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.
The radiation pressure of next generation high-intensity lasers could efficiently accelerate ions to GeV energies. However, nonlinear quantum-electrodynamic effects play an important role in the interaction of these lasers with matter. We show that these quantum-electrodynamic effects lead to the production of a critical density pair-plasma which completely absorbs the laser pulse and consequently reduces the accelerated ion energy and efficiency by 30-50%.
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.
The applications of laser wake field accelerators (LWFA) rely heavily on the quality of produced high energy electron beams and X-ray sources. We present our recent progress on this issue. Firstly we propose a bichromatic laser ionization injection scheme for obtaining high quality electron beams. With the laser pulse combinations of 800 nm and 267 nm, or 2400 nm and 800 nm in wavelengths, electron beams with energy spread of 1% or lower can be produced. Secondly we propose polarization tunable X-ray sources based on LWFA. By shooting a laser pulse into a preformed plasma channel with a skew angle referring to the channel axis, the plasma channel can act as a helical undulator for elliptically polarized X-rays.
KEYWORDS: Electrons, X-rays, Electron beams, Plasmas, Argon, Simulation of CCA and DLA aggregates, Pulsed laser operation, Physics, X-ray sources, Synchrotrons
Betatron radiation from the transverse oscillation of laser-wakefield accelerated electrons is very promising for a wide range of applications. Currently, the main limitation of this radiation source is the x-ray photon yield. We present our recent progress in achieving higher photon flux using a clustering gas target instead of the normal gas jet, leading to a 10-fold enhancement. Moreover, we observed monoenergetic electron beams and bright x-rays simultaneously, an occurrence which is considered contradictory, and succeeded in using the betatron radiation as a probe in the evolution of bubble dynamics. These breakthroughs are of great significance for pushing the use of betatron radiation source toward new applications.
By employing a pair of partially overlapped supersonic gas jets, we separated injection and acceleration stages of laser wakefield acceleration to produce stable, monoenergetic, and tunable electron beams. The first jet (injector) utilized a He/N2 mixture and resulted in electrons injected into the wake via ionization-assisted injection. These electrons were then accelerated in the second jet (accelerator) using pure He. By changing length and plasma density of the accelerator jet, we were able to tune electron energy in the 50 – 300 MeV range with energy spread of 10-30% and 20 pC charge. Simulations show that the injection region is limited within the overlap of the jets.
We propose to use a combination of Hermite-Gaussian laser modes to generate a nonlinear ring bubble with a large longitudinal accelerating field and a transverse focusing field suitable for positron beam focusing and acceleration. The nonlinear bubble can provide higher accelerating gradients compared with a linear plasma wake. PIC simulations have been used to demonstrate control of the focusing force by changing the relative intensity ratio of the two laser modes, enabling matched positron beam propagation for emittance preservation. In addition, in order to improve phase space characteristics of laser wakefield produced electron beams, in particular, to reduce their transverse emittance, we propose to generate femtosecond electron beams with ultralow emittance using a two-color laser-ionization injection method. By controlling the amplitude and the duration of the injection pulse, the emittance can be controlled to the level of 10-2 mm mrad.
A remarkable conversion efficiency from laser to protons is demonstrated by particle simulations in a laser-foil
interaction. The total laser-proton conversion efficiency becomes 16.7% in an optimized multi-hole target, though a
conventional plane foil target serves a low efficiency. When an intense short-pulse laser illuminates the thin foil target,
the foil electrons are accelerated around the target by the intense laser. The hot electrons generate a strong electric field,
which accelerates the foil protons, and the proton beam is generated. In our previous study, we found that multihole thin-foil
target was efficient for the energy conversion from laser to protons [Phys. Rev. E 78, 046401 (2008)], and the energy
conversion efficiency was 9.3%. In this paper the results clarify the role of the target hole thickness and depth in the
laser-proton energy conversion. The optimized multi-hole foil target provides a remarkable increase in the laser-proton
energy conversion efficiency.
The interaction of ultrashort intense laser pulses with plasma can produce electromagnetic radiation of ultra-broad
spectra ranging from terahertz (THz) radiation to keV x-rays and beyond. Here we present a review of our recent
theoretical and numerical investigation on high power THz generation from tenuous plasma or gas targets irradiated by
ultrashort intense laser pulses. Three mechanisms of THz emission are addressed, which include the linear mode
conversion from laser wakefields in inhomogeneous plasma, transient current emission at the plasma-vacuum
boundaries, and the emission from residual transverse currents produced by temporally-asymmetric laser pulses passing
through gas or plasma targets. Since there is no breakdown limit for plasma under the irradiation of high power lasers, in
principle, all these mechanisms can lead to terahertz pulse emission at the power of beyond megawatt with the field
strength of MV/cm, suitable for the study of high THz field physics and other applications.
Terahertz (THz) radiation from the interaction of ultrashort laser pulses with gases is studied both theoretically and experimentally.
We theoretically study the THz generation based on transient ionization current model and give the relation
between the final THz field and the initial transient ionization current. Recent experimental results on optimization of THz
radiation in laser air interaction are also shown. We find by use of a simple aperture to change the laser field distribution,
the terahertz wave amplitudes can be enhanced by more than eight times than those of aperture-free cases. We use two
dimensional particle-in-cell codes to simulate the experiments and give possible explanations.
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