Intensive studies on the plasma focus have been performed with the aim of identifying the design parameters and lows, which show the reasonable relations between the most device characteristics [1,2]. In spite of these efforts, design parametric criteria including the insulator sleeve length characteristic have not been presented. The insulator that surrounds the first part of the inner electrode has an important role in the plasma focus dynamics. In this paper we have point out that Mather-type plasma focus device operated in the optimum regimes have remarkably low variation in a design parameter related to the insulator-electrode geometry. The quantity of this parameter which has been defined as the ratio of insulator sleeve length and radial distance between the electrodes L_ins/(b-a), has an approximately value of 1.4. It was found that the insulator sleeve length strongly affects the filling pressure and hence the neutron yields, corresponding to the other works [3,4].
Furthermore, we have concluded that there is an optimum insulator sleeve length (L_opt) in which, the optimum pressure corresponding to the maximum neutron yield, is increased with increase in the insulator length up to L_opt and suddenly decrease using a high value of insulator length (L_ins>L_opt).
This result is in agreement with that; no good focused plasma is formed in the too high filling pressure (P >10 _torr).Finally it is resulted that no good pinch is occurred for the insulator design parameter values of 1.8<L_ins/(b-a)<1.

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