  We present a non-invasive, resonant cavity based approach to beam emittance
measurement of a shot-to-shot non-circular beam pulse of multi-bunches. In a
resonant cavity, desired field components can be enhanced up to Q_L_lambda/pi,
where Q_L_lambda is the loaded Q of the resonance mode lambda, when the cavity
resonant mode matches with the beam operating frequency. In particular, a
Quad-cavity, with its quadrupole mode at beam operating frequency, extracts the
beam quad-moment exclusively, utilizing the symmetry of the cavity and some
simple networks to suppress common modes. Six successive beam quadrupole moment
measurements, performed at different betatron phases in a linear transport
system, allow us to determine the beam emittance, i.e., the beam size and shape
in the beam's phase space. One measurement alone provides the rms-beam size if
the beam position is given, for instance, by nearby beam-position-monitors.
This paper describes the basic design and analysis of a Quad-cavity beam
monitoring system.
