טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentPalacio Jorge
SubjectTheoretical Investigation of Hollow Cathode Plasma Physics
DepartmentDepartment of Physics
Supervisor Professor Yakov Krasik
Full Thesis textFull thesis text - English Version


Abstract

Hollow Cathodes (HCs) are devices able to produce high density plasmas (> 1021 m-3) and high electron current densities (>103 A/cm2) at low potential drop (100<V). HCs are used in different fields as laser technology, material processing, vacuum microelectronics, atomic spectroscopy, Ion Thrusters and others. In HCs the entire plasma (or almost all of the plasma) is surrounded by the cathode in such a way that the plasma electrons are confined by the cathode fall. The confinement of the electrons leads to the pendulum effect in which the electrons oscillate inside the Hollow Cathode (HC). These oscillations of the electrons lead to efficient ionization and excitation processes. This is the so-called Hollow Cathode Effect (HCE) which is responsible for the high density plasma formed inside the HC. The most common HCs consist of a metal tube and the anode is in general located outside of the HC.

In common applications the HC has small radius (~1mm) and the plasma inside the HC is not uniform. In particular, in the radial direction there will be a distribution with the maximum plasma density at the axis. One of the purposes of this work was to investigate the radial distribution of the parameters of the plasma inside cylindrical HCs and to find expressions allowing rapid estimation of the plasma parameters without using computer simulations. In the present work, an expression that gives explicitly the electron temperature for known values of the Paschen parameter was found. This expression includes the effects of ion inertia and ion-neutral charge exchange collisions. The radial distribution of plasma density, plasma potential, ion drift velocity and neutral gas temperature were calculated. An expression for the axis-to-wall neutral temperature ratio was rigorously derived, and it was shown that the neutral gas can reaches on-axis temperature greater than that of the HC wall depending on the density of the inner plasma column.

An expression determining the dependence of the edge to axis plasma density ratio on the pressure regime of operation was found. The effect of the electron current density emitted by the cathode on the plasma behavior was analyzed in detail. In particular, the values of the plasma density and ion velocity at the sheath edge were found to be practically insensitive to the emitted current.