By combining the filtering procedure and the usual step by step method, a numerical method that can solve improperly posed Cauchy problem of the flux function equation for plasma equilibrium, has been obtained. By the new method, the drastic instability appearing in the usual step by step calculation, which was unsuccessfully applied to the calculation of the external flux function, has been overcome, and the stable numerical results have been obtained.

The present paper aims at setting up a stable numerical method for the inverse problem with noncircular cross-section plasma equilibrium. BY means of a discretization operator, the filtering is applied to determine the external solution for the inverse problem. In the paper, an effective explicit step by step filtering algorithm for numerical calculation is also presented and some of results obtained by this technique for noncircular cross-saction plasma is given.The numerical solution is compared with the analytic one in the case of circular cross-section plasma.