Maurizio Ottaviani

Electron transport in Tore Supra with fast wave electron heating

The hot electron plasmas (Te>2Ti) in Tore Supra (Equipe Tore Supra (presented by R. Aymar) in Plasma Physics and Controlled Nuclear Fusion Research [Proc. 12th Int. Conf., Nice, 1988 (IAEA, Vienna, 1989), Vol. 1, p. 9]) driven by fast wave electron heating (FWEH) are analyzed for thermal transport. Both neoclassical and anomalous transport processes are taken into account. The dominant power flow is through the electron channel of anomalous thermal diffusivity. The electron and ion temperature gradient driven instabilities are analyzed for a well documented discharge and shown to explain the diffusivities inferred from the steady state power balance analysis. The discharges are maintained in a quasi-steady state for periods up to 100 global energy replacement times. A large Tore Supra database is tested against two models for the turbulent electron thermal conductivity. Good correlation is obtained with an updated version of the collisionless skin depth formula. The electrostatic turbulence-based formula ...

Analysis of the critical electron temperature gradient in Tore Supra

The Tore Supra [Equipe Tore Supra presented by R. Aymar in Plasma Physics and Controlled Nuclear Fusion Research, Proc. 12th Int. Conf., Nice, 1988 (IAEA, Vienna, 1989), Vol. 1, p. 9] database of fast wave electron heating discharges is analyzed with respect to the role of the critical electron temperature gradient. The experimental evidence for the critical gradient is presented from power balance thermal flux qe(∇Te) relation extrapolated to zero in the measured gradient ∇Te. The fluctuation spectra are also known to extrapolate to low levels versus ∇Te. The inferred critical gradient (∇Te)c is defined by the offset linear extrapolation of the thermal flux to zero values. Histograms of the anomalous fluxes before and after being normalized to the theoretical models are constructed. For a wide range of heating powers, the electron power balance radial heat flux is shown to be well described by Te3/2 (1/LTe−1/Lc), where Lc=(Te/∇Te)c is the gradient scale length from linear eigenmodes in a sheared magnetic...

Asymmetric tearing mode in the presence of viscosity

The linear stability of the tearing mode (TM) in a plasma column is investigated in the presence of viscosity and finite equilibrium current density gradients (i.e., asymmetries). It is shown that for low β, both effects are essential in order to properly describe the mode behaviour close to marginality. In particular, the theory introduces a critical threshold for the destabilization, such that the perturbation grows only if Δ′>Δ′cr. The value of Δ′cr depends on the equilibrium configuration and on the plasma parameters. Most importantly, Δ′cr can take negative values, thus allowing unstable tearing modes for Δ′ < 0 (even in the absence of bootstrap current).

Finite system size effects on drift wave stability

Unstable electrostatic resistive modes, driven by density gradients, are identified in a bounded sheared slab. The boundary conditions play a crucial role and are shown to change the nature of the problem, which is related to so called “universal” mode. The dispersion relation and the structure of the eigenmodes of the instability are derived and are shown to depend on a limited set of dimensionless parameters. The occurrence and possible impact of these modes on numerical simulations and actual plasmas are discussed.

Turbulent impulsive magnetic energy release from electron scale reconnection

Magnetic reconnection may occur as bursts of nonlinear plasma dynamics on the electron collisionless skin length scale de=c∕ωpe, during which a large fraction of the magnetic energy is converted to plasma thermal energy and plasma flow energy. An example of such a bursty energy release event is given with a simple set of electron Hall equations. The energization mechanism is the cross-field compression of the electron gas between interacting magnetic islands. The electron energization appears to be consistent with the rapid electron energy flux changes measured by the Cluster spacecraft crossing thin current sheets at −17RE in the geotail. The analysis is relevant to planned spacecraft missions for measuring electron scale magnetic reconnection events in the geomagnetic tail.