Tailored terahertz pulses from a laser-modulated electron beam

We present a new method to generate steady and tunable, coherent, broadband terahertz radiation from a relativistic electron beam modulated by a femtosecond laser. We have demonstrated this in the electron storage ring at the Advanced Light Source. Interaction of an electron beam with a femtosecond laser pulse copropagating through a wiggler modulates the electron energies within a short slice of the electron bunch with about the same duration of the laser pulse. The bunch develops a longitudinal density perturbation due to the dispersion of electron trajectories, and the resulting hole emits short pulses of temporally and spatially coherent terahertz pulses synchronized to the laser. We present measurements of the intensity and spectra of these pulses. This technique allows tremendous flexibility in shaping the terahertz pulse by appropriate modulation of the laser pulse.     

A model describing stable coherent synchrotron radiation in storage rings

We present a model describing high power stable broadband coherent synchrotron radiation (CSR) in the terahertz frequency region in an electron storage ring. The model includes distortion of bunch shape from the synchrotron radiation (SR), which enhances higher frequency coherent emission, and limits to stable emission due to an instability excited by the SR wakefield. It gives a quantitative explanation of several features of the recent observations of CSR at the BESSY II storage ring. We also use this model to optimize the performance of a source for stable CSR emission.     

Observation of frequency-locked coherent terahertz Smith-Purcell radiation

We report the observation of enhanced coherent Smith-Purcell radiation (SPR) at terahertz (THz) frequencies from a train of picosecond bunches of 15 MeV electrons passing above a grating. SPR is more intense than other sources, such as transition radiation, by a factor of Ng, the number of grating periods. For electron bunches that are short compared with the radiation wavelength, coherent emission occurs, enhanced by a factor of Ne, the number of electrons in the bunch. The electron beam consists of a train of Nb bunches, giving an energy density spectrum restricted to harmonics of the 17 GHz bunch train frequency, with an increased energy density at these frequencies by a factor of Nb. We report the first observation of SPR displaying all three of these enhancements, NgNeNb. This powerful SPR THz radiation can be detected with a high signal to noise ratio by a heterodyne receiver.     

Observation of broadband self-amplified spontaneous coherent terahertz synchrotron radiation in a storage ring

Bursts of coherent synchrotron radiation at far-infrared and millimeter wavelengths have been observed at several storage rings. A microbunching instability has been proposed as the source for the bursts. However, the microbunching mechanism has yet to be elucidated. We provide the first evidence that the bursts are due to a microbunching instability driven by the emission of synchrotron radiation in the bunch. Observations made at the Advanced Light Source are consistent with the values predicted by the proposed microbunching model. These results demonstrate a new instability regime for high energy synchrotron radiation sources and could impact the design of future sources.     

High power THz source based on coherent radiation of picosecond relativistic electron bunch train

Tunable and compact high power terahertz (THz) radiation based on coherent radiation (CR) of the picosecond relativistic electron bunch train is under development at the Tsinghua accelerator lab. Coherent synchronization radiation (CSR) and coherent transition radiation (CTR) are researched based on an S-band compact electron linac, a bending magnet or a thin foil. The bunch train’s form factors, which are the key factor of THz radiation, are analyzed by the PARMELA simulation. The effects of electron bunch trains under different conditions, such as the bunch number, bunch charges, micro-pulses inter-distance, and accelerating gradient of the gun are investigated separately in this paper. The optimal radiated THz power and spectra should take these factors as a whole into account.     

基于相对论电子束的太赫兹源

太赫兹辐射在基础科学和产业应用中具有重要的应用前景,但传统的电子学和光学方法难以在1～10 THz产生相干的高功率、窄带且连续可调的太赫兹辐射。基于相对论性超短电子束和预调制电子束序列的加速器太赫兹源将能在上述范围内产生可调的高能谱强度窄带太赫兹辐射。综述了清华大学加速器实验室近年来在基于相对论电子束的加速器太赫兹源方面的理论和实验进展,以及与加速器太赫兹源一起发展起来的太赫兹辐射测量、束流诊断和先进加速技术。     

Coherent THz synchrotron radiation from a storage ring with high-frequency RF system

The generation of brilliant, stable, and broadband coherent synchrotron radiation (CSR) in electron storage rings depends strongly on ring rf system properties such as frequency and gap voltage. We have observed intense coherent radiation at frequencies approaching the THz regime produced by the MIT-Bates South Hall Ring, which employs a high-frequency S-band rf system. The measured CSR spectral intensity enhancement with 2 mA stored current was up to 10,000 times above background for wave numbers near 3 cm(-1). The measurements also uncovered strong beam instabilities that must be suppressed if such a very high rf frequency electron storage ring is to become a viable coherent THz source.     

Measurement of the time structure of a coherent synchrotron radiation burst in NewSUBARU

The time structure of a burst of coherent synchrotron radiation (CSR) emitted from a high-peak-current beam in the electron storage ring NewSUBARU has been investigated. The source of this CSR burst is fine time structure in a bunch produced by longitudinal beam instability. This burst is unstable but very easy to obtain, so it can be used for some experimental applications with appropriate averaging of data. With an averaging period of 10 ms, the fluctuation of the integrated power was about 10%.     

An intense terahertz radiation source at the Compact ERL

The Compact ERL is an energy recovery LINAC (ERL) test facility that is planned for KEK. The circumference of the recirculation path will be 70 m. Initially, the beam energy will be about 65 MeV and the current about 10 mA. Although the primary purpose of the machine is to aid the development of the key technologies that are essential for building an ultra-brilliant new synchrotron light source based on an ERL, the Compact ERL itself has great potential as an intense source of terahertz radiation. To generate the intense terahertz radiation, an electron bunch of a very short bunch length is required and bunch compression is inevitable. We discuss the parameters of the Compact ERL, present the results of a simulation of bunch compression, and make an estimate of the generated coherent synchrotron radiation.     

CIRCE: a dedicated storage ring for coherent THz synchrotron radiation

We present the concepts for an electron storage ring dedicated to and optimized for the production of stable coherent synchrotron radiation (CSR) over the far-infrared terahertz wavelength range from 200 μm to about 1 cm. CIRCE (Coherent InfraRed CEnter) will be a 66 m circumference ring located on top of the ALS booster synchrotron shielding tunnel and using the existing ALS injector. This location provides enough floor space for both the CIRCE ring, its required shielding, and numerous beamlines. We briefly outline a model for CSR emission in which a static bunch distortion induced by the synchrotron radiation field is used to significantly extend the stable CSR emission towards higher frequencies. This model has been verified with experimental CSR results. We present the calculated CIRCE photon flux where a gain of 6–9 orders of magnitude is shown compared to existing far-IR sources. Additionally, the particular design of the dipole vacuum chamber has been optimized to allow an excellent transmission of these far-infrared wavelengths. We believe that the CIRCE source can be constructed for a modest cost.     

磁压缩器中相干同步辐射对束团品质的影响

在进行束团压缩时,相干同步辐射导致束团能量再分配,这会引入一定类型的非线性,目前普遍认为该非线性会导致束团发射度的急剧增加.本文通过模拟计算发现该结论只在一定条件下成立,同时还发现相干同步辐射引入固定类型的非线性,在特定参数下,该非线性能在一定程度上抵消束团原有的非线性,最终可能对束团品质有改善作用.     

Bursts of coherent synchrotron radiation in electron storage rings: a dynamical model

Evidence of coherent synchrotron radiation has been reported recently at the electron storage rings of several light source facilities. The main features of the observations are (i) a radiation wavelength short compared to the nominal bunch length, and (ii) a coherent signal showing recurrent bursts of duration much shorter than the radiation damping time, but with spacing equal to a substantial fraction of the damping time. We present a model of beam longitudinal dynamics that reproduces these features.     

Simultaneous Amplification of Terahertz Difference Frequencies by an Injection-Seeded Semiconductor Laser Amplifier at 850 nm

Two-frequency operation of an 850 nm semiconductor optical amplifier was achieved by simultaneously injection seeding it with two diode lasers. The two frequencies could be independently amplified without strong interference when they were separated by more than 10 GHz, and the spectral purity was preserved by the amplification process. At frequency differences below 10 GHz, unbalanced two-frequency output was observed, which can be explained by a two-mode interaction driven by the refractive index modulation at the beat frequency. The laser system is suitable for the difference-frequency generation of coherent terahertz radiation in ultra-fast photoconductors or nonlinear optical media.     

A Compact-Intracavity Frequency-Doubling Nd:YAG Laser with 1.3 W Dual-Wavelength (659.6 nm/669.4 nm) Light Output

Terahertz wave radiation has important applications in medical instrumentation, optical communication, and so on. In this paper, we demonstrate a dual-wavelength (659.6 nm/669.4 nm) light output of a compact intracavity frequency-doubling Nd:YAG laser. We achieve a maximum output power of 1.3 W at central wavelengths of 659.6 and 669.4 nm with linewidths of 57 and 74 pm, respectively. The power instability at the maximum output power is less than 1%, and the beam quality is 1.5. This dual-wavelength laser provides a potential source for generating a coherent terahertz wave radiation of 6.46 THz by the nonlinear optical difference frequency method.     

IKNO,a user facility for coherent terahertz and UV synchrotron radiation

IKNO (Innovation and KNOwledge) is a proposal for a multi‐user facility based on an electron storage ring optimized for the generation of coherent synchrotron radiation (CSR) in the terahertz frequency range, and of broadband incoherent synchrotron radiation ranging from the IR to the VUV. IKNO can be operated in an ultra‐stable CSR mode with photon flux in the terahertz frequency region up to nine orders of magnitude higher than in existing third‐generation light sources. Simultaneously to the CSR operation, broadband incoherent synchrotron radiation up to VUV frequencies is available at the beamline ports. The main characteristics of the IKNO storage and its performance in terms of CSR and incoherent synchrotron radiation are described in this paper. The proposed location for the infrastructure facility is Sardinia, Italy.     

Producing terahertz coherent synchrotron radiation at the Hefei Light Source

This paper theoretically proves that an electron storage ring can generate coherent radiation in the THz region using a quick kicker magnet and an AC sextupole magnet. When the vertical chromaticity is modulated by the AC sextupole magnet, the vertical beam collective motion excited by the kicker produces a wavy spatial structure after a number of longitudinal oscillation periods. The radiation spectral distribution was calculated from the wavy bunch parameters at the Hefei Light Source(HLS). When the electron energy is reduced to 400 Me V, extremely strong coherent synchrotron radiation(CSR) at 0.115 THz should be produced.     

Observation of fine structures in laser-driven electron beams using coherent transition radiation

We have measured the coherent optical transition radiation emitted by an electron beam from laser-plasma interaction. The measurement of the spectrum of the radiation reveals fine structures of the electron beam in the range 400-1000 nm. These structures are reproduced using an electron distribution from a 3D particle-in-cell simulation and are attributed to microbunching of the electron bunch due to its interaction with the laser field. When the radiator is placed closer to the interaction point, spectral oscillations have also been recorded, signature of the interference of the radiation produced by two electron bunches delayed by 74 fs. The second electron bunch duration is shown to be ultrashort to match the intensity level of the radiation. Whereas transition radiation was used at longer wavelengths in order to estimate the electron bunch length, this study focuses on the ultrashort structures of the electron beam.     

Coherent transition radiation from thin targets irradiated by high intensity laser pulses

A theoretical examination on coherent transition radiations (CTR) from the surface of thin solid density target irradiated by high intensity laser is presented. The theory is extended to consider the expansion dynamics of thin foils. The motion of target surfaces leads to the modulation on the temporal structure of micro bunches in the electron beam as well as the spectrum of CTR. The spectral shifts of radiation are owing to the enhancement of electron bunch separation and the relativistic Doppler effects. The radiation power distribution is strongly affected by the temporal coherence of electron beam structure, so thus the electron temperature and velocity dispersions. With these effects accounted for, the spectral properties of coherent transition radiation can provide insights into the expansion of thin foil targets irradiated by intense laser pulse as well as the fast electron transport through it.     

Measurement of bunch length evolution in electron beam macropulse of S-band linac using coherent edge radiation

Intense coherent edge radiation (CER) with a power of 0.10 mW was observed in a straight line with an undulator of Kyoto University Free Electron Laser (KU-FEL). To investigate the evolution of a bunch length in a macropulse of an electron beam, a technique was developed using the CER beam. The measured air-extracted CER beam profile has a hollow structure resembling the first-order Laguerre–Gaussian mode with asymmetric intensity in horizontal direction. It roughly agrees with the profile calculated considering the effective area of a deflection mirror employed to extract the beam from the FEL optical cavity into air. The root-mean-squared (RMS) bunch length determined from a measured spectrum of the CER beam is 60 μm for an electron-beam macropulse. Changes in the CER intensity in the electron-beam macropulse were measured by diode detectors with short time constants at two frequencies. The evolution of the RMS bunch length was determined using the ratio of the CER intensity at the two frequencies. It could be concluded that the evolution correlated with the macropulse structure of the FEL power. Therefore, measuring the evolution of the CER intensity at multiple frequencies proved useful to control FEL macropulses.     

Electron bunch interaction with a monochromatic electromagnetic wave

A technique of modulation of the charge density in a bunch when it interacts with a linearly polarized laser beam is developed. Solution of the problem is based on the numerical method for determination of interaction time of different electrons in the bunch flying through the interaction region, with use of an integral of motion of an electron in the monochromatic electromagnetic field. It is shown that in the terahertz frequency range, at high values of the field strength, sufficiently harddriving modulation of the charge density in the bunch may be obtained. High efficiency of the bunch charge density modulation is demonstrated in plots for different electrodynamical parameters.