Nikolai A. Yampolsky

Limiting effects on laser compression by resonant backward Raman scattering in modern experimentsa)

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.

Comparisons between nonlinear kinetic modelings of simulated Raman scattering using envelope equations

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.

New X-ray free-electron laser architecture for generating high fluxes of longitudinally coherent 50 keV photons

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.

Exploring Minimal Scenarios to Produce Transversely Bright Electron Beams Using the Eigen-Emittance Concept

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 eigenemittances 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 eects are not too large. We study how such a beam may be produced using minimal linear correlations. We nd it is theoretically possible to produce such a beam, however it may be more dicult to realize in practice. We identify linear correlations that may lead to physically realizable emittance schemes and discuss promising future avenues.

Using Emittance Partitioning Instead of a Laser Heater to Suppress the Microbunch Instability

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.

20.2: Testing of the omniguide traveling-wave tube

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.

Double emittance exchanger as a bunch compressor for the MaRIE XFEL electron beam line at 1 GeV

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.

Development of the two-stream instability in a single bunch

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.