摘要:

The free-electron laser (FEL) is a device that is capable of high peak and average power, with broad tunability, and with a time structure that can be as short as one picosecond. In the far infrared (λ = 100 − 1000μm) portion of the spectrum the energy of the electron beam is not too high, there are few alternative bright sources, and there are a variety of interesting applications. Thus, the far infrared (FIR) is a likely region for the development of a table-top FEL that would be a useful laboratory instrument. In this paper the design of a FIR FEL is presented, and its performance characteristics are evaluated.

ISSN:1042-0150    

DOI:10.1080/10420159108211493

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Using the Weisskopf-Wigner technique, a self consistent quantum electrodynamic (SCQED) theory of spontaneous emission of radiation and single photon small signal gain is developed for high voltage free electron lasers (FEL). Excellent agreement is obtained simultaneously to our knowledge for the first time between the predictions and the experimental observations for lineshift, linewidth and gain. The SCQED theory predicts lineshift and broadening due to quantum mechanical effects for linear, helical and tapered undulator FELs which are not predicted by the classical/conventional FEL theories, but which have been observed4,5,18,22,23,45,46. Excellent agreement is obtained between the SCQED theory predicted spontaneous emission spectra and the 1980–81 ACO FEL4,18, ACO Optical Klystron FEL45,46, Stanford 10.6 μm FEL22and Stanford 3.4 μm FEL23experimental spectra. This agreement is much better than the prediction from the classical/conventional FEL theory which gives errors of many tens of percent. We show that the spontaneous emission spectrum obtained from classical/conventional FEL theories is valid only in the limit of a short undulator containing a small number of periods. The small signal gain derived from the SCQED theory is shown to reduce to Colson's gain formula12,34in the classical limit. However, the SCQED theory predicts significant reductions in the small signal gain which agree well with the ACO gain data5, and are not predicted well by Colson's formula. Due to the non-neglible finite electron state lifetime, it is discovered that a fundamental physical gain limit exists which is universal to all types of FELs within the limits of the single photon transition scheme considered (i.e. if multiphoton effects are ignored). Finally, the implications of the theoretically obtained results are discussed for practical conditions of experimental interest. It is shown that under practical experimental conditions quantum effects can be quite important in the FEL.

ISSN:1042-0150    

DOI:10.1080/10420159108211494

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The behavior of electromagnetic processes in strong magnetic fields is currently of great interest in high-energy astrophysics. Observations of neutron stars indicate that magnetic fields larger than 1012Gauss exist in nature. In fields this strong, where electrons behave much as if they were in bound atomic states, familiar processes undergo profound changes and exotic processes become important. Strong magnetic fields affect the physics in several fundamental ways: energies prependicular to the field are quantized, transverse momentum is not conserved and electron/positron spin is important. The relaxation of transverse mometum conservation allows first order processes and their inverses: one-photon pair production and annihilation, synchrotron/cyclotron radiation and absorption, which are kinematically forbidden under field-free conditions. The first two are essentially quantum-mechanical and hence significant only in fields whose strength approaches the critical field,Bcr= 4.414 × 1013Gauss. One-photon pair production is likely to be the dominant source ofe+-e−pairs in fields exceeding 1012Gauss. While synchrotron radiation and absorption are observable as classical electromagnetic processes in weak fields, they are considerably different in high fields, where the classical synchrotron radiation formulae can violate conservation of energy, and predict too large an emissivity and electron energy loss rate. The second-order processes: two-photon pair production and annihilation and Compton Scattering, are also modified in strong fields. The discreteness ofe+- e−pair states causes resonant behavior in the cross sections and decreases the second-order rates from their free-space values. These processes play an important role in modelling high energy emission from pulsars and gamma-ray bursts.

ISSN:1042-0150    

DOI:10.1080/10420159108211495

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

ISSN:1042-0150    

DOI:10.1080/10420159108211496

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

ISSN:1042-0150    

DOI:10.1080/10420159108211497

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

ISSN:1042-0150    

DOI:10.1080/10420159108211498

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The photodetachment of negative ions in a static electric field exhibits some new characteristic features and has beer considered in various theortical approaches.1Most of them, however, neglect the short-range interaction between the escaping electron and the atomic core, and must be modified to describe various resonant effects. Experiments2have shown very rich resonant structure in a dc-field, which can be attributed to the mixing of different excited states in the negative ion, to competition between elastic and inelastic decay channels, and to tunneling effects induced by the field. It is known that various resonant structures in Photoprocesses can be successfully described within standard multichannel quantum defect theory (MQDT). We present a modified MQDT frame transformation approach to extend the standard method to long-range potentials with nonspherical symmetry. In our treatment both the electron-field and electron-atom interactions are treated nonperturbatively and on an equal footing. The resulting theoretical calculations are compared with experimental data on field-modified H−photodetachment in the vicinity of then= 2 resonances.

ISSN:1042-0150    

DOI:10.1080/10420159108211499

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The definition and characteristics of the strong-field environment for an atom in a laser field are specified in terms of the relevant intensity parameters. The limits of perturbation theory are set, and it is emphasized that this must be done in terms of laser field energy, not electric field strength. The formal basis and special features of the SFA (strong-field approximation) are reviewed, and it is pointed out that the three methods encompassed in the so-called KFR (Keldysh-Faisal-Reiss) technique are actually quite different. Validity conditions and some applications of the SFA are given.

ISSN:1042-0150    

DOI:10.1080/10420159108211500

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

We extend our model for two-color above threshold ionization to include an infinite number of continua, which are labeled by their angular momenta. Results are presented for the photoelectron spectra and the angular photoelectron distributions.

ISSN:1042-0150    

DOI:10.1080/10420159108211501

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

We have calculated the high-order ac Stark Shift, multiphoton ionization rates, and nonlinear susceptibilities for high harmonic generation for the hydrogen atom in a radiation field. The calculations are done in the framework of Rayleigh-Schrödinger perturbation theory applied to a complex-rotated Hamiltonian. Our intention is to investigate the limitations of perturbation theory in calculations of multiphoton processes. Comparisons are made with results from nonperturbative calculations. For some frequencies the results of lowest-order perturbation theory are found to disagree with nonperturbative calculations even at moderate to low intensities (I ∼ 1011W cm−2) and in the absence of resonances. We find that the high-order perturbation expansion theory is not a reliable predictor of the behavior of hydrogen atoms in radiation fields with intensities greater than ∼ 1012W cm−2.

ISSN:1042-0150    

DOI:10.1080/10420159108211502

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor