Low Aspect Ratio Stellarator Gallery

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The design of compact (or low aspect ratio) stellarators is one of the new, challenging research areas of interest in magnetic confinement fusion in recent years. The potential advantages of compactness are twofold: (a) for near-term experimental devices it results in a larger plasma (i.e., larger minor radius) for a fixed cost; this allows a better separation between edge and core transport physics and less energy loss from charge exchange reactions from neutral particles entering from the plasma surface; and (b) for reactor extrapolations compactness can lead to smaller, less expensive, modular power plants. The challenge is that maintaining good transport and stability properties at low aspect ratio is generally more difficult than at high aspect ratios. However, careful control of the three-dimensional shape of the low aspect ratio stellarator appears to offer the flexibility to achieve good transport and stability characteristics. This control has been achieved by the development of sophisticated optimization codes which numerically explore 30-40 dimensional parameter spaces (the stellarator's outer flux surface shape is described in terms of 30-40 Fourier modes) and arrive at shapes which minimize a range of desired physics target functions.

With respect to the transport optimization, two approaches are being pursued: (a) direct minimization of the of the deviation of particle orbit trajectories away from magnetic flux surfaces - known as the Quasi-Omnigenous Stellarator or QOS, and (b) targeting of axisymmetry (i.e., symmetry the long way around the torus) in a particular set of magnetic coordinates in which the particle orbits depend only on the magnitude of the magnetic field - known as the Quasi-Axisymmetric Stellarator or QAS (this device then would share similar transport properties as the tokamak).

Quasi-Omnigenous Stellarator

The following images show different views of a low aspect ratio, modular coil stellarator under design at Oak Ridge National Laboratory along with collaborators at University of Texas and Princeton Plasma Physics Lab. The light blue coils provide the magnetic field, which produces the three-dimensional shape of the flux surface. The coil geometry is arrived at through a separate optimization step, which attempts to zero out the component of magnetic field perpendicular to the outer flux surface. The colored surface inside the white coils is the outermost flux surface. These are the surfaces, which would be formed if one traced out the magnetic field lines over many revolutions around the torus.

The coloration is proportional to the magnetic field strength with magenta corresponding the strongest fields, blue and light blue corresponding to the weakest fields. Although this device is not targeted to achieve any particular symmetry in its |B| spectrum, it is not far from helical symmetry, as may be seen from the helical variation of the colors on the magnetic flux surface.


Quasi-Axisymmetric Stellarator

Another approach towards the design of a low aspect ratio stellarator is the targeting of toroidal magnetic symmetry in magnetic coordinates. Such designs are being investigated at Princeton Plasma Physics Laboratory along with collaborators at Oak Ridge National Laboratory, University of Texas, Columbia University and several international labs. Views of the outer magnetic flux surface of such a device are shown below. The colors are isobars of magnetic field strength with red and magenta the highest fields and light blue the lowest. As may be seen, the bands of similar color approximately follow paths around the toroidal direction (i.e., the long way around the donut) corresponding to the symmetry, which is targeted by the QA optimization method.