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Three-dimensional reconnection and relaxation of merging spheromak plasmas
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View: Figures


Image of FIG. 1.
FIG. 1.

A cross section of the SSX device. The spheromaks are injected from the coaxial magnetized plasma guns from either end of the machine into the flux-conserver. The whole machine possesses cylindrical symmetry. The flux-conserver is illustrated by the dark line. Representative flux surfaces of the two initial spheromaks are also shown.

Image of FIG. 2.
FIG. 2.

A cartoon illustrating how the two spheromaks merge. In (a), the initial axisymmetric orientation of the two spheromaks is shown. The magnetic field in the layer of interaction between the two spheromaks is codirected everywhere, with a field null at the center. In (b), the spheromaks start to tilt in the same direction allowing the poloidal fields to now reconnect, shown by the dashed line. In (c), the original poloidal flux has fully reconnected, becoming the toroidal field of the final state. The original toroidal flux reconfigures in (d).

Image of FIG. 3.
FIG. 3.

Magnetic field lines from the HiFi simulation, illustrating the two spheromaks, are shown here. Peak in is represented by the surface in the center of the volume. The top row and bottom row both show the same point in time; the images in the bottom row are an orthogonal view to the ones in the top row. Starting at the left, the spheromaks have already begun to tilt at this point in the simulation. As time advances, the spheromaks tilt progressively more, shown in the middle two images, and begin to reconnect, as shown in the images on the right. It is important to note that reconnection is already occurring in the images on the left and continues to occur after the images on the right.

Image of FIG. 4.
FIG. 4.

Comparison of -field measurements in equivalent planes from the SSX experiment (top) and the HiFi simulation (bottom) during the fast relaxation through null-point magnetic reconnection. Note the magnetic null at the bottom-right of the center frame and top-center of the frame. The contours in the HiFi data represent .

Image of FIG. 5.
FIG. 5.

Time-traces from the simulation of (a) reconnection -field and over the duration of the simulation (zero is suppressed on the axis) and (b) radial location of the primary reconnection region and the -field null during the most intense reconnection (zoomed-in time-scale). The reconnection region is observed to track the magnetic null through the bulk of the plasma. Markers in panel (b) denote times for which the structure of the null is shown in Fig. 7 .

Image of FIG. 6.
FIG. 6.

Radial profiles of from the experiment at two times during the same shot, 52.8 and . These two times are during the merging phase. The field null-point is clearly visible at at . Slightly later in time, at , as the reconnection continues, the field null-point has moved out radially to . The flux-conserver is located at 20 cm. The last probe is located at the flux-conserver wall in the diagnostic gap, causing the small values of at this location.

Image of FIG. 7.
FIG. 7.

Evolving structure of field lines from the simulation around the null-point, as it is radially moving through the domain. Streamlines of -field originate on a small sphere surrounding and tracking the null-point, showing the dynamics of spine-fan reconnection.

Image of FIG. 8.
FIG. 8.

Image of the simulated 3D reconnection region during the most intense reconnection . Streamlines show the magnetic field, arrows show jets of Alfvénic plasma outflow , and the contour shows the region of peaked .

Image of FIG. 9.
FIG. 9.

Ion Doppler spectrometer measurements. During the merging [shown in (a)], two component flows appear, one moving toward the spectrometer and one moving away, at approximately ±25 km/s. The magnitude of the velocity shifts and the total photon counts of the two peaks change rapidly during the merging. After merging is completed (b), the two component structure disappears. Gaussian fits are indicated by the solid line. Data are from a single shot.

Image of FIG. 10.
FIG. 10.

The top plot (a), from the simulation, shows the time evolution of magnetic energy in cylindrical Fourier modes and integrated over the full cylindrical domain . The bottom plot (b), from the experiment, shows the total magnetic energy and the components. The plasma has reached its relaxed state at .


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Three-dimensional reconnection and relaxation of merging spheromak plasmas