We report a new regime of single-surface multipactor that was observed during high-power testing of an 11.424-GHz alumina-based dielectric-loaded accelerating structure. Previous experimental observations of single-surface multipactor on a dielectric occurred in cases for which the rf electric field was tangential and the rf power flow was normal to the dielectric surface (such as on rf windows) and found that the fraction of power absorbed at saturation is approximately 1%, independent of the incident power. In this new regime, in which strong normal and tangential rf electric fields are present and the power flow is parallel to the surface, the fraction of power absorbed at saturation is an increasing function of the incident power, and more than half of the incident power can be absorbed. A simple model is presented to explain the experimental results.

Collinear wakefield acceleration has been long established as a method capable of generating ultrahigh acceleration gradients. Because of the success on this front, recently, more efforts have shifted towards developing methods to raise the transformer ratio (TR). This figure of merit is defined as the ratio of the peak acceleration field behind the drive bunch to the peak deceleration field inside the drive bunch. TR is always less than 2 for temporally symmetric drive bunch distributions and therefore recent efforts have focused on generating asymmetric distributions to overcome this limitation. In this Letter, we report on using the emittance-exchange method to generate a shaped drive bunch to experimentally demonstrate a TR≈5 in a dielectric wakefield accelerator.

Dielectric-loaded power extraction is a method for the generation of high-power radio frequency (rf) waves under development for future particle accelerators. In this method, a high-charge electron beam drives a wakefield in a dielectric-loaded waveguide (the decelerator) and an rf output coupler extracts the rf power into an external waveguide. We report on the experimental demonstration of a 7.8 GHz dielectricloaded power extractor at the Argonne Wakefield Accelerator facility. We have generated more than 30 MW of rf power with a pulse length of approximately 1.7 ns by passing a single 66 nC electron bunch through the power extractor. We have also used a train of 4 electron bunches to show a clear signature of field superposition. Test results are in good agreement with predictions.

Polyimide/silica (PI/silica) nanocomposite films were successfully prepared via in situ dispersive polymerisation and thermal imidisation. In order to obtain homogeneous nanoscale dispersibility and good compatibility with the PI matrix, hydrophobic aerosil was selected as the nanosilica precursor. 4,4-Bis(3-aminophenoxy)biphenyl (4,3-BAPOBP) was used as diamine to improve the processability of PI. The PI/silica nanocomposite films were characterised using Fourier transform infrared spectroscopy, scanning electron microscopy and differential scanning calorimetry. The mechanical and dielectric properties of the films were also measured. The results demonstrate that the tensile strength and breakdown strength of films can be markedly improved by the addition of appropriate amounts of silica to the PI matrix. At a silica content of 4.0 wt-%, the tensile strength and the breakdown strength of the films increased by 21 and 13%, respectively, compared with the neat PI. Thus, it is feasible to use nanosilica to improve the properties of PI.

We report on a design of a TM-mode based metamaterial-loaded waveguide. Network analyzer measurements demonstrated a left-handed propagation region for the TM11 mode at around 10 GHz. A beamline experiment was performed with the metamaterial-loaded waveguide. In this experiment, a 6 MeV electron beam passes through the waveguide and generates a wakefield via the Cherenkov radiation mechanism. We detected a signal in the left-handed frequency band at 10 GHz. This is an indirect demonstration of reverse Cherenkov radiation as predicted in the work of Veselago [Sov. Phys. Usp. 10, 509 (1968)] and discussed in the works of Lu et al. [Opt. Express 11, 723 (2003)], Averkov and Yakovenko [Phys. Rev. B 72, 205110 (2005)], and Tyukhtin et al. [IEEE, Proceedings of the PAC, 2007 (unpublished), pp. 4156–4158]. Cherenkov radiation in artificially constructed materials [metamaterials (MTMs)] can provide unusual engineered features that can be advantageous for particle detector design.