Parametric study of density down-ramp injection in laser wakefield acceleration

In laser wakefield acceleration (LWFA) a high intensity laser pulse is used to excite a plasma density wave with an associated electric field. This electric field can be used to accelerate electrons. However, to be accelerated the electrons first of all need to enter the plasma wave. This process is called injection. In this thesis a scheme for injecting electrons into a laser wakefield accelerator is studied. Focus lies on particle-in-cell simulations performed on a computer cluster. A parametric scan is performed where a density down-ramp’s slope and length is varied. A linear relation between the density down-ramp length and injected charge is shown. Furthermore a small density difference is shown to yield higher electron energies. A logarithmic relation between the density down-ramp slope and injected charge is shown. The slope can be optimised to control the spatial distribution of injected electrons within the plasma wave. A high peak current is shown to preserve a mono-energetic distribution over the acceleration length. A number of simulations is performed to explain experimental results where a large variation of injected charge is shown. A second injection mechanism is identified as the source of a large variation in injected charge. An imaging diagnostic system with a resolution of 2 μm looking at the Thomson scattered light from the laser pulse is designed and implemented. The Thomson scattered light is proportional to the background density of the electrons, and could therefore be used to detect density gradients

Numerical and Experimental Studies of Wakefield Accelerators

This thesis is based on work done by the author on the development of laser wakefield accelerators.Wakefield acceleration in plasmas is a promising technique to provide the next generation of accelerating structures and particle beams. Plasmas can sustain electric fields that are many orders of magnitude stronger than those possible in conventional accelerators. Other benefits of wakefield accelerators are that electron beams produced inside the plasma can be generated with high peak current and ultra-low emittance.These strongly accelerating structures can reduce the size of particle accelerators, making them more available, for example in hospitals, or to increase the energy in particle colliders.In wakefield acceleration, a driver is used to excite a plasma wave.The acceleration of charged particles takes place in a plasma wave excited by, and co-propagating with, the driver. The driver can be a laser pulse or a bunch of charged particles.However, many technical challenges remain to be solved before a reliable particle source can be realized based on this technology.This thesis describes numerical studies performed using particle-in-cell simulations and experimental work using high-intensity laser pulses, with the aim of improving our knowledge on wakefield accelerators. The work presented here focuses on three different topics: trapping mechanisms, achieving higher electron energies and improvement of the betatron X-rays generated.In particular, trapping in a density down-ramp, ionization induced trapping, and trapping by colliding pulses have been investigated numerically and experimentally.A novel guidance technique for high-intensity laser pulses is suggested, the merging of two laser wakefields is experimentally demonstrated and suggested as a possible means of staging wakefield accelerators, and the possibility of carrying out a beam-driven plasma wakefield experiment is investigated through simulations. An improved X-ray source based on laser wakefield acceleration and enhancement of the betatron oscillations through direct laser acceleration is investigated and two applications are demonstrated

A tunable electron beam source using trapping of electrons in a density down-ramp in laser wakefield acceleration

One challenge in the development of laser wakefield accelerators is to demonstrate sufficient control and reproducibility of the parameters of the generated bunches of accelerated electrons. Here we report on a numerical study, where we demonstrate that trapping using density down-ramps allows for tuning of several electron bunch parameters by varying the properties of the density down-ramp. We show that the electron bunch length is determined by the difference in density before and after the ramp. Furthermore, the transverse emittance of the bunch is controlled by the steepness of the ramp. Finally, the amount of trapped charge depends both on the density difference and on the steepness of the ramp. We emphasize that both parameters of the density ramp are feasible to vary experimentally. We therefore conclude that this tunable electron accelerator makes it suitable for a wide range of applications, from those requiring short pulse length and low emittance, such as the free-electron lasers, to those requiring high-charge, large-emittance bunches to maximize betatron X-ray generation

Optimization of soft X-ray phase-contrast tomography using a laser wakefield accelerator

X-ray phase-contrast imaging allows for non-invasive analysis in low-absorbing materials, such as soft tissue. Its application in medical or materials science has yet to be realized on a wider scale due to the requirements on the X-ray source, demanding high flux and small source size. Laser wakefield accelerators generate betatron X-rays fulfilling these criteria and can be suitable sources for phase-contrast imaging. In this work, we present the first phase-contrast images obtained by using ionization injection-based laser wakefield acceleration, which results in a higher photon yield and smoother X-ray beam profile compared to self-injection. A peak photon yield of 1.9 × 1011 ph/sr and a source size of 3 μm were estimated. Furthermore, the current laser parameters produce an X-ray spectrum mainly in the soft X-ray range, in which laser-plasma based phase-contrast imaging had yet to be studied. The phase-contrast images of a Chrysopa lacewing resolve features on the order of 4 μm. These images are further used for a tomographic reconstruction and a volume rendering, showing details on the order of tens of μm

Simultaneous X-ray absorption and two-photon LIF for imaging the spray formation region

Imaging the spray formation region of atomizing sprays is particularly challenging due to the presence of a variety of irregular liquid structures such as ligaments, liquid blobs, droplets, liquid sheets and a possible liquid core. The number and concentration of those liquid bodies dictate the presence of liquid/air interfaces, which are responsible to undesired scattering effects. The resulting images are blurred, ultimately concealing the real structure of the spray formation region. Due to both, scattering effects and the presence of highly irregular 3D liquid structures, the only reliable measurement of liquid mass in the spray formation region is obtained using X-ray radiography. The generation of collimated X-rays pulsed has been done, in the past, by means of a synchrotron, thus limiting the number of studies that can be performed.In parallel to the use of X-rays, progresses in advanced laser imaging techniques for suppressing multiple scattering issues have been particularly important over the past decade. A very recent solution consists in using 2-photon excitation LIF laser sheet imaging.In this paper, we report for the first time the possibility of simultaneously imaging an atomizing spray using X-ray absorption and 2-photon LIF planar imaging, where the simultaneous single-shot recordings are made over a ~20mmx20mm viewed area. The spray is generated from a commercial fuel port injection system from which, water was injected. The unique illumination/detection scheme proposed here was made possible thanks to the use of X-rays emitted from a laser plasma accelerator (Betatron radiation). For this experiment, we use the High Intensity Laser system at Lund University that provides on target 800mJ, 38fs laser pulses. The emitted X-ray radiation is ranging from 1 to 10keV and peaking at ~2keV. It propagates outside of the vacuum chamber where an X-ray camera records the shadow of the liquid jet. In addition to that, a fraction of the laser pulse ~10mJ is directed on the liquid jet and focuses with a cylindrical lens where it induces fluorescence from a 2-photon excitation process in a dye -here, fluorescein- added to the liquid. The 2p-LIF images provide details on the size and shape of the liquid structures, optically sectioned by the light sheet, while the integrated liquid mass is extracted from the X-ray radiography. This is making the two imaging techniques highly complementary for the characterization of spray systems as well as for further understanding the physics related to liquid atomization

Driver-witness-bunches for plasma-wakefield acceleration at the MAX IV Linear Accelerator

Beam-driven plasma-wakefield acceleration is an acceleration scheme promising accelerating fields of at least two to three orders of magnitude higher than in conventional radiofrequency accelerating structures. The scheme relies on using a charged particle bunch (driver) to drive a non-linear plasma wake, into which a second bunch (witness) can be injected at an appropriate distance behind the first, yielding a substantial energy gain of the witness bunch particles. This puts very special demands on the machine providing the particle beam. In this article, we use simulations to show that, if driver-witness-bunches can be generated in the photocathode electron gun, the MAX IV Linear Accelerator could be used for plasma-wakefield acceleration

Beamline Design for Plasma-Wakefield Acceleration Experiments at MAX IV

The MAX IV Laboratory is a synchrotron radiation user facility located just outside the city of Lund, Sweden. The facility is made up of two storage rings, at 3 GeV and 1.5 GeV, respectively, and a linear accelerator, serving as a full-energy injector for the rings as well as a driver for the Short-Pulse Facility (SPF) located downstream of the extraction point to the 3 GeV ring. Recently, as part of the Soft X-ray Laser (SXL) project, a design study towards using the linac as a soft X-ray free-electron laser (FEL) driver was started. Part of the study is the design and commissioning of a diagnostics beamline based on a Transverse Deflecting Structure (TDS). Moreover, the PlasMAX collaboration is working towards using the MAX IV linac also for beam-driven plasma-wakefield (PWFA) experiments. Therefore, the design of the diagnostics beamline is being done to also accommodate an interaction chamber and final focusing, located upstream of the TDS. This proceeding details the current status of the beamline design and shows some preliminary single- and double-bunch current measurements

Low-divergence femtosecond X-ray pulses from a passive plasma lens

Electron and X-ray beams originating from compact laser-wakefield accelerators have very small source sizes that are typically on the micrometre scale. Therefore, the beam divergences are relatively high, which makes it difficult to preserve their high quality during transport to applications. To improve on this, tremendous efforts have been invested in controlling the divergence of the electron beams, but no mechanism for generating collimated X-ray beams has yet been demonstrated experimentally. Here we propose and realize a scheme where electron bunches undergoing focusing in a dense, passive plasma lens can emit X-ray pulses with divergences approaching the incoherent limit. Compared with conventional betatron emission, the divergence of this so-called plasma lens radiation is reduced by more than an order of magnitude in solid angle, while maintaining a similar number of emitted photons per electron. This X-ray source offers the possibility of producing brilliant and collimated few-femtosecond X-ray pulses for ultra-fast science, in particular for studies based on X-ray diffraction and absorption spectroscopy. X-ray pulses with low divergences are produced in a laser-wakefield accelerator by focusing electron bunches in a dense passive plasma lens

Channeling Acceleration in Crystals and Nanostructures and Studies of Solid Plasmas: New Opportunities

International audiencePlasma wakefield acceleration (PWA) has shown illustrious progress over the past two decades of active research and resulted in an impressive demonstration of O(10 GeV) particle acceleration in O(1 m) long single structures. While already potentially sufficient for some applications, like, e.g., FELs, the traditional laser- and beam-driven acceleration in gaseous plasma faces enormous challenges when it comes to the design of the PWA-based O(1-10 TeV) high energy $e^+e^-$ colliders due to the complexity of energy staging, low average geometric gradients, and unprecedented transverse and longitudinal stability requirements. Channeling acceleration in solid-state plasma of crystals or nanostructures, e.g., carbon nanotubes (CNTs) or alumna honeycomb holes, has the promise of ultra-high accelerating gradients O(1-10 TeV/m), continuous focusing of channeling particles without need of staging, and ultimately small equilibrium beam emittances naturally obtained while accelerating