130,771 research outputs found
Pepper-pot emittance measurement of laser-plasma wakefield accelerated electrons
No full textThe transverse emittance is an important parameter governing the brightness of an electron beam. Here we present the first pepper-pot measurement of the transverse emittance for a mono-energetic electron beam from a laser-plasma wakefield accelerator, carried out on the Advanced Laser-Plasma High Energy Accelerators towards X-Rays (ALPHA-X) beam line. Mono-energetic electrons are passed through an array of 52 mu m diameter holes in a tungsten mask. The pepper-pot results set an upper limit for the normalised emittance at 5.5 +/- 1 pi mm mrad for an 82 MeV beam
Chirped pulse Raman amplification in plasma: high gain measurements
No full textHigh power short pulse lasers are usually based on chirped pulse amplification (CPA), where a frequency chirped and temporarily stretched ``seed'' pulse is amplified by a broad-bandwidth solid state medium, which is usually pumped by a monochromatic ``pump'' laser. Here, we demonstrate the feasibility of using chirped pulse Raman amplification (CPRA) as a means of amplifying short pulses in plasma. In this scheme, a short seed pulse is amplified by a stretched and chirped pump pulse through Raman backscattering in a plasma channel. Unlike conventional CPA, each spectral component of the seed is amplified at different longitudinal positions determined by the resonance of the seed, pump and plasma wave, which excites a density echelon that acts as a "chirped'" mirror and simultaneously backscatters and compresses the pump. Experimental evidence shows that it has potential as an ultra-broad bandwidth linear amplifier which dispenses with the need for large compressor gratings
An ultrashort pulse ultra-violet radiation undulator source driven by a laser plasma wakefield accelerator
No full textNarrow band undulator radiation tuneable over the wavelength range of 150–260 nm has been produced by short electron bunches from a 2 mm long laser plasma wakefield accelerator based on a 20 TW femtosecond laser system. The number of photons measured is up to 9 × 106 per shot for a 100 period undulator, with a mean peak brilliance of 1 × 1018 photons/s/mrad2/mm2/0.1% bandwidth. Simulations estimate that the driving electron bunch r.m.s. duration is as short as 3 fs when the electron beam has energy of 120–130 MeV with the radiation pulse duration in the range of 50–100 fs
Wavebreaking as a limit for Raman amplification
No full textThe influence of wavebreaking on Raman amplification is investigated. A phenomenological modification is added to a set of slowly varying envelope equations, and found to give good agreement to particle-in-cell simulations for cold plasma in the wavebreaking regime. For warm plasma, good agreement is not found using the warm wavebreaking limit. However, the PIC simulations show that the decreased wavebreaking limit does have a significant impact on amplification. The limitations of our model are discussed, and possible future work outlined
Theory of short pulse FEL oscillators
No full textA simple model for the nonlinear low gain evolution of a short pulse Compton free electron laser (FEL) oscillator is derived. An analysis of the small signal regime allows the calculation of the shape of the produced optical pulse. For small excess over threshold, a Landau-Ginzburg equation is obtained, which allows us to show that a strong superradiant efficiency enhancement occurs at small detuning. Furthermore, the knowledge of the eigenfrequencies in the small signal regime allows one to infer a scale law for the period of limit cycles occurring at larger excess over threshold. Theoretical results are compared to experimental results from FELIX
Photon frequency up-shifting by an amplified plasma density wake due to two co-propagating laser pulses
No full textAn analytical study of significant photon acceleration (frequency up-shift) in a plasma density wake produced by two laser pulses in the mildly relativistic and linearized regime is presented. The wake amplitude is amplified and its phase controlled using two coaxially, co-propagating laser pulses, which are considered to be identical but separated by a fixed time. A third probe pulse, with a variable delay, is considered as ``test particle'' or quasi-photon propagating through the amplified density wake, which experiences significant photon acceleration because of the local temporal and spatial variation of the permittivity. The evolution of the ``photon'' is studied using Hamiltonian theory. The significant frequency up-shift is much larger than that produced by the wake of a single relativistic laser pulse in the highly relativistic nonlinear wake regime. Our study demonstrates that the inter-pulse separation between the ``controlling'' pulse and the ``driver'' pulse, producing the amplified density wake, can provide an additional degree of freedom for tuning the maximum up-shift of the probe photon frequency
Practical considerations for the ion channel free-electron laser
No full textThe ion-channel laser (ICL) has been proposed as an alternative to the free-electron laser (FEL), replacing the deflection of electrons by the periodic magnetic field of an undulator with the periodic betatron motion in an ion channel. Ion channels can be generated by passing dense energetic electron bunches or intense laser pulses through plasma. The ICL has potential to replace FELs based on magnetic undulators, leading to very compact coherent X-ray sources. In particular, coupling the ICL with a laser plasma wakefield accelerator would reduce the size of a coherent light source by several orders of magnitude. An important difference between FEL and ICL is the wavelength of transverse oscillations: In the former it is fixed by the undulator period, whereas in the latter it depends on the betatron amplitude, which therefore has to be treated as variable. Even so, the resulting equations for the ICL are formally similar to those for the FEL with space charge taken into account, so that the well-developed formalism for the FEL can be applied. The amplitude dependence leads to additional requirements compared to the FEL, e.g. a small spread of betatron amplitudes. We shall address these requirements and the resulting practical considerations for realizing an ICL, and give parameters for operation at UV fundamental wavelength, with harmonics extending into X-rays
MeSH term explosion and author rank improve expert recommendations
No full textInformation overload is an often-cited phenomenon that reduces the productivity, efficiency and efficacy of scientists. One challenge for scientists is to find appropriate collaborators in their research. The literature describes various solutions to the problem of expertise location, but most current approaches do not appear to be very suitable for expert recommendations in biomedical research. In this study, we present the development and initial evaluation of a vector space model-based algorithm to calculate researcher similarity using four inputs: 1) MeSH terms of publications; 2) MeSH terms and author rank; 3) exploded MeSH terms; and 4) exploded MeSH terms and author rank. We developed and evaluated the algorithm using a data set of 17,525 authors and their 22,542 papers. On average, our algorithms correctly predicted 2.5 of the top 5/10 coauthors of individual scientists. Exploded MeSH and author rank outperformed all other algorithms in accuracy, followed closely by MeSH and author rank. Our results show that the accuracy of MeSH term-based matching can be enhanced with other metadata such as author rank
Analytical theory of short-pulse free-electron laser oscillators
No full textA simple model for the nonlinear evolution of a short-pulse free-electron laser oscillator in the small gain regime is derived. An analysis of the linearized system allows the definition and calculation of the eigenmodes characterizing the small signal regime. An arbitrary solution of the nonlinear system can then be expanded in terms of these supermodes. In the single-supermode approximation, the system reduces to a Landau-Ginzburg equation, which allows the efficiency and saturated power to be obtained as functions of cavity detuning and cavity losses. In the limit of small cavity detuning, electrons emit superradiantly, with an efficiency inversely proportional to the number of radiation wavelengths within the optical pulse, and power proportional to the square of the bunch charge. In the multisupermode regime, limit cycles and period doubling behavior are observed and interpreted as a competition between supermodes. Finally, the analytical and numerical results are compared with the experimental observations from the Free-Electron Laser for Infrared eXperiments experiment
Observation of superradiance in a short-pulse fel oscillator
No full textSuperradiance has been experimentally studied, in a short-pulse free-electron laser (FEL) oscillator. Superradiance is the optimal way of extracting optical radiation from an FEL and can be characterised by the following scale laws: peak optical power P, scales as the square of electron charge, Q, (P, &unknown; Q2); the optical pulse duration, z, scales as the inverse of the square root of the charge, (z &unknown; 1/Q); the efficiency, , scales as the inverse of optical pulse length ( &unknown; 1/z &unknown; Q), which also implies that the relative spectral brightness defined by /(/) remains constant and close to 0.86. To characterise the properties of the superradiant emission, we have measured the efficiency, optical pulse energy, pulse duration and spectral width as functions of electron beam current and cavity loss for the optimum cavity length detuning. The efficiency has been deduced from measurements of electron beam energy spectra. The optical pulse duration has been determined from second-order autocorrelation measurements and the optical spectra determined using a grating spectrometer. We show that the superradiance in the oscillator has properties similar to that in a high-gain amplifier and discuss the links with spikes created by synchrotron instabilities
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