One disadvantage of conventional iris-loaded accelerating structures is the
high ratio of the peak surface electric field to the peak axial electric f
ield useful for accelerating a beam. Typically this ratio E-s/E(a)greater t
han or equal to2. The high surface electric field relative to the accelerat
ing gradient may prove to be a limitation for realizing technologies for ve
ry high gradient accelerators. In this article, we present a scheme that us
es a hybrid dielectric and iris-loaded periodic structure to reduce E-s/E-a
to near unity, while the shunt impedance per unit length r and the quality
factor Q compare favorably with conventional metallic structures. The anal
ysis based on MAFIA simulations of such structures shows that we can lower
the peak surface electric field close to the accelerating gradient while ma
intaining high acceleration efficiency as measured by r/Q. Numerical exampl
es of X-band hybrid accelerating structures are given. (C) 2001 American In
stitute of Physics.