Calculation of flux coordinates from discretized field maps in magnetic islands

We describe a method to build a straight-field-line coordinate system from discretized magnetic field maps. This method, valid for generic plasma domains with conserved, nested flux surfaces, is particularly useful to calculate the metric tensor coefficients, the Jacobian, and the flux surface average of any plasma quantity in magnetic islands. In this work, we will show examples for tokamak, heliotron, and reversed field pinch plasmas with m=1 islands in low beta plasmas.