Beam Diagnostic by outside Beam Chamber Fields*

Fields induced by a beam and penetrated outside the beam pipe can be used for a beam diagnostic. Wires placed in longitudinal slots in the outside wall of the beam pipe can work as a beam pickup. This has very small beam-coupling impedance and avoids complications of having a feed-through. The signal can be reasonably high at low frequencies. We calculate the beam-coupling impedance due to a long longitudinal slot in the resistive wall and the signal induced in a wire placed in such a slot and shielded by a thin screen from the beam. We present a field waveform at the outer side of a beam pipe, obtained as a result of calculations and measurements. Such kind of diagnostic can be used in storage rings, synchrotron light sources, and free electron lasers, like LINAC coherent light source. FIELDS OUTSIDE THE BEAM PIPE