Session TO7: X Ray and Intense Beam Generation

Using x-ray-free-electron lasers to generate and probe high-energy-density matter

The world's first hard x-ray free electron laser, the Linac Coherent Light Source (LCLS) has become available, providing 10 to 500 fs x-ray pulses in the 2.5 and 0.15 nm wavelength range and 4 mJ pulse energies. Utilizing this laser, we studied ultrafast processes in warm dense matter, including ionization, energy transfer, and atomic motion. We used the unique high peak-brightness radiation to heat carbon solids isochorically to up to 50 electron volts. We then used the LCLS pulses to probe the state of the material via by Bragg and x-ray Thomson scattering. Elastic Bragg scattering provides ionic properties and structural information about the crystal, while the inelastic Compton and plasmon scattering spectrum reflects the electrical/optical properties, and further provides temperature and density information. In this presentation, we will report on the first experimental results and compare them to model calculations   

Generation of uniform relativistic electron layer and linear and nonlinear coherent Thomson scattering

A novel, multi-layer target [H.-C. Wu \textit{et al.}, PRL 104, 234801 (2010)] is proposed to generate uniform relativistic electron layers for coherent Thomson backscattering (CTS). A few-cycle laser pulse produces an electron layer from an ultrathin foil and then a second foil reflects the laser pulse, but lets the electrons pass through unperturbed. 2D-PIC simulations show that after interacting with the drive and reflected laser pulses, the electrons form a very uniform flyer that propagates in the direction of laser propagation. Such a flyer backscatters light with a full Doppler shift factor of $4\gamma ^2$. Nonlinear CTS theory for relativistic laser intensity shows that compared with linear CTS, a relativistically intense laser induces transverse motion of the electron layer and decreases the linear Doppler shift by a factor $1+a_0^2 $. Consequently, in order to obtain the same x-ray photon energy as linear CTS, the nonlinear case needs higher electron energy. Theory also shows that CTS efficiency reaches saturation when laser amplitude$a_0 >2$. PIC simulations show that a powerful x-ray pulse (1 keV, 10 attoseconds, and GW power) can be generated and that diffraction-limited focusing may boost the intensity to of order $10^{24}$W/cm$^2$.   

LLNL's Precision Compton Scattering Light Source: Status \& Applications

A precision, tunable, monochromatic ($<$ 0.4{\%} rms spectral width) source driven by a compact, high-gradient X-band linac designed in collaboration with SLAC is under construction at LLNL. High-brightness (250 pC, 3.5 ps, 0.4 mm.mrad), relativistic electron bunches will interact with a Joule-class, 10 ps, diode-pumped laser pulse to generate tunable $\ge $-rays in the 0.5-2.5 MeV photon energy range. This $\ge $-ray source will be used to excite nuclear resonance fluorescence (NRF) in various isotopes, of interest for homeland security, stockpile science and surveillance, nuclear fuel assay, and waste imaging and assay. The source current status will be discussed, along with important applications, including NRF and\textit{ in situ} ps thermal measurements. This work performed under the auspices of the U.S. DoE by LLNL under Contract DE-AC52-07NA27344, and funded by the DHS DNDO.   

Inverse Compton Scattering from Laser Accelerated Quasi-Monoenergetic Electrons

The progress of the laser accelerator shows us the possible applications to the industries, such as an inspection source for soft materials like as human bodies, plants foods and medicines. The inverse Compton scattering will realize such a novel inspection system. We demonstrate for the fist time that the laser-accelerated mono-energetic electrons inversely scatter the same counter laser beam to the Compton X-ray emissions. A Ti:sapphire laser (500mJ width 150fs) is divided into two beams. Main beam is focused to an edge of a helium gasjet to accelerate electrons to 13 and 22 MeV monoenergies, which inversely scattered the counter laser beam into 6 and 11 keV X-ray emissions in agreement with that calculated from the obtained electron spectra. The scattering is within 30 deg. around the main beam direction.   

Design and optimization of MeV class Compton scattering MEGa-ray sources

The design and optimization of a Mono-Energetic Gamma-Ray (MEGa-Ray) Compton scattering source are presented. A new precision source with up to 2.5 MeV photon energies, enabled by state of the art laser and x-band linac technologies, is currently being built at LLNL. Various aspects of the theoretical design, including dose and brightness optimization, will be presented. In particular, while it is known that nonlinear effects occur in such light sources when the laser normalized potential is close to unity, we show that these can appear at lower values of the potential. A three dimensional analytical model and numerical benchmarks have been developed to model the source characteristics, including nonlinear spectra. Since MEGa-ray sources are being developed for precision applications such as nuclear resonance fluorescence, assessing spectral broadening mechanisms is essential. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Generation of a few femtosecond keV X-ray pulse via interaction of a tightly-focused laser co-propagating with an relativistic electron bunch

The generation of ultrashort hard X-ray pulse is demonstrated in a series of numerical simulation of the Thomson scattering of high power femtosecond laser with a co-propagating electron bunch. When an electron bunch co-propagates with a focused laser pulse, the pulse width of the radiation is similar to it of the laser pulse because the relative velocity between laser pulse and electron bunch is almost zero and the interaction occurs only in the focused region. However, when the laser is loosely focused, the radiation power is much weaker than Compton backscattering or the interaction of other geometry. When a laser is focused tightly, high-order fields (HOFs) appear to satisfy the Maxwell equations. The HOFs drastically enhance the radiation power and the photon energy. This study shows that the co-propagating interaction can be photon source of 5 fs pulse width and 10-100 keV photon energy.   

LLNL Precision Compton Scattering Light Source: X-band RF Photoinjector and Accelerator Design

The design and optimization of a Mono-Energetic Gamma-Ray (MEGa-Ray) Compton scattering source are presented. This precision light source with up to 2.5 MeV photon energy is currently being built at LLNL using high gradient X-band accelerator technology in collaboration with SLAC. The design of a high brightness 5.59 cell X-band RF photoinjector will be presented. An ``early light'' machine has been designed using the 250 pC, <1mm-mrad electron bunches from the photoinjector, a single T53 traveling wave accelerator section, and Joule-class diode pumped laser pulses to produce diagnostic Compton scattered photons. Design of this ``early-light'' machine will be presented with discussion of the various components, layout considerations, and plans for the full 250 MeV linear accelerator.   

Optimum Plasma Density and High Transformer Ratio of Plasma Wakefield Accelerator in the Blowout Regime

In PWFA, accelerating gradient and transformer ratio are two import figures of merit of the wake excitation process.In this talk, simple theories based on a nonlinear wakefield theoretical framework in the blowout regime [1-2] will be presented to predict the optimum density and transformer ratio in the blowout regime. It is found that the peak beam current I{\_}p plays an important role in determining the optimum density and transformer ratio. We show that for narrow beams of low peak current (I{\_}p$<<$I{\_}A=17kA), the linear theory predictions work well. However for high peak current (I{\_}p$\sim $I{\_}A), the optimum density can be an order of magnitude larger than that predicted by linear theory. In this regime we show that a new condition n{\_}p$\sim $n{\_}b0 should be used, where n{\_}b0 is the peak beam density[3]. Theoretical arguments for this new condition are given and the predictions are confirmed by PIC simulations. It is also found that a high transformer ratio can be achieved for both symmetric and asymmetric shaped bunches. We show that even in the blowout regime wedge shaped beams provide the highest transformer ratio and the highest efficiency. [1] W. Lu et al., PRL,96, 165002 (2006) [2] W. Lu et al., PoP, 13, 5,056709 (2006) [3] W. Lu et al., NJP, Special issue, 2010, in press   

Optical Frequency Domain Visualization of Electron Beam Driven Plasma Wakefields

Beam-driven plasma wakefield accelerators (PWFA), such as the ``plasma afterburner,'' are a promising approach for significantly increasing the particle energies of conventional accelerators. The study and optimization of PWFA would benefit from an experimental correlation between the parameters of the drive bunch, the accelerated bunch and the corresponding, accelerating plasma wave structure. However, the plasma wave structure has not yet been observed directly in PWFA. We will report our current work on noninvasive optical Frequency Domain Interferometric (FDI) and Holographic (FDH) visualization of beam-driven plasma waves. Both techniques employ two laser pulses (probe and reference) co-propagating with the particle drive-beam and its plasma wake. The reference pulse precedes the drive bunch, while the probe overlaps the plasma wave and maps its longitudinal and transverse structure. The experiment is being developed at the BNL/ATF Linac to visualize wakes generated by two and multi-bunch drive beams.   

Status of NDCX-II, a short-pulse ion accelerator for ion beam-driven physics studies

Construction of the Neutralized Drift Compression Experiment-II (NDCX-II) is underway at LBNL; completion is due March, 2012. This ion induction accelerator will enable studies of Warm Dense Matter and basic target physics for heavy-ion-driven Inertial Fusion Energy. NDCX-II compresses and accelerates a 20-50 nC Li+ pulse to 1.2-3 MeV, then shortens it to sub-ns duration in a neutralizing plasma and focuses it onto a target.\footnote{A. Friedman, et al., Phys. Plasmas 17, 056704 (2010).} Extensive simulations optimized the design and adapted it to induction waveforms generated on a test stand; ensembles of runs established tolerances and expected performance. NDCX-II is extensible and reconfigurable; we describe the baseline design and variants, and the status of the project.   

Effects of Errors of Velocity Tilt on Maximum Longitudinal Compression During Neutralized Drift Compression of Intense Beam Pulses

Neutralized drift compression offers an effective means for particle beam focusing and current amplification. In neutralized drift compression, a linear longitudinal velocity tilt is applied to the beam pulse, so that the beam pulse compresses as it drifts in the focusing section. The beam intensity can increase more than a factor of 100 in the longitudinal direction. We have performed an analytical study of how errors in the velocity tilt acquired by the beam in the induction bunching module limits the maximum longitudinal compression. It is found in general that the compression ratio is determined by the relative errors in the velocity tilt. That is, one-percent errors may limit the compression to a factor of one hundred. However, part of pulse where the errors are small may compress to much higher values determined by the initial thermal spread of the beam pulse. Examples of slowly varying and rapidly varying errors compared to the beam pulse duration are studied.