X-ray Grating Talbot Interferometer (XGTI) is already routinely used for quantitative phase contrast imaging of soft tissue samples. XGTI can be realized using various measurement techniques, depending on the X-ray source used, the required spatial resolution and the speed of acquisition. The phase-stepping measurement technique, which is commonly used for XGTI data acquisition, needs multiple acquisitions for a single projection. For fast imaging the Moiré technique, a single-shot technique, is often preferred. However, it requires two gratings which increases the dose on the sample. We have therefore examined the Spatial Harmonic Imaging (SHI) technique which is low-dose and single-shot, using the I13-2 Diamond-Manchester Beamline at Diamond Light Source (DLS). The DLS I13-2 beamline is equipped with a Double Crystal Monochromator (DCM) and a Multi-Layer Monochromator (MLM) to deliver monochromatic beam, which work at the energy bandwidths (ΔE/E) of 10-4 and 10-2, respectively. However, the disadvantage of using these monochromators, especially for fast imaging, is loss of X-ray flux. It has already been shown that XGTI can work with an energy bandwidth (ΔE/E) of 10-1. Our aim is to develop a single phase grating interferometer with pink beam from an undulator source, with X-ray mirror optics and multiple absorption filters, to obtain maximum possible flux with sufficient coherence and monochromaticity. We demonstrate performance with optimized beamline parameters for a photon energy of 15 keV with some demonstrative image reconstructions.
The DIAMOND beamline I13L is dedicated to multi-scale and multi-modal imaging in real and reciprocal space. The beamline consists of two independently operating experimental stations, located at a distance of more than 200 m from the source. The Imaging Branch performs micro-tomography with in-line phase contrast in the 6-30 keV energy range. In addition, a grating interferometry setup and a full-field microscope for nano-tomography are currently implemented. Other techniques providing high-resolution three-dimensional information, in particular coherent X-ray diffraction, are hosted on the Coherence Branch. All imaging methods are tested to operate with large energy bandwidths and therefore shorter exposure times. To this end, two options are currently used: the so-called ‘pink-beam’ mode using a reflecting mirror and X-ray filters and monochromatic mode using a multilayer monochromator. The operation mode enables science for in-situ and operando studies across a wide range of scientific areas.
The Diamond Beamline I13L is dedicated to micro- and nano- imaging, with two independently operating branchlines. The imaging branch preforms imaging in real space, with In-line phase contrast imaging and grating interferometry at micrometre resolution and full-field transmission microscopy up to 50nm spatial resolution. Highest spatial resolution is achieved on the coherence branchline, where diffraction imaging methods such as Ptychography and Bragg-CDI are performed. The article provides an update about the experimental capabilities at the beamline with an emphasis on the rapidly evolving ptychography capabilities. The latter has evolved to an user-friendly method with non-expert users able to explore their science without any specific a-priory knowledge.
The increasing demand for high laser powers is placing huge demands on current laser technology. This is now reaching a limit, and to realise the existing new areas of research promised at high intensities, new cost-effective and technically feasible ways of scaling up the laser power will be required. Plasma-based laser amplifiers may represent the required breakthrough to reach powers of tens of petawatts to exawatts, because of the fundamental advantage that amplification and compression can be realised simultaneously in a plasma medium, which is also robust and resistant to damage, unlike conventional amplifying media. Raman amplification is a promising method, where a long pump pulse transfers energy to a lower frequency, short duration counter-propagating seed pulse through resonant excitation of a plasma wave that creates a transient plasma echelon, which backscatters the pump into the probe. While very efficient, this comes at the cost of noise amplification (from plasma density fluctuations) that needs to be controlled. Here we present the results of an experimental campaign where we have demonstrated chirped pulse Raman amplification (CPRA) at high intensities. We have used a frequency chirped pump pulse to limit the growth of noise amplification, while trying to maintain the amplification of the seed. In non-optimised conditions we show that indeed noise amplification can be controlled but reducing noise scattering also limits the seed amplification factor. Finally, we show that the gross efficiency is a few percent, consistent with previous measurements of CPRA obtained in capillaries with pump pulses of duration of a few hundred picoseconds.
The laser wake-field accelerator (LWFA) traditionally produces high brightness, quasi-monoenergetic electron beams with Gaussian-like spatial and angular distributions. In the present work we investigate the generation of ultra-relativistic beams with ring-like structures in the blowout regime of the LWFA using a dual stage accelerator. A density down-ramp triggers injection after the first stage and is used to produce ring-like electron spectra in the 300 - 600 MeV energy range. These well defined, annular beams are observed simultaneously with the on-axis, high energy electron beams, with a divergence of a few milliradians. The rings have quasi-monoenergetic energy spectra with an RMS spread estimated to be less than 5%. Particle-in-cell simulations confirm that off-axis injection provides the electrons with the initial transverse momentum necessary to undertake distinct betatron oscillations within the plasma bubble during their acceleration process.
One challenge in X-ray optics is how to focus hard X-rays. The possibility of achieving a wide-range of spot sizes would be an advantage for many applications, such as semiconductor lithography, nuclear fusion diagnostics or focused X-ray beams for cancer therapy. Focused X-rays could deliver precise high doses to tumours while sparing the surrounding tissue. Currently, a promising approach is using a Laue lens. However, they are limited by the diffraction capability of crystals and the complex mm-size arrangement of the optical elements. In addition, they are restricted to sub-MeV photon beams, because for higher energies the Bragg condition is not satisfied. We present an efficient and cost-effective method to extend this range up to tens of MeV using an electron-photon converter in the stream of focused high energy electrons. The emerging X-rays follow the trajectories of the electrons and the focus can be simply adjusted by modifying the focus of the electron beam. A bending magnet can be used to remove electrons, if necessary, however a mixed radiation could be an additional option. Our solution can also be adapted to a multiple beam arrangement as an effective alternative to Gamma Knife without the need for handling radioactive sources.
Here we explore ways of transforming laser radiation into incoherent and coherent electromagnetic radiation using laserdriven plasma waves. We present several examples based on the laser wakefield accelerator (LWFA) and show that the electron beam and radiation from the LWFA has several unique characteristics compared with conventional devices. We show that the energy spread can be much smaller than 1% at 130-150 MeV. This makes LWFAs useful tools for scientists undertaking time resolved probing of matter subject to stimuli. They also make excellent imaging tools. We present experimental evidence that ultra-short XUV pulses, as short as 30 fs, are produced directly from an undulator driven by a LWFA, due to the electron bunches having a duration of a few femtoseconds. By extending the electron energy to 1 GeV, and for 1-2 fs duration pulses of 2 nm radiation peak powers of several MW per pC can be produced. The increased charge at higher electron energies will increase the peak power to GW levels, making the LWFA driven synchrotron an extremely useful source with a spectral range extending into the water window. With the reduction in size afforded by using LWFA driven radiation sources, and with the predicted advances in laser stability and repletion rate, ultra-short pulse radiation sources should become more affordable and widely used, which could change the way science is done.
Preserving the coherence and wavefront of a diffraction limited x-ray beam from the source to the experiment poses stringent quality requirements on the production processes for X-ray optics. In the near future this will require on-line and in-situ at-wavelength metrology for both, free electron lasers and diffraction limited storage rings. A compact and easy to move X-ray grating interferometry (XGI) setup has been implemented by the Beamline Optics Group at PSI in order to characterize x-ray optical components by determining the aberrations from reconstructing the x-ray wavefront. The XGI setup was configured for measurements in the moire mode and tested with focusing optic at Swiss Light Source, Diamond Light Source and LCLS. In this paper measurements on a bendable toroidal mirror, a zone plate, a single and a stack of beryllium compound refractive lenses (CRL) are presented. From these measurements the focal position and quality of the beam spot in terms of wavefront distortions are determined by analysing the phase-signal obtained from the XGI measurement. In addition, using a bendable toroidal mirror, we directly compare radius of curvature measurements obtained from XGI data with data from a long-trace profilometer, and compare the CRL wavefront distortions with data obtained by ptychography.
The Diamond Beamline I13L is dedicated to imaging on the micro- and nano-lengthsale, operating in the energy range
between 6 and 30keV. For this purpose two independently operating branchlines and endstations have been built. The
imaging branch is fully operational for micro-tomography and in-line phase contrast imaging with micrometre
resolution. Grating interferometry is currently implemented, adding the capability of measuring phase and small-angle
information. For tomography with increased resolution a full-field microscope providing 50nm spatial resolution with a
field of view of 100μm is being tested. The instrument provides a large working distance between optics and sample to
adapt a wide range of customised sample environments. On the coherence branch coherent diffraction imaging
techniques such as ptychography, coherent X-ray diffraction (CXRD) are currently developed for three dimensional
imaging with the highest resolution.
The imaging branch is operated in collaboration with Manchester University, called therefore the Diamond-Manchester
Branchline. The scientific applications cover a large area including bio-medicine, materials science, chemistry geology
and more. The present paper provides an overview about the current status of the beamline and the science addressed.
The phase-stepping (PS) mode of X-ray Grating Talbot interferometer (XGTI) data processing technique can yield
high-spatial resolution images, albeit with lower throughput since each projection of a tomogram requires at least five
phase-stepping images. To overcome this problem, we can use a setup with only a single phase grating in combination
with a high-resolution detector system and a Spatial Harmonic Imaging (SHI) technique. The latter technique makes use
of the fact that a Talbot interferogram consists of carrier frequency spectrum of the grating overlapping with the sample
spectrum and by Fourier transforming the interferogram we can separate the two. The disadvantage of this is that the
spatial resolution is inferior. In this manuscript we will compare these two single grating data processing techniques
using a single data set measured with mouse embryo heart and discuss advantages and disadvantages of each technique.
These two techniques can be used as complementary; one for high resolution, and the other for high-speed imaging.
The Diamond Beamline I13L is designed to imaging on the micron- and nano-lengthsale with X-rays of energies between 6 and 30 keV [1]. Two independently operating branchlines and endstations have been built at distance of more than 200m from the source for this purpose. The imaging branch is dedicated for imaging in real space, providing In-line phase contrast imaging and grating interferometry with micrometre resolution and full-field transmission microscopy with 50nm spatial resolution.
On the coherence branch coherent diffraction imaging techniques such as ptychography, coherent X-ray diffraction (CXRD) and Fourier-Transform holography are currently developed. Because of the large lateral coherence length available at I13, the beamline hosts numerous microscopy experiments. The coherence branchline in particular contains a number of unique features. New instrumental designs have been employed such as a robot arm for the detector in diffraction experiments and a photon counting detector for diffraction experiments. The so-called ‘mini-beta’ layout in the straight section of the electron storage ring permits modulating the horizontal source size and therefor the lateral coherence length.
We will present the recent progress in coherent imaging at the beamline and the sciences addressed with the instrumental capabilities.
Reference:
[1] C. Rau, U. Wagner, Z. Pesic, A. De Fanis Physica Status Solidi (a) 208 (11). Issue 11 2522-2525, 2011, 10.1002/pssa.201184272
The increasing demand for high laser powers is placing huge demands on current laser technology. This is now reaching a limit, and to realise the existing new areas of research promised at high intensities, new cost-effective and technically feasible ways of scaling up the laser power will be required. Plasma-based laser amplifiers may represent the required breakthrough to reach powers of tens of petawatt to exawatt, because of the fundamental advantage that amplification and compression can be realised simultaneously in a plasma medium, which is also robust and resistant to damage, unlike conventional amplifying media. Raman amplification is a promising method, where a long pump pulse transfers energy to a lower frequency, short duration counter-propagating seed pulse through resonant excitation of a plasma wave that creates a transient plasma echelon that backscatters the pump into the probe. Here we present the results of an experimental campaign conducted at the Central Laser Facility. Pump pulses with energies up to 100 J have been used to amplify sub-nanojoule seed pulses to near-joule level. An unprecedented gain of eight orders of magnitude, with a gain coefficient of 180 cm−1 has been measured, which exceeds high-power solid-state amplifying media by orders of magnitude. High gain leads to strong competing amplification from noise, which reaches similar levels to the amplified seed. The observation of 640 Jsr−1 directly backscattered from noise, implies potential overall efficiencies greater than 10%.
Advances in laser technology have driven the development of laser-wakefield accelerators, compact devices that are capable of accelerating electrons to GeV energies over centimetre distances by exploiting the strong electric field gradients arising from the interaction of intense laser pulses with an underdense plasma. A side-effect of this acceleration mechanism is the production of high-charge, low-energy electron beams at wide angles. Here we present an experimental and numerical study of the properties of these wide-angle electron beams, and show that they carry off a significant fraction of the energy transferred from the laser to the plasma. These high-charge, wide-angle beams can also cause damage to laser-wakefield accelerators based on capillaries, as well as become source of unwanted bremsstrahlung radiation.
X-ray phase contrast imaging (X-PCi) is a very promising method of dramatically enhancing the contrast of X-ray images of microscopic weakly absorbing objects and soft tissue, which may lead to significant advancement in medical imaging with high-resolution and low-dose. The interest in X-PCi is giving rise to a demand for effective simulation methods. Monte Carlo codes have been proved a valuable tool for studying X-PCi including coherent effects. The laser-plasma wakefield accelerators (LWFA) is a very compact particle accelerator that uses plasma as an accelerating medium. Accelerating gradient in excess of 1 GV/cm can be obtained, which makes them over a thousand times more compact than conventional accelerators. LWFA are also sources of brilliant betatron radiation, which are promising for applications including medical imaging. We present a study that explores the potential of LWFA-based betatron sources for medical X-PCi and investigate its resolution limit using numerical simulations based on the FLUKA Monte Carlo code, and present preliminary experimental results.
The Advanced Laser-Plasma High-Energy Accelerators towards X-rays (ALPHA-X) programme is developing laserplasma accelerators for the production of ultra-short electron bunches with subsequent generation of coherent, bright, short-wavelength radiation pulses. The new Scottish Centre for the Application of Plasma-based Accelerators (SCAPA) will develop a wide range of applications utilising such light sources. Electron bunches can be propagated through a magnetic undulator with the aim of generating fully coherent free-electron laser (FEL) radiation in the ultra-violet and Xrays spectral ranges. Demonstration experiments producing spontaneous undulator radiation have been conducted at visible and extreme ultra-violet wavelengths but it is an on-going challenge to generate and maintain electron bunches of sufficient quality in order to stimulate FEL behaviour. In the ALPHA-X beam line experiments, a Ti:sapphire femtosecond laser system with peak power 20 TW has been used to generate electron bunches of energy 80-150 MeV in a 2 mm gas jet laser-plasma wakefield accelerator and these bunches have been transported through a 100 period planar undulator. High peak brilliance, narrow band spontaneous radiation pulses in the vacuum ultra-violet wavelength range have been generated. Analysis is provided with respect to the magnetic quadrupole beam transport system and subsequent effect on beam emittance and duration. Requirements for coherent spontaneous emission and FEL operation are presented.
The laser-plasma wakefield accelerator is a novel ultra-compact particle accelerator. A very intense laser pulse focused onto plasma can excites plasma density waves. Electrons surfing these waves can be accelerated to very high energies with unprecedented accelerating gradients in excess of 1 GV/cm. While accelerating, electrons undergo transverse betatron oscillations and emit synchrotron-like x-ray radiation into a narrow on-axis cone, which is enhanced when electrons interact with the electromagnetic field of the laser. In this case, the laser can resonantly drive the electron motion, lading to direct laser acceleration. This occurs when the betatron frequency matches the Doppler down-shifted frequency of the laser. As a consequence, the number of photons emitted is strongly enhanced and the critical photon energy is increases to 100’s of keV.
Both the laser-plasma wakefield accelerator (LWFA) and X-ray phase-contrast imaging (XPCi) are promising technologies that are attracting the attention of the scientific community. Conventional X-ray absorption imaging cannot be used as a means of imaging biological material because of low contrast. XPCi overcomes this limitation by exploiting the variation of the refraction index of materials. The contrast obtained is higher than for conventional absorption imaging and requires a lower dose. The LWFA is a new concept of acceleration where electrons are accelerated to very high energy (~150 MeV) in very short distances (mm scale) by surfing plasma waves excited by the passage of an ultra-intense laser pulse (~1018 Wcm-2) through plasma. Electrons in the LWFA can undergo transverse oscillation and emit synchrotron-like (betatron) radiation in a narrow cone around the propagation axis. The properties of the betatron radiation produced by LWFA, such as source size and spectrum, make it an excellent candidate for XPCi. In this work we present the characterization of betatron radiation produced by the LWFA in the ALPHA-X laboratory (University of Strathclyde). We show how phase contrast images can be obtained using the betatron radiation in a free-space propagation configuration and we discuss the potential and limitation of the LWFA driven XPCi.
Proc. SPIE. 8079, Laser Acceleration of Electrons, Protons, and Ions; and Medical Applications of Laser-Generated Secondary Sources of Radiation and Particles
KEYWORDS: Electron beams, Spectroscopy, Particles, Electrons, Laser applications, Laser scattering, Adaptive optics, Quality measurement, Free electron lasers, Plasma
The normalised transverse emittance is a measure of the quality of an electron beam from a particle accelerator. The
brightness, parallelism and focusability are all functions of the emittance. Here we present a high-resolution single shot
method of measuring the transverse emittance of a 125 ± 3 MeV electron beam generated from a laser wakefield
accelerator (LWFA) using a pepper-pot mask. An average normalised emittance of εrms,x,y = 2.2 ± 0.7, 2.3 ± 0.6 π-mmmrad
was measured, which is comparable to that of a conventional linear accelerator. The best measured emittance was
εrms,x,=1.1 ± 0.1 π-mm-mrad, corresponding to the resolution limit of our system. The low emittance indicates that this
accelerator is suitable for driving a compact free electron laser.
The Advanced Laser-Plasma High-Energy Accelerators towards X-rays (ALPHA-X) programme is developing laserplasma
accelerators for the production of ultra-short electron bunches with subsequent generation of high brilliance,
short-wavelength radiation pulses. Ti:sapphire laser systems with peak power in the range 20-200 TW are coupled into
mm- and cm-scale plasma channels in order to generate electron beams of energy 50-800 MeV. Ultra-short radiation
pulses generated in these compact sources will be of tremendous benefit for time-resolved studies in a wide range of
applications across many branches of science. Primary mechanisms of radiation production are (i) betatron radiation due
to transverse oscillations of the highly relativistic electrons in the plasma wakefield, (ii) gamma ray bremsstrahlung
radiation produced from the electron beams impacting on metal targets and (iii) undulator radiation arising from
transport of the electron beam through a planar undulator. In the latter, free-electron laser action will be observed if the
electron beam quality is sufficiently high leading to stimulated emission and a significant increase in the photon yield.
All these varied source types are characterised by their high brilliance arising from the inherently short duration (~1-10
fs) of the driving electron bunch.
The 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 μm diameter holes in
a tungsten mask. The pepper-pot results set an upper limit for the normalised emittance at 5.5 ± 1 π mm mrad
for an 82 MeV beam.
The Advanced Laser-Plasma High-Energy Accelerators towards X-rays (ALPHA-X) programme is developing laserplasma
accelerators for the production of ultra-short electron bunches with subsequent generation of incoherent radiation
pulses from plasma and coherent short-wavelength radiation pulses from a free-electron laser (FEL). The first
quantitative measurements of the electron energy spectra have been made on the University of Strathclyde ALPHA-X
wakefield acceleration beam line. A high peak power laser pulse (energy 900 mJ, duration 35 fs) is focused into a gas jet
(nozzle length 2 mm) using an F/16 spherical mirror. Electrons from the laser-induced plasma are self-injected into the
accelerating potential of the plasma density wake behind the laser pulse. Electron beams emitted from the plasma have
been imaged downstream using a series of Lanex screens positioned along the beam line axis and the divergence of the
electron beam has been measured to be typically in the range 1-3 mrad. Measurements of the electron energy spectrum,
obtained using the ALPHA-X high resolution magnetic dipole spectrometer, are presented. The maximum central energy
of the monoenergetic beam is 90 MeV and r.m.s. relative energy spreads as low as 0.8% are measured. The mean central
energy is 82 MeV and mean relative energy spread is 1.1%. A theoretical analysis of this unexpectedly high electron
beam quality is presented and the potential impact on the viability of FELs driven by electron beams from laser
wakefield accelerators is examined.
Electron acceleration using plasma waves driven by ultra-short relativistic intensity laser pulses has
undoubtedly excellent potential for driving a compact light source. However, for a wakefield accelerator to
become a useful and reliable compact accelerator the beam properties need to meet a minimum standard. To
demonstrate the feasibility of a wakefield based radiation source we have reliably produced electron beams
with energies of 82±5 MeV, with 1±0.2% energy spread and 3 mrad r.m.s. divergence using a 0.9 J, 35 fs 800
nm laser. Reproducible beam pointing is essential for transporting the beam along the electron beam line. We
find experimentally that electrons are accelerated close to the laser axis at low plasma densities. However, at
plasma densities in excess of 1019 cm-3, electron beams have an elliptical beam profile with the major axis of
the ellipse rotated with respect to the direction of polarization of the laser.
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