Experimental Method

In order to calculate the helicity and magnetic energy, it is necessary to define the volume in which the integration ( and ) will take place. Theoretically this is not a problem, the entire plasma volume can be chosen. Experimentally however, it is not possible to position probes throughout the entire volume without irreversibly disturbing the process being studied. Therefore, there is no straightforward way to define the volume. It is thus necessary to make a few assumptions about the problem. Assumptions:

1. Cylindrical Symmetry
2. The B - field profiles fit a hyperbolic tangent function

The condition of cylindrical symmetry is assumed because of the geometry of the machine. The reason for using the function is that it can be obtained theoretically as the form of the B - field in a plasma in the diffusion region.