A method for tuning dielectric loaded accelerating structures

We present a method to vary the resonant frequency of a dielectric loaded accelerating (DLA) structure driven by either the wakefield of a beam or an external rf source. The structure consists of a thick ceramic layer backed by a thin layer of ferroelectric. The overall frequency of the DLA structure is tuned by applying a DC bias voltage to the ferroelectric layer in order to vary its permittivity. This scheme is needed to compensate for frequency shifts in DLA structures due to machining imperfections and dielectric constant heterogeneity. We have identified BST ferroelectric-oxides compounds as a suitable material for this application; it has a relative dielectric constant that can be tuned from 300 to 500. From this, we calculate that the overall frequency of the structure can be tuned over a range of (2/spl divide/4)% for an X-band DLA structure. In this paper, a detailed model of the DLA structure is given and an experimental test is proposed. We present cold test measurements for an 11.424 GHz planar tunable DLA structure.