Excitation of wakefields from a short charge bunch moving parallel to the axis of a dielectric-lined cylindrical waveguide is analyzed. This situation amounts to generation of Cerenkov radiation in a transversely bounded system. Wakefields are expanded into an orthonormal set of hybrid electric-magnetic eigenfunctions for this waveguide geometry. The orthonormalization relations for this system are obtained, evidently for the first time, both for a stationary source and for a localized moving source such as a charge bunch; it is shown that these orthonormalization relations differ. Forces arising from wakefields are found, valid within and behind a distributed bunch. Deviation of bunch distribution from axisymmetry leads to generation of dipole modes of significant amplitude that may lead to instability. Poynting's theorem is examined for this system, and it is shown that convected Coulomb field energy must be subtracted from the Poynting flux to obtain the radiation power. This power, which balances drag on the bunch as calculated directly from the fields, is shown to flow in a direction opposite to that of the charge bunch. The results are easily generalized to bunches of arbitrary length and charge distribution, and to a train of such bunches. Numerical examples are presented for monopole, dipole, and quadrupole wakefield forces, and sample electric field patterns are shown to assist in understanding the unusual nature of this type of Cerenkov radiation. For a 2-nC rectangular drive bunch of length 0. 20 mm, moving along the axis of an alumina-lined waveguide (varepsilon=9.50) with inner and outer radii of 0.50 and 5.0 mm, a peak accelerating gradient behind the bunch of 155 MeV/m is predicted. This relatively high magnitude of accelerating gradient suggests that a simple uniform dielectric pipe could be the basis for the structure of a future high-gradient electron/positron linear accelerator, once low-emittance, kiloampere, subpicosecond electron bunches are available in the laboratory.

A subtle balance between competing interactions in iron-based superconductors (FeSCs) can be tipped by additional interfacial interactions in a heterostructure, often inducing exotic phases with unprecedented properties. Particularly when the proximity-coupled layer is magnetically active, rich phase diagrams are expected in FeSCs, but this has not been explored yet. Here, using high-accuracy 75As and 51V nuclear magnetic resonance measurements, we investigate an electronic phase that emerges in the FeAs layer below T 0 ~ 155 K of Sr2VO3FeAs, a naturally assembled heterostructure of an FeSC and a Mott-insulating vanadium oxide. We find that frustration of the otherwise dominant Fe stripe and V Neel fluctuations via interfacial coupling induces a charge/orbital order in the FeAs layers, without either static magnetism or broken C 4 symmetry, while suppressing the Neel antiferromagnetism in the SrVO3 layers. These findings demonstrate that the magnetic proximity coupling stabilizes a hidden order in FeSCs, which may also apply to other strongly correlated heterostructures.

A recently developed analytic theory for wakefields generated when a charge bunch, or train of bunches, passes along a dielectric-lined waveguide is applied to examine stability issues for this system. Such a configuration could be the key element in a two-beam accelerator to create colliding TeV-range electron and positron beams. It is shown that a 587 MV/m peak acceleration gradient can be achieved when a train of five 2 nC, 1.0 GeV, 0.20 mm long drive bunches passes along the axis of an alumina-lined waveguide (ε=9.5) with inner and outer radii 0.50 and 5.0 mm. The energy gain of a 0.2 nC test bunch in a 4 m long stage under these conditions is shown to be 1.48 GeV, for an average acceleration gradient of 370 MV/m. However, when a drive bunch is injected parallel to, but displaced from, the waveguide axis, transverse (mainly dipole) forces cause the tail of the bunch to swerve sharply toward the waveguide wall (head-to-tail instability), and to intersect the wall after traversing a relatively short distance. These results suggest that successful exploitation of the appealing strong wakefields generated by a train of drive bunches will require some means of transverse stabilization, such as an array of externally applied focusing and defocusing quadrupoles.