Linac-driven Free Electron Laser Research Articles

Absolute and non-invasive determination of the electron bunch length in a free electron laser using a bunch compressor monitor

In a linac driven Free Electron Laser (FEL), the shot-to-shot and non-invasive monitoring of the electron bunch length is normally ensured by Bunch Compressor Monitors (BCMs). The bunch-length dependent signal of a BCM results from the detection and integration—over a given frequency band—of the temporal coherent enhancement of the radiation spectral energy emitted by the electron beam while experiencing a longitudinal compression. In this work, we present a method that permits to express the relative variation of the bunch length as a function of the relative statistical fluctuations of the BCM and charge signals. Furthermore, in the case of a BCM equipped with two detectors simultaneously operating in two distinct wavelength bands, the method permits an absolute determination of the electron bunch length. The proposed method is beneficial to a FEL. Thanks to it, the machine compression feedback can be tuned against the absolute measurement of the bunch length rather than a bunch-length dependent signal. In a CW-superconducting-linac driven FEL, it can offer the precious opportunity to implement a fully non-invasive and absolute diagnostics of the bunch length.

Machine learning based longitudinal virtual diagnostics at SwissFEL.

The bunch length in a linac driven Free Electron Laser (FEL) is a major parameter to be characterized to optimize the final accelerator performance. In linear machines, this observable is typically determined from the beam imaged on a screen located downstream of a Transverse Deflecting Structure (TDS) used to impinge a time dependent kick along the longitudinal coordinate of the beam. This measurement is typically performed during the machine setup and only sporadically to check the beam duration, but it cannot be continuously repeated because it is time consuming and invasive. A non-invasive method to determine the electron bunch length has already been presented in the past. This method is based on the analysis of the synchrotron radiation light spot emitted by the bunch passing through a magnetic chicane, provided that the energy chirp impinged on the bunch by the upstream radio frequency structures is known. In order to overcome a systematic discrepancy affecting the synchrotron radiation monitor based results compared to the absolute TDS based ones, we implemented and optimized a machine learning approach to predict the bunch length downstream of the two SwissFEL compression stages-from about 10fs up to about 2 ps-as well as the beam longitudinal profile at the first one.

Effective DC acceleration of charged particles in a circular ring and its potential application

A novel scheme is proposed for realizing effective DC acceleration of charged particles in a circular ring. The key is to use an induction acceleration cell with multi-insulating gaps that allow forced short circuit using solid-state switching devices such as SiC-MOSFET or unforced short circuit using diodes. A baseline design of the induction acceleration cell is shown. A coasting beam is accelerated in the circular ring without bunching. As its possible application, a continuous-wave terahertz free-electron laser (FEL) is proposed with numerical parameters, where this effective DC acceleration compensates for energy loss of the electron beam occupying the entire isochronous storage ring due to synchrotron radiation and FEL interaction. As a result, FEL microbunching of a coasting beam can be created, resulting in a high average output power that has not been possible with existing linac-driven FELs or RF storage-ring FELs.

Bridging the gap of storage ring light sources and linac-driven free-electron lasers

High-gain free-electron lasers (FELs) are driven by short, high-charge density electron beams as only produced at dedicated single pass or recirculating linear accelerators. We describe new conceptual, technical, and modeling solutions to produce subpicosecond, up to $\ensuremath{\sim}100\text{ }\text{ }\ensuremath{\mu}\mathrm{J}$-energy extreme ultra-violet and soft x-ray FEL pulses at high- and tunable repetition rates, from diffraction-limited storage ring light source. In contrast to previously proposed schemes, we show that lasing can be simultaneous to the standard multibunch radiation emission from short insertion devices, and that it can be obtained with limited impact on the storage ring infrastructure. By virtue of the high-average power but moderate pulse energy, the storage ring-driven high-gain FEL would open the door to unprecedented accuracy in time-resolved spectroscopic analysis of matter in the linear response regime, in addition to inelastic scattering experiments.

Matrix model for collective phenomena in electron beam\u2019s longitudinal phase space

The possibility to predict, characterize and minimize the presence of spurious harmonic content in the longitudinal profile of high brightness electron beams, namely the microbunching instability, has become vital to ensure accurate modeling and reliable operation of radiofrequency and plasma-based linear accelerators such as those driving free-electron lasers. Recently, the impact of intrabeam scattering (IBS) on the instability has been experimentally demonstrated by the authors. This work complements that experimental study by extending existing theories in a self-consistent, piece-wise calculation of IBS in single pass linacs and multi-bend transfer lines. New expressions for the IBS are introduced in two different semi-analytical models of microbunching. The accuracy of the proposed models and the range of beam parameters to which they apply is discussed. The overall modeling turns out to be a fast comprehensive tool for the optimization of linac-driven free-electron lasers.

Control and application of beam microbunching in high brightness linac-driven free electron lasers

We review physical mechanisms and driving forces behind the microbunching instability in modern free-electron lasers. Laser heater and alternative methods to fight the instability are presented and evaluated. Advanced uses of the laser heater setup for temporal and spectral controls of the FEL and for intense THz production are discussed. A recent idea of using the microbunching instability for coherent generation of UV radiation is described. Noise suppression in relativistic beams is briefly reviewed.

Microbunching instability study in a linac-driven free electron laser spreader beam line

Suppression of the microbunching instability is a priority for free electron lasers (FELs) whose goal is production of intense coherent radiation with very narrow spectral bandwidth. The instability can have large gain in multibend switchyard lines that connect the accelerator to multiple undulator lines. This study provides practical guidelines for switchyard optics designs that are largely immune to microbunching growth. A case study of the FERMI FEL switchyard is illustrated, resulting in an improved optics design that provides a unity microbunching gain while preserving the electron beam brightness. The analytical computation of the gain for the improved optics is supported by simulations and by experimental results.

Passive Linearization of the Magnetic Bunch Compression Using Self-Induced Fields.

In linac-driven free-electron lasers, colliders, and energy recovery linacs, a common way to compress the electron bunch to kiloampere level is based upon the implementation of a magnetic dispersive element that converts particle energy deviation into a path-length difference. Nonlinearities of such a process are usually compensated by enabling a high harmonic rf structure properly tuned in amplitude and phase. This approach is however not straightforward, e.g., in C-band and X-band linacs. In this Letter we demonstrate that the longitudinal self-induced field excited by the electron beam itself is able to linearize the compression process without any use of high harmonic rf structure. The method is implemented at the FERMI linac, with the resulting high quality beam used to drive the seeded free-electron laser during user experiments.

Experimental Demonstration of Electron Longitudinal-Phase-Space Linearization by Shaping the Photoinjector Laser Pulse

Control of the electron-beam longitudinal-phase-space distribution is of crucial importance in a number of accelerator applications, such as linac-driven free-electron lasers, colliders and energy recovery linacs. Some longitudinal-phase-space features produced by nonlinear electron beam self- fields, such as a quadratic energy chirp introduced by geometric longitudinal wakefields in radio-frequency (rf) accelerator structures, cannot be compensated by ordinary tuning of the linac rf phases nor corrected by a single high harmonic accelerating cavity. In this Letter we report an experimental demonstration of the removal of the quadratic energy chirp by properly shaping the electron beam current at the photoinjector. Specifically, a longitudinal ramp in the current distribution at the cathode linearizes the longitudinal wakefields in the downstream linac, resulting in a flat electron current and energy distribution. We present longitudinal-phase-space measurements in this novel configuration compared to those typically obtained without longitudinal current shaping at the FERMI linac.

Radiative regime of linear colliders, high repetetion rate free electron lasers and associated accelerating structures

A review of various scalings of high energy and high luminosity electron–positron colliders in the highly radiative regime is presented together with the electromagnetic and mechanical limitations of various associated acceleration structures to support operation with high gradients and high repetition rates. The potential for operation of normal conducting linacs in a mode of high repetition rate of 1 kHz or higher, relevant for today's linac-driven free electron lasers, is also discussed.

Overview on superconducting photoinjectors

The success of most of the proposed energy recovery linac (ERL) based electron accelerator projects for future storage ring replacements (SRR) and high power IR--free-electron lasers (FELs) largely depends on the development of an appropriate source. For example, to meet the FEL specifications [J. W. Lewellen, Proc. SPIE Int. Soc. Opt. Eng. 5534, 22 (2004)] electron beams with an unprecedented combination of high brightness, low emittance ($0.1\text{ }\text{ }\ensuremath{\mu}\mathrm{mrad}$), and high average current (hundreds of mA) are required. An elegant way to create a beam of such quality is to combine the high beam quality of a normal conducting rf photoinjector with the superconducting technology, i.e., to build a superconducting rf photoinjector (SRF gun). SRF gun R programs based on different approaches have been launched at a growing number of institutes and companies (AES, Beijing University, BESSY, BNL, DESY, FZD, TJNAF, Niowave, NPS, Wisconsin University). Substantial progress was achieved in recent years and the first long term operation was demonstrated at FZD [R. Xiang et al., in Proceedings of the 31st International Free Electron Laser Conference (FEL 09), Liverpool, UK (STFC Daresbury Laboratory, Warrington, 2009), p. 488]. In the near future SRF guns are expected to play an important role for linac-driven FEL facilities. In this paper we will review the concepts, the design parameters, and the status of the major SRF gun projects.

FEL cavity length measurement with an external laser

The nature of linac-driven free electron laser (FEL) operation requires that the length of the laser optical resonator be matched to the repetition rate of the accelerator in order for the returning optical pulse to overlap the newly arriving electron bunch. The range of cavity lengths over which oscillation will occur is limited in typical FEL designs to a few tens of microns, while the total resonator length is often many meters. The task of correctly setting the length of a new FEL resonator is therefore quite difficult, particularly if the cavity geometry contains more than a single line segment. We have demonstrated that such an absolute length measurement with micron accuracy over many meters in any cavity configuration is possible by establishing resonances in the cavity with an external laser of known repetition frequency. This technique was successfully applied in the development of two new FELs at the Stanford FEL Center. We will discuss the advantages of this method over conventional measurement schemes.

The Michelson resonator free-electron laser. I. Passive mode structure and mode decay

A general analysis of the evolution of phase locking in an RF linac-driven Michelson resonator free-electron laser (FEL) is presented. By providing a delay of one RF period in the secondary arm of the interferometer, successive optical pulses can be coupled at the beamsplitter, so that they build up from noise with a definite phase relationship. Phase locking is described in terms of longitudinal mode decay by extracting the mode losses directly from the passive frequency response of the resonator. The analyses predict significant mode structure simplification in microsecond macropulses, so that high-resolution spectroscopy is feasible on RF linac-driven FELs. Simulations of the perfectly tuned Michelson resonator FEL are described from spontaneous radiation to saturation, using a one-dimensional pulse propagation code. Excellent agreement with the analytical results in the small-signal regime is demonstrated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Radio-frequency linac-driven free-electron laser configurations

Recent developments in accelerator technology and the optical guiding property of the free-electron laser (FEL) radiation indicate that rf linacs may be a strong contender for driving high average power FEL amplifiers. We discuss two rf linac-driven FEL configurations: the master oscillator power amplifier (MOPA) and the single-state power amplifier (SSPA). Our numerical results are obtained using the source-dependent expansion (SDE) method in the computer code shera. The SDE method and the code shera are discussed and illustrations of the MOPA and SSPA configurations presented.

The physics of free electron lasers and applications to electron cyclotron heating

An induction linac-driven free electron laser (FEL) is considered as a potential source of high-average power millimeter radiation for heating tokamaks. The basic physics of the FEL is reviewed, the state of high-power millimeter experiments is summarized, and a preliminary-design induction linac-driven FEL for auxiliary heating is described.

Pulse-laser-irradiated high-brightness photoelectron source

Linac-driven free-electron lasers, new powerful synchrotron radiation sources, and advanced high-power microwave devices will require very bright emitters of electrons to operate efficiently. By irradiating a Cs3Sb photocathode with a frequency-doubled, Nd:glass laser, we have obtained a very high normalized electron beam brightness of over 1011 A/m2/rad2. Peak currents emitted from the 1 cm2 surface in 50 ns pulses ranged up to 80 A. Measured normalized emittances were between 5 and 9 π mm mrad.