Role of Insulator Length in Plasma Focus Operation

Summary form only given. he plasma focus has been studied for several decades, but significant aspects of this device are still not well understood. One example is the importance and role of the insulator length as a parameter for machine operation. Generally, the community knows that the first few nanoseconds are critical in the formation of the plasma sheath, which may be the most critical time in the overall operation. Historically, the insulator length has either been used for fine tuning the timing of a particular electrode geometry for the capacitive energy source or selected on the basis of other considerations that are not directly related to the plasma processes. In a series of experiments that were performed for other purposes, we appear to have resolved the direct role of the insulator length for operations with a Mather-geometry plasma focus. There are two distinct electric fields associated with the insulator in the coaxial gun geometry, namely the electric field radially through the insulator and the electric field coaxially along the insulator surface. The radial field serves the purpose of initiating the discharge from the cathode. The axial field, directly related to the insulator length, is much smaller than the radial field, but its effect has not been clear. In our machine, we use a field enhancement initiation electrode, similar to the one developed by Nardi´s group several years ago. In the experiments that we were performing, the behavior of several different gases in the focus were being evaluated. For gases with progressively lower ionization states and the same electrical conditions, the focus operation shifted modes and even ceased to form a workable plasma sheath. We realized that the difference in operating mode resulted from the discharge striking directly through the fill gas, as opposed to initiating along the insulator surface. Further analysis of the effects of this parameter will be presented.