free-electron lasers (FELs)

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The detrimental effect of spontaneous emission in quantum free electron lasers : a discrete Wigner model

The detrimental effect of spontaneous emission in quantum free electron lasers : a discrete Wigner model

Free electron lasers (FELs) using highly relativistic electron beams passing through very long magnetic undulators are currently operating as high-intensity coherent x-ray radiation sources, with many interesting applications [1, 2]. A proposed extension of these machines includes the use of laser wigglers [3] or micro-undulators [4], in order to make such devices more compact and flexible. In these new schemes, the quantum recoil associated with emission of each photon starts to play an important role, since the photon recoil can be comparable with the fraction of the electron momentum transferred to the radiation. From this perspective, the Quantum FEL (QFEL) concept [5, 6] is attractive as a potential source of intense, quasi-monochromatic radiation at wavelengths in the Angstrom even sub-Angstrom range.

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Selftrapping, biomolecules and free electron lasers

Selftrapping, biomolecules and free electron lasers

For a solid-state physicist, the controlled movement of energy and charge are central phenomena. Novel materials have often pointed to new mechanisms: self-trapping in halides giving energy localization, the incoherent motion of small polarons in colossal magnetoresistance oxides, solitons in conducting polymers, magnetic polarons in magnetic semiconductors, the quantum motion of electrons in mesoscopic metals, the regimes of both coherent and incoherent quantum propagation of muons in solids, and so on. The issues of energy and charge transport are equally crucial in living matter, but are understood less well. How can the energy from light or chemical processes be moved around and used efficiently? The important paper by Austin et al (2003) in this special issue shows that new experimental tools, using the ultra short, tunable laser pulses from free electron lasers, open up the possibility of resolving outstanding problems in understanding the energy and charge processes which underpin life itself. In biological physics, one of the most intriguing ideas is that of the soliton, whose coherent motion is believed to enable efficient energy transport. Here we shall discuss the connection between these ideas of solitons and similar concepts occurring in more conventional condensed matter systems.

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Towards zeptosecond-scale pulses from x-ray free electron lasers

Towards zeptosecond-scale pulses from x-ray free electron lasers

An alternative might be to disregard FEL amplification to establish microbunching, and instead use an external source (though this itself may be a FEL) to induce mi- crobunching (or a single sharp current spike) over a region only a few cycles in length and then make it radiate in an undulator, as shown in Figure 2. There are several propos- als to do this, such as by Zholents and Fawley [31] or by Xiang et al. [32], though again the slippage has a limiting effect. If the number of undulator periods in the radiator is greater than the number of cycles in the microbunched re- gion, then the slippage effect dominates and lengthens the pulse. The undulator must therefore be similarly short - also a few periods - otherwise slippage of the radiation rel- ative to the electrons broadens the pulse. As a consequence proposals for this type of technique predict relatively low power compared to FEL saturation, however the power could potentially be increased by future improvements in electron beam brightness. Requiring the microbunching to be imposed by an external source may also present difficul- ties in scaling such techniques to the shortest wavelengths of FELs in some cases.

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Modelling elliptically polarised free electron lasers

Modelling elliptically polarised free electron lasers

In a planar undulator, the electrons have a fast axial ‘jitter’ motion at twice the undulator period as they propagate along the undulator axis. In addition to the coupling of the electrons to the fundamental radiation wavelength, the jitter motion allows coupling to odd harmonics of the fundamental, which can also experience gain. A commonly used model used for simulating the FEL interaction is the ‘averaged’ model which, as the name suggests, averages the governing Maxwell and Lorentz equations describing the electron / radiation coupling over an undulator period [12]. The averaging of the jitter motion introduces coupling terms described by a difference of Bessel functions which depend upon both the undulator strength and the harmonic [ 12, 13 ] . For an helical undulator, there is no electron jitter and the difference of Bessel functions coupling terms become a constant for the fundamental and zero for all harmonics, i.e in an helical undulator there is no gain coupling to harmonics.

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Electromagnetic field measurements inside photonic crystals developing a photonic free electron laser

Electromagnetic field measurements inside photonic crystals developing a photonic free electron laser

In a photonic free electron lasers (pFEL) electrons propagate with ve- locities much smaller then the speed of light through a photonic crystal (PhC). The electrons emit radiation inside the PhC through the Cherenkov effect. Cerenkov radiation is emitted whenever charge particles move faster than phase velocity 3 of the radiation. Therefore, Cerenkov radiation is only emitted when the particles move through a medium, as in a vacuum the radiation always moves faster than the electrons. When the medium is a photonic crystal (PhC), one can take full advantage of the PhC to engineer the propagation of the light. For example, the light can be slowed down considerably [1] and this allows the use of low energy electrons i.e. the ve- locity of electrons is a small fraction of the speed of light in vacuum. Before discussing the two main physics principles behind the pFEL we will first

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Chirped and modulated electron pulse free electron laser techniques

Chirped and modulated electron pulse free electron laser techniques

greater traditional linear accelerators, which offers the po- tential to reduce the total length of the FEL. Electron pulses used in free electron lasers can exhibit a large energy chirp (greater than 1 % of mean electron beam energy) which can degrade the FEL interaction. Linear energy chirps have been previously studied in [1] the results of this work have been recreated here using Puffin [2] an unaveraged 3D parallel FEL simulator. The results of these chirped pulse simula- tions are in good agreement with [1] showing the flexibility of Puffin. Electron pulses from plasma accelerators are lim- ited by a large energy spread, this is also issue with older accelerators were energy spread is sacrificed for a larger rho (a measure of FEL efficiency) and higher pulse ener- gies. A method that may allow the free electron laser to operate with a large energy spread is proposed, simulations were performed using Puffin. In this method a chirped elec- tron pulse is split in a number of chirped electron beams or beamlets. To sustain the FEL interaction radiation is passed from beamlet to beamlet by applying a series of chicane slip-

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Aspects of phase retrieval in x ray crystallography

Aspects of phase retrieval in x ray crystallography

The advent of X-ray free electron lasers, with their extreme brightness, ultra-short pulses and megahertz repetition rate is offering new opportunities for the study of a variety of new samples (2D crystals, fibers, nanocrystals, and single particles) and the development of new techniques (SFX, SPI) in protein X-ray crystallography. The phase problem is not exempt from this revolution, and the problem can be potentially eased if continuous diffraction can be measured along one or more dimensions in reciprocal space. For the case of 2D crystals, the additional intensity data falls short of rendering the solution to the phase problem unique, so that additional real space data or knowl- edge on the sample is still needed. A parameter, denoted by Λ 0 2dc , which depends on the shape of the molecular envelope and the resolution was defined and proved more useful than the usual constraint ratio Ω 2dc for determining the uniqueness of the solution.

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Spatial Meaning of Quantum Mechanics

Spatial Meaning of Quantum Mechanics

energy and energy transformation without the necessity of dealing with the force directly. The energy does not belong to the electron alone, but both the electron and the electric field. For Schrödinger equation, it is an inappropriate perspective to separate the electron from the Hamiltonian and assign the energies only to the electron, regardless of the electric field. A space consisting of electron and electric field is different from vacuum, and such a space may be called a field. As such, it is the space (or the field) rather than the electron exhibits a wave phenomenon. The wave like light wave can be made quantization to give particles, none of which is the electron that is a material particle. It can be understood since de Broglie’s work antedated Schrödinger equation and Dirac radiation field theory, and his postulate was not able to ascribe the wave properties to the space containing electrons. It is the continuous mo- tion of the electron that causes the wave. It seems that Schrödinger did not perceive that the wave originates in the electron’s motion, and the wave functions can change with time even if for a steady system. The wave mo- tion requires each wave function to vibrate at its own site. The wave hence is a disturbance of the wave func- tions in the space. A wave function at a position transfers its physical state in this way to the wave function at the next position. The wave as a whole is to be made quantization to produce particles to be like photons. Even though Schrödinger equation for single electron gives perfectly accurate solution, there is still more or less defi- ciency of dynamical characteristics for the wave functions. His time-dependent wave equation is a second-order differential in the three spatial coordinates but only a first-order differential in time. Actually, the wave motion is a classical phenomenon satisfying a classical dynamical equation with a second-order differential in time. In addition, the wave function of a system of N electrons is a function in a 3N-dimensional configuration space, in which the Born’s probabilistic interpretation cannot be used. It is difficult to imagine that a physical reality ex- ists in such an abstract multidimensional configuration space. The double-slit experiment is a successful exam- ple of the probabilistic interpretation [14] [20]. The behavior of an electron in the space between the slit and the detector shows the characteristics of a free electron. For these reasons, in the third section of this paper, the wave functions for the space involving a free electron are obtained by means of a dynamical equation. Then, the wave quantization for such space will be made, which will give a theoretical evidence of the probabilistic inter- pretation. In the fourth section we discuss different meanings of wave-particles for photons and for electron. The relativistic effect will not be discussed in this paper.

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Influence of doping density on electron dynamics in GaAs/AlGaAs quantum cascade lasers

Influence of doping density on electron dynamics in GaAs/AlGaAs quantum cascade lasers

The subband energies and wave functions were calcu- lated by solving the envelope function Schrödinger equation in an effective mass approximation with conduction band dispersion nonparabolicity taken into account via Kane’s two-band model of the energy-dependent effective mass. The Schrödinger equation was solved for three full QCL periods. Clearly, the states confined mostly in the central period are calculated with better accuracy than states in other periods, owing to distant boundary conditions, and therefore these states have been taken to form a period state set. Based on the space and energy shift invariance, they were afterwards used to create the states of all other periods. The relevant scattering mechanisms that have been taken into account are based on electron-phonon, electron-electron, and electron- impurity interactions. The latter of which can be important at high doping levels. The scattering rates were calculated us- ing Fermi’s golden rule and averaged over the in-plane wave vector assuming Fermi-Dirac distributions over subbands. For calculating the electron-LO-phonon scattering, bulk pho- non modes were assumed, which is widely used in the litera- ture owing to a good agreement with the experiment for QCL structures. 34,38,40,47 Single subband screening of the

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Frequency modulated free electron laser

Frequency modulated free electron laser

For example, it has been shown via simulations that equally spaced frequency modes may be generated in a single-pass FEL amplifier [4, 5] by introducing a series of delays to the electron beam with respect to the co-propagating radiation field (e.g. by using magnetic chicanes placed between undulator modules). These radiation modes are formally identical to those created in an oscillator cavity. Analogously with a mode-locked conventional laser oscillator, a modulation of the electron beam energy [4, 5] or current [6] at the mode spacing can phase-lock the modes and amplify them to generate a train of short, high power pulses.

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The Role of Using Ultra Short Pulsed Lasers in Determining Dynamical Dependent of Free Electron Density in the Skin Tissue for Treatment of Skin Cancer

The Role of Using Ultra Short Pulsed Lasers in Determining Dynamical Dependent of Free Electron Density in the Skin Tissue for Treatment of Skin Cancer

Abstract: We study on the new advances in laser cell surgery for skin cancer treatment and investigate on the working mechanisms of femtosecondlaser. Also, influences of the critical electron density and radiation intensity on the free- electron density for ablation on the epidermis and dermis tissues are investigated. Our studies on this work, show that if the amount of radiation intensity of pulsed laser is increased then the distance of focal spot is decreased also the amount of time dependent free- electron density become more and more, such that this case is proper for skin cancer treatment. Also, our calculations for skin tissue show that optimum time dependent free electron density for dermis layer at angle 16˚ and 22˚ which is a function of wavelength, beam width, beam radius, amplitude of the beam radiation strength and pulse duration is more than for epidermis layer.

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An extended model of the quantum free-electron laser

An extended model of the quantum free-electron laser

profile is used. This has a discontinuous current at either end of the pulse which, in previous classical models, has been shown to act as a strong source of Coherent Spontaneous Emission (CSE) [27]. CSE effects have not previously been investigated in the QFEL regime, requiring the extended model used here. While top-hat or other current profiles with sufficiently large current gradients to generate significant CSE may not yet be achievable, progress in this field is being made [30] and cannot be ruled out in future developments. We note that the top-hat current profile used here means that, due to the current discontinuities at either ends of the pulse, there are large electron momentum uncertainties and so a large energy spread in these regions due to the Heisenberg uncertainty principle [31]. Such energy spreads do not inhibit the generation of CSE however, as such emission is spontaneous and independent of the FEL interaction [27].

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The quantum free electron laser

The quantum free electron laser

Free-electron lasers (FELs) have demonstrated the ability to produce coherent radiation ranging from the microwave to the X-ray region of the electromagnetic spectrum. Although the first theoretical studies of the FEL involved a quantum mechanical analysis (e.g., [1]), it is generally accepted that FEL experiments to date are well described by classical models in which an electron beam consisting of classical particles interacts with a classical electromagnetic radiation field. Until relatively recently, most theoretical studies using a quantum analysis have been restricted to the regime of low gain (e.g., [2–6]). More recently, however, there has been a revival of interest in the role of quantum effects in the FEL interaction (e.g., [7–9]), stimulated in part by the development of short-wavelength, high-gain FEL amplifiers producing progressively higher-energy photons (see, e.g., Ref. [10] for a review of X-ray FELs). In this article, methods for describing the quantum regime of high-gain FEL operation are outlined and some of the effects which are beyond description by the usual classical models are discussed.

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The free electron response in reflectance anisotropy spectra

The free electron response in reflectance anisotropy spectra

The optical anisotropy in a free-electron gas may thus be caused by anisotropic values of the effective masses 共replac- ing the free-electron mass 兲 and/or scattering rates. As a first approximation, the experimental bulk plasma frequency is used 共implying an isotropic effective mass兲, while different values for the scattering rate are assumed here for the two orthogonal directions probed with the RAS. This seems rea- sonable, as scattering in metals at room temperature is a nanoscale phenomenon, allowing surface and interface scat- tering from anisotropic nanoscale structures to produce an- isotropic scattering rates. This will be discussed further in Sec. VII. The set of parameters 共 ␻ p , ␥ x , ␥ y 兲 appears sufficient

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Strong field physics and QED experiments with ELI-NP 2×10PW laser beams

Strong field physics and QED experiments with ELI-NP 2×10PW laser beams

Abstract. The ELI-NP facility will focus a 10 PW pulsed laser beam at intensities of ~10 23 W/cm 2 for the first time, enabling investigation of the new physical phenomena at the interfaces of plasma, nuclear and particle physics. The electric field in the laser focus has a maximum value of ~10 15 V/m at such laser intensities. In the ELI-NP Experimental Area E6, we propose the study of Radiation Reaction, Strong Field Quantum Electrodynamics (QED) effects and resulting production of Ultra-bright Sources of Gamma-rays which could be used for nuclear activation. Two powerful, synchronized 10 PW laser beams will be focused in the E6 Interaction Chamber on either gas or solid targets. One 10 PW beam is the Pump-beam and the other is the Probe-beam. The focused Pump beam accelerates the electrons to relativistic energies. The accelerated electron bunches interact with the very high electro-magnetic field of the focused Probe beam. The layout of the experimental area E6 will be presented with several options for the experimental configurations.

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Dense GeV electron–positron pairs generated by lasers in near-critical-density plasmas

Dense GeV electron–positron pairs generated by lasers in near-critical-density plasmas

State-of-art laser systems 20 are capable of delivering a laser pulse with intensity up to 2 10 22 W cm 2 . The next-generation multi-PW lasers (for example, the XCELS and ELI facilities (the next generation of laser facilities, such as Exawatt Center for Extreme Light Studies (XCELS) and Extreme Light Infrastructure (ELI). Available at http://www.xcels.iapras.ru and http://www. eli-np.ro)) are expected to reach B10 24 W cm 2 and beyond. This opens the door for studying light–matter interactions as well as QED effects in unexplored domains 1,4,21,22 . Diverse schemes have been proposed for energetic e e þ pairs production via the BW process using ultrarelativistic lasers 23–31 . It is shown that using multiple colliding lasers 25 for pair cascades in vacuum can reduce the required laser intensity down to B10 26 W cm 2 . This intensity is significantly smaller than the Schwinger value. An alternative scheme 26,27 relies on the energetic electrons from a laser-driven gas jet or thin solid target by using either two counter-propagating lasers or a single laser. The positron beam produced is very bright and energetic. However, the required laser intensity is as high as B10 24 W cm 2 , still two orders of magnitude higher than that of the available lasers. Another challenge is the target transparence 27 to the incident super intense lasers, which leads to the low efficiency of the BW process. By

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A high average current electron source for the Jefferson Laboratory free electron laser

A high average current electron source for the Jefferson Laboratory free electron laser

The JLab FEL injector routinely demonstrates that it can deliver 1nC electron bunches, but both the maximum repetition rate of the laser, and the high volt- age (HV) power supply limit the maximum current that can be drawn from the gun. Increasing the current available from the gun is not limited by the tech- nology of the laser or HV supply, as the requirements are within the bounds of that available. Adverse effects may occur by increasing the repetition rate of the laser, as this increases the power loading on the cathode and hence the temperature rises as more laser power is absorbed. This in turn will increase the thermal emittance of the emitted electrons. Heating inside the gun chamber will also serve to degrade the vacuum quality which subsequently will destroy the quantum efficiency of the cathode. With a poor QE, more laser power is required to deliver the same bunch charge. In this way it is possible to envisage a situation that would progressively worsen. The JLab gun design has no ac- tive cooling of the cathode in the design at present, but a modification may be necessary if the temperature rise due to the incident laser is too much.

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Ten fold enhancement of InAs nanowire photoluminescence emission with an InP passivation layer

Ten fold enhancement of InAs nanowire photoluminescence emission with an InP passivation layer

The main challenge in producing high-performance InAs NWs-based optoelectronic and photonic devices is that good optical properties are hard to achieve. This is caused by poor material quality and high levels of surface states emission due to large surface-to-volume ratio of NWs and defect states near surfaces. 30 While high surface electron density of InAs is a desirable characteristic for

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Nonequilibrium electron heating in inter-subband terahertz lasers

Nonequilibrium electron heating in inter-subband terahertz lasers

The effect is also less significant for lower lattice tem- peratures as the cooling effect of intra-subband LO phonon emission is more significant, resulting in electron distribu- tions which are more localized near the subband centers. It also affects the lower laser subband significantly less than the upper, because the overlap occurs for an energy range ~a tail! where the ground subband looks less hot. In general, though, this effect competes with other processes, and also depends on the relative populations of the two subbands in- volved. This tail heating could be reduced by a larger energy separation between the upper and ground subbands, so as to minimize the overlap. However, this would pay a penalty both in terms of increased inefficiency: the larger pump en- ergy would still be producing only THz output; and there would be increased heating as LO emission processes from the upper subbands would add more kinetic energy than be- fore.

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Velocity dispersion of correlated energy spread electron beams in the free electron laser

Velocity dispersion of correlated energy spread electron beams in the free electron laser

There is also great interest in reducing the size and cost of future FELs by utilising novel accelerator technology. Plasma accelerators are considered a promising future driver of FELs, with their high accelerating gradients and large peak currents. The electron beams typical of plasma accelerators possess small emittance, a large energy spread, and are very short compared to beams from more conventional linac sources. These characteristics provide challenges in beam transport both to and through the undulator. With regard to the FEL gain, the large energy spread is potentially the most deleterious feature at first glance, but measurements and simulations imply that a large proportion of the energy spread is corellated with the temporal bunch coordinate.

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