This is a photo of a brushless exciter rotor and its diodes. The diodes, basically, converts the AC output current of the exciter generator to DC, which is then feed to the rotor of the Main generator.
The output power of the exciter in a brushless excitation system is in the rotor, thus the diodes rotate with the rotor. That is why, sometimes, the diodes are called “rotating diodes”. The output DC current of the diodes is directly feed to the rotor winding of the main generator.
The exciter generator in this photo is rated 125kV, 120V, 3 phase and 240Hz.