3,830 research outputs found
Four wave mixing with self-phase matching due to collective atomic recoil
We describe a method for non-degenerate four-wave mixing in a cold sample of
4-level atoms. An integral part of the four-wave mixing process is a
collective instability which spontaneously generates a periodic density
modulation in the cold atomic sample with a period equal to half of the
wavelength of the generated high-frequency optical field. Due to the generation
of this density modulation, phase-matching between the pump and scattered
fields is not a necessary initial condition for this wave-mixing process to
occur, rather the density modulation acts to "self phase-match" the fields
during the course of the wave-mixing process. We describe a one-dimensional
model of this process, and suggest a proof-of-principle experiment which would
involve pumping a sample of cold Cs atoms with three infra-red pump fields to
produce blue light.Comment: to appear in Physical Review Letter
Inducing strong density modulation with small energy dispersion in particle beams and the harmonic amplifier free electron laser
We present a possible method of inducing a periodic density modulation in a particle beam with little increase in the energy dispersion of the particles. The flow of particles in phase space does not obey Liouville's Theorem. The method relies upon the Kuramoto-like model of collective synchronism found in free electron generators of radiation, such as Cyclotron Resonance Masers and the Free Electron Laser. For the case of an FEL interaction, electrons initially begin to bunch and emit radiation energy with a correlated energy dispersion which is periodic with the FEL ponderomotive potential. The relative phase between potential and particles is then changed by approximately 180 degrees. The particles continue to bunch, however, there is now a correlated re-absorption of energy from the field. We show that, by repeating this relative phase change many times, a significant density modulation of the particles may be achieved with only relatively small energy dispersion. A similar method of repeated relative electron/radiation phase changes is used to demonstrate supression of the fundamental growth in a high gain FEL so that the FEL lases at the harmonic only
Towards Zeptosecond-Scale Pulses from X-Ray Free-Electron Lasers
The short wavelength and high peak power of the present generation of
free-electron lasers (FELs) opens the possibility of ultra-short pulses even
surpassing the present (tens to hundreds of attoseconds) capabilities of other
light sources - but only if x-ray FELs can be made to generate pulses
consisting of just a few optical cycles. For hard x-ray operation (~0.1nm),
this corresponds to durations of approximately a single attosecond, and below
into the zeptosecond scale. This talk will describe a novel method to generate
trains of few-cycle pulses, at GW peak powers, from existing x-ray FEL
facilities by using a relatively short 'afterburner'. Such pulses would enhance
research opportunity in atomic dynamics and push capability towards the
investigation of electronic-nuclear and nuclear dynamics. The corresponding
multi-colour spectral output, with a bandwidth envelope increased by up to two
orders of magnitudes over SASE, also has potential applications.Comment: Submitted to 35th International Free Electron Laser Conference, New
York, 201
Start-to-end modelling of a mode-locked optical klystron free electron laser amplifier
A free electron laser (FEL) in a mode-locked optical klystron (MLOK) configuration is modelled using start-to-end simulations that simulate realistic electron beam acceleration and transport before input into a full three-dimensional FEL simulation code. These simulations demonstrate that the MLOK scheme is compatible with the present generation of radiofrequency accelerator designs. A train of few-optical cycle pulses is predicted with peak powers similar to those of the equivalent conventional FEL amplifier. The role of electron beam energy modulation in these results is explained and the limitations of some simulation codes discussed. It is shown how seeding the FEL interaction using a High Harmonic seed laser can improve the coherence properties of the output
An extended model of the quantum free-electron laser
Previous models of the quantum regime of operation of the Free Electron Laser
(QFEL) have performed an averaging and the application of periodic boundary
conditions to the coupled Maxwell - Schrodinger equations over short, resonant
wavelength intervals of the interaction. Here, an extended, one-dimensional
model of the QFEL interaction is presented in the absence of any such averaging
or application of periodic boundary conditions, the absence of the latter
allowing electron diffusion processes to be modeled throughout the pulse. The
model is used to investigate how both the steady-state (CW) and pulsed regimes
of QFEL operation are affected. In the steady-state regime it is found that the
electrons are confined to evolve as a 2-level system, similar to the previous
QFEL models. In the pulsed regime Coherent Spontaneous Emission (CSE) due to
the shape of the electron pulse current distribution is shown to be present in
the QFEL regime for the first time. However, unlike the classical case, CSE in
the QFEL is damped by the effects of quantum diffusion of the electron
wavefunction. Electron recoil from the QFEL interaction can also cause a
diffusive drift between the recoiled and non-recoiled parts of the electron
pulse wavefunction, effectively removing the recoiled part from the primary
electron-radiation interaction.Comment: Submitted to Optics Expres
The Electrosphere of Macroscopic "Quark Nuclei": A Source for Diffuse MeV Emissions from Dark Matter
Using a Thomas-Fermi model, we calculate the structure of the electrosphere
of the quark antimatter nuggets postulated to comprise much of the dark matter.
This provides a single self-consistent density profile from ultrarelativistic
densities to the nonrelativistic Boltzmann regime that use to present
microscopically justified calculations of several properties of the nuggets,
including their net charge, and the ratio of MeV to 511 keV emissions from
electron annihilation. We find that the calculated parameters agree with
previous phenomenological estimates based on the observational supposition that
the nuggets are a source of several unexplained diffuse emissions from the
Galaxy. As no phenomenological parameters are required to describe these
observations, the calculation provides another nontrivial verification of the
dark-matter proposal. The structure of the electrosphere is quite general and
will also be valid at the surface of strange-quark stars, should they exist.Comment: 20 Pages, REVTeX4.
Transform-limited X-ray pulse generation from a high-brightness self-amplified spontaneous-emission free-electron laser
A method to achieve high-brightness self-amplified spontaneous emission (HB-SASE) in the free-electron laser (FEL) is described. The method uses repeated nonequal electron beam delays to delocalize the collective FEL interaction and break the radiation coherence length dependence on the FEL cooperation length. The method requires no external seeding or photon optics and so is applicable at any wavelength or repetition rate. It is demonstrated, using linear theory and numerical simulations, that the radiation coherence length can be increased by approximately 2 orders of magnitude over SASE with a corresponding increase in spectral brightness. Examples are shown of HB-SASE generating transform-limited FEL pulses in the soft x-ray and near transform-limited pulses in the hard x-ray. Such pulses may greatly benefit existing applications and may also open up new areas of scientific research
A fully 3D unaveraged non-localized electron, parallelized-computational model of the FEL
A new unaveraged 3D parallelized numerical model has been developed that will allow investigation of previously unexplored FEL physics. Unaveraged models are required to describe such effects as amplification of Coherent Spontaneous Emission and non-localised electron dynamics (see e.g. [1] and refs therein). A previous parallelized 3D model [2] was based upon a mixed finite element/Fourier method, however, there were some limitations in the parallel algorithm and numerical routines. These limitations are removed in the new model presented here by using only transforms in Fourier space enabling more effective data or-ganization across multiple parallel processors and therefore allowing larger, more complex FEL systems to be studied. Furthermore, unlike the previous 3D model, which uses commercial numerical packages, the new simulation code uses only open-source routines
Start to end simulations of the ERL prototype at Daresbury Laboratory
Daresbury Laboratory is currently building an Energy Recovery Linac Prototype (ERLP) that will serve as a research and development facility for the study of beam dynamics and accelerator technology important to the design and construction of the proposed 4th Generation Light Source (4GLS) project. Two major objectives of the ERLP are the demonstration of energy recovery and of energy recovery from a beam disrupted by an FEL interaction as supplied by an infrared oscillator system. In this paper we present start-to-end simulations of the ERLP including such an FEL interaction. The beam dynamics in the highbrightness injector, which consists of a DC photocathode Gun and a superconducting booster, have been modelled using the particle tracking code ASTRA. After the booster the particles have been tracked with the code elegant. The 3D code GENESIS 1.3 was used to model the FEL interaction with the electron beam at 35 MeV. A brief summary of impedance and wakefield calculations for the whole machine is also given
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