In what pattern does the electrons get emitted from cathode in thermionic emission?
Thermionic emission says to me that electrons &quot;boil&quot; off the cathode. If anode was not at +ve potential, a &quot;cloud&quot; of electrons would exist near the cathode (as @jsotola suggests) while the cathode is hot.
A pattern of electron emission is achieved by cold cathodes, where no electrons are emitted until a large anode potential is created. Electrons are ripped away, in a pattern determined by cathode shape (where electric field is high). Cold cathodes might have a sharp point, where electrons are emitted.