[Show abstract][Hide abstract] ABSTRACT: The generation of GeV-scale electron beams in a gas-filled capillary discharge waveguide with good reproducibility is discussed. Beams of electrons with energies above 900 MeV, and with root-mean-square divergences of 3.5 mrad, are observed for a plasma density of 2.2 × 1018 cm−3 and a peak input laser power of 55 TW. The variation of the maximum electron energy with the plasma density is measured and found to agree well with simple models. Injection and acceleration of electrons at the to date lowest plasma density of 3.2 × 1017 cm−3 are reported. The energy spectra of the generated electron beams exhibit good shot-to-shot reproducibility, with the observed variations attributable to the measured shot-to-shot jitter of the laser parameters. Two methods for correcting the effect of beam pointing variations on the measured energy spectrum are described.
Full-text · Article · Apr 2013 · New Journal of Physics
[Show abstract][Hide abstract] ABSTRACT: We use coherent optical transition radiation (COTR) to measure the
transverse profile of laser-accelerated electron bunches. The retrieved
electron beam profiles are compared to scintillator-based beam profile
[Show abstract][Hide abstract] ABSTRACT: The generation of GeV-scale electron beams in the plasma channel formed
in a gas-filled capillary discharge waveguide is investigated. Electron
beams with energies above 900 MeV and with root-mean-square divergence
of 3.5 mrad are observed for plasma densities of 2.15 × 1018 cm-3
and a peak input laser power of only 55 TW. The variation of the
electron energy with the plasma density is measured and found to exhibit
a maximum at plasma densities for which the dephasing length
approximately matches the length of the plasma channel. Injection and
acceleration of electrons at the relatively low plasma density of 3.2
× 1017 cm-3 is observed. The energy spectra of the generated
electron beams are shown to exhibit good shot-to-shot reproducibility,
with the observed variations attributable to the measured shot-to-shot
jitter of the laser parameters. Two methods for correcting for the
effects on the measured energy spectrum of off-axis electron beam
propagation are investigated.
[Show abstract][Hide abstract] ABSTRACT: Introduction The acceleration gradients achieved in laser-driven plasma accelerators are three or more orders of magnitudes greater than provided by conventional accelerators, offering a route to a new class of compact accelerators. To date electron beams have been generated with energies up to the GeV level in accelerators a few cm long [1-3]. Having reached this milestone, attention is turning to increasing the energy gain per stage and improving the shot-to-shot reproducibility of the electron beams in order that they may be used to drive applications. In laser-driven plasma accelerators, the difference in the phase velocity of the plasma wave and the velocity of the electrons causes the accelerated electron bunch to move from an accelerating phase to decelerating one after the dephasing length √ , where and are the angular laser and plasma frequencies respectively, and is the normalized vector potential of the laser field. Since the peak accelerating electric field is proportional to the maximum energy gain per stage is proportional to , where is the plasma density. For these reasons there is considerable interest in driving plasma accelerators at lower plasma densities and over longer distances. In the experiments described here a hydrogen-filled capillary discharge waveguide [4,5] was used to form a plasma channel, extending the distance over which the plasma wave could be driven. In this report we describe experiments with the Astra-Gemini laser on acceleration driven with plasma channels formed in a capillary discharge waveguide. These show improved reproducibility and also injection and acceleration of electrons at the lowest plasma densities reported to date, a result which is promising for future experiments with greater energy gain per stage.
[Show abstract][Hide abstract] ABSTRACT: We report on the characterization of electron bunches, accelerated
within a laser-driven plasma wakefield, using incoherent transition
radiation (TR). TR is generated whenever a charged particle crosses an
interface between different materials. Incoherent TR is often used
within RF accelerators as a bunch diagnostic, although for short
bunch-durations coherence effects restrict its usefulness. Usually this
coherent TR has been used for the measurement of
laser-plasma-accelerated bunch-durations. Instead incoherent TR allows
simultaneous measurement of the transverse profile and charge of the
bunch, as well as the bunch-duration by a different method than from
coherent TR. Tailored gas targets were used with the high-power laser of
the Lund Laser Centre (1J, 35fs) to generate electron bunches through
ionization injection of electrons into a wakefield and their
acceleration. Transition radiation diagnostics were employed for
single-shot diagnosis of these bunches and the preliminary results are
[Show abstract][Hide abstract] ABSTRACT: Magnetic dipole spectrometers are widely used to measure the energy spectrum of electron beams generated in laser plasma accelerators. In essence they comprise: (i) a region of known magnetic field in which electrons are deflected according to the Lorentz force and (ii) an imaging screen on which the deflected electrons are detected. In laser wakefield acceleration experiments, the screens most often used are image plates (IPs) and Lanex scintillating screens. Imaging plates have the advantage of being absolutely calibrated for charge and high spatial resolution (< 10um). However, they are relatively expensive and the read-out time is long: after every exposure the IP has to be transported to an IP reader, read-out and blanked. Depending on the spatial resolution required, and size of the IP, this process can take between a few minutes to an hour. Further IPs have to be protected against room light since this erases the signal on the imaging plate. Lanex scintillating screens have the advantage of being cheap and readily available. With these screens, CCD cameras are used to image the phosphorescence excited by the incident electron beam. Lanex screens are therefore convenient since the signal they generate can be recorded in situ. However, calibrating the Lanex signal against charge requires either: accurate calibration of the light collecting efficiency and detection sensitivity of the imaging system and camera ; or cross-calibration against an image plate .
[Show abstract][Hide abstract] ABSTRACT: Tapered plasma channels are considered for controlling dephasing of a beam with respect to a plasma wave driven by a weakly relativistic, short-pulse laser. Tapering allows for enhanced energy gain in a single laser-plasma accelerator stage. Expressions are derived for the taper, or longitudinal plasma density variation, required to maintain a beam at a constant phase in the longitudinal and/or transverse fields of the plasma wave. In a plasma channel, the phase velocities of the longitudinal and transverse fields differ and, hence, the required tapering differs. The length over which the tapered plasma density becomes singular is calculated. Linear plasma tapering as well as discontinuous plasma tapering, which moves beams to adjacent plasma wave buckets, is also considered. The energy gain of an accelerated electron in a tapered laser-plasma accelerator is calculated and the laser pulse length to optimize the energy gain is determined.
[Show abstract][Hide abstract] ABSTRACT: In a laser-plasma-based accelerator, the laser-driven plasma wave phase velocity (determined in part by the intensity transport velocity and evolution of the short-pulse drive laser) sets the dephasing length of the plasma accelerating structure and, hence, the energy gain of an accelerated particle beam. The phase velocity of a plasma wave driven by a relativistically-intense, short-pulse laser propagating in a cold underdense plasma is investigated, as well as the drive laser evolution. A relativistic beam may be phase-locked to the plasma wave using a plasma density taper, increasing the single-stage energy gain. The expression for the density taper in a plasma channel to maintain a relativistic beam at a constant plasma wave phase is presented. The optimal laser pulse duration for maximizing energy gain in a tapered plasma channel is calculated. Novel quasi-periodic plasma tapering schemes are considered.