Session ZO2: Beams and Coherent Radiation III

Generation of Relativistic Electron Beam via Capillary Discharge Guided Laser Plasma Accelerator

Generation of semi-monoenergetic relativistic electron beam was demonstrated with capillary discharge guided laser plasma wakefield accelerator (g-LWFA) at LOASIS laser facility of Lawrence Berkeley National Laboratory (LBNL). With 40TW-40fs Ti:Sapphire laser and 33 mm length capillary, 1 GeV electron beam with 4\% energy spread (rms) was produced. Also, the stable generation of the 0.5 GeV electron beam was observed with 12TW-75fs laser. Capillary dimension, plasma density and discharge current dependence of the electron beam properties will be presented and electron self trapping within g-LWFA will be discussed.   

Conical electron seeding mechanism for the low density capillary-guiding electron laser acceleration

The low density laser electron acceleration was examined for a conical-guiding glass capillary target with sub ps pulse duration 200 TW laser system toward electron energy gain improvement. Dedicated experiments reveals that conical-tip glass capillary contributed relativistic electron beams generation unless no detectable high energy electrons were observed for a glass capillary target without any kind of guiding or seeding schemes. 1D-PIC simulation also reveals that hot electrons components were necessary to be trapped by laser-driven wake field for the presented experiments case. The conical target provided sufficient number of MeV order energy electrons and then the existence of glass capillary contributes to an extension of such electron energy spectrum up-to 50 MeV.   

Generating Multi-GeV Electron Bunches Using Laser Wakefield Acceleration in the Blowout Regime

The extraordinary ability of space-charge waves in plasmas to accelerate charged particles at gradients that are orders of magnitude greater than in current accelerators has been well documented. We show here that 100TW to 2000TW class lasers can excite large amplitude wakefields and be stably self-guided in very underdense plasmas to produce 1 to 10 GeV mono-energetic, self-injected electron beams with nCs of charge. For such powers the plasma wakes can be excited by the nearly complete blowout, i.e., expulsion, of plasma electrons by the radiation pressure of a short pulse laser. The proposed regime is distinct from the ``bubble regime'' in that it advocates using lower densities and wider spot sizes while keeping the intensity relatively constant in order to increase the output electron beam energy and keep the efficiency high. Our theoretical results are verified by three-dimensional particle-in-cell simulations.   

Ultrarelativistic electron generation during the intense laser pulse interaction with clusters

Collimated relativistic electrons up to 58 MeV with an electron charge of 2.1 nC were generated by the interaction of intense laser pulses with the Ar cluster target at the laser intensity of 3.5$\times $10$^{19}$ W/cm$^{2}$. The resulting spectrum does not fit a Maxwellian distribution, but is well described by a two-temperature Maxwellian, which indicates two mechanisms of the electron acceleration. Two dimensional particle-in-cell simulations demonstrate an important role of clusters. The higher energy electrons are injected when they are expelled from the clusters by the laser pulse field. They then gain their energy during the direct acceleration by the laser pulse, whose phase velocity in the underdense plasma is larger than speed of light in vacuum. The lower energy electrons, which are injected during the plasma wave breaking, are accelerated by the wakefield.   

Focusing and spectral control of laser-driven, picosecond ion beams

Laser-accelerated ion beams have the great advantage over conventional ion beams of retaining the ultrashort (sub-ps) pulse duration of the drive-laser. Together with the high beam current of kA-MA this makes them unique probes that enable new classes of experiments. In order to facilitate these applications, a better control over the divergence and spectral shape of the accelerated ion beams is desirable. We present results of controlling the shape of the energy spectrum both by target engineering and by using a laser-triggered plasma lens. This last device can also be used to reduce the divergence of a selected charge state at a specific energy range. We present first experimental results where transfered the technique from protons to heavier particles.   

The Implementation of Ultrahigh Intensity Laser Based Proton Accelerator for Proton Therapy

The ultrahigh-intensity-laser based proton acceleration has attracted numerous attentions in many research fields, particularly proton therapy for cancer treatment, due to the potential cost-effectiveness and compactness of the laser based proton accelerator. We have investigated the primary implementation of the ultrahigh-intensity-laser based proton accelerator for proton therapy. A 10$^{19 }$W/cm$^{2 }$peak intensity laser pulse with 5-order contrast is incident on aluminum targets at 30 degree. A 40 degree cone of protons is generated from the laser-overdense-plasma interaction with 13MeV maximum energy. Selected proton particles from a magnetic spectrometer are used to irradiate lung cancer cells situated on the surface of CR-39, which simultaneously detects the incident proton beam profile. Proton beams with different spectra, which involve different designs of the target and the particle selection methods, have been attempted to deliver to the cells. PIC model followed by Monte Carlo particle transport simulations are also performed to predict the possibility and the favorable conditions for the proton therapy implementation. This study provides the first design and predicts the optimal laser and target condition of proton therapy by ultrahigh-intensity-laser based proton accelerator.   

Dynamics of electric fields driving the laser acceleration of multi-MeV protons

We present the first direct experimental measurement of the electric fields driving the acceleration of high energy protons from a thin foil irradiated by an intense (I $\sim $ 3.5 $\times $ 10$^{18}$ W/cm$^{2})$ and short (t$_{l} \quad \sim $ 1.5 ps) laser pulse. The measurement was performed employing an auxiliary laser-accelerated proton beam as a transverse charged particle probe. The initial sheath field at the target-vacuum interface and the predicted late time peak of the accelerating field at the expanding ion front are observed. The experimental results are in good agreement with Particle In Cell and fluid simulations of the expansion of a thin plasma into a vacuum.   

Directed Coulomb explosion regime of proton acceleration by ultra-intense ultra-high contrast and ultra-short laser pulses.

Higher intensity and higher contrast are required for the ultra short pulses to accelerate protons up to therapeutic energies of over 200 MeV. 50 TW Hercules laser at the University of Michigan, which can be maintained up to 500 TW, can achieve 10$^{21}$ W/cm$^{2}$ intensity with the contrast ratio of 10$^{-11}$ between the peak intensity and the ASE. The performed under the anticipated experimental conditions PIC simulations show the maximum energy and the narrow peaked at high energies spectrum of accelerated protons that are of interest to medical applications. The results of PIC simulations are compared with the experimental ones. We found that the regime of directed Coulomb explosion, when the charge separation electric field is combined with the direct proton acceleration by the laser pulse, is the most efficient for proton acceleration by a tightly focused laser pulse.   

Proton acceleration along the laser axis in a high contrast laser-matter interaction

We have used the 10 TW ALLS laser system (300 mJ, 30 fs, 10 Hz) with frequency doubling to achieve high intensites on solid targets with high a contrast ratio laser pulse. The 400 nm laser pulse is focused at intensities between $10^{17}$-$10^{18}$ W/$cm^{-2}$ and at an incident angle of $45^{\circ}$ onto a 12 $\mu$m thick polyimide moving foil. Proton acceleration has been observed in front of and behind the foil surface. A maximum energy of 11 MeV is measured in the direction of the laser beam propagation. We discuss experimental results by using 2D PIC simulations. At the ALLS Canadian facility the new 200 TW laser system (5 J, 25 fs, 10 Hz) is currently starting operation. The laser high contrast ratio of $10^{10}$ at the 800 nm fundamental frequency combined with a high repetition rate of 10 Hz should allow us to produce 50 MeV protons with a high mean current.   

Spectral Control of Laser-Accelerated MeV Proton Beams

One of the phenomena associated with ultra-intense laser-solid interactions is the emission of multi-MeV proton beams from the target. These proton beams may find a number of important applications from medicine to Fast Ignition ICF. However until recently it has not been clear how to control the energy spectrum. Typically experimental observations have been of broad, quasi-exponential spectra. In the past year there have been four separate observations of proton or ion spectra with quasi-monoenergetic features in the energy spectrum. We have studied the generation of quasi-monoenergetic features in the ion spectrum. The simulations have been performed using 1D1P Vlasov codes and a fully 3D particle (tree) code, and the results of these calculations will be presented. We have focused on the role of \textit{target composition} in generating quasi-monoenergetic features in the spectrum. These spectral peaks during the plasma expansion because of the electrostatic shock associated with the separation of the protons and the heavier ion species [1]. This was initially investigated in 1D, and we have recently studied this in 3D, particularly in the context of microdot targets. This is very relevant to recent experiments. Alternative methods of generating quasi-monoenergetic features may be discussed. [1] A.P.L.Robinson et al, Phys.Rev.Lett., 96, 035005 (2006)   

Online real time characterization of fast protons induced by a repetitive high-intensity laser-foil interaction

We report the result on a novel online analysis of fast ions generated in an ultraintense laser-foil interaction. Fast protons are observed by a time-of-flight (TOF) detector, which is precisely calibrated using proton beams from an ion accelerator as to its detection efficiency depending on the proton energy. The TOF detector provides shot-to-shot energy distributions of protons immediately after the irradiation of a high-intensity laser pulse of $\sim $10$^{18}$ W/cm$^{2}$. Definite correlations are found between the prepulse intensity and the high energy cutoff of protons as well as the conversion efficiency of the laser energy into the proton energy, governing the stability of the repetitive proton generation.   

PIC Simulations of Particle Acceleration by Colliding Ultra-intense Laser Pulses

We have extended the PIC simulation of the recently proposed electron acceleration scheme using colliding laser pulses to irradiate an over-dense plasma [1] to 2 and 3 dimensions, to study the effects of finite laser spot sizes and incident angles. We have applied this acceleration scheme to electron-ion plasmas of different densities and thicknesses, and studied the effects of different laser parameters. Proposals for future experiments to study this acceleration mechanism will be discussed. \newline [1] E. Liang, Phys. of Plasmas 13, 064506 (2006).   

Coherence-based Transverse Measurement of Laser-Synchrotron X-ray Radiation and Laser-Accelerated Electrons

A narrow divergence (2$^{\circ}$), broad-spectrum of keV x-rays results from the interaction of a fs intense laser pulse (I$\geq10^{18}$ W/cm$^{2}$) with He gas$^ {1}$. The x-rays originate from transverse oscillations of laser-accelerated electrons within the plasma accelerator. We present measurement of single edge diffraction from this source at center energy 4.5 keV. Filters characterize the source spectrum. Calculation of the radiation profile based on mechanism shows that x-ray transverse dimension acts as upper limit on that of the radiating electron beam. The measured full-width-half-maximum (FWHM) x-ray source size, assuming Gaussian profile, of 5$\pm2$ $\mu$m agrees with the near Gaussian profile of electrons at the plasma exit in three-dimensional particle-in-cell simulation (FWHM=4 $\mu$m). Such measurements are fundamental to both the x-ray radiation coherence and emittance of the electron beam. Supported by EU (Contract Nos. HPRI-CT-1999-00086, HPRI-CT-2000-40016, and HPRI-CT-1999-50004 (FAMTO project)). R.S.supported by NSF (Grant No. 0502281) and CNRS.\\ $^{1}$A. Rousse {\it et al.}, Phys. Rev. Lett. {\bf 93}, 135005 (2004).   

Wave-breaking limits for relativistic electrostatic waves in a warm plasma

The propagation of electrostatic plasma waves having elativistic phase speed and amplitude has been studied. The plasma is described as a warm, relativistic, collisionless, nonequilibrium electron fluid. Wave breaking limits for the electrostatic field are calculated for non-relativistic initial plasma temperatures and arbitrary phase velocities. Particular care is given to the ultra-relativistic regime ($\gamma_\varphi^2 kT_0/m_e c^2 \gg 1 $), since conflicting results for this regime have been published in literature. It is shown here that the ultra- relativistic wave-breaking limit will reach arbitrarily large values for $\gamma_\varphi\rightarrow\infty$ and fixed initial temperature. Previous studies claiming that this limit is bounded even in the limit $\gamma_\varphi\rightarrow\infty$ are shown to suffer from incorrect application of the relativistic fluid equations and higher, more realistic wave breaking limits are appropriate. These results will have important consequences for the study of background electron trapping and acceleration in laser-plasma and beam-plasma interactions.   

Development of a high-brightness photoinjector for Compton scattering x-ray sources

Compton scattering of intense laser pulses with ultra-relativistic electron beams has proven to be an attractive source of high-brightness x-rays with keV to MeV energies. This type of x-ray source requires the electron beam brightness to be comparable with that used in x-ray free-electron lasers and laser and plasma based advanced accelerators. We describe the development of a 1.6 cell RF photoinjector for use in Compton scattering experiments at LLNL. RF cavity design, beam dynamics simulations, emittance diagnostic development, and results of sputtered magnesium photo-cathode experiments are discussed. The photo-cathode drive laser is described and injector performance goals are presented.