Towards single-cycle attosecond light from accelerators

The Free-Electron Laser (FEL) is a cutting-edge, accelerator-based instrument that has the potential to provide simultaneous access to the spatial and temporal resolution of the atomic world. In a FEL, ultra-short electron bunches from an accelerator are passed through a long undulator magnet to generate coherent light. Recently, scientists from SLAC demonstrated the first generation of attosecond hard X-ray pulses, using the Linac Coherent Light Source. Now, as described in the review article by Alan Mak et al. [1], researchers are proposing developments that will make the FEL a fully coherent, singlecycle (attosecond) X-ray laser. The new concepts build upon a strong nexus between linear accelerators, FELs and quantum lasers, to produce extreme attosecond pulses with controllable waveforms

Attosecond single-cycle undulator light : a review

Research at modern light sources continues to improve our knowledge of the natural world, from the subtle workings of life to matter under extreme conditions. Free-electron lasers, for instance, have enabled the characterization of biomolecular structures with sub-angstrom spatial resolution, and paved the way to controlling the molecular functions. On the other hand, attosecond temporal resolution is necessary to broaden our scope of the ultrafast world. Here we discuss attosecond pulse generation beyond present capabilities. Furthermore, we review three recently proposed methods of generating attosecond x-ray pulses. These novel methods exploit the coherent radiation of microbunched electrons in undulators and the tailoring of the emitted wavefronts. The computed pulse energy outperforms pre-existing technologies by three orders of magnitude. Specifically, our simulations of the proposed Soft X-ray Laser at MAX IV (Lund, Sweden) show that a pulse duration of 50-100 as and a pulse energy up to 5 μJ is feasible with the novel methods. In addition, the methods feature pulse shape control, enable the incorporation of orbital angular momentum, and can be used in combination with modern compact free-electron laser setups

Matter manipulation with extreme terahertz light: Progress in the enabling THz technology

Terahertz (THz) light has proven to be a fine tool to probe and control quasi-particles and collective excitations in solids, to drive phase transitions and associated changes in material properties, and to study rotations and vibrations in molecular systems. In contrast to visible light, which usually carries excessive photon energy for collective excitations in condensed matter systems, THz light allows for direct coupling to low-energy (meV scale) excitations of interest, The development of light sources of strong-field few-cycle THz pulses in the 2000s opened the door to controlled manipulation of reactions and processes. Such THz pulses can drive new dynamic states of matter, in which materials exhibit properties entirely different from that of the equilibrium. In this review, we first systematically analyze known studies on matter manipulation with strong-field few-cycle THz light and outline some anticipated new results. We focus on how properties of materials can be manipulated by driving the dynamics of different excitations and how molecules and particles can be controlled in useful ways by extreme THz light. Around 200 studies are examined, most of which were done during the last five years. Secondly, we discuss available and proposed sources of strong-field few-cycle THz pulses and their state-of-the-art operation parameters. Finally, we review current approaches to guiding, focusing, reshaping and diagnostics of THz pulses. (C) 2019 The Author(s). Published by Elsevier B.V

Single-cycle undulator light

The past decade has witnessed a sharp rise of interest in coherent terahertz (THz) light sources for applications in condensed-matter physics. These sources are a powerful tool for studying collective excitations in solid-state systems: THz light can directly couple to low-energy excitations on the meV-scale such as the collective excitations of spins and phonons. Furthermore, coherent excitation of the material spin or phonon subsystem by a THz light pulse allows for tailoring the material’s macroscopic properties. This enables the creation of materials with new dynamic functionalities. To fully exploit the potential of the control of materials’ properties, a new generation of versatile sources of intense short-pulse THz light is needed. This thesis addresses the principles of generation of intense single-cycle THz pulses in an accelerator-based light source. The overarching principle is the phase-locked coherent emission of frequency-chirped waveforms from a specially prepared train of electron bunches inside a tapered undulator. The first part of the thesis (Ch. 1-2) motivates the THz light source development. It surveys the available light sources and scientific applications in the field of low-energy electrodynamics. Looking at a wide selection of THz-induced phenomena, the desired parameters of the proposed undulator-based THz source are determined. The second part (Ch. 3-4) focuses on the technicalities of the accelerator-based THz light source. It addresses the questions of electron beam requirements, photocathode gun performance, dynamics of electrons in an RF gun and in a superconducting linear accelerator. The beam dynamics simulations are carried out and the required characteristics of the electron bunch train are demonstrated. In what follows, the process of waveform-controlled single-cycle emission from an undulator is described: starting from the case of a single electron bunch and then proceeding to the single-cycle emission by the electron bunch train. The last part (Ch. 5) introduces a concept of tunable focusing of THz light. Specifically, it presents the model of an optical magnetic lens, based on a two-dimensional magneto-optical material immersed into a non-uniform magnetic field. To sum up, the formation of the electron bunch train with necessary spatiotemporal properties, single-cycle emission in a matching tapered undulator and tunable focusing of THz light are addressed in the thesis.

Lattices for a 4th-Generation Synchrotron Light Source

Inspired by light source upgrades, such as ESRF-EBS (Extremely Brilliant Source) and APS-U, I present some modern lattices for a medium-sized 4th-generation synchrotron radiation source. They incorporate new elements, such as anti-bend magnets. The composed lattices are optimized using a simple double-objective algorithm. Its goal is to minimize the natural emittance and absolute chromaticities simultaneously. Then, the lattices are analyzed and compared to a version of the ESRF-EBS lattice scaled down in size. The design is performed to meet the needs of the user community of the Siberian Synchrotron and Terahertz Radiation Centre under the umbrella of the Budker Institute of Nuclear Physics

Single-cycle undulator light

The past decade has witnessed a sharp rise of interest in coherent terahertz (THz) light sources for applications in condensed-matter physics. These sources are a powerful tool for studying collective excitations in solid-state systems: THz light can directly couple to low-energy excitations on the meV-scale such as the collective excitations of spins and phonons. Furthermore, coherent excitation of the material spin or phonon subsystem by a THz light pulse allows for tailoring the material’s macroscopic properties. This enables the creation of materials with new dynamic functionalities. To fully exploit the potential of the control of materials’ properties, a new generation of versatile sources of intense short-pulse THz light is needed. This thesis addresses the principles of generation of intense single-cycle THz pulses in an accelerator-based light source. The overarching principle is the phase-locked coherent emission of frequency-chirped waveforms from a specially prepared train of electron bunches inside a tapered undulator. The first part of the thesis (Ch. 1-2) motivates the THz light source development. It surveys the available light sources and scientific applications in the field of low-energy electrodynamics. Looking at a wide selection of THz-induced phenomena, the desired parameters of the proposed undulator-based THz source are determined. The second part (Ch. 3-4) focuses on the technicalities of the accelerator-based THz light source. It addresses the questions of electron beam requirements, photocathode gun performance, dynamics of electrons in an RF gun and in a superconducting linear accelerator. The beam dynamics simulations are carried out and the required characteristics of the electron bunch train are demonstrated. In what follows, the process of waveform-controlled single-cycle emission from an undulator is described: starting from the case of a single electron bunch and then proceeding to the single-cycle emission by the electron bunch train. The last part (Ch. 5) introduces a concept of tunable focusing of THz light. Specifically, it presents the model of an optical magnetic lens, based on a two-dimensional magneto-optical material immersed into a non-uniform magnetic field. To sum up, the formation of the electron bunch train with necessary spatiotemporal properties, single-cycle emission in a matching tapered undulator and tunable focusing of THz light are addressed in the thesis.

Emittance self-compensation in blow-out mode

We report an unusual regime of emittance self-compensation in an electron bunch generated in the blow-out mode by a radio-frequency photocathode gun. This regime is observed for a strong space-charge field on the cathode reaching around 30%-35% of the accelerating field. Simulations clearly show an initial growth and a subsequent self-compensation of projected emittance in a divergent electron bunch originating from the effects of: (a) strong space-charge forces of mirror charges on the cathode, (b) an energy chirp in the bunch and (c) substantial re-shaping of the electron bunch. Furthermore, we show analytically and numerically how a complex interplay between these effects leads to emittance self-compensation at the gun exit-the effect that is normally observed only in the presence of focusing fields

Nanometre-scale emittance beams from a continuous-wave RF gun

The operation of Ultrafast Electron Diffractometers (UEDs) and Free-Electron Lasers (FELs) relies on high-brightness electron beams produced by radio-frequency (RF) photocathode guns. The next generation of high-repetition rate UEDs and FELs requires electron beams with a high average brightness. To this end, we introduce a continuous wave RF photocathode gun at 325 MHz with an APEX-like geometry. The gun allows for the production of electron beams with very high both peak and average 5D brightness while having moderate RF power consumption. The gun is operated in blowout regime with an energy gain of 0.4 MeV and a peak cathode field of 35 MV/m. Via massive numerical simulations, we exemplify three regimes of the gun operation: (i) 160 fC electron beams with a 5-nm-scale emittance for UEDs, (ii) 1.6 pC beams with a 20-nm-scale emittance for table-top FELs and dielectric-based accelerators, and (iii) 16 pC beams with a 50-nm-scale emittance for inverse Compton sources and other accelerator-based photon sources. We introduce a simple analytical model for the formation of the virtual cathode - the onset of the suppression of photoemission current due to space-charge forces. The model accounts for the laser pulse duration. Furthermore, our extensive numerical simulations indicate a well-pronounced maximum in the 5D beam brightness for the laser spot radius approximately 150% of that corresponding to the onset of the virtual cathode. The finding does not support the common approach in the literature that in the blowout regime the laser spot radius must be much larger than the critical radius corresponding to the virtual cathode onset

Optical magnetic lens : towards actively tunable terahertz optics

As we read this text, our eyes dynamically adjust the focal length to keep the image in focus on the retina. Similarly, in many optics applications the focal length must be dynamically tunable. In the quest for compactness and tunability, flat lenses based on metasurfaces were introduced. However, their dynamic tunability is still limited because their functionality mostly relies upon fixed geometry. In contrast, we put forward an original concept of a tunable Optical Magnetic Lens (OML) that focuses photon beams using a subwavelength-thin layer of a magneto-optical material in a non-uniform magnetic field. We applied the OML concept to a wide range of materials and found out that the effect of OML is present in a broad frequency range from microwaves to visible light. For terahertz light, OML can allow 50% relative tunability of the focal length on the picosecond time scale, which is of practical interest for ultrafast shaping of electron beams in microscopy. The OML based on magneto-optical natural bulk and 2D materials may find broad use in technologies such as 3D optical microscopy and acceleration of charged particle beams by THz beams