The Slotted Coax as a Beam Electrode

Abstract

The slot-coupled TEM line has been employed at CERN as a pick-up electrode in the GHz range. It is a compact and broad band device and will prove itself commercially attractive once its coupling efficiency is competitive with an array of quarter wave loops. The slotted coupler presented in this paper is comprised of two tubes joined along their length. The bottom tube is the beam pipe and the top tube is the TEM line which picks up the electromagnetic energy from the beam particles. Energy is transferred from the beam to the TEM line through a series of slots which are mutual to the two tubes and which occur where the tubes are joined. In this device the signal voltages add in phase when the beam velocity matches the line velocity. The pick-up impedance can thus be large if the structure is long. It turns out, however, that the signal in the synchronous case is weaker than the signal obtained from an equal length array of loop couplers. In this paper the authors derive the response of the slotted line in a general non-synchronous case and explore the possible advantages of designing the coupler with reduced velocity to increasemore » the coupling efficiency. Other properties as well as a theoretical analysis of the TEM line are presented.« less