[ian gostling (OP)]

In principle,why is it not possible to electrostatically charge a van de graaff style conducting sphere,by using thermionic emission inside the sphere? leaving aside the engineering  requirements for now.

[wolfekeeper]

Probably would arc across onto the wire leading to the dome.

[alancalverd]

Electrons are emitted from a hot metal cathode. Then what? If there is an insulating gas around the cathode, and no potential gradient, the electron cloud won't disperse.

The Van de Graaff principle creates a potential gradient by moving charges mechanically from near-ground potential. The simplest engineering approach is to induce charge onto the belt triboelectrically at  ground potential and "scrape it off" inside the sphere. You might get a more efficient charge transfer if the "scraper" comb were heated or illuminated but in practice the gas in the sphere is adequately ionised if the comb has sharp teeth.

[evan_au]

by using thermionic emission inside the sphere?

I'm afraid not.

Thermionic emission has been used for over a century in "thermionic valves" or "vacuum tubes", like old-style TV screens.
See: https://en.wikipedia.org/wiki/Thermionic_emission

If you just heat the cathode (with no voltage on the anode), a small fraction of electrons will attain enough energy to overcome the work function and jump off. These electrons will immediately jump back onto the cathode, as their departure left the cathode with a positive charge.

To produce a charge separation, you need a current to flow; a thermionic cathode has no net current, and cannot generate a voltage.

To make a current flow through a thermionic tube, you need a positive voltage on the anode. This attracts thermionic electrons away from the cathode. For the current to continue to flow, you need a power source to replenish the electrons on the cathode.

So if you have a voltage source you can produce a charge separation.
But a thermionic source can't produce a charge separation by itself.

Photoelectric Effect
With a source of ultraviolet light, you can knock electrons out with more energy than the work function, but this only produces a few volts. This is far more effectively than a thermionic cathode - but not enough to make your hair stand on end!
See: https://en.wikipedia.org/wiki/Photoelectric_effect

To generate more volts from the photoelectric effect you need to use X-rays - but then an X-Ray machine contains a voltage source with thousands of volts.

In fact, Van der Graaf made the first million-volt X-Ray machine - I wonder how he generated such high voltages to power it?
https://en.wikipedia.org/wiki/X-ray_generator