionosphere by the region-1 currents. In the absence of solar wind driving and ionospheric dissipation the system is a three-degree-of-freedom Hamiltonian system. The large ion gyroradius conductance of the quasineutral sheet produced the energization of the central plasma sheet (CPS) while the unloading is triggered when the net geotail current or current density exceeds a critical value. For a steady southward IMF the model predicts an irregular sequence of substorms with a mean recurrence period of about 1 hr as in the Klimas et al. (1992) Faraday loop model. Here we use the new model as a nonlinear prediction filter on the Bargatze et al. (1985) database. Starting with physics calculations of the 13 physical parameters of the model we show that the average relative variance (ARV) is comparable to that obtained with data-based prediction filters. To obtain agreement between the predicted AL and the database AL it is essential to include the nonlinear increase of the ionospheric conductance with power deposited in the ionosphere.

To be submitted to J. Geophys. Res., December 1996
AGU Indices: Magnetospheric Physics: (MS) 2740, 2788, 7839

In this work a new model of the ion thermal transport, based on the long wave-length behavior of the ion-temperature-gradient-driven turbulence, is introduced. A relevant property of the model is that the thermal conductivities grow with minor radius, as a consequence of the positive dependence of the correlation length and of the inverse of the correlation time on the ion temperature gradient. The model is assessed by comparing the predicted profiles of temperature and effective conductivity with the experimental ones of several machines.

P.A.C.S. 52.25.Fi, 52.35.Ra, 52.55.Dy

Low-order mode coupling equations are derived to describe recent computer simulations of the toroidal ion-temperature-gradient turbulent convection with steady and pulsating sheared mass flows in the transport barrier zone. The three convective transport states are identified with the tokamak confinement regimes called low mode (L-mode), high mode (H-mode), and barrier localized modes (BLMs) when the transport barrier is in the core plasma. The L-mode limit cycle is analytically derived and a bifurcation diagram showing L to H and H to BLM transitions in confinement is constructed numerically. Markovian closure procedures are sought to further reduce the dimensionality of the nonlinear system. First an exact expression is given for the energy transfer rate from the fluctuations to the sheared mass flow through the triplet velocity correlation function. Then the time scale expansion required to derive the Markovian closure formula is given. Markovian closure formulas form the basis for the thermodynamic-like L-H bifurcation models.

PACS nos: 52.25.Fi, 52.35.Kt, 52.35.Ra, 52.55.Fa