As far as I can find out, the capacitor excited brushless alternator was invented by
Sakutaro Nonaka.

I assume you are familiar with the construction but it is basically a 2 phase stator as in an induction motor, one winding is the power circuit and the other has a capacitor connected across it. The rotor is a salient pole wound construction with a diode connected across the winding.

Excitation builds up as in a self excited induction generator.

You can regard a single phase induction motor as having two fields rotating in opposite directions, there is no starting torque but if you spin the rotor in either direction, one field will accelerate the rotor and the rotor will run at slip speed. The other component then becomes a pulsating component at twice supply frequency.

Now in the case of this alternator when you drive the rotor at synchronous speed, one component rotates at the same speed as the rotor and is ineffective. The other component rotates backwards to the rotor and the field cuts the field winding at twice line frequency. The emf from this component of the field is rectified by the diode and produces a dc current in the rotor.

Unlike an induction generator, these machines are a true synchronous generator and run without slip.

Voltage stability is mainly provided by saturation of the iron circuit in the winding connected to the exciting capacitor. The value of exciting capacitor and the speed are critical. Departure from nominal speed badly affects the voltage and performance is not good unless the device driving it has stable speed characteristics.

The things are cheap and reliable and good enough for most purposes but they are not as well regulated as an alternator with an avr and generally the performance is affected by load power factor, making them a poor choice for driving welders or starting large motors.