This PDF file contains the front matter associated with SPIE Proceedings Volume 7359, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

When a highly relativistic electron is injected off-axis into an ion channel, the restoring force of the radial field of the ions will cause the electron to accelerate towards the axis, overshoot, and begin to undergo oscillations about the ioncolumn axis at a characteristic frequency; the betatron frequency. This so-called betatron motion will cause the electron to radiate hard x-rays in the forward direction. In two recent experiments at the Stanford Linear Accelerator Center (SLAC), betatron x-rays in the 1-20kV range and in the 1-50MV range were produced with an electron beam with an energy of 28.5 GeV for ion densities of about 1 x 10¹⁴ cm^-3 and 1 x 10¹⁷cm^-3, respectively. To make such an x-ray source more compact, the 3km long SLAC linac would be replaced by a source of electrons from a Laser Wakefield accelerator (LWFA). To increase the efficiency of converting laser into photons at high photon energies, we propose adding a second stage where the LWFA electrons radiate via a second ion channel, independent of the accelerating process. This two stage concept allows one to control the critical frequency of the emitted radiation as well as the efficiency of the process.

Recent years have seen rapid improvement in the quality of electron beams produced by wakefields in plasmas. The electron beams produced have inherently short durations and high peak current. To further shorten the pulse duration of these beams for future applications, an experiment is proposed to produce a single sub-femtosecond slice of electrons via an Inverse Free Electron Laser interaction (IFEL) with a few cycle laser pulse. The IFEL is followed by a combined function, permanent magnet quadrupole triplet chicane that both disperses the beam transversely while simultaneously focusing, allowing for efficient energy collimation to select the attosecond slice. Simulations are presented showing the expected electron slice characteristics.

The relativistic Doppler effect offers a fundamental means of frequency upconverting electromagnetic radiation. In 1993, Esarey et al.¹ mentioned the possibility of scattering light at fast moving electrons to upconvert its frequency. For the process to be efficient, one needs to have a highly compressed bunch of electrons, since only then the scattering process can become coherent. The condition for coherency is, that the scale length of the electron bunch or its density gradient needs to be on the order of the wavelength to be generated or smaller. This is much tinier than what can be reached by commonly known techniques, including conventional accelerators as well as laser-plasma accelerators. Therefore, electrons are extracted from a small droplet or a thin foil by a highly relativistic driver laser (a₀ = eA₀/mc² ⪆⪆ 1). The electron bunch becomes accelerated and at the same time compressed by the forces of the laser field. The acceleration can be achieved either by the relativistic ponderomotive force of a conventional laser pulse, as suggested in,⁶ or by the longitudinal field on the optical axis of a radially polarized pulse, as suggested in.8 In both cases, the bunch is compressed because of the fundamental snowplough effect of the co-moving force, i.e. the laser pulse. Spacecharge forces are counteracting the compression, thus limiting the amount of charge to be compressed. Nevertheless, in a wide range of parameters the edges of the electron bunches density profile remain sharp, enabling coherent Thomson scattering. We use analytic models and PIC simulations to describe and analyze thoroughly the effects occurring and finally estimate the conversion efficiency that can be reached by this scheme. Techniques to increase the efficiency and gain further control over the generated radiation are suggested and discussed. Reaching best possible control over temporal envelope of the driver pulse appears to be the most important issue here.

The transverse emittance is an important parameter governing the brightness of an electron beam. Here we present the first pepper-pot measurement of the transverse emittance for a mono-energetic electron beam from a laser-plasma wakefield accelerator, carried out on the Advanced Laser-Plasma High Energy Accelerators towards X-Rays (ALPHA-X) beam line. Mono-energetic electrons are passed through an array of 52 μm diameter holes in a tungsten mask. The pepper-pot results set an upper limit for the normalised emittance at 5.5 ± 1 π mm mrad for an 82 MeV beam.

Compact tuneable sources of ultrashort hard x-ray pulses can be realized by Thomson scattering, taking advantage of the comparatively short wavelength of a scattered laser pulse with respect to the period length of conventional undulators. Here, we present a detailed analysis and optimization of the efficiency of linear and non-linear Thomson scattering when the process is driven with relativistic laser pulses and when the conventional accelerator is replaced by a laser-plasma wakefield accelerator.

An ultra-bright high-power X-ray and γ-ray source is proposed. A relativistic flying mirror reflects a counterpropagating electromagnetic radiation causing its frequency multiplication and intensification, while the role of the mirror is played by a solid-density thin plasma slab accelerating in the radiation pressure dominant regime. Frequencies of high harmonics generated at the flying mirror by a relativistically strong counter-propagating radiation undergo multiplication with the same factor as the fundamental frequency of the reflected radiation, approximately equal to the quadruple of the square of the mirror Lorentz factor. The theory of the reflectivity of a moving thin plasma slab is presented.

Photon Landau damping of electron plasma waves with relativistic phase velocity has been rst considered in 1997, in the frame of geometric optics [1]. Here we consider more recent results based on a kinetic model where photon recoil is taken into account [2]. By photon recoil we mean the change of a nite amount of momentum by photons, due to the emission or the absorption of electron plasma waves. This gives a surprising quantum avor to a purely classical description. Our approach is based on an exact form of the wave kinetic equation. Kinetic and uid regimes of photon beam instabilities, and their relevance to particle acceleration and new radiation sources are discussed. Quasi-lnear results leading to a photon Boltzmann equation are also discussed. Diusion in the photon momentum space are shown to be a consequence of photon-plasmon collisions, taken in the geometric optics limit. A brief discussion of photon trapping by the plasma wave potential is also included. Our theoretical discussion will be illustrated with the description of recent experimental results using intense laser plasma interactions, as well as with a new experimental proposal.

The temporal characteristics of the harmonic emission from solid targets irradiated with intense laser pulses is examined in detail. In the case where the CoherentWake Emission mechanism is dominant it is found that indeed the harmonics thus produced possess a frequency chirp resulting in non Fourier-Transform-Limited pulses. A simple model explains the underlying physics while Particle-In-Cell simulations support the conclusions drawn.

When an intense ultrashort laser pulse impinges on an initially-solid target, it creates a dense plasma at the surface, which reflects a large fraction of the incident light. At high enough intensities, high-order harmonics of the incident laser frequency, associated in the time domain to trains of attosecond pulses, are generated in the light beam specularly reflected by this "plasma mirror". The mechanisms leading to this generation are now relatively well-established, and the first experimental evidence for attosecond pulses generated on plasma mirrors has recently been reported. An accurate characterization of the temporal structure of the light reflected by plasma mirrors, down to the attosecond scale, however remains an experimental challenge. In this paper, we describe three different methods that could be used for such temporal measurements, from the femtosecond to the attosecond time scale. Two of them are interferometric techniques which only require measurements of photons, while the third one is a new configuration of a now well-established method, developed for attosecond pulses generated in gases, and based on photoelectron spectroscopy.

The interaction of relativistically intense (Iλ²>>1.3 10¹⁸Wcm^-2μm²) laser pulses with a near step-like plasma density profile results in relativistic oscillations of the reflection point. This process results in efficient conversion of the incident laser to a phase-locked high harmonic spectrum, which allows the generation of attosecond pulses and pulse trains. Recent experimental results on efficiency scaling, highest harmonic generated and beam quality suggest that very high focused intensities can be achieved opening up the possibility of ultra-intense attosecond X-ray interactions for the first time.

We study the possibility of producing short-wavelength magnetostatic structures in plasmas by exciting a plasma magnetic mode in the collision of light pulses with relativistic ionization fronts. Results from PIC simulations demonstrate the generation of these structures with existing state-of-the-art laser systems. We analyze the feasibility of using the magnetic structure associated with the plasma magnetic mode as an undulator for compact synchrotron radiation sources, illustrating the generation of ultrashort-wavelength radiation.

Electromagnetic wave generation in the extreme ultraviolet (XUV) and infrared (IR) wavelength range occurs during the interaction of intense short laser pulses with underdense plasmas. XUV pulses are generated through laser light reflection from relativistically moving electron dense shells (flying mirrors). A proof-of-principle and an advanced experiment on flying mirrors are presented. Both of the experiments demonstrated light reflection and frequency upshift to the XUV wavelength range (14-20 nm). The advanced experiment with a head-on collision of two laser pulses exhibited the high reflected photon number. IR radiation, which is observed in the forward direction, has the wavelength of 5 μm and dominantly the same polarization as the driving laser. The source of the IR radiation is attributed to emission from relativistic solitons formed in the underdense plasma.

An analytical study of significant photon acceleration (frequency up-shift) in a plasma density wake produced by two laser pulses in the mildly relativistic and linearized regime is presented. The wake amplitude is amplified and its phase controlled using two coaxially, co-propagating laser pulses, which are considered to be identical but separated by a fixed time. A third probe pulse, with a variable delay, is considered as "test particle" or quasi-photon propagating through the amplified density wake, which experiences significant photon acceleration because of the local temporal and spatial variation of the permittivity. The evolution of the "photon" is studied using Hamiltonian theory. The significant frequency up-shift is much larger than that produced by the wake of a single relativistic laser pulse in the highly relativistic nonlinear wake regime. Our study demonstrates that the inter-pulse separation between the "controlling" pulse and the "driver" pulse, producing the amplified density wake, can provide an additional degree of freedom for tuning the maximum up-shift of the probe photon frequency.

The role of thermal effects on Raman amplification are investigated. The direct effects of damping on the process are found to be limited, leading only to a decrease from the peak output intensity predicted by cold plasma models. However, the shift in plasma resonance due to the Bohm-Gross shift can have a much larger influence, changing the required detuning between pump and probe and introducing an effective chirp through heating of the plasma by the pump pulse. This "thermal chirp" can both reduce the efficiency of the interaction and alter the evolution of the amplified probe, avoiding the increase in length observed in the linear regime without significant pump depletion. The influence of this chirp can be reduced by using a smaller ratio of laser frequency to plasma frequency, which simultaneously increases the growth rate of the probe and decreases the shift in plasma resonance. As such, thermal effects only serve to suppress the amplification of noise at low growth rates. The use of a chirped pump pulse can be used to suppress noise for higher growth rates, and has a smaller impact on the peak output intensity for seeded amplification. For the parameter ranges considered, Landau damping was found to be negligible, as Landau damping rates are typically small, and the low collisionality of the plasma causes the process to saturate quickly.

Energy Energy transfer between a long (3-10 ps) "pump" pulse and a short (400 fs) "seed" one, both at a wavelength of 1.057μm quasi counterpropagating in an underdense preformed plasma and produced from the ionization of a gas jet, was observed. Numerical simulations reveal that the energy transfer is due to the coupling involving ion acoustic waves excited in the Stimulated Brillouin Backscattering in the strong coupling regime. The plasma characteristics were tailored using a high-energy ionization laser beam and the plasma density was controlled using a Thomson scattering diagnostic. The energy exchange was observed for different gas (ion) types, pressures (plasma densities), polarization and intensities of the interacting beams.

The nonlinear regime of Raman amplification has been studied including the combined effects of relativistic and ponderomotive nonlinearities. The study is important for interaction of mildly relativistic pump and probe laser pulses. Nonlinear coupled temporal evolution of fields and density in Raman amplification is analyzed. It is shown that the saturation amplitude and time of the probe pulse in nonlinear regime depends upon the intensity of the electromagnetic waves and the density of the medium. Further in the nonlinear regime the probe laser pulse gain is severely affected by changes in both the electromagnetic wave amplitude and the plasma density.

Stimulated Raman backscattering in plasma has been suggested as a way to amplify short laser pulses to intensities not limited by damage thresholds as in chirped pulse amplification using conventional media. Energy is transferred between two transverse electromagnetic waves, pump and probe, through the parametric interaction with a longitudinal Langmuir wave that is ponderomotively excited by their beat wave. The increase of the plasma temperature due to collisional absorption of the pump wave modifies the dispersion of the Langmuir wave: firstly, its resonance frequency rises (Bohm-Gross shift), and secondly, Landau damping sets in. The frequency shift acts in a similar way to a chirp of the pump frequency, or a density ramp: different spectral components of the probe satisfy the resonance condition at different times. This limits their growth, while increasing the bandwidth of the amplifier, thus leading to superradiant amplification. Landau damping may shorten the probe pulse, but reduces the amplification efficiency. We investigate these effects analytically and using numerical simulations in order to assess their importance in experimental demonstrations, and the possibility of applications.

Raman backscattering (RBS) in plasma is an attractive source of intense, ultrashort laser pulses, which has the potential asa basic for a new generation of laser amplifiers.1 Taking advantage of plasma, which can withstand extremely high power densities and can offer high efficiencies over short distances, Raman amplification in plasma could lead to significant reductions in both size and cost of high power laser systems. Chirped laser pulse amplification through RBS could be an effective way to transfer energy from a long pump pulse to a resonant counter propagating short probe pulse. The probe pulse is spectrally broadened in a controlled manner through self-phase modulation. Mechanism of chirped pulse Raman amplification has been studied, and features of supperradiant growth associated with the nonlinear stage are observed in the linear regime. Gain measurements are briefly summarized. The experimental measurements are in qualitative agreement with simulations and theoretical predictions.

High power short pulse lasers are usually based on chirped pulse amplification (CPA), where a frequency chirped and temporarily stretched "seed" pulse is amplified by a broad-bandwidth solid state medium, which is usually pumped by a monochromatic "pump" laser. Here, we demonstrate the feasibility of using chirped pulse Raman amplification (CPRA) as a means of amplifying short pulses in plasma. In this scheme, a short seed pulse is amplified by a stretched and chirped pump pulse through Raman backscattering in a plasma channel. Unlike conventional CPA, each spectral component of the seed is amplified at different longitudinal positions determined by the resonance of the seed, pump and plasma wave, which excites a density echelon that acts as a "chirped" mirror and simultaneously backscatters and compresses the pump. Experimental evidence shows that it has potential as an ultra-broad bandwidth linear amplifier which dispenses with the need for large compressor gratings.

The dynamics of relativistic electrons in a laser driven plasma cavity are studied via measurements of their radiation. For ultrashort laser pulses at comparatively low focused laser intensities (3 < a₀ < 10), low density and long f-number of 10, electrons are predominantly accelerated in the wakefield leading to quasi-monoenergetic collimated electron beams and well collimated (< 12 mrad) beams of comparatively soft x-rays (1-10 keV) with unprecedented small source size (2-3 μm). For laser pulses with increasing laser intensity (10 < a₀ < 30), density and short f-number (< 5), electrons are accelerated directly by the laser, leading to divergent quasimaxwellian electron beams and divergent (50-95°) beams of hard x-rays (20-50 keV) with relatively large source size (> 100 μm). In both cases, the measured x-rays are well described in the synchrotron asymptotic limit of electrons oscillating in a plasma channel. At low laser intensity transverse oscillations are small as the electrons are predominantly accelerated axially by the laser generated wakefield. At high laser intensity, electrons are directly accelerated by the laser. A betatron resonance leads to a tenfold increase in transverse oscillation amplitude and electrons enter a highly radiative regime with up to 5% of their energy converted into x-rays.

In 3D simulations, PIC codes cannot resolve the radiation of short wavelength compared to the grid spacing, which raises challenges in multi-dimensional simulations because of memory constraints. However, in many plasma physics scenarios (e.g. laser wakefield acceleration) the radiation mechanisms can cover several orders of magnitude in energy/frequency (from the THz range, associated with transition radiation of relativistic electron beams, to gamma-rays, associated with the betatron radiation of self-injected electrons in the bubble or blow-out regime). We describe a massivelly parallel post-processing radiation diagnostic that takes the track information from 3D/2D particle-in-cell simulations and determines the full radiation spectrum of the corresponding particle( s). Benchmark examples with cyclotron/synchrotron radiation as well as betatron radiation are presented and compared with the analytical predictions. Special emphasis is given to the numerical properties of the diagnostic, in particular the resolution of the particle tracks, the diagnostic spectral and spatial resolutions, as well as the different aproximations on the numerical calculation of the radiation integral over the trajectory of the particles. We then use this diagnostic to probe different scenarios, taking advantage of the spatial, temporal and frequency resolved characteristics of the diagnostic.

In this paper, we present the first temporal characterization of betatron X-ray radiation. Results obtained from time resolved x-ray diffraction experiments, for which the ultra fast phase transition of non thermal melting of InSb was used, indicates that the x-ray pulse duration is less than 1 ps. We then propose a novel technique to improve the spectral and flux properties of the x-ray source. The energy and the flux can be enhanced when the electron beam propagates and oscillates in a tailored plasma density profile.

The interaction of ultrashort intense laser pulses with plasma can produce electromagnetic radiation of ultra-broad spectra ranging from terahertz (THz) radiation to keV x-rays and beyond. Here we present a review of our recent theoretical and numerical investigation on high power THz generation from tenuous plasma or gas targets irradiated by ultrashort intense laser pulses. Three mechanisms of THz emission are addressed, which include the linear mode conversion from laser wakefields in inhomogeneous plasma, transient current emission at the plasma-vacuum boundaries, and the emission from residual transverse currents produced by temporally-asymmetric laser pulses passing through gas or plasma targets. Since there is no breakdown limit for plasma under the irradiation of high power lasers, in principle, all these mechanisms can lead to terahertz pulse emission at the power of beyond megawatt with the field strength of MV/cm, suitable for the study of high THz field physics and other applications.

Electromagnetically induced transparency (EIT) is a well-known quantum phenomena that electromagnetic wave controls the refractive index of medium. It enables us to create a passband for low frequency electromagnetic wave in a dense plasma even if the plasma is opaque for the electromagnetic wave. This technique can be used to prove the ion acoustic wave because the ion acoustic frequency is lower than the plasma frequency. We have investigated a feasibility of electromagnetic radiation at THz region corresponding to the ion acoustic frequency from a dense plasma. We confirmed that the passband is created at about 7.5 THz corresponding to the ion acoustic frequency in the plasma (10²¹ cm^-3) with a Ti:Sapphire laser (800 nm, 10¹⁷ W/cm²). The estimated radiation power is around 1 MW, which is expected to be useful for nonlinear THz science and applications.

Terahertz (THz) radiation from the interaction of ultrashort laser pulses with gases is studied both theoretically and experimentally. We theoretically study the THz generation based on transient ionization current model and give the relation between the final THz field and the initial transient ionization current. Recent experimental results on optimization of THz radiation in laser air interaction are also shown. We find by use of a simple aperture to change the laser field distribution, the terahertz wave amplitudes can be enhanced by more than eight times than those of aperture-free cases. We use two dimensional particle-in-cell codes to simulate the experiments and give possible explanations.

The Advanced Laser-Plasma High-Energy Accelerators towards X-rays (ALPHA-X) programme is developing laserplasma accelerators for the production of ultra-short electron bunches with subsequent generation of incoherent radiation pulses from plasma and coherent short-wavelength radiation pulses from a free-electron laser (FEL). The first quantitative measurements of the electron energy spectra have been made on the University of Strathclyde ALPHA-X wakefield acceleration beam line. A high peak power laser pulse (energy 900 mJ, duration 35 fs) is focused into a gas jet (nozzle length 2 mm) using an F/16 spherical mirror. Electrons from the laser-induced plasma are self-injected into the accelerating potential of the plasma density wake behind the laser pulse. Electron beams emitted from the plasma have been imaged downstream using a series of Lanex screens positioned along the beam line axis and the divergence of the electron beam has been measured to be typically in the range 1-3 mrad. Measurements of the electron energy spectrum, obtained using the ALPHA-X high resolution magnetic dipole spectrometer, are presented. The maximum central energy of the monoenergetic beam is 90 MeV and r.m.s. relative energy spreads as low as 0.8% are measured. The mean central energy is 82 MeV and mean relative energy spread is 1.1%. A theoretical analysis of this unexpectedly high electron beam quality is presented and the potential impact on the viability of FELs driven by electron beams from laser wakefield accelerators is examined.

Electron acceleration using plasma waves driven by ultra-short relativistic intensity laser pulses has undoubtedly excellent potential for driving a compact light source. However, for a wakefield accelerator to become a useful and reliable compact accelerator the beam properties need to meet a minimum standard. To demonstrate the feasibility of a wakefield based radiation source we have reliably produced electron beams with energies of 82±5 MeV, with 1±0.2% energy spread and 3 mrad r.m.s. divergence using a 0.9 J, 35 fs 800 nm laser. Reproducible beam pointing is essential for transporting the beam along the electron beam line. We find experimentally that electrons are accelerated close to the laser axis at low plasma densities. However, at plasma densities in excess of 10¹⁹ cm^-3, electron beams have an elliptical beam profile with the major axis of the ellipse rotated with respect to the direction of polarization of the laser.

Focussing ultra-short electron bunches from a laser-plasma wakefield accelerator into an undulator requires particular attention to be paid to the emittance, electron bunch duration and energy spread. Here we present the design and implementation of a focussing system for the ALPHA-X beam transport line, which consists of a triplet of permanent magnet quadrupoles and a triplet of electromagnetic quadrupoles.

We reported the production of a plasma channel in a capillary discharge-produced plasma. Plasma parameters of its channel were observed by use of both a laser interferometer and a hydrogen plasma spectrum. A time-resolved electron temperature was measured, and its maximum temperature of 3 eV with electron densities of the order of 1017 cm^-3 was observed at a discharge time of 150 ns and a maximum discharge current of 400 A. Intense laser pulse was guided over many vacuum Rayleigh lengths using its channel.

Pulse compression through filamentation in a free-space argon gas-filled cell has been demonstrated by use of the high energy laser pulse. Compression and splitting of the optical laser pulse due to multiple filamentation in an argon gas-filled cell were observed. A 130-fs pulse was compressed to less than 60 fs (full width at half-maximum) with the output energy of 16 mJ at the argon gas pressure of 25 kPa.

A wide-band spectral diagnostic system based on dispersion property of the Zinc Selenide prism, a crystalline material highly dispersive in the near-to-far infrared spectral range, has been studied and developed for the laser wakefield acceleration experiment at LOA for the measurement of few femto-seconds long electron beam. The extensive PIC simulation studies of the colliding-beam LWFA have shown very short electron beam duration of less than 10 femtoseconds. The prism spectrometer diagnostic with highly sensitive Mercury Cadmium Telluride infrared detector and the diffraction-grating spectrometer with a high-resolution imaging visible camera together have the spectral range coverage and resolution capable of detecting ultra-short Coherent Transition Radiation (CTR) generated by interaction of bunch charges with a 100 microns thickness aluminum foil. The beam profile of asymmetric shape then could be extracted from the CTR spectrum by inverse Fourier transformation with Kramers-Kronig relation. The diagnostic system has been tested and calibrated for characterization of blackbody source spectrum and spectral responsivity. The measurement of electron beam duration of few femtoseconds has yet been convincingly shown with high resolution, and the measurements of this kind have important implication in understanding and subsequent successful operation of the future FEL light source with a highly mono-energetic LWFA beam source.