The dynamics of plasma electrons in the focus of a petawatt laser beam are studied via measurements of their x-ray synchrotron radiation. With increasing laser intensity, a forward directed beam of x ...rays extending to 50 keV is observed. The measured x rays are well described in the synchrotron asymptotic limit of electrons oscillating in a plasma channel. The critical energy of the measured synchrotron spectrum is found to scale as the Maxwellian temperature of the simultaneously measured electron spectra. At low laser intensity transverse oscillations are negligible as the electrons are predominantly accelerated axially by the laser generated wakefield. At high laser intensity, electrons are directly accelerated by the laser and enter a highly radiative regime with up to 5% of their energy converted into x rays.
A bright μm-sized source of hard synchrotron x-rays (critical energy Ecrit > 30 keV) based on the betatron oscillations of laser wakefield accelerated electrons has been developed. The potential of ...this source for medical imaging was demonstrated by performing micro-computed tomography of a human femoral trabecular bone sample, allowing full 3D reconstruction to a resolution below 50 μm. The use of a 1 cm long wakefield accelerator means that the length of the beamline (excluding the laser) is dominated by the x-ray imaging distances rather than the electron acceleration distances. The source possesses high peak brightness, which allows each image to be recorded with a single exposure and reduces the time required for a full tomographic scan. These properties make this an interesting laboratory source for many tomographic imaging applications.
Beam profile measurements of laser-wakefield accelerated electron bunches reveal that in the monoenergetic regime the electrons are injected and accelerated at the back of the first period of the ...plasma wave. With pulse durations ctau >or= lambda(p), we observe an elliptical beam profile with the axis of the ellipse parallel to the axis of the laser polarization. This increase in divergence in the laser polarization direction indicates that the electrons are accelerated within the laser pulse. Reducing the plasma density (decreasing ctau/lambda(p)) leads to a beam profile with less ellipticity, implying that the self-injection occurs at the rear of the first period of the plasma wave. This also demonstrates that the electron bunches are less than a plasma wavelength long, i.e., have a duration <25 fs. This interpretation is supported by 3D particle-in-cell simulations.
The spectra of energetic electrons produced by a laser interaction with underdense plasma have been measured at intensities >3×10^20 W cm^-2. Electron energies in excess of 300 MeV have been ...observed. Measurements of the transmitted laser spectrum indicate that there is no correlation between the acceleration of electrons and plasma wave production. Particle-in-cell simulations show that the laser ponderomotive force produces an ion channel. The interaction of the laser field with the nonlinear focusing force of the channel leads to electron acceleration. The majority of the electrons never reach the betatron resonance but those which gain the highest energies do so. The acceleration process exhibits a strong sensitivity to initial conditions with particles that start within a fraction of a laser wavelength following completely different trajectories and gaining markedly different energies.
Ultrafast shadowgraphy is a new experimental technique that uses few-cycle laser pulses to image density gradients in a rapidly evolving plasma. It enables structures that move at speeds close to the ...speed of light, such as laser driven wakes, to be visualized. Here we study the process of shadowgraphic image formation during the propagation of a few cycle probe pulse transversely through a laser-driven wake using three-dimensional particle-in-cell simulations. In order to construct synthetic shadowgrams a near-field snapshot of the ultrashort probe pulse is analyzed by means of Fourier optics, taking into account the effect of a typical imaging setup. By comparing synthetic and experimental shadowgrams we show that the generation of synthetic data is crucial for the correct interpretation of experiments. Moreover, we study the dependence of synthetic shadowgrams on various parameters such as the imaging system aperture, the position of the object plane and the probe pulse delay, duration and wavelength. Finally, we show that time-dependent information from the interaction can be recovered from a single shot by using a broadband, chirped probe pulse and subsequent spectral filtering.
As new laser facilities are developed with intensities on the scale of 10 22 - 10 24 W cm − 2 , it becomes ever more important to understand the effect of strong field quantum electrodynamic ...processes, such as quantum radiation reaction, which will play a dominant role in laser-plasma interactions at these intensities. Recent all-optical experiments, where GeV electrons from a laser wakefield accelerator encountered a counter-propagating laser pulse with a0 > 10, have produced evidence of radiation reaction, but have not conclusively identified quantum effects nor their most suitable theoretical description. Here we show the number of collisions and the conditions required to accomplish this, based on a simulation campaign of radiation reaction experiments under realistic conditions. We conclude that while the critical energy of the photon spectrum distinguishes classical and quantum-corrected models, a better means of distinguishing the stochastic and deterministic quantum models is the change in the electron energy spread. This is robust against shot-to-shot fluctuations and the necessary laser intensity and electron beam energies are already available. For example, we show that so long as the electron energy spread is below 25%, collisions at a0 = 10 with electron energies of 500 MeV could differentiate between different quantum models in under 30 shots, even with shot-to-shot variations at the 50% level.
A number of theoretical calculations have studied the effect of radiation-reaction forces on radiation distributions in strong-field counterpropagating electron-beam–laser interactions, but could ...these effects—including quantum corrections—be observed in interactions with realistic bunches and focusing fields, as is hoped in a number of soon-to-be-proposed experiments? We present numerical calculations of the angularly resolved radiation spectrum from an electron bunch with parameters similar to those produced in laser-wakefield-acceleration experiments, interacting with an intense, ultrashort laser pulse. For our parameters, the effect of radiation damping on the angular distribution and energy distribution of photons is not easily discernible for a realistic moderate-emittance electron beam. However, experiments using such a counterpropagating beam–laser geometry should be able to measure these effects using current laser systems through measurement of the electron-beam properties. In addition, the brilliance of this source is very high, with peak spectral brilliance exceeding 1029photonss−1mm−2mrad−2(0.1%bandwidth)−1 with an approximately 2% conversion efficiency and with a peak energy of 10 MeV.
Ion acceleration by the interaction of an ultraintense short-pulse laser with an underdense-plasma has been studied at intensities up to 3 x 10(20) W/cm(2). Helium ions having a maximum energy of ...13.2+/-1.0 MeV were measured at an angle of 100 degrees from the laser propagation direction. The maximum ion energy scaled with plasma density as n(0.70+/-0.05)(e). Two-dimensional particle-in-cell simulations suggest that multiple collisionless shocks are formed at high density. The interaction of shocks is responsible for the observed plateau structure in the ion spectrum and leads to an enhanced ion acceleration beyond that possible by the ponderomotive potential of the laser alone.
Model-independent inference of laser intensity Blackburn, T. G.; Gerstmayr, E.; Mangles, S. P. D. ...
Physical review. Accelerators and beams,
06/2020, Letnik:
23, Številka:
6
Journal Article
Recenzirano
Odprti dostop
An ultrarelativistic electron beam passing through an intense laser pulse emits radiation around its direction of propagation into a characteristic angular profile. Here, we show that measurement of ...the variances of this profile in the planes parallel and perpendicular to the laser polarization, and the mean initial and final energies of the electron beam, allows the intensity of the laser pulse to be inferred in a way that is independent of the model of the electron dynamics. The method presented applies whether radiation reaction is important or not, and whether it is classical or quantum in nature, with an accuracy of a few percent across 3 orders of magnitude in intensity. It is tolerant of electron beams with a broad energy spread and finite divergence. In laser-electron-beam collision experiments, where spatiotemporal fluctuations cause the alignment of the beams to vary from shot to shot, this permits inference of the laser intensity at the collision point, thereby facilitating comparisons between theoretical calculations and experimental data.
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