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.Comment: 12 pages, 11 figures, submitted to Phys. Rev. ST Accel. Beam

Longitudinal Bunch Diagnostics using Coherent Transition Radiation Spectroscopy

The generation and properties of transition radiation (TR) are thoroughly treated. The spectral energy density, as described by the Ginzburg-Frank formula, is computed analytically, and the modifications caused by the finite size of the TR screen and by near-field diffraction effects are carefully analyzed. The principles of electron bunch shape reconstruction using coherent transition radiation are outlined. Spectroscopic measure- ments yield only the magnitude of the longitudinal form factor but not its phase. Two phase retrieval methods are investigated and illustrated with model calculations: analytic phase computation by means of the Kramers- Kronig dispersion relation, and iterative phase retrieval. Particular attention is paid to the ambiguities which are unavoidable in the reconstruction of longitudinal charge density profiles from spectroscopic data. The origin of these ambiguities has been identified and a thorough mathematical analysis is presented. The experimental part of the paper comprises a description of our multichannel infrared and THz spectrometer and a selection of measurements at FLASH, comparing the bunch profiles derived from spectroscopic data with those determined with a transversely deflecting microwave structure. A rigorous derivation of the Kramers-Kronig phase formula is presented in Appendix A. Numerous analytic model calculations can be found in Appendix B. The differences between normal and truncated Gaussians are discussed in Appendix C. Finally, Appendix D contains a short description of the propagation of an electromagnetic wave front by two-dimensional fast Fourier transformation. This is the basis of a powerful numerical Mathematica code THzTransport, which permits the propagation of electromagnetic wave fronts through a beam line consisting of drift spaces, lenses, mirrors and apertures.Comment: 60 page, 59 figure

Expression of different L1 isoforms of Mastomys natalensis papillomavirus as mechanism to circumvent adaptive immunity

Although many high-risk mucosal and cutaneous human papillomaviruses (HPVs) theoretically have the potential to synthesize L1 isoforms differing in length, previous seroepidemiological studies only focused on the short L1 variants, co-assembling with L2 to infectious virions. Using the multimammate mouse Mastomys coucha as preclinical model, this is the first study demonstrating seroconversion against different L1 isoforms during the natural course of papillomavirus infection. Intriguingly, positivity with the cutaneous MnPV was accompanied by a strong seroresponse against a longer L1 isoform, but to our surprise, the raised antibodies were non-neutralizing. Only after a delay of around 4 months, protecting antibodies against the short L1 appeared, enabling the virus to successfully establish an infection. This argues for a novel humoral immune escape mechanism that may also have important implications on the interpretation of epidemiological data in terms of seropositivity and protection of PV infections in general.Fil: Fu, Yingying. German Cancer Research Center; AlemaniaFil: Cao, Rui. German Cancer Research Center; AlemaniaFil: Schäfer, Miriam. German Cancer Research Center; AlemaniaFil: Stephan, Sonja. German Cancer Research Center; AlemaniaFil: Braspenning Wesch, Ilona. German Cancer Research Center; AlemaniaFil: Schmitt, Laura. German Cancer Research Center; AlemaniaFil: Bischoff, Ralf. German Cancer Research Center; AlemaniaFil: Müller, Martin. German Cancer Research Center; AlemaniaFil: Schäfer, Kai. German Cancer Research Center; AlemaniaFil: Vinzon, Sabrina Eugenia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Bioquímicas de Buenos Aires. Fundación Instituto Leloir. Instituto de Investigaciones Bioquímicas de Buenos Aires; ArgentinaFil: Rösl, Frank. German Cancer Research Center; AlemaniaFil: Hasche, Daniel. German Cancer Research Center ; Alemani

Controlled density-downramp injection in a beam-driven plasma wakefield accelerator

This paper describes the utilization of beam-driven plasma wakefield acceleration to implement a high-quality plasma cathode via density-downramp injection in a short injector stage at the FLASHForward facility at DESY. Electron beams with charge of up to 105 pC and energy spread of a few percent were accelerated by a tunable effective accelerating field of up to 2.7 GV/m. The plasma cathode was operated drift-free with very high injection efficiency. Sources of jitter, the emittance and divergence of the resulting beam were investigated and modeled, as were strategies for performance improvements that would further increase the wide-ranging applications for a plasma cathode with the demonstrated operational stabilityComment: 11 pages, 9 figure

Echtzeitspektroskopie von Ferninfrarotstrahlung zur Strahldiagnose an Linearbeschleunigern - Real-time far-infrared spectroscopy as diagnostic tool for linear accelerators

Zusammenfassung: Freie-Elektronen-Laser für den Röntgenbereich,in denen ein hochenergetischer Elektronenstrahlals Lasermedium dient, erfordern Strahlparameterweitjenseits denen herkömmlicher Beschleunigeranlagen.Von zentraler Bedeutung ist hierbei die Länge der relativistischbeschleunigten Elektronenpakete („bunche“),die in speziellen Magnetanordnungen auf deutlich unter100 μm komprimiert werden. Dabei werden für einige100 fs Spitzenströme imkA-Bereich erzeugt. Durch diverseProzesse kann die Abstrahlung von breitbandiger Infrarotstrahlungim Millimeter- bis Mikrometerbereich erzwungenwerden. Die Intensität und die spektrale Zusammensetzungdieser Strahlung geben Auskunft über das Stromprofilder Elektronenpakete. In den letzten Jahren wurdenerhebliche Fortschritte erzielt, diese Strahlung spektralaufgelöst zu messen und somit leistungsfähige Methodenzur Überwachung des Beschleunigungs- und Kompressionsprozessesbereitzustellen.Schlüsselwörter: Ferninfrarot, Spektroskopie, Echtzeit, linearBeschleuniger.Abstract: X-ray Free-Electron Lasers, driven by highenergyelectron linear accelerators, require beam parametersbeyond those of conventional machines. Of criticalimportance is the length of the accelerated electron packets(„bunches“) which are compressed by special magnetsequences towell below 100 μm, resulting in peak currentsof several kA. Monitoring and control of the longitudinalcurrent distribution is one of the key challenges at thesemachines. Different phenomena lead to the emission ofbroad-band infrared radiation in the millimeter tomicrometerregime whose intensity and spectral composition indirectlyreveals the longitudinal current profile of the radiatingbunches. During the past years, substantial progresshas been made to develop powerful methods to measureand spectroscopically resolve this radiation in a way thaton-line monitoring of the acceleration and compressionprocess became possible.Keywords: Far infrared, spectroscopy, real time, electronbunch, diagnostic

THz Spectroscopy with MHz Repetition Rates for Bunch Profile Reconstructions at European XFEL

The European X-ray Free-Electron Laser generates most powerful and brilliant X-ray laser pulses. Exact knowledge about the longitudinal electron bunch profile is crucial for the operation of the linear accelerator as well as for photon science experiments. The only longitudinal diagnostic downstream of the main linac is based on spectroscopy of diffraction radiation (DR). The spectral intensity of the DR in the THz and infrared regime is monitored by a four-staged grating spectrometer and allows non-invasive bunch length characterization based on form factor measurements in the range 0.7 - 60 THz. As the readout and signal shaping electronics of the spectrometer allow MHz readout rates, the longitudinal bunch profile of all bunches inside the bunch train can be characterized non-invasively and simultaneously to FEL operation. In this paper, form factor measurements along the bunch train will be described and presented as well as the resulting reconstructed current profiles

Benchmarking Coherent Radiation Spectroscopy as a Tool for High-Resolution Bunch Shape Reconstruction at Free-Electron Lasers

We present a systematic comparison of two complementary methods for determining the longitudinal charge density profile of the compressed electron bunches in the soft x-ray free-electron laser FLASH: a frequency-domain technique—coherent transition radiation (CTR) spectroscopy—and a time-domain technique—streaking of the electron beam with a transversely deflecting microwave structure (TDS). While the direct time-profile measurement with a TDS is a well-established method invented at SLAC, our group has pioneered high-resolution bunch shape analysis based on coherent radiation spectroscopy. We have developed a broadband spectrometer covering the wavelength range from 5 μm to 433 μm with two sets of remotely interchangeable staged reflection gratings. The measured spectral intensity allows to compute the absolute magnitude of the bunch form factor but not its phase which, however, is needed to retrieve the bunch profile. Two phase retrieval methods are investigated in detail: analytic phase computation by means of the Kramers-Kronig dispersion relation, and iterative phase retrieval. Several computational techniques are compared and evaluated in view of their applicability and efficiency. For a large variety of bunch shapes, the time profiles derived from the spectroscopic data are compared with the TDS profiles, and generally excellent agreement is observed down to the 10 fs level. For part of the measurements, two independent CTR spectrometer systems have been available, yielding almost identical bunch shapes. In summary, we demonstrate that using well calibrated and broadband spectroscopy data, a fast and reliable phase reconstruction algorithm leads to bunch profiles competitive to high resolution TDS measurements

Noninvasive THz spectroscopy for bunch current profile reconstructions at MHz repetition rates

X-ray free-electron lasers based on superconducting accelerator technology deliver ultrashort photon pulses with unprecedented peak brilliance at high repetition rates. Continuous and noninvasive monitoring of the current profile of the electron bunches is essential for the operation and control of the accelerator. Longitudinal diagnostics based on coherent radiation have already shown their potential at various free-electron laser facilities, and the multi-GeV electron beams of x-ray free-electron lasers are powerful sources for the generation of broadband coherent diffraction radiation. We present noninvasive current profile measurements with a few femtoseconds resolution based on spectroscopy of coherent diffraction radiation in the frequency range 0.7–58 THz. The current profiles, reconstructed from the spectroscopic data with an advanced phase retrieval method, are compared with measurement results obtained with a transverse deflecting structure. For the first time, bunch-resolved current profiles have been recorded simultaneously to user operation at European XFEL for all bunches in the bunch train at MHz repetition rates

Non-invasive Longitudinal Profile Measurements of Electron Bunches Simultaneously to FEL Operation at MHz Rates

Hard X-ray FELs require ultra-short electron bunches with peak currents of several kiloamperes. Therefore, longitudinal bunch profile characterization with femtosecond resolution is essential for a successful operation and control of the accelerator as well as a wide field of photon experiments. The high electron beam energies of hard X-ray FELs enable non-invasive longitudinal form factor monitoring down to a few micrometers utilizing coherent diffraction radiation spectroscopy. For this purpose, a 4-staged grating spectrometer has been recently installed at European XFEL. Here, current profiles are reconstructed with femtosecond time resolution based on phase retrieval algorithms which are in excellent agreement to results obtained with a transverse deflecting structure. The fast pyroelectric detectors allow, for the first time, to measure the current profile of all bunches inside the bunch train with repetition rates of up to 2.2 MHz. The low latency electronic readout of the 120 channels of the spectrometer provides high potential for fast compression feedbacks and machine learning applications

A Non-Invasive THz Spectrometer for Bunch Length Characterization at European XFEL

The European X-ray Free-Electron Laser provides one of the most powerful X-ray laser pulses to a wide range of experiments. These experiments strongly benefit from the exact knowledge of the electron bunch current profile and demand for stable and shortest-possible pulse lengths. During the 2018 summer shutdown, the 4-staged grating spectrometer CRISP* has been installed at a diffraction radiation (DR) beamline just upstream of the undulator beamline switchyard. The DR at final electron beam energies of up to 17.5 GeV enables non-invasive bunch length characterization based on form factor measurements down to a few micrometers. Fast detectors and electronics allow for the characterization of the whole bunch train with repetition rates above 1 MHz. This contribution will present commissioning results of the THz beamline as well as first measured form factors and reconstructed electron current profiles