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Sources of negative ions such as H

Knowledge of longitudinal electron bunch profiles is vital to optimize the performance of plasma wakefield accelerators and x-ray free electron laser linacs. Because of their importance to these novel applications, noninvasive frequency domain techniques are often employed to reconstruct longitudinal bunch profiles from coherent synchrotron, transition, or undulator radiation measurements. In this paper, we detail several common reconstruction techniques involving the Kramers–Kronig phase relationship and Gerchberg–Saxton algorithm. Through statistical analysis, we draw general conclusions about the accuracy of these reconstruction techniques and the most suitable candidate for reconstructing well-isolated longitudinal bunch profiles from spectroscopic data.

The recent development of compact, relativistic electron accelerators [1] might open up a new era of active experiments in space, driven by important scientific and national security applications [2] . Examples include using electron beams to trace magnetic field lines and establish causality between physical processes occurring in the magnetosphere and those in the ionosphere [2] . Another example is the use of electron beams to trigger waves in the near-Earth environment [3] . Waves could induce pitch-angle scattering and precipitation of energetic electrons, acting as an effective radiation belt remediation scheme.

A novel source for broadband millimeter-wave RF is being designed at Los Alamos National Laboratory utilizing the two-stream instability. This RF source offers the potential for consistent output power over a large bandwidth on a single device. The source is designed to use two co-axial electron beams coupled into solenoidal magnetic fields. This is accomplished using two cathodes independently modulated nested in each other. The electron beam energy from the innermost cathode will fall in the range between 15 and 20 keV with the beam energy from the outer cathode being 75%–95% of the inner beam energy. In order to efficiently match the beam from the outer cathode into the solenoidal magnetic field, shields are used around the cathodes to shape the electric field in the gun region to extend the electrostatic focal length of the beams for better coupling into the magnetic field. We present the design of electron guns and expected performance using Trak simulations.

We develop a universal framework that allows for quickly solving a wide class of problems for longitudinal space charge effects in beams and plasmas in cylindrical geometry. We introduce the longitudinal dielectric permittivity for the beam of charged particles, which describes its collective space charge response. The analysis yields an effective plasma frequency, which depends on the transverse geometry of the system. This dielectric permittivity mirrors the dielectric permittivity of plasma and matches the one dimensional expression once the transverse size of the beam is large. Several particle species can be included as additive terms describing susceptibility of each specie. The developed approach allows to study stability criteria for collective beam–beam and beam–plasma instabilities for arbitrary transverse distributions in particle densities.

Electromagnetic fields induced by the space charge in relativistic beams play an important role in Accelerator Physics. They lead to emittance growth, slice energy change, and the microbunching instability. Typically, these effects are modeled numerically since simple description exists only in the limits of large- or small-scale current variations. In this paper we consider an axially symmetric charged beam inside a round pipe and find the solution of the space charge problem that is valid in the full range of current variations. We express the solution for the field components in terms of Green's functions, which are fully determined by just a single function. We then find that this function is an on-axis potential from a charged disk in a round pipe, with transverse charge density $\rho_\perp(r)$, and it has a compact analytical expression. We finally provide an integrated Green's function based approach for efficient numerical evaluation in the case when the transverse charge density stays the same along the beam.

A novel source for millimeter-wave RF is being designed at Los Alamos National Laboratory (LANL) utilizing the two-stream instability. This source has the potential for consistent output power over a large bandwidth on a single device [1]. The characteristic length of longitudinal bunching due to the two stream instability is dependent along with current, on the energy difference of the two beams[6]. The source is optimized for 1–5kV energy differential between the electron beams coupled into a single solenoid using electrostatic focusing in the electron gun region. The source will use co-axial electron beams to evaluate the longitudinal bunching of the at millimeter wavelengths.

Sources of negative ions such as H− are essential elements of proton accelerators and tokamaks. They have limited lifetime. The replacement of an ion source is a costly process causing delays in the operation of the entire machine. The hot tungsten filament is the key element that limits the lifetime of the H− ion source at the Los Alamos Neutron Science Center (LANSCE) facility. An accurate model, which describes the filament physics and provides a reliable estimate on its lifetime, may significantly improve the operation of the facility. Here, we describe such a model, including a comprehensive list of relevant physical processes from first principles. The model can be used to describe different regimes of operation using diagnostics data as input parameters. It has been benchmarked against the data collected during the production cycle at LANSCE and shows good agreement with experimental data.

General formalism for describing dynamics of modulated beams along linear beamlines is developed. We describe modulated beams with spectral distribution function which represents Fourier transform of the conventional beam distribution function in the 6-dimensional phase space. The introduced spectral distribution function is localized in some region of the spectral domain for nearly monochromatic modulations. It can be characterized with a small number of typical parameters such as the lowest order moments of the spectral distribution. We study evolution of the modulated beams in linear beamlines and find that characteristic spectral parameters transform linearly. The developed approach significantly simplifies analysis of various schemes proposed for seeding X-ray free electron lasers. We use this approach to study several recently proposed schemes and find the bandwidth of the output bunching in each case.

One of the options for increasing longitudinal coherency of X-ray free electron lasers (XFELs) is seeding with a microbunched electron beam. Several schemes leading to significant amplitude of the beam bunching at X-ray wavelengths were recently proposed. All these schemes rely on beam optics having several magnetic dipoles. While the beam passes through a dipole, its energy spread increases due to quantum effects of synchrotron radiation. As a result, the bunching factor at small wavelengths reduces since electrons having different energies follow different trajectories in the bend. We rigorously calculate the reduction in the bunching factor due to incoherent synchrotron radiation while the beam travels in an arbitrary beamline. We apply general results to estimate reduction of harmonic current in common schemes proposed for XFEL seeding.

A future capability in dynamic mesoscale materials science is needed to study the limitations of materials under irreversible and extreme conditions, where these limitations are caused by nonuniformities and defects in the mesoscale. This capability gap could potentially be closed with an X-ray free-electron laser (XFEL), producing 5 &times, 1010 photons with an energy of 42 keV, known as the Matter&ndash, Radiation Interactions in Extremes (MaRIE) XFEL. Over the last few years, researchers at the Los Alamos National Laboratory have developed a preconceptual design for a MaRIE-class XFEL based on existing high-brightness beam technologies, including superconducting L-band cryomodules. However, the performance of a MaRIE-class XFEL can be improved and the risk of its operation reduced by investing in emerging high-brightness beam technologies, such as the development of high-gradient normal conducting radio frequency (RF) structures. Additionally, an alternative XFEL architecture, which generates a series of high-current microbunches instead of a single bunch with uniformly high current along it, may suppress the most important emittance degradation effects in the accelerator and in the XFEL undulator. In this paper, we describe the needed dynamic mesoscale materials science capability, a MaRIE-class XFEL, and the proposed microbunched XFEL accelerator architecture in detail.

At the Linac Coherent Light Source (LCLS) X-ray free-electron laser, a laser “heater” is used to generate an uncorrelated 20-keV energy spread on an electron beam to suppress the microbunching instability in downstream bunch compressors. Here we describe an alternative approach using emittance partitioning, where the increase in energy spread is generated by moving phase space volume from the transverse dimensions into the longitudinal dimension. For LCLS-relevant beam parameters, about a factor of six reduction in the product of both transverse emittances is feasible with the same amount of induced energy spread, with additional improvements possible with an optimized setup.

The Eighteenth Workshop on Advanced Accelerator Concepts (AAC 2018) was held at the Beaver Run Resort and Conference Center in Breckenridge, Colorado, USA on August 12–17, 2018. This paper prefaces a set of 67 conference proceedings published by the presenters and attendees of the Workshop and summarizes the proceedings and the Workshop.

A number of electron beam optics elements allow for substantial control over the phase space of the relativistic bunches. We study a scheme capable of generating a beam distribution consisting of several well-separated energy bands. Such a distribution is unstable and the two-stream instability driven by the space charge develops. We investigate the instability analytically and numerically.

We demonstrate an alternative realization of a bunch compressor (specifically, the second bunch compressor for the MaRIE XFEL beamline, 1GeV electron energy) using a double emittance exchanger (EEX) and a telescope in the transverse phase space. We compare our results with a traditional bunch compressor realized via a chicane, taking into account the nonlinear dynamics, Coherent Synchrotron Radiation (CSR) and Space Charge (SC) effects. In particular, we use the Elegant code for tracking particles through the beamline, and analyze the evolution of the eigen-emittances to separate the influence of the CSR/SC effects from the nonlinear dynamics effects. We optimize the scheme parameters to reach a desirable compression factor and minimize the emittance growth. We observe dominant CSR effects in our scheme, resulting in critical emittance growth, and introduce an alternative version of an emittance exchanger with a reduced number of bending magnets to minimize the impact of CSR effects.

In this note we discuss the possibility of extracting the 3D information about samples using a single x-ray beam diagnostics.

In this report we estimate the effect of the microbunching instability in the MaRIE XFEL linac. The reference design for the linac is described in a separate report. The parameters of the L1, L2, and L3 linacs as well as BC1 and BC2 bunch compressors were the same as in the referenced report. The beam dynamics was assumed to be linear along the accelerator (which is a reasonable assumption for estimating the effect of the microbunching instability). The parameters of the bunch also match the parameters described in the referenced report. Additionally it was assumed that the beam radius is equal to R = 100 m and does not change along linac. This assumption needs to be revisited at later studies. The beam dynamics during acceleration was accounted in the matrix formalism using a Matlab code. The input parameters for the linacs are: RF peak gradient, RF frequency, RF phase, linac length, and initial beam energy. The energy gain and the imposed chirp are calculated based on the RF parameters self-consistently. The bunch compressors are accounted in the matrix formalism as well. Each chicane is characterized by the beam energy and the R56 matrix element. It was confirmed that themore » linac and beam parameters described previously provide two-stage bunch compression with compression ratios of 10 and 20 resulting in the bunch of 3kA peak current.« less

MaRIE x-ray free electron laser (FEL) is envisioned to deliver a burst of closely spaced in time pulses for enabling the capability of studying the dynamic processes in a sample. MaRIE capability can be largely enhanced using the superconducting undulator, which has the capability of doubling its period. This technology will allow reaching the photon energy as low as ~200-500 eV. As a result, the MaRIE facility will have a broader photon energy range enabling a larger variety of experiments. The soft x-ray capability is more likely to achieve the 3D imaging of dynamic processes in noncrystal materials than the hard x-ray capability alone.

Materials science needs to study dynamic properties of high-Z materials lead to a unique and challenging set of requirements for future X-ray free-electron lasers (XFELs), with single-pulse fluxes of up to 1012 50 keV X-rays that are both transversely and longitudinally coherent. These parameters cannot be met through an extension of the beam and FEL technologies used at existing and currently planned X-ray FEL facilities. We describe a novel technique to achieve higher fluxes by reducing the transverse beam emittance of high bunch charges and another to achieve longitudinal coherency by pre-modulating the electron beam current before it reaches the undulator. These techniques are investigated numerically and analytically, and also hold potential for increasing performance and decreasing cost of soft X-ray FELs.

The backward Raman amplification (BRA) of short laser pulses in plasma is studied numerically in two space dimensions. A high quality prefocused seed pulse is shown to remain well-focused through the entire process of the seed amplification by a high quality pump. In addition, it is shown that the BRA is not sensitive to a broad class of pump and seed fluctuations. The BRA length at which self-focusing and self-phase-modulation effects appear is determined numerically.

Harmonic generation becomes increasingly important as x-ray free-electron lasers push to shorter wavelengths. Recent studies have pointed to the possibility of enhancing harmonic generation by detuning the fundamental. In x-ray free-electron lasers, the wiggler line is composed of multiple wiggler segments with magnetic quadrupoles in the gaps to provide for increased focusing. In this paper, we study the effect on harmonic generation in simulation by (1) varying the gap lengths between the wiggler segments and (2) varying the electron beam $\ensuremath{\beta}$ function. In studying the harmonic we find enhanced harmonic generation is periodic in the gap length and peaks are found as the wiggler separation varies by $\ensuremath{\lambda}/3$ (where $\ensuremath{\lambda}$ is the fundamental wavelength), which corresponds to a phase shift of $2\ensuremath{\pi}/3$. As a consequence, enhanced harmonic generation is found both when the fundamental emission is strong by the nonlinear harmonic generation mechanism and by linear harmonic generation when the fundamental is detuned.

A general architecture of an emittance exchanger (EEX) is considered, where the horizontal and longitudinal phase spaces are exchanged. A family of designs is described which can lead to extremely short final longitudinal lengths, even subfemtosecond. Using higher-order particle simulations, a preferred configuration is found, which has better compression capability and less emittance growth than the standard EEX design at high beam energy. An alternative design is also found which eliminates any final energy-phase coupling. Features of compression using an EEX are significantly different than with a chicane because the final longitudinal phase space is decoupled from the initial longitudinal phase space. Advantages of using an EEX for compression include less susceptibility to the coherent synchrotron radiation (CSR) microbunch instability, less susceptibility to bunch length broadening from CSR effects, and elimination of the initial energy-phase correlation that is needed for compression using a chicane as well as any residual energy-phase correlation after compression. A key disadvantage of using an EEX is that the final horizontal emittance tends to strongly depend on the initial bunch length and beam energy.

The flat-beam transform (FBT) for round symmetric beams can be extended using the concept of eigenemittances. By tailoring the initial beam conditions at the cathode, including adding arbitrary correlations between any two dimensions, this extension can be used to provide greater freedom in controlling the beam’s final emittances. In principle, this technique can be used to generate extraordinarily transversely bright electron beams. Examples are provided where an equivalent FBT is established between the horizontal and the longitudinal beam dimensions.

Next generation hard X-ray free electron lasers require electron beams with low transverse emittance. One proposal to achieve these low emittances is to exploit the eigen-emittance values of the beam. The eigen-emittances are invariant under linear beam transport and equivalent to the emittances in an uncorrelated beam. If a correlated beam with two small eigen-emittances can be produced, removal of the correlations via appropriate optics will lead to two small emittance values, provided non-linear effects are not too large. We study how such a beam may be produced using minimal linear correlations. We find it is theoretically possible to produce such a beam, however it may be more difficult to realize in practice. We identify linear correlations that may lead to physically realizable emittance schemes and discuss promising future avenues., Comment: 7 pages, 2 figures, to appear in NIM A

We have designed, fabricated and tested with low power a novel W-band TWT based on a slow-wave cylindrically-symmetric PBG dielectric structure, or an “omniguide”. PBG TWT structures have great potential for very large bandwidth and linear dispersion. A gain experiment is under way at Los Alamos with the structure being driven by a 2 A, 110 kV electron beam. In this presentation we will report the design of the omniguide TWT structure, theoretical computations of the gain and the results of S-parameters measurements. We will also describe the electron beam test stand setup and report first results from the gain measurement experiment.

In the process of backward Raman amplification (BRA), the leading layers of the seed laser pulse can shadow the rear layers, thus weakening the effective seeding power and affecting parameters of output pulses in BRA. We study this effect numerically and also analytically by approximating the pumped pulse by the ''*-pulse'' manifold of self-similar solutions. We determine how the pumped pulse projection moves within the *-pulse manifold, and describe quantitatively the effective seeding power evolution. Our results extend the quantitative theory of BRA to regimes where the effective seeding power varies substantially during the amplification. These results might be of broader interest, since the basic equations, are general equations for resonant 3-wave interactions.

We describe a hybrid free-electron laser (FEL)/inverse Compton scattering (ICS) system that can be operated at very high repetition rates and with higher average gamma-ray fluxes than possible from ICS systems driven by J/kHz laser systems. Also, since the FEL system can generate 100 mJ class photon pulses at UV wavelengths, the electron beam energy can be lower than for systems driven with ~micron wavelength lasers for attaining gamma rays of similar energy.

In this paper, we compare two recent models [N. A. Yampolsky and N. J. Fisch, Phys. Plasmas 16, 072104 (2009); D. Benisti, D. J. Strozzi, L. Gremillet, and O. Morice, Phys. Rev. Lett. 103, 155002 (2009)] introduced to predict the nonlinear growth of stimulated Raman scattering in the kinetic regime, and providing moreover a nonlinear description of the collisionless, Landau-like, damping rate of the driven electron plasma wave. We first recall the general theoretical framework common to these two models, based on the derivation of the imaginary part of the electron susceptibility, χi, and then discuss in detail why the two approaches differ. By comparing the theoretical predictions for χi to those derived from test particle or Vlasov simulations, we moreover discuss the range of validity of the two models.

Through resonant backward Raman scattering, the plasma wave mediates the energy transfer between long pump and short seed laser pulses. These mediations can result in pulse compression at extraordinarily high powers. However, both the overall efficiency of the energy transfer and the duration of the amplified pulse depend upon the persistence of the plasma wave excitation. At least with respect to the recent state-of-the-art experiments, it is possible to deduce that at present the experimentally realized efficiency of the amplifier is likely constrained mainly by two effects, namely, the pump chirp and the plasma wave wavebreaking.

A plasma wave can mediate laser coupling in a plasma-based resonant backward Raman amplifier for high power amplification of short laser pulses. The resonant nature of amplification requires a long lifetime of the plasma wave. However, the plasma wave can be heavily Landau damped in warm plasma. On the other hand, Landau damping can be saturated in the presence of a strong plasma wave. We study backward Raman amplifier in the nonlinear regime of Landau damping using a simplified fluid model. We find the regime in which initially high linear Landau damping can be significantly saturated. Because of the saturation effect, higher temperatures can be tolerated in achieving efficient amplification. The plasma temperature can be as much as 50% larger compared to the case of unsaturated Landau damping.

The nonlinear interaction of a plasma wave with resonant electrons results in a plateau in the electron distribution function close to the phase velocity of the plasma wave. As a result, Landau damping of the plasma wave vanishes and the resonant frequency of the plasma wave downshifts. However, this simple picture is invalid when the external driving force changes the plasma wave fast enough so that the plateau cannot be fully developed. A new model to describe amplification of the plasma wave including the saturation of Landau damping and the nonlinear frequency shift is proposed. The proposed model takes into account the change of the plasma wave amplitude and describes saturation of the Landau damping rate in terms of a single fluid equation, which simplifies the description of the inherently kinetic nature of Landau damping. A proposed fluid model, incorporating these simplifications, is verified numerically using a kinetic Vlasov code.

A plasma-based resonant backward Raman amplifier/compressor for high power amplification of short laser pulses might, under ideal conditions, convert as much as 90% of the pump energy to the seed pulse. While the theoretical highest possible efficiency of this scheme has not yet been achieved, larger efficiencies than ever before obtained experimentally (6.4%) are now being reported, and these efficiencies are accompanied by strong pulse compression. Based on these recent extensive experiments, it is now possible to deduce that the experimentally realized efficiency of the amplifier is likely constrained by two factors, namely the pump chirp and the plasma wavebreaking, and that these experimental observations may likely involve favorable compensation between the chirp of the laser and the density variation of the mediating plasma. Several methods for further improvement of the amplifier efficiency in current experiments are suggested.

The enhancement of stimulated Raman backscattering (SRBS) amplification was demonstrated by introducing a plasma density gradient along the pump and the seed interaction path and by a novel double-pass design. The energy transfer efficiency was significantly improved to a level of 6.4%. The seed pulse was amplified by a factor of more than 20 000 from the input in a 2mm long plasma, which also exceeded the intensity of the pump pulse by 2 orders of magnitude. This was accompanied by very effective pulse compression, from 500fsto90fs in the first pass measurements and in the second pass down to approximately 50fs, as it is indicated by the energy-pulse duration relation. Further improvements to the energy transfer efficiency and the SRBS performance by extending the region of resonance is also discussed where a uniform ∼4mm long plasma channel for SRBS was generated by using two subsequent laser pulses in an ethane gas jet.

A new scheme of terahertz radiation (T-ray) amplification based on four-wave coupling in plasma is proposed. Three stages of the amplification, namely, linear, initial nonlinear, and advanced nonlinear are analyzed. It is shown that (1) during the advanced nonlinear stage, a series of T-ray pulses with fixed energies is generated, and (2) the duration of each pulse decreases during the amplification, while the pulse amplitude grows. Possible parameters of the actual experimental apparatus are also suggested.