Soft-x-ray coherent radiation using a single-cascade free-electron laser

Coherent harmonic generation using single-pass free-electron lasers is a promising method for generating coherent radiation in the vacuum ultraviolet and x-ray spectral region. We propose a simple scheme allowing one to generate powerful coherent radiation in the soft x-ray region by making use of present available technology. The method relies on the possibility of creating substantial bunching in a relativistic electron beam, while limiting the growth of its energy spread. The validity of the scheme is demonstrated using a simple one-dimensional model. Results are confirmed by three-dimensional simulations.     

Steady-state microbunching in a storage ring for generating coherent radiation

Synchrotrons and storage rings deliver radiation across the electromagnetic spectrum at high repetition rates, and free electron lasers produce radiation pulses with high peak brightness. However, at present few light sources can generate both high repetition rates and high brightness outside the optical range. We propose to create steady-state microbunching (SSMB) in a storage ring to produce coherent radiation at a high repetition rate or in continuous wave mode. In this Letter we describe a general mechanism for producing SSMB and give sample parameters for extreme ultraviolet lithography and submillimeter sources. We also describe a similar arrangement to produce two pulses with variable spacing for pump-probe experiments. With technological advances, SSMB could reach the soft x-ray range (<10 nm).     

Maximum entropy principle and coherent harmonic generation using a single-pass free-electron laser

Single-pass free-electron lasers constitute an example of systems with long-range interactions. The light-particle interplay leading to the power growth and successive relaxation towards a quasi-stationary state is governed by the Vlasov equation. A maximum entropy principle inspired to Lynden-Bell's theory of “violent relaxation" for the Vlasov equation can be invoked to analytically characterize the behaviour of the saturated system. In particular, we here concentrate on the case of coherent harmonic generation obtained from an externally seeded free-electron laser and provide a simple strategy to predict the laser intensity as well as the final electron-beam energy distribution.     

Autocorrelation measurements of free-electron laser radiation using a two-photon QWIP

The two-photon QWIP comprises three equidistant subbands, namely two bound states localized in the quantum well and an extended state in the continuum. This device is very promising for quadratic autocorrelation measurements of pulsed mid-infrared lasers due to its resonantly enhanced optical nonlinearity and sub-ps time resolution. We report on interferometric autocorrelation measurements of ps optical pulses from a free-electron laser (FEL). The intense FEL radiation further allows us to study the saturation properties of two-photon QWIPs at liquid nitrogen temperature and their detection properties at 300 K. The device is well suited for standard diagnostics of the FEL pulse shape via interferometric autocorrelation.     

Plasma-aided radiation guiding in a free-electron laser

The introduction of a plasma in a free-electron laser (FEL) helps radiation guiding via nonlinear refraction. At high-radiation power density, when oscillatory electron velocity is comparable to the electron thermal velocity, the radiation pushes plasma radially out, forming a depleted plasma duct and guiding the radiation. The radius of the self-trapped laser is ~c/ω_po, where ω_po is the unperturbed plasma frequency and c is the velocity of light in vacuum     

Laser seeding of the storage-ring microbunching instability for high-power coherent terahertz radiation

We report the first observation of laser seeding of the storage-ring microbunching instability. Above a threshold bunch current, the interaction of the beam and its radiation results in a coherent instability, observed as a series of stochastic bursts of coherent synchrotron radiation (CSR) at terahertz frequencies initiated by fluctuations in the beam density. We have observed that this effect can be seeded by imprinting an initial density modulation on the beam by means of laser "slicing." In such a situation, most of the bursts of CSR become synchronous with the pulses of the modulating laser and their average intensity scales exponentially with the current per bunch. We present detailed experimental observations of the seeding effect and a model of the phenomenon. This seeding mechanism also creates potential applications as a high-power source of CSR at terahertz frequencies.     

Doppler-free optical double resonance spectroscopy using a single-frequency laser and modulation sidebands

Principles and applications are described for a form of Doppler-free optical double resonance spectroscopy which uses amplitude modulation sidebands (v _L ±v) imposed on a single laser frequency (v _L ). The sidebands are generated by passing the carrier radiationv _L through an electro-optic modulator, driven at a radiofrequency ν, which enables the intensity and polarization characteristics of the emerging radiation to be varied for enhancement of selected double-resonance processes. The technique has been applied to infrared-infrared double-resonance studies of the Stark effects of a variety of molecules—¹³CH₃F,¹²CH₃F, PH₃,¹⁵NH₃, GeH₄, SiH₄, and CH₃D—for which physical results are presented and discussed. These results include determination of extremely small electric dipole moments (10⁻³–10⁻⁵ debye) for GeH₄ and CH₃D and, for the dipole moment of PH₃, a vibrational state dependence which is extremely small (Δμ=0.0028(5) debye for ∣Δv ₂∣=1) and a rotational state dependence which is of an unexpected sign. The spectra recorded in some cases display unusual polarization and optical saturation effects which deviate markedly from the predictions of a simple three-wave polarization theory.     

Fully coherent x-ray pulses from a regenerative-amplifier free-electron laser

We propose and analyze a regenerative-amplifier free-electron laser (FEL) to produce fully coherent, hard x-ray pulses. The method makes use of narrow-bandwidth Bragg crystals to form an x-ray feedback loop around a relatively short undulator. Self-amplified spontaneous emission (SASE) from the leading electron bunch in a bunch train is spectrally filtered by the Bragg reflectors and is brought back to the beginning of the undulator to interact repeatedly with subsequent bunches in the bunch train. The FEL interaction with these short bunches regeneratively amplifies the radiation intensity and broadens its spectrum, allowing for effective transmission of the x rays outside the crystal bandwidth. The spectral brightness of these x-ray pulses is about 2 to 3 orders of magnitude higher than that from a single-pass SASE FEL.     

Characterization of a chaotic optical field using a high-gain, self-amplified free-electron laser

We report on a characterization of the chaotic optical field from a high-gain, self-amplified spontaneous-emission (SASE) free-electron laser. The temporal structure of the amplitude and phase are measured in a single-shot mode, with a resolution well below the coherence length, and the statistics over multiple pulses is determined. The measurement is in excellent quantitative agreement with the prediction based on analysis of random noise, and further verifies the chaotic nature of the SASE optical field.     

Spatially coherent radiation from a coaxial free-electron laser with a resonator composed of one-dimensional and two-dimensional Bragg mirrors

It is shown that the application of a resonator composed of one-dimensional and two-dimensional coaxial Bragg mirrors provides a spatially coherent radiation from a hollow electron beam with a transverse size several orders of magnitude larger than the wavelength. The two-dimensional Bragg mirror placed at the cathodic end of the resonator synchronizes the radiation across the hollow electron beam. The standard one-dimensional Bragg mirror placed at the collector end closes the feedback loop and reduces ohmic losses to as low as 5–10% of the lasing power.     

Absorption spectroscopy using a coherent transition radiation millimeter wave light source

The characteristics and applicability of the absorption spectroscopy system using the coherent transition radiation light source have been investigated. The configurations of the system are relatively simple. The operational conditions of an electron linear accelerator have been evaluated. The stability of the light intensity has been within ±2–3% in a wavenumber range of 4–13 cm⁻¹. The light intensity has been found to be more than six orders of magnitude higher than that for the lowest limit for detection.     

Evidence for a percolation-driven transition to coherent surface superconductivity

Above the upper critical field we have investigated the field dependences of the surface conductance, G'-iG" and the critical current J(c) of an electropolished pure niobium cylinder. The low frequency limits of G', G", and J(c) display power-law singularities, defining a transition to coherent surface superconductivity at H(c)(c3). The critical exponents as well as the dynamical scaling of G'-iG" are consistent with predictions for a two-dimensional percolation transition. Relating H(c)(c3) to the conventional onset field, we find H(c)(c3)/H(c3)=0.81, and, surprisingly, this ratio turns out to be independent of significant variations of H(c3) due to differently structured surfaces.     

光学渡越辐射测量中能量分辨精度分析

基于光学渡越辐射原理的用于高能强流电子束束流参数在线测量及诊断系统,具有时间响应快、分辨率高等特点,可以测量电子束的束剖面、发散角、能量等多个参数。分析了测量系统的结构参数（包括了透镜的焦距、成像面位置、CCD像元尺寸）对电子束能量测量精度的影响,并在理论上模拟了电子束的发散角的影响。还根据系统数据的特点,阐述了数据噪声对能量测量结果精度的影响,指出了光学渡越辐射测量中电子束能量分辨精度受到多种因素的影响,需要在数据处理时考虑修正。     

光学渡越辐射测量中能量分辨精度分析

 基于光学渡越辐射原理的用于高能强流电子束束流参数在线测量及诊断系统,具有时间响应快、分辨率高等特点,可以测量电子束的束剖面、发散角、能量等多个参数。分析了测量系统的结构参数（包括了透镜的焦距、成像面位置、CCD像元尺寸）对电子束能量测量精度的影响,并在理论上模拟了电子束的发散角的影响。还根据系统数据的特点,阐述了数据噪声对能量测量结果精度的影响,指出了光学渡越辐射测量中电子束能量分辨精度受到多种因素的影响,需要在数据处理时考虑修正。