A separatrix shape of a field-reversed configuration (FRC) plasma is determined approximately by an excluded flux measurement. However, it is known that the method does not coincide with the real plasma especially at end regions. In order to measure and control the position and the shape of the separatrix, a new method (iterative method) determining the separatrix shape with high accuracy is developed in this report; the separatrix shape of the FRC plasma is determined by comparing iteratively measured magnetic fluxes with the solution of the Grad-Shafranov equation.
A magnetic reconnection point of the bias field with the main field and an axial movement of the X-point are clarified by using the iterative method at the formation phase. The X-point is also measured directly using internal magnetic probes. Both axial positions agree well with each other. The amount of an edge-layer plasma is discussed at the quiescent phase and the unstable phase. It is found that the beta value at the separatrix is 0.6-0.7 and the thickness of the edge-layer plasma is 4-5 times the ion gyro-radius.
Internal structures of the FRC plasma can be determined by solving the Grad-Shafranov equation using the measured separatrix shape and the beta value at the separatrix. It is found that the magnetic islands are observed near the field null at the formation phase. Although they are coalesced once by the axial contraction of the plasma, they appear again at the quiescent phase.
In order to confirm the formation of the magnetic islands, a radiation profile of the FRC is measured. The profile is analyzed by the Abel inversion method to obtain a radial profile of the radiation power density. If uniformity of the plasma temperature inside the separatrix is assumed, it is obtained that the beta value at the separatrix is 0.76. From the radiation outside the separatrix, the thickness of the edge-layer plasma is 6 times the ion gyro-radius. These values are close to the above values by the iterative method at the midplane. A contour of the radiation intensity in the r-z plane, which measured the same method at several positions on axis, is compared to the internal structure of the FRC.