| OVERVIEW : As highlighted in Professor Rod Latham's technical reference books, "High
Voltage Vacuum Insulation (HVVI): Basic Concepts and Technological Practice"
(Academic Press 1981 and 1995), the central challenge faced in the design of all
vacuum tube devices is to achieve operating fields in excess of 5 MV/m. At
one extreme there are such large-scale complex systems as particle
accelerators, whilst at the other there are the microscopic structures used
in flat panel field emission display modules. In between, there are such
devices as X-ray tubes, magnetrons, klystrons travelling wave tubes, vacuum
capacitors and vacuum switches. Over-arching these terrestrial applications,
there is the whole domain of space technology where the requirement of
stable electric fields is of vital importance. For all of the above devices
and systems, their ultimate performance is ultimately limited by the
presence of electron pin-holes in electrode surfaces: i.e. microscopic sites
from which electrons can escape from the metal into the vacuum gap by a
complex field emission mechanism under anomalously low field conditions. The
presence of these electronic defects manifest themselves in two practical
ways. Firstly, they can give rise to what are variously referred to as dark,
leakage, or prebreakdown currents. Secondly, they can trigger the
catastrophic "nightmare" phenomenon of vacuum breakdown: i.e. where the
insulating properties of the vacuum gap are spontaneously lost as the result
of an electrical discharge between the electrodes. The design challenge is
therefore to employ effective technological procedures to minimise the
occurrence of electron pin-holes, and to incorporate remedial technological
measures for suppressing their residual propensity to trigger breakdown. |
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