Single-Shot Multi-keV X-Ray Absorption Spectroscopy Using an Ultrashort Laser-Wakefield Accelerator Source.

Single-shot absorption measurements have been performed using the multi-keV x rays generated by a laser-wakefield accelerator. A 200 TW laser was used to drive a laser-wakefield accelerator in a mode which produced broadband electron beams with a maximum energy above 1 GeV and a broad divergence of ≈15 mrad FWHM. Betatron oscillations of these electrons generated 1.2±0.2×10^{6} photons/eV in the 5 keV region, with a signal-to-noise ratio of approximately 300∶1. This was sufficient to allow high-resolution x-ray absorption near-edge structure measurements at the K edge of a titanium sample in a single shot. We demonstrate that this source is capable of single-shot, simultaneous measurements of both the electron and ion distributions in matter heated to eV temperatures by comparison with density functional theory simulations. The unique combination of a high-flux, large bandwidth, few femtosecond duration x-ray pulse synchronized to a high-power laser will enable key advances in the study of ultrafast energetic processes such as electron-ion equilibration.

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.

Experimental measurements of electron-bunch trains in a laser-plasma accelerator.

Spectral measurements of visible coherent transition radiation produced by a laser-plasma-accelerated electron beam are reported. The significant periodic modulations that are observed in the spectrum result from the interference of transition radiation produced by multiple bunches of electrons. A Fourier analysis of the spectral interference fringes reveals that electrons are injected and accelerated in multiple plasma wave periods, up to at least 10 periods behind the laser pulse. The bunch separation scales with the plasma wavelength when the plasma density is changed over a wide range. An analysis of the spectral fringe visibility indicates that the first bunch contains most of the charge.

Comparison of measured with calculated dose distribution from a 120-MeV electron beam from a laser-plasma accelerator.

PURPOSE: To evaluate the dose distribution of a 120-MeV laser-plasma accelerated electron beam which may be of potential interest for high-energy electron radiation therapy. METHODS: In the interaction between an intense laser pulse and a helium gas jet, a well collimated electron beam with very high energy is produced. A secondary laser beam is used to optically control and to tune the electron beam energy and charge. The potential use of this beam for radiation treatment is evaluated experimentally by measurements of dose deposition in a polystyrene phantom. The results are compared to Monte Carlo simulations using the geant4 code. RESULTS: It has been shown that the laser-plasma accelerated electron beam can deliver a peak dose of more than 1 Gy at the entrance of the phantom in a single laser shot by direct irradiation, without the use of intermediate magnetic transport or focusing. The dose distribution is peaked on axis, with narrow lateral penumbra. Monte Carlo simulations of electron beam propagation and dose deposition indicate that the propagation of the intense electron beam (with large self-fields) can be described by standard models that exclude collective effects in the response of the material. CONCLUSIONS: The measurements show that the high-energy electron beams produced by an optically injected laser-plasma accelerator can deliver high enough dose at penetration depths of interest for electron beam radiotherapy of deep-seated tumors. Many engineering issues must be resolved before laser-accelerated electrons can be used for cancer therapy, but they also represent exciting challenges for future research.

Mapping the x-ray emission region in a laser-plasma accelerator.

The x-ray emission in laser-plasma accelerators can be a powerful tool to understand the physics of relativistic laser-plasma interaction. It is shown here that the mapping of betatron x-ray radiation can be obtained from the x-ray beam profile when an aperture mask is positioned just beyond the end of the emission region. The influence of the plasma density on the position and the longitudinal profile of the x-ray emission is investigated and compared to particle-in-cell simulations. The measurement of the x-ray emission position and length provides insight on the dynamics of the interaction, including the electron self-injection region, possible multiple injection, and the role of the electron beam driven wakefield.

Author Correction: Laser-wakefield accelerators for high-resolution X-ray imaging of complex microstructures.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Laser-wakefield accelerators for high-resolution X-ray imaging of complex microstructures.

Laser-wakefield accelerators (LWFAs) are high acceleration-gradient plasma-based particle accelerators capable of producing ultra-relativistic electron beams. Within the strong focusing fields of the wakefield, accelerated electrons undergo betatron oscillations, emitting a bright pulse of X-rays with a micrometer-scale source size that may be used for imaging applications. Non-destructive X-ray phase contrast imaging and tomography of heterogeneous materials can provide insight into their processing, structure, and performance. To demonstrate the imaging capability of X-rays from an LWFA we have examined an irregular eutectic in the aluminum-silicon (Al-Si) system. The lamellar spacing of the Al-Si eutectic microstructure is on the order of a few micrometers, thus requiring high spatial resolution. We present comparisons between the sharpness and spatial resolution in phase contrast images of this eutectic alloy obtained via X-ray phase contrast imaging at the Swiss Light Source (SLS) synchrotron and X-ray projection microscopy via an LWFA source. An upper bound on the resolving power of 2.7 ± 0.3 µm of the LWFA source in this experiment was measured. These results indicate that betatron X-rays from laser wakefield acceleration can provide an alternative to conventional synchrotron sources for high resolution imaging of eutectics and, more broadly, complex microstructures.

Quantitative study of 10 Hz operation of a soft x-ray laser-energy stability and target considerations.

A soft x-ray laser from Ni-like Mo, pumped in grazing incidence (GRIP), is analyzed with regard to high repetition rate operation. Reliable lasing is obtained, but with significant energy fluctuations attributed mainly to beam pointing jitter from the pump laser. Two modes of operation are compared: continuously moving target and stationary target. With a moving target the soft X-ray output is constant on average, whereas the repeated use of the same target position leads to a pulse energy which increases for several tens of shots. This effect might be caused by improved guiding of the pump laser in the formed groove and the removal, through laser ablation, of the oxide layer on the target surface.

Influence of shock waves on laser-driven proton acceleration.

The influence of shock waves, driven by amplified spontaneous emission (ASE), on laser-accelerated proton beams is investigated. A local deformation, produced by a cold shock wave launched by the ablation pressure of the ASE pedestal, can under oblique laser irradiation significantly direct the proton beam toward the laser axis. This can be understood in the frame of target normal sheath acceleration as proton emission from an area of the target where the local target normal is shifted toward the laser axis. Hydrodynamic simulations and experimental data show that there exists a window in laser and target parameter space where the target can be significantly deformed and yet facilitate efficient proton acceleration. The dependence of the magnitude of the deflection on target material, foil thickness, and ASE pedestal intensity and duration is experimentally investigated. The deflection angle is found to increase with increasing ASE intensity and duration and decrease with increasing target thickness. In a comparison between aluminum and copper target foils, aluminum is found to yield a larger proton beam deflection. An analytic model is successfully used to predict the proton emission direction.

Active manipulation of the spatial energy distribution of laser-accelerated proton beams.

The spatial energy distributions of beams of protons accelerated by ultrahigh intensity (>10(19)Wcm2) picosecond laser pulse interactions with thin foil targets are investigated. Using separate, low intensity (<10(13)Wcm2) nanosecond laser pulses, focused onto the front surface of the target foil prior to the arrival of the high intensity pulse, it is demonstrated that the proton beam profile can be actively manipulated. In particular, results obtained with an annular intensity distribution at the focus of the low intensity beam are presented, showing smooth proton beams with a sharp circular boundary at all energies, which represents a significant improvement in the beam quality compared to irradiation with the picosecond beam alone.

Highly efficient angularly resolving x-ray spectrometer optimized for absorption measurements with collimated sources.

Highly collimated betatron radiation from a laser wakefield accelerator is a promising tool for spectroscopic measurements. Therefore, there is a requirement to create spectrometers suited to the unique properties of such a source. We demonstrate a spectrometer which achieves an energy resolution of <5 eV at 9 keV (E∕ΔE>1800) and is angularly resolving the x-ray emission allowing the reference and spectrum to be recorded at the same time. The single photon analysis is used to significantly reduce the background noise. Theoretical performance of various configurations of the spectrometer is calculated by a ray-tracing algorithm. The properties and performance of the spectrometer including the angular and spectral resolution are demonstrated experimentally on absorption above the K-edge of a Cu foil backlit by a laser-produced betatron radiation x-ray beam.

Shock assisted ionization injection in laser-plasma accelerators.

Ionization injection is a simple and efficient method to trap an electron beam in a laser plasma accelerator. Yet, because of a long injection length, this injection technique leads generally to the production of large energy spread electron beams. Here, we propose to use a shock front transition to localize the injection. Experimental results show that the energy spread can be reduced down to 10 MeV and that the beam energy can be tuned by varying the position of the shock. This simple technique leads to very stable and reliable injection even for modest laser energy. It should therefore become a unique tool for the development of laser-plasma accelerators.

Guiding of relativistic electron beams in solid targets by resistively controlled magnetic fields.

Guided transport of a relativistic electron beam in solid is achieved experimentally by exploiting the strong magnetic fields created at the interface of two metals of different electrical resistivities. This is of substantial relevance to the Fast Ignitor approach to fusion energy production [M. Tabak, Phys. Plasmas 12, 057305 (2005)10.1063/1.1871246], since it allows the electron deposition to be spatially tailored-thus adding substantial design flexibility and preventing inefficiencies due to electron beam spreading. In the experiment, optical transition radiation and thermal emission from the target rear surface provide a clear signature of the electron confinement within a high resistivity tin layer sandwiched transversely between two low resistivity aluminum slabs. The experimental data are found to agree well with numerical simulations.

Laser-driven plasma waves in capillary tubes.

The excitation of plasma waves over a length of up to 8 cm is demonstrated using laser guiding of intense laser pulses through hydrogen-filled glass capillary tubes. The plasma waves are diagnosed by spectral analysis of the transmitted laser radiation. The dependence of the spectral redshift-measured as a function of filling pressure, capillary tube length, and incident laser energy-is in excellent agreement with simulation results. The longitudinal accelerating field inferred from the simulations is in the range of 1-10 GV/m.

Radiochromic film imaging spectroscopy of laser-accelerated proton beams.

This article reports on an experimental method to fully reconstruct laser-accelerated proton beam parameters called radiochromic film imaging spectroscopy (RIS). RIS allows for the characterization of proton beams concerning real and virtual source size, envelope- and microdivergence, normalized transverse emittance, phase space, and proton spectrum. This technique requires particular targets and a high resolution proton detector. Therefore thin gold foils with a microgrooved rear side were manufactured and characterized. Calibrated GafChromic radiochromic film (RCF) types MD-55, HS, and HD-810 in stack configuration were used as spatial and energy resolved film detectors. The principle of the RCF imaging spectroscopy was demonstrated at four different laser systems. This can be a method to characterize a laser system with respect to its proton-acceleration capability. In addition, an algorithm to calculate the spatial and energy resolved proton distribution has been developed and tested to get a better idea of laser-accelerated proton beams and their energy deposition with respect to further applications.

Optimization toward a high-average-brightness soft-x-ray laser pumped at grazing incidence.

We report the near-field imaging characterization of a 10 Hz Ni-like 18.9 nm molybdenum soft-x-ray laser pumped in a grazing incidence pumping (GRIP) geometry with a table-top laser driver. We investigate the effect of varying the GRIP angle on the spatial behavior of the soft-x-ray laser source. After multiparameter optimization, we were able to find conditions to generate routinely a high-repetition-rate soft-x-ray laser with an energy level of up to 3 microJ/pulse and to 6x10(17) photons/s/mm2/mrad2/(0.1% bandwidth) average brightness and 1x10(28) photons/s/mm2/mrad2/(0.1% bandwidth) peak brightness.

Lateral electron transport in high-intensity laser-irradiated foils diagnosed by ion emission.

An experimental investigation of lateral electron transport in thin metallic foil targets irradiated by ultraintense (>or=10(19) W/cm2) laser pulses is reported. Two-dimensional spatially resolved ion emission measurements are used to quantify electric-field generation resulting from electron transport. The measurement of large electric fields ( approximately 0.1 TV/m) millimeters from the laser focus reveals that lateral energy transport continues long after the laser pulse has decayed. Numerical simulations confirm a very strong enhancement of electron density and electric field at the edges of the target.

Laser-wakefield acceleration of monoenergetic electron beams in the first plasma-wave period.

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.

Laser-accelerated protons with energy-dependent beam direction.

The spatial distribution of protons, accelerated by intense femtosecond laser pulses interacting with thin target foils under oblique irradiation are investigated. Under certain conditions, the proton beams are directed away from the target normal. This deviation is towards the laser forward direction, with an angle that increases with the level and duration of the amplified spontaneous emission pedestal before the main laser pulse. In addition, for a given laser pulse, this beam deviation increases with proton energy. The observations are discussed in terms of different electron acceleration mechanisms and target normal sheath acceleration, in combination with a laser-controllable shock wave locally deforming the target rear surface.