Transport Bifurcation Research Articles

Dynamic Modeling of Stepwise Internal Transport Barrier Formation in DIII-D Negative-Central-Shear Discharges

The GLF23 transport model is used to dynamically follow bifurcations in the energy and toroidal momentum confinement in DIII-D discharges with an internal transport barrier. The temperatures and toroidal velocity profiles are evolved while self-consistently computing the effects of E x B shear stabilization during the formation and expansion of internal transport barriers. The barrier is predicted to form in a stepwise fashion through a series of sudden jumps in the core-electron and ion temperatures and toroidal rotation velocity. These results are consistent with experimental observations. In the simulations, the step transitions are a direct result of local E x B driven transport bifurcations.

Transport barrier formation and bifurcation patterns ofpotential profiles in the compact helical systemheliotron/torsatron

In the compact helical system, several bifurcation patterns havebeen observed in potential profiles ofelectron-cyclotron-heated plasmas. In a bifurcation pattern witha strong shear of radial electric field, an internal transportbarrier is confirmed to be established for electron thermalenergy. Probabilistic treatment with realization of the patternor internal transport barrier is discussed.

Transport Dynamics of Collapse in a Giant ELM

Dynamics of collapse in a giant ELM (Edge Localized Mode) is presented. A model of transport bifurcation between the H-mode and the magnetic braiding mode (M-mode) is applied to the simulational study of edge plasma dynamics. The bifurcation contains the hysteresis characteristics. The pressure profile development, the propagation of the bifurcation (transition) fronts and the resultant bursting fluxes are obtained. The appearance of a pivot point in the pressure profile, an avalanche followed to the transition front propagation, and the structure of the burst are shown.

Poloidal Rotation in TFTR Reversed Shear Plasmas

A bifurcation in the core poloidal rotation of carbon impurity ions in the Tokamak Fusion Test Reactor (TFTR) has been observed prior to the transport bifurcation associated with enhanced reverse shear plasmas. In a narrow radial region of the plasma, the impurity ion poloidal rotation reverses direction. This poloidal flow is associated with the establishment of a large negative radial electric field with strong shear. The measured poloidal velocities before, during, and after this precursor differ from neoclassical predictions.

1D simulation of sawtooth crash based on transport bifurcation

The crash of the temperature profile associated with a propagating transport bifurcation between the L-mode and the magnetic braiding mode (M-mode) in a tokamak plasma is presented. The magnetic braiding mode is induced by the magnetic stochasticity. If the pressure gradient exceeds a threshold value a magnetic stochasticity sets in and the heat conductivity is much enhanced (M-mode). The back transition to the L-mode occurs when the pressure gradient decreases to another threshold value. The model of the transport bifurcation is included in the 1D transport code. The crash of the temperature profile and the propagation of the crash front (avalanche) are realized by this model. The collapse without a precursor oscillation is revealed.

Enhanced reverse shear bifurcation in tokamak

A transport bifurcation model for enhanced reverse shear (ERS) mode recently observed in tokamaks is given. The model includes effects due to evolution of magnetic shear and bootstrap current. It is shown that these effects substantially reduce the power threshold for ERS transition.

On the dynamics of turbulent transport near marginal stability

A general methodology for describing the dynamics of transport near marginal stability is formulated. Marginal stability is a special case of the more general phenomenon of self-organized criticality. Simple, one field models of the dynamics of tokamak plasma self-organized criticality have been constructed, and include relevant features such as sheared mean flow and transport bifurcations. In such models, slow mode (i.e., large-scale, low-frequency transport events) correlation times determine the behavior of transport dynamics near marginal stability. To illustrate this, impulse response scaling exponents (z) and turbulent diffusivities (D) have been calculated for the minimal (Burgers’) and sheared flow models. For the minimal model, z=1 (indicating ballistic propagation) and D∼(S20)1/3, where S20 is the noise strength. With an identically structured noise spectrum and flow with shearing rate exceeding the ambient decorrelation rate for the largest-scale transport events, diffusion is recovered with z=2 and D∼(S20)3/5. This indicates a qualitative change in the dynamics, as well as a reduction in losses. These results are consistent with recent findings from dimensionless scaling studies. Several tokamak transport experiments are suggested.

Advances in the simulation of toroidal gyro-Landau fluid model turbulence

The gyro-Landau fluid (GLF) model equations for toroidal geometry [R. E. Waltz, R. R. Dominguez, and G. W. Hammett, Phys. Fluids B 4, 3138 (1992)] have been recently applied to study ion temperature gradient (ITG) mode turbulence using the three-dimensional (3-D) nonlinear ballooning mode representation (BMR) outlined earlier [R. E. Waltz, G. D. Kerbel, and J. Milovich, Phys. Plasmas 1, 2229 (1994)]. The present paper extends this work by treating some unresolved issues concerning ITG turbulence with adiabatic electrons. Although eddies are highly elongated in the radial direction, long time radial correlation lengths are short and comparable to poloidal lengths. Although transport at vanishing shear is not particularly large, transport at reverse global shear, is significantly less. Electrostatic transport at moderate shear is not much affected by inclusion of local shear and average favorable curvature. Transport is suppressed when critical E×B rotational shear is comparable to the maximum linear growth rate with only a weak dependence on magnetic shear. Self-consistent turbulent transport of toroidal momentum can result in a transport bifurcation at sufficiently large r/(Rq). However, the main thrust of the new formulation in the paper deals with advances in the development of finite beta GLF models with trapped electrons and BMR numerical methods for treating the fast parallel field motion of the untrapped electrons.

An Experimental Investigation of Free Surface Transport, Bifurcation, and Adhesion Phenomena as Related to a Hollow Glass Ampule and a Metallic Conductor

An experimental investigation of free surface transport and subsequent bifurcation and adhesion for a hollow glass ampule is presented in this work. Detailed phenomenological features of the process are displayed and discussed. This experimental investigation, which is generic in nature, provides the much needed phenomenological information on free surface transport, glass processing as related to optical fiber production, and glass-to-metal sealing processes. Detailed images of the actual sealing process provide valuable information on identifying and isolating the key regimes in the process and mapping out process defects, and contribute to a basic understanding of the physical mechanisms involved in the sealing process.