Dmitry Yu. Shchegolkov

First experimental demonstration of a photonic band gap channel-drop filter at 240 GHz

We have designed, fabricated, and tested a novel photonic band gap (PBG) channel-drop filter (CDF) operating at around 240 GHz. A PBG CDF is a device that allows the channeling of selected frequencies from continuous spectra into separate waveguides through select defects in a PBG structure. It is compact and configurable, and thus, it can be employed for millimeter-wave spectrometry with applications in communications, radio astronomy, and radar receivers for remote sensing and nonproliferation. In this paper we present the design, modeling, and fabrication methods used to produce a silicon-based PBG CDF, and demonstrate its ability to filter the frequency of 240 GHz with a linewidth of approximately 1 GHz and transmission of 25 dB above background.

A proposed measurement of the Reverse Cherenkov Radiation effect in a metamaterial-loaded circular waveguide

We have recently proposed an experiment on verification of the Reverse Cherenkov Radiation (RCR) effect in a Left-Handed-Material-loaded waveguide [1]. Applications of the RCR effect may range from novel higher-order-mode suppressors in microwave and millimeter-wave sources to improved particle detectors for satellite non-proliferation missions. The experimental configuration includes a circular waveguide filled with an artificial metamaterial with simultaneously negative permittivity and permeability, in which the electromagnetic wave with a frequency of 95 GHz will interact with an electron beam. We have demonstrated that for certain values of effective permittivity and permeability only the backward-propagating mode can be exited by the electron beam. At the conference we will present some newly developed metamaterial designs, which we plan to employ for producing the proper effective medium parameters for this experiment.

Emittance Effects on Gain in

We consider the main effects of beam emittance on W-band traveling-wave tube (TWT) performance and gain. Specifically, we consider a representative dielectric TWT structure with ~5 dB/cm of gain driven by a 5-A, 20-keV, sheet electron beam that is focused by a wiggler magnetic field. The normalized beam transverse emittance must be about 1 μm or lower to ensure that both the transport is stable and the gain is not degraded by the effective energy spread arising from the emittance. This emittance limit scales roughly inversely with frequency.

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We report results from recent 2.1 GHz superconducting radio frequency (SRF) photonic band gap (PBG) resonator experiments at Los Alamos. Two 2.1 GHz PBG cells with elliptical rods were fabricated and tested at high power in a liquid helium bath at the temperatures of 4 K and below 2 K. The described SRF PBG cells were designed with a particular emphasis on changing the shape of the PBG rods to reduce peak surface magnetic fields and at the same time to preserve its effectiveness at damping higher-order-modes. The superconducting PBG cavities have great potential for damping long-range wakefields in SRF accelerator structures without affecting the fundamental accelerating mode. The cells performed in accordance with simulations predictions and the maximum achieved accelerating gradient was 18.3 MV/m. This represents a 30% increase over gradients previously demonstrated in superconducting PBG cavities with round rods.

-Band TWTs

We study experimentally and numerically the electro-optic (EO) effect in a single metamaterial cell incorporating a nonlinear crystal. A metal split-ring fabricated on a ZnTe substrate resonating at 3 GHz was excited with a tunable microwave source. A linearly polarized laser beam probed the EO effect. Enhanced EO modulation was observed in the split-gap at the resonant frequency proportional to Q-factor. The electric field surrounding the metamaterial element in the ZnTe was mapped by the EO effect and agrees well with simulation. We show that high-Q metamaterials incorporating an EO crystal in the split-gap could lead to improved EO devices.

Raising gradient limitations in 2.1 GHz superconducting photonic band gap accelerator cavities

We present the results of our investigations of fabrication technologies for ceramic photonic band gap (PBG) structures for mm-wave traveling-wave tubes (TWTs). There is a need for a high-bandwidth, high output power TWT operating at relatively low electron-beam voltages (20 keV). The key advance needed for this technology is the development of a novel high-frequency TWT structure. We proposed to put together a TWT with a sheet electron beam in an elliptical wide-bandwidth dielectric RF structure. PBG structures can be designed to be mode-selective and enable the required wide bandwidth and output power for the TWT. We conducted a feasibility study on fabrication of dielectric PBG structures in a high-epsilon ceramic material, designed suitable RF structure, drilled holes in high-dielectric ceramic blanks, measured them and studied the effect of holes size variations and misalignments on the RF mode.

Direct observation of electro-optic modulation in a single split-ring resonator

We describe a conceptual proposal to combine the Dielectric Wakefield Accelerator (DWA) with the Emittance Exchanger (EEX) to demonstrate a high-brightness DWA with a gradient of above 100 MV/m and less than 0.1% induced energy spread in the accelerated beam. We currently evaluate the DWA concept as a performance upgrade for the future LANL signature facility MaRIE with the goal of significantly reducing the electron beam energy spread. The preconceptual design for MaRIE is underway at LANL, with the design of the electron linear accelerator being one of the main research goals. Although generally the baseline design needs to be conservative and rely on existing technology, any future upgrade would immediately call for looking into the advanced accelerator concepts capable of boosting the electron beam energy up by a few GeV in a very short distance without degrading the beams quality. Scoping studies have identified large induced energy spreads as the major cause of beam quality degradation in high-grad...

Fabrication of ceramic structures for MM-wave traveling wave tubes

We have designed, fabricated and tested with low power a novel W-band TWT based on a slow-wave cylindrically-symmetric PBG dielectric structure, or an “omniguide”. PBG TWT structures have great potential for very large bandwidth and linear dispersion. A gain experiment is under way at Los Alamos with the structure being driven by a 2 A, 110 kV electron beam. In this presentation we will report the design of the omniguide TWT structure, theoretical computations of the gain and the results of S-parameters measurements. We will also describe the electron beam test stand setup and report first results from the gain measurement experiment.

Possibility for ultra-bright electron beam acceleration in dielectric wakefield accelerators

We present simulations of shaped electron beam production from diamond field emitter array (DFEA) cathodes [1, 2]. DFEAs are arrays of diamond pyramids with bases of the order of 10 microns that produce high current densities. These arrays can be fabricated in arbitrary shapes such as a triangle or a double triangle, so that they produce an inherently shaped beam. These transversely shaped beams can be put through an emittance exchanger (EEX) [3-7] to produce a longitudinally shaped electron beam distribution for use with high-transformer ratio wakefield accelerators. Simulations are conducted with MICHELLE. We design cathodes and focusing systems that preserve the beam’s shape while transporting it to the emittance exchanger.

20.2: Testing of the omniguide traveling-wave tube

A number of electron beam optics elements allow for substantial control over the phase space of the relativistic bunches. We study a scheme capable of generating a beam distribution consisting of several well-separated energy bands. Such a distribution is unstable and the two-stream instability driven by the space charge develops. We investigate the instability analytically and numerically.