What I understand is that the electrons get attracted to the anode and during their passage through the buncher gap, they regroup into bunches due to differing potentials imposed on different electrons by the RF signal. Then they reach the catcher cavity and transfer their energy to the cavity therefore amplifying the RF signal.
But then I came across the term buncher gap transit angle \$\theta_g\$ which is equated as
$$\theta_g = \omega \tau$$ where \$\omega\$ denotes the angular frequency of the RF signal and \$\tau\$ is the buncher gap transit time. I don't understand whether the parameter transit angle is really an angle between two physical dimensions inside the klystron or is it a mathematically convenient notation. Is there any intuition which the parameter tries to deliver?