Noah Hershkowitz contribution to sheath experiment, problem in the sheath theory and my encounters with Noah

 

This presentation consists of three parts. 1) A century ago, Irving Langmuir put forward concept of plasma and sheath and formulated the kinetic model for a bounded, low-temperature, collisionless plasma. The solution of this model resulted in a continuous structure consisting of a quasi-neutral plasma and sheaths at the plasma boundaries. According to Langmuir, the sheath is a thin, an of order of Debye length electron depleted area where the plasma ions are accelerated to the wall. In the following decades, the science of the sheath evolved in probe diagnostics, cathode and anode electrode sheaths, the interaction of rf field with sheath in rf plasma and rf probes. These activities were primarily theoretical, with a few experimental works dealing mainly with the sheath’s integral characteristics. Experiments on the sheath structure and the sheath edge condition were especially important due to hot discussion of controversy in the sheath perception during the last decades. One among the significant contributions of Noah Hershkowitz is comprehensive experimental study of the sheath potential distributions with emissive probes developed by Noah with his collaborates. This area of Noah’s activity will be elucidated in this session by his colleagues. 2) It is shown in this presentation that an analog of the Tonks-Langmuir model for a bounded fully ionized plasma in the fluid approximation boils down to a set of equation that ultimately coincides with those derived by Bohm for collisionless sheath, from which the Bohm criterion for the sheath existence were obtained. The full solution of this system with postulated by Bohm boundary conditions resulted in the sheath and an infinitely large neutral plasma, and the “sheath” Bohm criterion is the condition of the steady-state plasma without ionization. This becomes evident after the second integration of the system that is missing in Bohm derivation and in textbooks reproducing his results. 3) My encounters with Noah will be shared during the session.