Coherent Optical Transition Radiation Research Articles

Revealing the three-dimensional structure of microbunched plasma-wakefield-accelerated electron beams

Plasma wakefield accelerators use tabletop equipment to produce relativistic femtosecond electron bunches. Optical and X-ray diagnostics have established that their charge concentrates within a micrometre-sized volume, but its sub-micrometre internal distribution, which critically influences gain in free-electron lasers or particle yield in colliders, has proven elusive to characterize. Here, by simultaneously imaging different wavelengths of coherent optical transition radiation that a laser-wakefield-accelerated electron bunch generates when exiting a metal foil, we reveal the structure of the coherently radiating component of bunch charge. The key features of the images are shown to uniquely correlate with how plasma electrons injected into the wake: by a plasma-density discontinuity, by ionizing high-Z gas-target dopants or by uncontrolled laser–plasma dynamics. With additional input from the electron spectra, spatially averaged coherent optical transition radiation spectra and particle-in-cell simulations, we reconstruct coherent three-dimensional charge structures. The results demonstrate an essential metrology for next-generation compact X-ray free-electron lasers driven by plasma-based accelerators.

Microbunching of thermionic cathode rf gun beams in the Advanced Photon Source s-band linac

We report on measurements of beams from thermionic cathode (TC) rf guns in the Advanced Photon Source S-Band Linac. These measurements include the macropulse out of both new and existing TC guns as well as the observation of microbunching within the micropulses of these beams. A gun chopper limits the macropulse FWHM duration to the 10-ns range. Our objectives were to analyse the new TC gun and investigate microbunching within a TC-rf-gun-generated beam. Our diagnostics elucidated longitudinal beam structures from the ns to the fs time scales. Coherent transition radiation (CTR) interferometers responding to far-infrared wavelengths were employed after each compression stage to provide the autocorrelations of the sub-ps micropulse durations. The first compression stage is an alpha magnet and the second a chicane. A CCD camera was used to image the beam via optical transition radiation from an Al screen at the end of the linac and also employed to measure coherent optical transition radiation (COTR) in the visible range. The COTR diagnostic observations, implying microbunching on a fs time scale, are presented and compared with a longitudinal space-charge impedance model.

Partial Focusing of Coherent Optical Transition Radiation and Measurement of Transverse Size of Femtosecond Electron Bunches

Coherent optical transition radiation (COTR) from femtosecond electron bunches is simulated using an earlier proposed model. It is shown that partial screening of the COTR field followed by focusing with a lens allows increasing the spatial resolution and makes it possible to measure transverse bunch sizes accurate within a few microns. It is demonstrated that the most reasonable lens screening ratio equals to 50%. Given this, it is possible to eliminate the ring-like structure typically observed on bunch images as a result of coherence.

Неполная фокусировка когерентного оптического переходного излучения и измерение поперечных размеров фемтосекундных электронных сгустков

Неполная фокусировка когерентного оптического переходного излучения и измерение поперечных размеров фемтосекундных электронных сгустков

Coherent Optical Signatures of Electron Microbunching in Laser-Driven Plasma Accelerators.

We report observations of coherent optical transition radiation interferometry (COTRI) patterns generated by microbunched ∼200-MeV electrons as they emerge from a laser-driven plasma accelerator. The divergence of the microbunched portion of electrons, deduced by comparison to a COTRI model, is ∼9× smaller than the ∼3 mrad ensemble beam divergence, while the radius of the microbunched beam, obtained from COTR images on the same shot, is <3 μm. The combined results show that the microbunched distribution has estimated transverse normalized emittance ∼0.4 mm mrad.

Image of the transverse bunch profile via coherent optical transition radiation

The use of optical transition radiation (OTR) monitors is a standard technique to measure transverse beam profiles at many electron accelerators. With modern accelerator technology it is possible to produce and accelerate even ultrashort electron bunches with sub--femtosecond duration. Such bunches interacting with the OTR target generate coherent optical transition radiation (COTR). For the COTR case, a reconstruction of the bunch profile from a recorded image using a conventional optical scheme is a task with inconclusive solution. In this paper an approach is proposed which is based on the strict propagation of COTR fields through a focusing lens. As result a linear dependence of the measured rms image size on the bunch size is obtained.

Coherent optical transition radiation imaging for compact accelerator electron-beam diagnostics

Application of coherent optical transition radiation (COTR) diagnostics to compact accelerators has been demonstrated for the laser-driven plasma accelerator case recently. It is proposed that such diagnostics for beam size, beam divergence, microbunching fraction, spectral content, and bunch length would be useful before and after any subsequent acceleration in crystals or nanostructures. In addition, there are indications that under some scenarios a microbunched beam could resonantly excite wake fields in nanostructures that might lead to an increased acceleration gradient.

Correlated electron beam microbunching and shot-noise characterization with near- and far-field optical transition radiation

In this work we demonstrate how to analyze the dynamics of the correlated longitudinal current noise above and below the Poissonian shot-noise level in an e-beam, by calculating the emitted transition radiation (OTR) using a complex vector field formulation which is exact in the near and far fields. We simulate a non-zero emittance electron beam, uncorrelated in the transverse cross section, but with different types of longitudinal correlations, resulting in coherent optical transition radiation. We show how this method is applied on the opposite case (suppression), by tracking simulated beam particles that correlate due to quarter-plasma oscillation during drift, and show how the OTR tracks the dynamics of the electron beam noise level.

Near-threshold electron injection in the laser–plasma wakefield accelerator leading to femtosecond bunches

The laser–plasma wakefield accelerator is a compact source of high brightness, ultra-short duration electron bunches. Self-injection occurs when electrons from the background plasma gain sufficient momentum at the back of the bubble-shaped accelerating structure to experience sustained acceleration. The shortest duration and highest brightness electron bunches result from self-injection close to the threshold for injection. Here we show that in this case injection is due to the localized charge density build-up in the sheath crossing region at the rear of the bubble, which has the effect of increasing the accelerating potential to above a critical value. Bunch duration is determined by the dwell time above this critical value, which explains why single or multiple ultra-short electron bunches with little dark current are formed in the first bubble. We confirm experimentally, using coherent optical transition radiation measurements, that single or multiple bunches with femtosecond duration and peak currents of several kiloAmpere, and femtosecond intervals between bunches, emerge from the accelerator.

Transverse profile imager for ultrabright electron beams

A transverse profile imager for ultrabright electron beams is presented, which overcomes resolution issues in present designs by observing the Scheimpflug imaging condition as well as the Snell-Descartes law of refraction in the scintillating crystal. Coherent optical transition radiation emitted by highly compressed electron bunches on the surface of the crystal is directed away from the camera, allowing to use the monitor for profile measurements of electron bunches suitable for X-ray free electron lasers. The optical design has been verified by ray tracing simulations, and the angular dependency of the resolution has been verified experimentally. An instrument according to the presented design principles has been used in the SwissFEL Injector Test Facility, and different scintillator materials have been tested. Measurements in conjunction with a transverse deflecting radiofrequency structure and an array of quadrupole magnets demonstrate a normalized slice emittance of 25 nm in the core of a 30 fC electron beam at a pulse length of 10 ps and a particle energy of 230 MeV.

Laser heater commissioning at an externally seeded free-electron laser

FERMI is the first user facility based upon an externally seeded free-electron laser (FEL) and was designed to deliver high quality, transversely and longitudinally coherent radiation pulses in the extreme ultraviolet and soft x-ray spectral regimes. The FERMI linear accelerator includes a laser heater to control the longitudinal microbunching instability, which otherwise is expected to degrade the quality of the high brightness electron beam sufficiently to reduce the FEL output intensity and spectral brightness. In this paper, we present the results of the FERMI laser heater commissioning. For the first time, we show that optimizing the electron beam heating at an upstream location (beam energy, 100 MeV) leads to a reduction of the incoherent energy spread at the linac exit (beam energy, 1.2 GeV). We also discuss some of the positive effects of such heating upon the emission of coherent optical transition radiation and the FEL output intensity.

Electron beam profile imaging in the presence of coherent optical radiation effects

High-brightness electron beams with low energy spread at existing and future x-ray free-electron lasers are affected by various collective beam self-interactions and microbunching instabilities. The corresponding coherent optical radiation effects, e.g., coherent optical transition radiation, render electron beam profile imaging impossible and become a serious issue for all kinds of electron beam diagnostics using imaging screens. Furthermore, coherent optical radiation effects can also be related to intrinsically ultrashort electron bunches or the existence of ultrashort spikes inside the electron bunches. In this paper, we discuss methods to suppress coherent optical radiation effects both by electron beam profile imaging in dispersive beamlines and by using scintillation imaging screens in combination with separation techniques. The suppression of coherent optical emission in dispersive beamlines is shown by analytical calculations, numerical simulations, and measurements. Transverse and longitudinal electron beam profile measurements in the presence of coherent optical radiation effects in non-dispersive beamlines are demonstrated by applying a temporal separation technique.

Long-Range Persistence of Femtosecond Modulations on Laser-Plasma-Accelerated Electron Beams

Laser plasma accelerators have produced femtosecond electron bunches with a relative energy spread ranging from 100% to a few percent. Simulations indicate that the measured energy spread can be dominated by a correlated spread, with the slice spread significantly lower. Measurements of coherent optical transition radiation are presented for broad-energy-spread beams with laser-induced density and momentum modulations. The long-range (meter-scale) observation of coherent optical transition radiation indicates that the slice energy spread is below the percent level to preserve the modulations.

Microscopic kinetic analysis of space-charge induced optical microbunching in a relativistic electron beam

Longitudinal space-charge forces from density fluctuations generated by shot noise can be a major source of microbunching instability in relativistic high brightness electron beams. The gain in microbunching due to this effect is broadband, extending at least up to optical frequencies, where the induced structure on the beam distribution gives rise to effects such as coherent optical transition radiation. In the high-frequency regime, theoretical and computational analyses of microbunching formation require a full three-dimensional treatment. In this paper we address the problem of space-charge induced optical microbunching formation in the high-frequency limit when transverse thermal motion due to finite emittance is included for the first time. We derive an analytical description of this process based on the beam's plasma dielectric function. We discuss the effect of transverse temperature on the angular distribution of microbunching gain and its connection to the physics of Landau damping in longitudinal plasma oscillations. Application of the theory to a relevant experimental scenario is discussed. The analytical results obtained are then compared to the predictions arising from high resolution three-dimensional molecular dynamics simulations.

Characterization and mitigation of coherent-optical-transition-radiation signals from a compressed electron beam

The Advanced Photon Source (APS) injector complex includes an option for rf photocathode (PC) gun beam injection into the 450-MeV S-band linac. At the 150-MeV point, a four-dipole chicane was used to compress the micropulse bunch length from a few ps to sub-0.5 ps (FWHM). Noticeable enhancements of the optical transition radiation (OTR) signal sampled after the APS chicane were then observed as has been reported in the Linac Coherent Light Source (LCLS) injector commissioning. A far-infrared (FIR) coherent transition radiation detector and interferometer were used to monitor the bunch compression process and correlate the appearance of localized spikes of OTR signal (5 to 10 times brighter than adjacent areas) within the beam-image footprint. We have performed spectral-dependency measurements at 375 MeV with a series of bandpass filters centered in 50-nm increments from 400 to 700 nm and with an imaging spectrometer and observed a broadband enhancement in these spikes. Mitigation concepts of the observed coherent OTR, which exhibits an intensity enhancement in the red part of the visible spectrum as compared to incoherent OTR, are described.

Publisher’s Note: Coherent optical transition radiation and self-amplified spontaneous emission generated by chicane-compressed electron beams [Phys. Rev. ST Accel. Beams12, 040704 (2009)

Publisher’s Note: Coherent optical transition radiation and self-amplified spontaneous emission generated by chicane-compressed electron beams [Phys. Rev. ST Accel. Beams<b>12</b>, 040704 (2009)

Coherent optical transition radiation and self-amplified spontaneous emission generated by chicane-compressed electron beams

Observations of strongly enhanced optical transition radiation (OTR) following significant bunch compression of photoinjector beams by a chicane have been reported during the commissioning of the Linac Coherent Light Source accelerator and recently at the Advanced Photon Source (APS) linac. These localized transverse spatial features involve signal enhancements of nearly a factor of 10 and 100 in the APS case at the 150-MeV and 375-MeV OTR stations, respectively. They are consistent with a coherent process seeded by noise and may be evidence of a longitudinal space charge microbunching instability which leads to coherent OTR emissions. Additionally, we suggest that localized transverse structure in the previous self-amplified spontaneous emission (SASE) free-electron laser (FEL) data at APS in the visible regime as reported at FEL02 may be attributed to such beam structure entering the FEL undulators and inducing the SASE startup at those ``prebunched'' structures. Separate beam structures 120 microns apart in $x$ and 2.9 nm apart in wavelength were reported. The details of these observations and operational parameters will be presented.

Collective-Interaction Control and Reduction of Optical Frequency Shot Noise in Charged-Particle Beams

We present a theoretical model for longitudinal collective Coulomb interactions in a charged-particle beam. It suggests a possibility to control and reduce optical frequency shot-noise current in accelerated electron beams. For short interaction lengths, the model describes well coherent optical transition radiation effects observed in SLAC LCLS and in other laboratories. For longer interaction lengths (quarter plasma oscillation period) the model predicts the possibility to reduce the beam current noise below the classical shot-noise level, an effect not yet observed experimentally at optical frequencies.

Observation of two-dimensional longitudinal-transverse correlations in an electron beam by laser-electron interactions

During the preparatory work for the optical-replica synthesizer experiment in the free-electron laser FLASH at DESY, we were able to superimpose a short, approximately 200 fs long pulse from a frequency-doubled mode-locked erbium laser with titanium-sapphire amplifier and an approximately 20 ps long electron bunch in an undulator. This induces an energy modulation in a longitudinal slice of the electron bunch. A magnetic chicane downstream of the undulator converts the energy modulation into a density modulation within the slice that causes the emission of coherent optical transition radiation from a silver-coated silicon screen. Varying the relative timing between electron and laser, we use a camera to record two-dimensional images of the slices as a function of the longitudinal position within the electron bunch.

Production and characterization of attosecond electron bunch trains

We report the production of optically spaced attosecond electron microbunches produced by the inverse free-electron-laser (IFEL) process. The IFEL is driven by a Ti:sapphire laser synchronized with the electron beam. The IFEL is followed by a magnetic chicane that converts the energy modulation into the longitudinal microbunch structure. The microbunch train is characterized by observing coherent optical transition radiation (COTR) at multiple harmonics of the bunching. Experimental results are compared with 1D analytic theory showing good agreement. Estimates of the bunching factors are given and correspond to a microbunch length of 410 attosec FWHM. The formation of stable attosecond electron pulse trains marks an important step towards direct laser acceleration.