Laser-driven Acceleration Research Articles

Vlasov simulation of laser-driven shock acceleration and ion turbulence

We present a Vlasov, i.e. a kinetic Eulerian simulation study of nonlinear collisionless ion-acoustic shocks and solitons excited by an intense laser interacting with an overdense plasma. The use of the Vlasov code avoids problems with low particle statistics and allows a validation of particle-in-cell results. A simple, original correction to the splitting method for the numerical integration of the Vlasov equation has been implemented in order to ensure the charge conservation in the relativistic regime. We show that the ion distribution is affected by the development of a turbulence driven by the relativistic ‘fast’ electron bunches generated at the laser-plasma interaction surface. This leads to the onset of ion reflection at the shock front in an initially cold plasma where only soliton solutions without ion reflection are expected to propagate. We give a simple analytical model to describe the onset of the turbulence as a nonlinear coupling of the ion density with the fast electron currents, taking the pulsed nature of the relativistic electron bunches into account.

Laser acceleration in argon clusters and gas media

We experimentally investigated the generation of high-energy electron beams from laser-driven wakefield acceleration in argon gas jets by using tens of terawatt 30 fs ultrafast laser pulses that were focused to a relatively large-spot size, unmatched with the laser–plasma parameters. In this interaction, and depending on the Ar gas jet density, we could distinguish two different regimes for electron acceleration in the argon medium. In the high-density argon gas jet where argon clusters are formed, upon interaction with the laser electron beams having as high a charge as 3nC are generated. However, the energy spectra of those electron beams were continuous. On the other hand, high-quality quasi-mono-energetic electron beams with a modest charge of tens of pC are observed at much lower argon gas jet densities. The generation of such a high-quality electron beam is attributed to the ionization injection mechanism in which the electron injection takes place over only a few hundred micrometers of the laser–plasma interaction length, leading to the generation of high-quality electron beams.

A.116 - Dosimetric characterization of a laser-driven acceleration system

A.116 - Dosimetric characterization of a laser-driven acceleration system

SINBAD—The accelerator R&D facility under construction at DESY

The SINBAD facility (Short INnovative Bunches and Accelerators at DESY) is a long-term dedicated accelerator research and development facility currently under construction at DESY. It will be located in the premises of the old DORIS accelerator complex and host multiple independent experiments cost-effectively accessing the same central infrastructure like a central high power laser. With the removal of the old DORIS accelerator being completed, the refurbishment of the technical infrastructure is currently starting up.The presently ongoing conversion of the area into the SINBAD facility and the currently foreseen layout is described. The first experiment will use a compact S-band linac for the production of ultra-short bunches at hundred MeV. Once established, one of the main usages will be to externally inject electrons into a laser-driven plasma wakefield accelerator to boost the energy to GeV-level while maintaining a usable beam quality, ultimately aiming to drive an FEL. The second experiment already under planning is the setup of an attosecond radiation source with advanced technology. Further usage of the available space and infrastructure is revised and national and international collaborations are being established.

Matching sub-fs electron bunches for laser-driven plasma acceleration at SINBAD

We present theoretical and numerical studies of matching sub-femtosecond space-charge-dominated electron bunch into the Laser-plasma Wake Field Accelerator (LWFA) foreseen at the SINBAD facility. The longitudinal space-charge (SC) effect induced growths of the energy spread and longitudinal phase-space chirp are major issues in the matching section, which will result in bunch elongation, emittance growth and spot size dilution. In addition, the transverse SC effect would lead to a mismatch of the beam optics if it were not compensated for. Start-to-end simulations and preliminary optimizations were carried out in order to understand the achievable beam parameters at the entrance of the plasma accelerator.

Laser-driven acceleration of subrelativistic electrons near a nanostructured dielectric grating: From acceleration via higher spatial harmonics to necessary elements of a dielectric accelerator

The experimental setup that allows for the observation of energy gain of electrons interacting with Dielectric Laser Accelerators (DLAs) is reviewed. Moreover, recent results, including acceleration due to electron interaction with third, fourth and fifth spatial harmonics of a nanostructured grating are discussed and an extended outlook is given.

Laser-Driven Plasma Electron Acceleration and Radiation

Laser-driven plasma acceleration of electron beams is reviewed from the viewpoint of the underlying physics and recent progress in the experimental research. Betatron radiation cogenerated from laser plasma accelerators is mentioned in terms of electron beam dynamics and the radiation spectrum. At the end, future perspectives of possible applications are presented.

Modeling laser-driven electron acceleration using WARP with Fourier decomposition

WARP is used with the recent implementation of the Fourier decomposition algorithm to model laser-driven electron acceleration in plasmas. Simulations were carried out to analyze the experimental results obtained on ionization-induced injection in a gas cell. The simulated results are in good agreement with the experimental ones, confirming the ability of the code to take into account the physics of electron injection and reduce calculation time. We present a detailed analysis of the laser propagation, the plasma wave generation and the electron beam dynamics.

Measurement of power spectral density of broad-spectrum visible light with heterodyne near field scattering and its scalability to betatron radiation.

We exploit the speckle field generated by scattering from a colloidal suspension to access both spatial and temporal coherence properties of broadband radiation. By applying the Wiener-Khinchine theorem to the retrieved temporal coherence function, information about the emission spectrum of the source is obtained in good agreement with the results of a grating spectrometer. Experiments have been performed with visible light. We prove more generally that our approach can be considered as a tool for modeling a variety of cases. Here we discuss how to apply such diagnostics to broad-spectrum betatron radiation produced in the laser-driven wakefield accelerator under development at SPARC LAB facility in Frascati.

Laser-driven electron acceleration in an inhomogeneous plasma channel

We study the laser-driven electron acceleration in a transversely inhomogeneous plasma channel. We find that, in inhomogeneous plasma channel, the developing of instability for electron acceleration and the electron energy gain can be controlled by adjusting the laser polarization angle and inhomogeneity of plasma channel. That is, we can short the accelerating length and enhance the energy gain in inhomogeneous plasma channel by adjusting the laser polarization angle and inhomogeneity of the plasma channel.

Multi-MeV Electron Acceleration by Subterawatt Laser Pulses.

We demonstrate laser-plasma acceleration of high charge electron beams to the ∼10 MeV scale using ultrashort laser pulses with as little energy as 10mJ. This result is made possible by an extremely dense and thin hydrogen gas jet. Total charge up to ∼0.5 nC is measured for energies >1 MeV. Acceleration is correlated to the presence of a relativistically self-focused laser filament accompanied by an intense coherent broadband light flash, associated with wave breaking, which can radiate more than ∼3% of the laser energy in a ∼1 fs bandwidth consistent with half-cycle optical emission. Our results enable truly portable applications of laser-driven acceleration, such as low dose radiography, ultrafast probing of matter, and isotope production.

Terahertz-driven linear electron acceleration.

The cost, size and availability of electron accelerators are dominated by the achievable accelerating gradient. Conventional high-brightness radio-frequency accelerating structures operate with 30–50 MeV m−1 gradients. Electron accelerators driven with optical or infrared sources have demonstrated accelerating gradients orders of magnitude above that achievable with conventional radio-frequency structures. However, laser-driven wakefield accelerators require intense femtosecond sources and direct laser-driven accelerators suffer from low bunch charge, sub-micron tolerances and sub-femtosecond timing requirements due to the short wavelength of operation. Here we demonstrate linear acceleration of electrons with keV energy gain using optically generated terahertz pulses. Terahertz-driven accelerating structures enable high-gradient electron/proton accelerators with simple accelerating structures, high repetition rates and significant charge per bunch. These ultra-compact terahertz accelerators with extremely short electron bunches hold great potential to have a transformative impact for free electron lasers, linear colliders, ultrafast electron diffraction, X-ray science and medical therapy with X-rays and electron beams.

Enhanced laser-driven proton acceleration from a relativistically transparent transversely nano-striped target

Enhanced laser-driven proton acceleration occurs when an intense laser interacts with a transversely nano-striped target consisted of alternate gold and hydrogen stripes. The density of the laser-irradiated hydrogen stripes initially decreases to less than the critical density, enabling the laser field to penetrate into the hydrogen stripes and heat the electrons in the still-opaque gold stripes. The resulting increase in the electron Lorentz factor and the expansion of all the electrons ultimately make the entire target relativistically transparent to the latter part of the laser beam. Large numbers of hot electrons are expelled from the target because the laser volumetrically interacts with the whole target. A fairly high electrostatic field is then induced on the rear side of the target to accelerate the protons. When compared with double-layer or single-layer targets, the proton acceleration is much more efficient in the relativistically transparent nano-striped target presented here. Two spatial and three velocity dimensions (2D3V) particle-in-cell simulations show that with an W cm−2 laser, a proton beam with a maximum energy of approximately 190 MeV can be generated.

Enhanced electron yield from laser-driven wakefield acceleration in high-Z gas jets.

An investigation of the electron beam yield (charge) form helium, nitrogen, and neon gas jet plasmas in a typical laser-plasma wakefield acceleration experiment is carried out. The charge measurement is made by imaging the electron beam intensity profile on a fluorescent screen into a charge coupled device which was cross-calibrated with an integrated current transformer. The dependence of electron beam charge on the laser and plasma conditions for the aforementioned gases are studied. We found that laser-driven wakefield acceleration in low Z-gas jet targets usually generates high-quality and well-collimated electron beams with modest yields at the level of 10-100 pC. On the other hand, filamentary electron beams which are observed from high-Z gases at higher densities reached much higher yields. Evidences for cluster formation were clearly observed in the nitrogen gas jet target, where we received the highest electron beam charge of ∼1.7 nC. Those intense electron beams will be beneficial for the applications on the generation of bright X-rays, gamma rays radiations, and energetic positrons via the bremsstrahlung or inverse-scattering processes.

Science and applications of the coherent amplifying network (CAN) laser

Accelerators are today in every walk of science and life [1]. High intensity lasers drive frontiers of contemporary science. Lasers in the TW and PW regime have the potential to replace conventional accelerators with the distinct advantage to be dramatically shorter by a factor of a thousand or more [2]. For instance, electrons are accelerated to few GeV over only few centimeters, representing three to fours orders of magnitude higher accelerating gradients than traditional RF-based accelerators can offer. The approach was proposed in 1979 [3], where a strong laser pulse [4], moving in a plasma creates a wake in which electrons are trapped and violently accelerated. In addition, under ultra high intensity, high energy protons over 100 MeV have been demonstrated as well as high energy radiation greater that MeV [5]. Key to laser-driven accelerators, ion, X-Ray, or Gamma-Ray production are ultra high peak power lasers at the petawatt level [6]. However, current petawatt laser exhibits low repetition rates (state-of-the-art is about 1 Hz) due to thermo-optical character in their gain medium, resulting in low average powers in the 50 W range. In addition, a rather poor wall-plug efficiency (electrical power to optical power) of 10−3 % avoids any scaling perspectives. Hence, state-of-the-art high peak power laser system cannot pretend to be tomorrow's replacement to conventional RF Technology –a new class of ultrafast lasers is urgently needed. Under the ICFA-ICUIL [2] initiative, laser experts in the field of particle acceleration and high intensity lasers defined target parameters of a future laser system should deliver to pave the way for a new kind of accelerator technology revolutionizing fundamental science and applications. The following laser parameters are envisaged for what could be a future linear e–e+ collider: peak power in the PW regime, defined by a 10's of Joules of pulse energy and an ultrashort pulse duration below 50 fs, in combination with an unparalleled average power exceeding 100 kW even exceeding the megawatt level, implying repetition rates of >10 kHz. These extreme parameters should be contained in a beam of excellent spatial quality, featuring outstanding temporal stability and temporal contrast. An excellent wall-plug efficiency of >30% is an essential condition that such average powers are realized in a cost effective, economic and compact way. Overall, any known laser technology known today faces severe issues, with current performance orders of magnitude below these target parameters. Inspired by these ground-breaking challenges under the Eu leadership, the International Coherent Amplification Network (ICAN) group was formed. It combines the complementary expertise of science authorities in the field of high performance fiber amplifiers, theoretical and applied optics of optical systems and finally ultra high intensity lasers. ICAN aspired to study the fundamentals of interferometric amplification i.e. spatially separated amplification followed by coherent addition, of ultrashort laser pulses as the underlying concept of a breakthrough in laser physics. In detail ICAN has studied: 1) average/peak power and efficiency limits of coherently combined ultrafast laser systems 2) synchronization, spatial and temporal recombination of a large number of fibers amplifiers 3) temporal and spatial beam quality, combining efficiency of coherent addition, amplitude and phase stability as a function of the number of fibers and their individual performance 4) reduction of pulse duration and manipulation of pulse shape.

A New Scheme for High‐Intensity Laser‐Driven Electron Acceleration in a Plasma

AbstractWe propose a new approach to high‐intensity relativistic laser‐driven electron acceleration in a plasma. Here, we demonstrate that a plasma wave generated by a stimulated forward‐scattering of an incident laser pulse can be in the longest acceleration phase with injected relativistic beam electrons. This is why the plasma wave has the maximum amplification coefficient which is determined by the acceleration time and the breakdown (overturn) electric field in which the acceleration of the injected beam electrons occurs. We must note that for the longest acceleration phase the relativity of the injected beam electrons plays a crucial role in our scheme. We estimate qualitatively the acceleration parameters of relativistic electrons in the field of a plasma wave generated at the stimulated forward‐scattering of a high‐intensity laser pulse in a plasma. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A treatment planning study to assess the feasibility of laser-driven proton therapy using a compact gantry design.

Laser-driven proton acceleration is suggested as a cost- and space-efficient alternative for future radiation therapy centers, although the properties of these beams are fairly different compared to conventionally accelerated proton beams. The laser-driven proton beam is extremely pulsed containing a very high proton number within ultrashort bunches at low bunch repetition rates of few Hz and the energy spectrum of the protons per bunch is very broad. Moreover, these laser accelerated bunches are subject to shot-to-shot fluctuations. Therefore, the aim of this study was to investigate the feasibility of a compact gantry design for laser-driven proton therapy and to determine limitations to comply with. Based on a published gantry beam line design which can filter parabolic spectra from an exponentially decaying broad initial spectrum, a treatment planning study was performed on real patient data sets. All potential parabolic spectra were fed into a treatment planning system and numerous spot scanning proton plans were calculated. To investigate limitations in the fluence per bunch, the proton number of the initial spectrum and the beam width at patient entrance were varied. A scenario where only integer shots are delivered as well as an intensity modulation from shot to shot was studied. The resulting plans were evaluated depending on their dosimetric quality and in terms of required treatment time. In addition, the influence of random shot-to-shot fluctuations on the plan quality was analyzed. The study showed that clinically relevant dose distributions can be produced with the system under investigation even with integer shots. For small target volumes receiving high doses per fraction, the initial proton number per bunch must remain between 1.4 × 10(8) and 8.3 × 10(9) to achieve acceptable delivery times as well as plan qualities. For larger target volumes and standard doses per fraction, the initial proton number is even more restricted to stay between 1.4 × 10(9) and 2.9 × 10(9). The lowest delivery time that could be reached for such a case was 16 min for a 10 Hz system. When modulating the intensity from shot to shot, the delivery time can be reduced to 6 min for this scenario. Since the shot-to-shot fluctuations are of random nature, a compensation effect can be observed, especially for higher laser shot numbers. Therefore, a fluctuation of ± 30% within the proton number does not translate into a dosimetric deviation of the same size. However, for plans with short delivery times these fluctuations cannot cancel out sufficiently, even for ± 10% fluctuations. Under the analyzed terms, it is feasible to achieve clinically relevant dose distributions with laser-driven proton beams. However, to keep the delivery times of the proton plans comparable to conventional proton plans for typical target volumes, a device is required which can modulate the bunch intensity from shot to shot. From the laser acceleration point of view, the proton number per bunch must be kept under control as well as the reproducibility of the bunches.

Enhanced electron injection in laser-driven bubble acceleration by ultra-intense laser irradiating foil-gas targets

A scheme for enhancing the electron injection charge in a laser-driven bubble acceleration is proposed. In this scheme, a thin foil target is placed in front of a gas target. Upon interaction with an ultra-intense laser pulse, the foil emits electrons with large longitudinal momenta, allowing them to be trapped into the transmitted shaped laser-excited bubble in the gaseous plasma target. Two-dimensional particle-in-cell simulation is used to demonstrate this scheme, and an electron beam with a total electron number of 4.21×108 μm−1 can be produced, which is twice the number of electrons produced without the foil. Such scheme may be widely used for applications that require high electron yields such as positron and gamma ray generation from relativistic electron beams interacting with solid targets.

Performances of the Alpha-X RF gun on the PHIL accelerator at LAL

The Alpha-X RF-gun was designed to produce an ultra-short (<100fs rms), 100pC and 6.3MeV electron beam with a normalized rms transverse emittance of 1πmmmrad for a gun peak accelerating field of 100MV/m. Such beams will be required by the Alpha-X project, which aims to study a laser-driven plasma accelerator with a short wavelength accelerating medium. It has been demonstrated on PHIL (Photo-Injector at LAL) that the coaxial RF coupling, chosen to preserve the gun field cylindrical symmetry, is perfectly understood and allows reaching the required peak accelerating field of 100MV/m giving beam energy of 6.3MeV. Moreover, a quite low beam rms relative energy spread of 0.15% at 3.8MeV has been measured, completely agreeing with simulations. Dark current, quantum efficiencies and dephasing curves measurements have also been performed. They all show high values of the field enhancement factor β, which can be explained by the preparation of the photocathodes. Finally, measurements on the transverse phase-space have been carried out, with some limitations given by the difficult modelization of one of the PHIL solenoid magnets and by the enlargement of the beam transverse dimensions due to the use of YAG screens. These measurements give a normalized rms transverse emittance around 5πmmmrad, which does not fulfill the requirement for the Alpha-X project.

Laser–plasma acceleration of electrons for radiobiology and radiation sources

Laser-driven acceleration in mm-sized plasmas using multi-TW laser systems is now established for the generation of high energy electron bunches. Depending on the acceleration regime, electrons can be used directly for radiobiology applications or for secondary radiation sources. Scattering of these electrons with intense laser pulses is also being considered for the generation of X-rays or γ-rays and for the investigation of fundamental electrodynamic processes. We report on laser-plasma acceleration in the 10TW regime in two different experimental configurations aimed at generating either high charge bunches with properties suitable for radiobiology studies or high collimation bunches for secondary radiation sources with high quality and good shot-to-shot stability. We discuss the basic mechanisms and describe the latest experimental results on injection threshold and bunch properties.