Coherent Cerenkov Radiation Research Articles

Positron driven high-field terahertz waves via dielectric wakefield interaction

Advanced acceleration methods based on wakefields generated by high energy electron bunches passing through dielectric-based structures have demonstrated $>$GV/m fields, paving the first steps on a path to applications such as future compact linear colliders. For a collider scenario, it is desirable that, in contrast to plasmas, wakefields in dielectrics do not behave differently for positron and electron bunches. In this Letter, we present measurements of large amplitude fields excited by positron bunches with collider-relevant parameters (energy 20 GeV, and $0.7 \times 10^{10}$ particles per bunch) in a 0.4 THz, cylindrically symmetric dielectric structure. Interferometric measurements of emitted coherent Cerenkov radiation permit spectral characterization of the positron-generated wakefields, which are compared to those excited by electron bunches. Statistical equivalence tests are incorporated to show the charge-sign invariance of the induced wakefield spectra. Transverse effects on positron beams resulting from off-axis excitation are examined and found to be consistent with the known linear response of the DWA system. The results are supported by numerical simulations and demonstrate high-gradient wakefield excitation in dielectrics for positron beams.

Photonic Free-Electron Lasers

A photonic free-electron laser (pFEL) produces coherent Cerenkov radiation from a set of parallel electron beams streaming through a photonic crystal. The function of the crystal is to slow down the phase velocity of a copropagating electromagnetic wave, such that also mildly relativistic electrons (of about 10-keV energy) can emit coherent Cerenkov radiation. Starting from spontaneous emission, the feedback of the radiation on the electrons results in bunching of the electrons on the scale of the radiation wavelength, and consequently, coherent radiation can build up. The frequency of the coherent mode is set by the electron velocity and wave dispersion of the photonic crystal and can, a priori, be continuously varied by varying the electron energy. The scale invariance of Maxwell's equation allows operation from Gigahertz to Terahertz and possible infrared (IR) frequencies without the need to increase the electron beam energy. Therefore, the pFEL is a very attractive, compact, and coherent radiation source that has the potential to significantly enhance the power available in the THz domain.

Resonant excitation of coherent Cerenkov radiation in dielectric lined waveguides

We report the observation of coherent Cerenkov radiation in the terahertz regime emitted by a relativistic electron pulse train passing through a dielectric lined cylindrical waveguide. We describe the beam manipulations and measurements involved in repetitive pulse train creation including comb collimation and nonlinear optics corrections. With this technique, modes beyond the fundamental are selectively excited by use of the appropriate frequency train. The spectral characterization of the structure shows preferential excitation of the fundamental and of a higher longitudinal mode.

PRELIMINARY RESULTS FROM THE UCLA/SLAC ULTRA-HIGH GRADIENT CERENKOV WAKEFIELD ACCELERATOR EXPERIMENT

The first phase of an experiment to study the performance of dielectric Cerenkov wakefield accelerating structures at extremely high gradients in the GV/m range has been completed. This experiment takes advantage of the unique SLAC FFTB electron beam and its demonstrated ultra-short pulse lengths and high currents (e.g., σz = 20 μ m at Q = 3 nC ). The FFTB electron beam has been successfully focused down and sent through varying lengths of fused silica capillary tubing with two different sizes: ID = 200 μ m / OD = 325 μ m and ID = 100 μ m / OD = 325 μ m . The pulse length of the electron beam was varied in the range 20 μ m < σz < 100 μ m which produced a range of electric fields between 2 and 20 GV/m at the inner surface of the dielectric tubes. We observed a sharp increase in optical emissions from the capillaries in the middle part of this surface field range which we believe indicates the transition between sustainable field levels and breakdown. If this initial interpretation is correct, the surfaced fields that were sustained equate to on axis accelerating field of several GV/m. In future experiments we plan to collect and measure coherent Cerenkov radiation emitted from the capillary tube to gain more information about the strength of the accelerating fields.

Cerenkov radiation in photonic crystals.

In a conventional material, the coherent Cerenkov radiation due to a moving charged particle is associated with a velocity threshold, a forward-pointing radiation cone, and a forward direction of emission. We describe different behavior for the Cerenkov radiation in a photonic crystal. In particular, this radiation is intrinsically coupled with transition radiation and is observable without any threshold. Within one particle-velocity range, we found a radiation pattern with a backward-pointing radiation cone. In another velocity range, backward-propagating Cerenkov radiation can be expected. Potential applications include velocity-sensitive particle detection and radiation generation at selectable frequencies.

Observation of coherent cerenkov radiation from a solid dielectric with short bunches of electrons

Short bunches of 150-MeV electrons of a linear accelerator passed along the surface of a crystal quartz or a teflon and coherent Cerenkov radiation from the solid dielectrics has been observed in the wavelength range from 0.5 to 4 mm. Properties of the radiation have been experimentally investigated. The angular distribution of the observed radiation showed a maximum peak in the direction of the Cerenkov angle with several satellite peaks. The intensity increased linearly with increasing the length of the medium and was proportional to the square of the number of electrons in the bunch. The spectral intensity was enhanced by almost five orders of magnitude in comparison with the theoretical calculation of incoherent radiation.

Coherent Cerenkov radiation from the Spacelab 2 electron beam

During the Spacelab 2 mission, a spacecraft called the Plasma Diagnostics Package (PDP) was released from the space shuttle to investigate the surrounding plasma environment. During an interval when the shuttle and PDP were magnetically connected, a continuous 1‐keV/50‐mA electron beam was ejected along a field line from an electron generator on board the shuttle. As the PDP flew by the beam, the PDP plasma wave instrument detected intense whistler mode radiation originating from the beam. It is believed that coherent Cerenkov radiation from bunches of beam electrons is responsible for the whistler mode radiation, where an electrostatic beam‐plasma instability forms the coherently radiating bunches. In this paper a detailed model of the coherent Cerenkov emission process is presented. A one‐dimensional computer simulation of the beam is used to model the expected phase space structure of the electrons, and power emitted from Cerenkov radiation is computed using an analytical expression. The calculated power from the modeled 200‐m beam segment is ∼5 × 10−8 W/Hz, which can easily account for the measured whistler mode wave power. The inclusion of coherent effects in the beam increases the wave powers by nearly 90 dB above incoherent power levels. These calculations demonstrate that a spontaneous emission process, alone, can account for the observed whistler mode wave powers.

The stimulated Cerenkov interaction and its applications

Reviewed are the results of two experiments in which momentum modulation of a relativistic electron beam by laser fields using the stimulated Cerenkov interaction was measured, and coherent Cerenkov radiation from the optically bunched electron beam was observed. In the first experiment, light at 1.06 μm from a 30 MW Nd:YAG laser intersected 102 MeV electrons at an angle of 18 mrad in hydrogen gas, which was used as the phase-matching medium. The change in the electron-beam energy spectrum in the presence of the laser was measured, together with its functional dependence on the index of refraction of the phase-matching medium. In the second experiment, the same laser intersected 55.9 MeV electrons at an angle of 17 mrad, again in hydrogen gas. Coherent Cerenkov radiation at the second harmonic of the laser frequency (0.532 μm) was measured and indicated that the electrons were bunched on the order of the laser wavelength as a result of being velocity modulated by the laser. Applications of these results, such as development of optical klystrons and laser-driven particle accelerators, are considered. The characteristics of these devices and possible design configurations are discussed.

Observation of coherent Cerenkov radiation at optical frequencies

Coherent Cerenkov radiation at 0.532 μm was observed from an optically bunched relativistic electron beam. Bunching was induced by means of the inverse Cerenkov effect wherein, to achieve phase matching, light at 1.06 μm from a Nd:YAG laser intersected the electron beam at the Cerenkov angle within a gaseous medium. The pressure dependence of this radiation was verified.

Generation of coherent Cerenkov radiation with an intense relativistic electron beam

Theoretical predictions and preliminary experimental results for a microwave source consisting of a dielectric loaded waveguide and an intense relativistic electron beam are compared.

OPTICAL SECOND HARMONIC GENERATION IN FORM OF COHERENT CERENKOV RADIATION FROM A THIN-FILM WAVEGUIDE

We report optical second harmonic generation in form of coherent Cerenkov radiation. The fundamental wave at 1.06 μm propagates in a thin-film optical waveguide which is simply a ZnS film vacuum-deposited on a single-crystal ZnO substrate. The nonlinear polarization excited in the substrate has a phase velocity exceeding that of radiation propagating freely in the substrate material. It thus acts as the source of the observed Cerenkov radiation.

Coherent radio emission from cosmic ray air-showers in the turbulent atmosphere

Coherent electromagnetic Cerenkov radiation is produced by cosmic ray air showers passing through the atmosphere. This radiation is detected by radio telescopes. We demonstrate here that the effect of random spatial fluctuations in the refractive index of air, about a mean exceeding unity, causes the airshower to emit not only the spontaneous coherent radio emission described elsewhere by Kahn and Lerche, but also an induced radiation field which can exceed the spontaneous field in certain frequency bands. Further the conditions for emission of the coherent radio Cerenkov radiation are altered by the presence of the refractive index fluctuations. And the Earth's magnetic field gives rise to the dominant term in the far-field radiation, be it spontaneous or induced, since it causes a systematic separation of electrons and positrons in the shower which, for parameters currently acceptable for air showers, is the major factor in determining the far-field radiation pattern. Also we suggest that the coherent 500 Mc/sec radiation seen from occasional showers is probably a reflection of an atmospheric correlation length of order 15 cm at the time the shower passes through the atmosphere.