An inertial electrostatic confinement fusion (IECF) is the scheme of injecting the ions and electrons towards the spherical center, trapping both species in the electrostatic self-field and giving rise to fusion reactions in the dense core. R.L. Hirsch reported 10^9 n/s D-T neutron in 1967. Recently we obtained more than 10^6 n/s D-D neutrons in single grid IECF device, where the applied voltage is 45-kV and the discharge current is 15-mA.

Fusion reaction rates obtained by IECF experiments so far cannot be explained by the model of a simple potential well structures, since an electrical potential at the center prevents to make the dense core. R.L. Hirsch proposed a multi-well structure called "poissors" to explain the experiments.

The particle simulation we carried out shows that the potential inside the cathode becomes double well structure, unstable and oscillated periodically, which bring about high fusion reaction rate. Thus, the potential well structure may posses an important physics for understanding the principle of an IECF.

Experimental efforts to measure potential wells using Langmuir probe and through collimated proton measurement were carried out by several groups, but it is a difficult task to obtain enough resolution to show multi-well, because a size of the well may be so small and located inside the high potential cathode.

We, therefore, use an optical method to measure a local electrical field inside the cathode by a stark effect of He. First, we applied the method to light from a hollow-cathode lamp, and identified the electric field distribution inside the lamp. We are trying to apply this method to our IECF plasma, and also to measure high-energy protons produced by D-D reactions to clarify the structure of potential wells in the IECF.