With the increase of laser power at facilities reaching petawatt-level, there is a need for accurate electron beam diagnostics of the laser wakefield accelerator (LWFA), which are becoming important tools for a wide range of applications including high field physics. Electrons in the range of several 100s of GeV are expected at these power levels. Precise diagnostic systems are required to enable applications such as advanced radiation sources. Accurate measurement of the energy spread of electron beams will help pave the way towards LWFA based free-electron lasers and plasma based coherent radiation sources. We propose an innovative double dipole spectrometer suitable for characterizing bunches produced using a petawatt class laser.
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.
High energy attosecond electron bunches from the laser-plasma wakefield accelerator (LWFA) are potentially useful sources of ultra-short duration X-rays pulses, which can be used for ultrafast imaging of electron motion in biological and physical systems. Electron injection in the LWFA depends on the plasma density and gradient, and the laser intensity. Recent research has shown that injection of attosecond electron bunches is possible using a short plasma density ramp. For controlled injection it is necessary to keep both the laser intensity and background plasma density constant, but set to just below the threshold for injection. This ensures that injection is only triggered by an imposed density perturbation; the peak density should also not exceed the threshold for injection. A density gradient that only persists over a short range can lead to the injection of femtosecond duration bunches, which are then Lorentz contracted to attoseconds on injection. We consider an example of a sin2 shaped modulation where the gradient varies until the downward slope exceeds the threshold for injection and then reduces subsequently to prevent any further injection. The persistence above the threshold determines the injected bunch length, which can be varied. We consider several designs of plasma media including density perturbations formed by shaped Laval nozzles and present an experimental and theoretical study of the modulated media suitable for producing attosecond-duration electron bunches.
Laser plasma accelerators are highly versatile and are sources of both radiation and particle beams, with unique properties. The Scottish Centre for Application based Plasma Accelerators (SCAPA) 40 TW and 350 TW laser at the University of Strathclyde has been used to produce both soft and hard x-rays using a laser wakefield accelerator (LWFA). The inherent characteristics of these femtosecond duration pulsed x-rays make them ideal for probing matter and ultrafast imaging applications. To support the development of applications of laser plasma accelerators at the SCAPA facility an adjustable Kirkpatrick-Baez x-ray microscope has been designed to focus 50 eV - 10 KeV x-rays. It is now possible to produce high quality at silicon wafers substrates that can be used for x-ray optics. Platinum-coated (40 nm) silicon wafers have been used in the KB instrument to image the LWFA x-ray source. We simulate the source distribution as part of an investigation to determine the x-ray source size and therefore its transverse coherence and ultimately the peak brilliance. The OASYS SHAODOW-OUI raytracing and wave propagation code has been used to simulate the imaging setup and determine instrument resolution.
The Scottish Centre for the Application of Plasma-based Accelerators (SCAPA) is a research centre dedicated to providing high energy particle beams and high peak brightness radiation pulses for users across all scientific and engineering disciplines. A pair of Ti:Sapphire femtosecond laser systems (40 TW peak power at 10 Hz pulse repetition rate and 350 TW at 5 Hz, respectively) are the drivers for a suite of laser-plasma accelerator beamlines housed across a series of radiation shielded areas. The petawatt-scale laser delivers 45 W of average power that establishes it as the world leader in its class. The University of Strathclyde has had an operational laser wakefield accelerator since 2007 as the centrepiece of the ongoing Advanced Laser Plasma High-energy Accelerators towards X-rays (ALPHA-X) project. SCAPA, which is a multipartner venture under the auspices of the Scottish Universities Physics Alliance, continues the dedicated beamline approach pioneered by ALPHA-X and represents a significant expansion in the UK’s experimental capability at the university level in laser-driven acceleration. The new centre supports seven radiation beamlines across three concrete shielded bunkers that each nominally specialise in a different aspect of fundamental laser-plasma interaction physics and radiation sources: GeVscale electron beams, MeV/c proton and ion beams, X-rays, gamma rays and so on. Development of application programmes based on these sources cover a wide range of fields including nuclear physics, radiotherapy, space radiation reproduction, warm dense matter, high field physics and radioisotope generation.
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.
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.
Laser-plasma studies have been undertaken for 50 years using infra-red to ultra-violet lasers. We show that a new regime of laser-produced plasmas can be created with capillary discharge and free electron lasers operating in the extreme ultra-violet (EUV). For example, EUV radiation (wavelength < 50 nm) has a critical electron density above electron densities formed by ionization at solid material density and so potentially can penetrate to large depth into a solid density plasma. We explore here the importance of this penetration in ablating solid targets, in creating novel warm dense matter and in the diagnosis of plasmas.
The potential for coherent extreme ultra-violet (EUV) light in probing laser-produced plasmas is investigated. New
results are presented to demonstrate that EUV radiation can be employed to measure heat penetration into solid targets
from electrons using the signature of a change of opacity due to heating. We examine, in particular, the effects of hot
electron heating of targets. In addition, phase variations after transmission through a laser-irradiated target change the
subsequent propagation of the radiation, suggesting a simple diagnostic measuring the far-field footprint of coherent
EUV radiation can be a useful measurement of the uniformity of target heating.
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