A grid of equidistant electron stripes is generated during the collision of two laser pulses under a small angle in underdense plasma. Due to the oblique incidence, transverse standing wave in plasma is observed, in addition to the longitudinal traveling wave of the compound laser field. This standing wave results in the generation of plasma density grating. The ratio of the peak stripe density to background density can reach the value of 20:1. The grating period is determined by the interaction angle. Analytical theory of the compound electric fields is provided for plane waves. The grating formation is then verified via particle-in-cell simulations for short Gaussian laser pulses with typical experimental parameters. In addition, the interference pattern was also observed during experiments with Diocles laser. The results presented here are relevant for many laser-plasma applications, such as Raman scattering, inertial confinement fusion, plasma photonic crystals and laser wakefield acceleration.
We examine betatron radiation properties from the bubble regime of laser-wakefield acceleration for a tailored plasma density profile. Previous studies have already discussed enhancement of radiation properties by using various density modifications in later acceleration phases. This paper will focus on a density profile with a short linear up-ramp and compare it with a uniform density case. The process is studied for standard parameters feasible with current sub-100 TW laser systems by means of numerical particle-in-cell simulations. We show here that the critical energy and intensity of radiation increase when the plasma density increases. This enhancement is caused either by electron energy gain in the rear part of the bubble or by oscillation amplitude boost by fields behind the bubble.
It is usually assumed that ions are accelerated most efficiently in the case of non-expanded targets irradiated by femtosecond ultra-intense laser pulse, alternatively with only short scale preplasma on their front side. Here, we demonstrate that the ions in an expanded foil with near-critical density plasma before its interaction with the main petawatt pulse may be accelerated to higher energies than that from ultra-thin foils. In order to investigate the mechanisms responsible for the acceleration of the most energetic ions, we used particle tracking in particle-in-cell simulations. It is demonstrated that high-energy ions originate from a small region of the depth below 1 μm and the width about the laser focal spot size (3 - 4 μm) in the case of semi-expanded target (with gradually increasing density up to the maximum density from the front side) and of a thin foil. On the other hand, the length of this region exceeds 5 μm for the expanded target. When the laser pulse propagates through near-critical density targets, a high density electron bunch is formed and travels with the laser pulse behind the target. Behind this electron bunch, a relatively long longitudinal electric field is generated and this field accelerates ions. Longitudinal electric field can be also generated due to expanding transverse magnetic field, which is observed for the expanded target.
Electron acceleration with optical injection by a perpendicularly propagating and orthogonally polarized low intensity laser pulse into a nonlinear plasma wave driven by a short intense laser pulse was explored by particle- in-cell simulations. The scheme presented here provides an energetic electron bunch in the first ion cavity with a low energy spread. The electron bunch short and compact, with the mean energy about 400 MeV and a low energy spread about 10 MeV in time of 6 ps of acceleration. The injected charge is several tens of pC for the low intensity of the injection pulse. Initial positions of electrons forming the energetic bunch are shown and then these electrons are followed during the simulation in order to understand the injection process and determine electron bunch properties.
Relativistic electron beams accelerated by laser wakefield have the ability to serve as sources of collimated,
point-like and femtosecond X-ray radiation. Experimental conditions for generation of stable quasi-monoenergetic
electron bunches using a femtosecond few-terawatt laser pulse (600 mJ, 50 fs) were investigated as they are crucial
for generation of stable betatron radiation and X-ray pulses from inverse Compton scattering. A mixture of helium
with argon, and helium with an admixture of synthetic air were tested for this purpose using different backing
pressures and the obtained results are compared. The approach to use synthetic air was previously proven to stabilize
the energy and energy spread of the generated electron beams at the given laser power. The accelerator was operated
in nonlinear regime with forced self-injection and resulted in the generation of stable relativistic electron beams with
an energy of tens of MeV and betatron X-ray radiation was generated in the keV range. A razor blade was tested to
create a steep density gradient in order to improve the stability of electron injection and to increase the total electron
bunch charge. It was proven that the stable electron and X-ray source can be built at small-scale facilities, which
readily opens possibilities for various applications due to availability of such few-terawatt laser systems in many
laboratories around the world.
In a plasma wakefield accelerator, an intense laser pulse propagates in an under-dense plasma that drives a relativistic plasma wave in which electrons can be injected and accelerated to relativistic energies within a short distance. These accelerated electrons undergo betatron oscillation and emit a collimated X-ray beam along the direction of electron velocity. This X-ray source is characterised with a source size of the order of a micrometer, a pulse duration of the order of femtosecond, and with a high spectral brightness. This novel X-ray source provides an excellent imaging tool to achieve unprecedented high-resolution image through phase contrast imaging. The phase contrast technique has the potential to reveal structures which are invisible with the conventional absorption imaging. In the X-ray phase contrast imaging, the image contrast is obtained thanks to phase shifts induced on the X-rays passing through the sample. It involves the real part of refractive index of the object. Here we present high-resolution phase contrast X-ray images of two biological samples using laser-driven Betatron X-ray source.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.