Coherent Cherenkov-Cyclotron Radiation Excited by an Electron Beam in a Metamaterial Waveguide

An electron beam passing through a metamaterial structure is predicted to generate reversed Cherenkov radiation, an unusual and potentially very useful property. We present an experimental test of this phenomenon using an intense electron beam passing through a metamaterial loaded waveguide. Power levels of up to 5 MW are observed in backward wave modes at a frequency of 2.40 GHz using a one microsecond pulsed electron beam of 490 keV, 84 A in a 400 G magnetic field. Contrary to expectations, the output power is not generated in the Cherenkov mode. Instead, the presence of the magnetic field, which is required to transport the electron beam, induces a Cherenkov-cyclotron (or anomalous Doppler) instability at a frequency equal to the Cherenkov frequency minus the cyclotron frequency. Nonlinear simulations indicate that the Cherenkov-cyclotron mode should dominate over the Cherenkov instability at a lower magnetic field where the highest output power is obtained.United States. Air Force Office of Scientific Research. Multidisciplinary University Research Initiative (Grant FA9550-12-1-0489

Controlling Cherenkov angles with resonance transition radiation

Cherenkov radiation provides a valuable way to identify high energy particles in a wide momentum range, through the relation between the particle velocity and the Cherenkov angle. However, since the Cherenkov angle depends only on material's permittivity, the material unavoidably sets a fundamental limit to the momentum coverage and sensitivity of Cherenkov detectors. For example, Ring Imaging Cherenkov detectors must employ materials transparent to the frequency of interest as well as possessing permittivities close to unity to identify particles in the multi GeV range, and thus are often limited to large gas chambers. It would be extremely important albeit challenging to lift this fundamental limit and control Cherenkov angles as preferred. Here we propose a new mechanism that uses constructive interference of resonance transition radiation from photonic crystals to generate both forward and backward Cherenkov radiation. This mechanism can control Cherenkov angles in a flexible way with high sensitivity to any desired range of velocities. Photonic crystals thus overcome the severe material limit for Cherenkov detectors, enabling the use of transparent materials with arbitrary values of permittivity, and provide a promising option suited for identification of particles at high energy with enhanced sensitivity.Comment: There are 16 pages and 4 figures for the manuscript. Supplementary information with 18 pages and 5 figures, appended at the end of the file with the manuscript. Source files in Word format converted to PDF. Submitted to Nature Physic

Thermal and irradiation induced interdiffusion in Fe3O4/MgO(0 0 1) thin film

The interface reactions in an epitaxial 10 nm-thick Fe3O4/MgO(0 0 1) film were investigated by using Rutherford Backscattering spectrometry (RBS), channeling (RBS-C) and X-ray reflectometry (XRR). The as-grown film had a good crystallinity indicated by the minimum yield and the half-angle value for Fe, respectively, [chi]min(Fe) = 22% and [psi]1/2(Fe) = 0.62°. Annealing the films under partial argon pressure up to 600 °C led to a large enhancement of Mg out-diffusion into the film forming a wustite-type phase, but the total layer thickness did not change much. Ion irradiation of the film by 1 MeV Ar ion beam caused a strong Fe ion mixing resulting in a large interfacial zone with a thickness of 23 nm

Structure, composition and crystallinity of epitaxial magnetite thin films

Epitaxially-grown Fe3O4(0 0 1) thin films by reactive deposition on MgO(1 0 0) substrates were studied using low-energy electron diffraction (LEED), conversion electron Mössbauer spectroscopy (CEMS), Rutherford backscattering spectrometry (RBS), channeling (RBS-C) experiments and X-ray reflectometry (XRR). No visible influence from the ion irradiation of the samples on the CEMS spectra was found, while surface oxidation of the samples was observed after exposure to the atmospheric pressure. RBS analysis indicated the presence of magnesium with an average amount of 3% in the films. RBS-C experiments yielded a value of 22% for the minimum yield of Fe and a value of 0.62° for the half-angle for Fe in the film indicating a good crystal quality of the films. The value for film-thickness obtained from XRR is in a good agreement with that from RBS and the nominal value