I think the e-field effect observed could be some kind of resonance/reflection.
The reactor is made of different types of metal so when there is some signal near it it will work as a reflector or resonator = antenna.
I have the feeling there is something like a wifi or a cellphone connection and the reactor just alternates this field.
This also explains why the amplitude of the field changes when he is turning the reactor.
While changing the position of the reactor he changes the polarisation and/or direction of the "antenna"/reactor so the reactor reflects/resonates the signal better than before.
By moving it under the table he is moving the reactor/"antenna" out of the signal so there is nothing to see anymore.
I did not know that alumina can be used as an antenna. Can you cite an example of an already existing alumina antenna in current technology? I would be interested and surprised.
For another thing, I did not know that a wifi or a cell phone connection could produce a 140 V/M e-field. I thought the FCC had regulations to keep such powerful EMF production by that technology to minimum.