E. Hemsing

Resonant excitation of coherent Cerenkov radiation in dielectric lined waveguides

We report the observation of coherent Cerenkov radiation in the terahertz regime emitted by a relativistic electron pulse train passing through a dielectric lined cylindrical waveguide. We describe the beam manipulations and measurements involved in repetitive pulse train creation including comb collimation and nonlinear optics corrections. With this technique, modes beyond the fundamental are selectively excited by use of the appropriate frequency train. The spectral characterization of the structure shows preferential excitation of the fundamental and of a higher longitudinal mode.

Experimental observation of helical microbunching of a relativistic electron beam

Experimental observation of the microbunching of a relativistic electron beam at the second harmonic interaction frequency of a helical undulator is presented. The microbunching signal is observed from the coherent transition radiation of the electron beam and indicates experimental evidence of a dominantly helical electron beam density distribution. This result is in agreement with theoretical and numerical predictions and provides a proof-of-principle demonstration of proposed schemes designed to generate light with orbital angular momentum in high-gain free-electron lasers.

Virtual dielectric waveguide mode description of a high-gain free-electron laser. I. Theory

†Particle Beam Physics Laboratory, Department of Physics and AstronomyUniversity of California Los Angeles, Los Angeles, CA 90095, USA and‡Faculty of Engineering, Department of Physical Electronics,Tel-Aviv University, Ramat-Aviv 69978, Tel-Aviv, Israel(Dated: April 29, 2008)A set of mode-coupled excitation equations for the slowly-growing amplitudes of dielectric waveg-uide eigenmodes is derived as a description of the electromagnetic signal field of a high-gain free-electron laser, or FEL, including the effects of longitudinal space-charge. This approach of describingthe field basis set has notable advantages for FEL analysis in providing an efficient characterizationof eigenmodes, and in allowing a clear connection to free-space propagation of the input (seeding)and output radiation. The formulation describes the entire evolution of the radiation wave throughthe linear gain regime, prior to the onset of saturation, with arbitrary initial conditions. By virtueof the flexibility in the expansion basis, this technique can be used to find the direct coupling andamplification of a particular mode. A simple transformation converts the derived coupled differen-tial excitation equations into a set of coupled algebraic equations and yields a matrix determinantequation for the FEL eigenmodes. A quadratic index medium is used as a model dielectric waveg-uide to obtain an expression for the predicted spot size of the dominant system eigenmode, in theapproximation that it is a single gaussian mode.

Coherent transition radiation from a helically microbunched electron beam

The coherent transition radiation emitted from an electron beam with higher-order spatial microbunching is analyzed. The characteristic angular and phase dependence can be used to identify the dominant bunching structure of such beams, which can be generated during the harmonic interaction in optical klystron modulators and free-electron lasers, and used as tunable sources of coherent light with orbital angular momentum.

Three dimensional analysis of longitudinal plasma oscillations in a thermal relativistic electron beam

In this paper, we develop a three-dimensional kinetic theory of longitudinal plasma oscillations in a relativistic electron beam. Our analysis includes the effects of betatron motion, finite emittance, and energy spread as well as edge effects due to the finite size of the beam. The consequences of these effects are shown to be of significant interest, as they give rise to several distinct new phenomena. These include an emittance induced anisotropy between backward and forward propagating plasma waves as well as a beatwave effect between plasma and betatron oscillations caused by the coupling of betatron motion to the transverse structure of the charge perturbation. The experimental implications of these new results and their relevance to the beam and radiation physics of advanced light sources, such as free-electron lasers, are also discussed.

Quasicrystalline Beam Formation in RF Photoinjectors

The recent observation of coherent optical transition radiation at the LCLS has raised serious questions concerning the present model of beam dynamics in RF photoinjectors. We present here an analysis of what we term quasicrystalline beam formation. In this scenario, the low longitudinal temperature, in combination with strong acceleration and temporal rearrangement due to bending, allows the longitudinal beam dimension to become more regular, on the microscopic scale of optical wavelengths, than expected from equilibrium statistical properties. This beam distribution then may then display a strong degree of coherence in its optical transition radiation output. We discuss further experimental investigations of this phenomenon.

Chicane radiation measurements with a compressed electron beam at the BNL ATF

The radiation emitted from a chicane compressor has been studied at the Brookhaven national laboratory (BNL) accelerator test facility (ATF). Coherent edge radiation (CER) is emitted from a compressed electron beam as it traverses sharp edge regions of a magnet. The compression is accompanied by strong self-fields, which are manifested as distortions in the momentum space called beam bifurcation. Recent measurements indicate that the bunch length is approximately 150 fs rms. The emitted THz chicane radiation displays strong signatures of CER. This paper reports on the experimental characterization and subsequent analysis of the chicane radiation measurements at the BNL ATF with a discussion of diagnostics development and implementation. The characterization includes spectral analysis, far-field intensity distribution, and polarization effects. Experimental data is benchmarked to a custom developed start-to-end simulation suite.

Breaking the Attosecond, Angstrom and TV/m Field Barriers with Ultrafast Electron Beams

Recent initiatives at UCLA concerning ultra‐short, GeV electron beam generation have been aimed at achieving sub‐fs pulses capable of driving X‐ray free‐electron lasers (FELs) in single‐spike mode. This scheme uses very low charge beams, which may allow existing FEL injectors to produce few‐100 attosecond pulses, with very high brightness. Towards this end, recent experiments at the Stanford X‐ray FEL (LCLS, first of its kind, built with essential UCLA leadership) have produced ∼2 fs, 20 pC electron pulses. We discuss here extensions of this work, in which we seek to exploit the beam brightness in FELs, in tandem with new developments at UCLA in cryogenic undulator technology, to create compact accelerator/undulator systems that can lase below 0.15 A, or be used to permit 1.5 A operation at 4.5 GeV. In addition, we are now developing experiments which use the present LCLS fs pulses to excite plasma wakefields exceeding 1 TV/m, permitting a table‐top TeV accelerator for frontier high energy physics applica...

Seeded VISA: A 1064nm laser-seeded FEL amplifier at the BNL ATF

An experimental study of a seeded Free Electron Laser (FEL) using the VISA undulator and a Nd:YAG seed laser will be performed at the Accelerator Test Facility (ATF) at Brookhaven National Laboratory. The study is motivated by the demand for a short Rayleigh length FEL amplifier at 1 micron to allow for high power transmission with minimal damage of transport optics. Planned measurements include transverse and longitudinal coherence, angular distribution, and wavelength spectrum of the FEL radiation. The effects of detuning the electron beam energy will be studied, with an emphasis on control of the radiation emission angles and increase of the amplifier efficiency. Results of start-to-end simulations are also presented.

Beam Compression Experiments using the UCLA/ATF Compressor

We review recent experimental results from the BNL ATF using the compressor built by UCLA. The measurements discussed include: first observation of short wavelength coherent edge radiation angular/wavelength spectrum and spectrum, sub‐100 fsec pulse‐length coherent transition autocorrelation measurements, and longitudinal and transverse phase space distortions. Extension of these measurements, as well as those which can be made possible by a new X‐band traveling wave deflector being developed in an industrial collaboration, are examined.