Dynamic Devices: Pickups and Pickers

A large proportion of the dynamic devices used to interact with the charged-particle beams in accelerators or storage rings can be classified as pickups or kickers. These devices act through time-varying electromagnetic fields either to extract information about the particle's motion or to effect a change in that motion. A given configuration of electrodes may be used either as a pickup or as a kicker; that duality will be addressed in this paper. An example of a simple electrode is the loop antenna, which may be made in the shape of an electrical stripline at the side of a beam chamber. This electrode.picks up a signal from the beam current by intercepting time-varying magnetic flux and image charges of the beam. One can also understand that the difference signal from two such striplines placed on opposite sides of the beam will give information on the beam's transverse position. This same electrode, if externally excited as a kicker, can produce transverse forces through its magnetic field acting on moving charges; and its electric fields in the direction of the particle motion produce its effects as a longitudinal kicker. Electrodes for a particular application call for response over a particular range of frequencies. The stripline electrodes provide useful coupling over a range greater than one octave, centered about frequencies usually between 100 EIHz and 10 GHz. In this same frequency region, a narrow-band device is the r.f. cavity which can be employed as either a sensitive pickup or a high-power accelerating or deflecting electrode; the rf accelerating system of a particle accelerator is a specialized example of such a kicker. A particular wave form with short rise time, then long flat dwell, is called for in the pulsed magnet kicker used to inject beam onto a closed orbit. Depending on the application, analysis, and description, an electrode's performance may be in either the time domain or the frequency domain. relations between longitudinal and transverse effects, and between the responses as pickup and as kicker. We shall see that dynamic effects are entirely determined by the longitudinal electric fields in the direction of the beam current when the electrode is excited as a kicker; and as a corollary, absence of longitudinal electric fields guarantees no coupling to beam particles. Response functions that serve as figures of merit will be defined. We shall then analyze the responses of specific examples of pickups and kickers. Finally, an approach to the calculation of the transverse variation of coupling over the electrode aperture will be presented