Kenneth Gentle

Research Interests

            The Institue for Fusion Studies (IFS) conducts a wide range of experiments and related theoretical work.  The largest experimental programs are at the CMOD tokamak at MIT and the DIII-D tokamak at General Atomics in San Diego, as well as the JTEXT tokamak in Wuhan, China.  The focus of this work is transport in fusion plasmas and the turbulence that causes it.  I am involved with this work as well as with an experiment for basic studies of turbulence and transport, the Helimak.

             My personal research has concentrated on questions of particle and electron transport, perhaps the least understood of the transport channels.  Electron transport is especially challenging because it does not seem to be diffusive -- the thermal flux is not proportional to the temperature gradient.  One example of this is shown here, in which the variation in thermal flux can be measured as a function of the change in temperature gradient using modulated heating -- Electron cyclotron heating on DIII-D in this case.
Results are shown for various normlized radii -- 0 at the center, 1 at the edge.  In the core, transport is diffusive, but in the outer half of the plasma, transport is "stiff", the flux varies rapidly with gradinet and has a clear offset from zero.  .  Transport Article

     Another example of peculiarity in the electron channel is the "cold pulse", in which rapid cooling of the edge can produce a cooling transient that disappears as it moves toward the core and then inverts to cause temperature increases in the core.  This cannot happen with any sort of simple diffusion equation.

However, it is possible with suitably coupled diffusion equations.  The smooth, fine lines in the plot are from a model of coupled electrons and ions in a nonlinear critical gradient model.  Both electron and ion transport are sensitive to the electron-ion themperature ratio and carefully adjusted to match the experiment. The outstanding question is whether such a coupling is consistent with other aspects of electron and ion energy transport.

            I am also conducting experiments on a device here, the Helimak.   This is a model system for study of turbulence in a magnetized plasma.  It offers a simple geometry, essentially one-dimensional, but one in which the generic turbulence of hot magnetized plasmas occurs in a controlled fashion.  The operating parameters are chosen to facilitate detailed turbulence measurements.  The device permits control and variation of many of the key parameters of the theory, including magnetic curvature, magnetic shear, and flow shear.  For example, recent experiments have shown the applying radial bias of sufficient magnitude can stabilize the turbulence.   The operating parameters can also be chosen to model the scrape-off layer of confinement devices. 

        Plasmas with comparatively low density and temperature are often used to model phenomena in fusion-grade plasmas under conditions that can be more thoroughly measured.  The Helimak is unique among these for its size, which is important because the machine and plasma are large compared with the scale size of the plasma -- n/(dn/dr) -- and turbulence.  The turbulence is therefor not dominated or strongly constrained by the boundaries.  The origins and beginning of the project are described  in the Helimak Presentation in pdf.   The Texas Helimak is a technical paper describing the device, and the Bias Experiment is described in another paper.  A selection of presentations at meetings includes TTF2012, DDP2012, and TTF2013.   There is also a brief Movie of the device and a plasma discharge.