Flat thermionic cathode

A flat thermionic cathode, comprising:

• an insulating substrate;
• plural heating elements disposed on said substrate, each including a resistive current conductor arranged in serpentine configuration to produce a corresponding localized area of heat, said localized areas of heat tending to produce a temperature gradient directed from localized areas toward the perimeter of said substrate, whereby thermal stress is created along said perimeter of said substrate;
• sub-heating means disposed on said substrate and energizable to reduce said temperature gradient;
• and
• plural cathode members disposed at corresponding ones of said localized areas of heat, each cathode member including electron emissive material responsive to said heat.

A flat thermionic cathode structure adapted to cooperate with a grid member, comprising:

• a substrate;
• sub-heating means provided on said substrate to substantially define a heating area;
• main heating means provided on said substrate within said heating area to produce at least one substantially localized area of heat;
• cathode means disposed at said at least one localized area and including electron emissive material;
• a frame-shaped spacer of predetermined thickness having plural tab members extending inward to said frame to support said substrate;
• and
• a frame-shaped locking means having plural tab members cooperable with the tab members of said spacer to grip said substrate therebetween, said spacer and said locking means being adapted for insertion into a cup-shaped grid whereby said cathode means are positioned at a predetermined distance from an end wall of said grid.