FEL Radiation Research Articles

THEORETICAL ANALYSIS OF AN RAMAN FREE ELECTRON LASER EXPERIMENT WITHOUT AXIAL MAGNETIC FIELD

In this paper, the self-focusing processes and ability of helical wiggler, which is often used in Raman Free Electron Laser experiment,, are analyzed. And the conditions for steady beam transportation are given and compared with the Raman FEL experiment beam data. In addition, the theoretical analysis of EPA-74 FEL radiation is presented, which is in agreement with the experimental results.

Investigation of mechanism of free electron laser through spontaneous radiation spectrum

In this paper, the mechanism of free electron laser with a guide magnetic field is investigated by analyzing spontaneous radiation spectrum of electrons. The physical relation between spontaneous radiation and stimulated radiation in FEL with a guide magnetic field is studied. It is found that this relation in FEL is similar to that in quantum lasers. The spontaneous radiation spectrum is discrete. The frequency of stimulated radiation in FEL is oneselected and amplified in the radiation spectrum.

Theory of short-wavelength lasing from channeled projectiles: Nondegenerate dipole transitions

PHYSICAL REVIEW A VOLUME 35, NUMBER 8 APRIL 15, 1987 Theory of short-wavelength lasing from channeled projectiles: Nondegenerate dipole transitions G. Kurizki Chemical Physics Department, Weizmann Institute of Science, Rehovot 76100, Israel M. Strauss and J. Oreg The Negev Nuclear Research Center, Beer Sheva 84190, Israel N. Rostoker Physics Department, of California, Irvine, California 9271 7 (Received 28 August 1986) University A theory is developed for lasing action from ensembles of relativistically or subrelativistically propagating emitters whose motion is bound in the direction(s) transverse to the direction of propa- gation. These include relativistic electrons and positrons channeled in crystals or other hollow- channel structures, as well as fast ions wherein a bound electron is perturbed by the crystal potential or laser light. Apart from planar-channeled positrons in crystals, the confining potentials for all other emitters in this category are strongly anharmonic. Therefore, their spectral dipolar transitions are nondegenerate, each involving a different pair of nearly discrete levels of the confining potential. This implies that stimulated emission from such systems can exhibit coherence in the Glauber sense. The theoretical framework presented here consists of Heisenberg equations which have the Maxwell-Bloch form with modifications resulting from the high velocity of the emitters. Steady- state semiclassical solutions of these equations are obtained. It is shown that previous approaches, based on the assumption that the cross section for stimulated emission is uniform throughout the system, do not account for the spatial variation of the polarization at high velocities. As a result, these approaches do not yield the correct gain coefficient whenever the characteristic lengths for the dephasing of the dipole oscillation and for emission amplification are comparable. The latter condi- tions are realizable in structures composed of channels much wider than in crystals. Lasing schemes are investigated and the prospects for achieving gain in these schemes at wavelengths below 100 A are discussed. I. INTRODUCTION (vacuum uv Many types of stimulated short-wavelength and x-ray) radiation sources that have been suggested over the years employ beams of high-velocity (relativistic or subrelativistic) emitters. The appeal of such sources lies in the fact that the emission is continuously Doppler shifted towards shorter wavelengths and becomes progres- sively more collimated about the beam direction as the beam velocity is increased. Such sources may be grouped into two categories. (a) Free electron lasers (FE-L's), encompassing structures wherein a spatially periodic perturbation acts either on the electron beam (as in magnetic undulators, ' light and crystals serving as coherent bremsstrah- wigglers, or on the emitted radiation (due to lung amplifiers) periodic changes in the refraction index, e.g. , in superlat- devices tices acting as transition-radiation ). The emis- sion features in FEL's are determined by transfer of discrete momentum (wiggler quanta or reciprocal lattice vectors) to the structure from the emitter or the field. Generally, many quantum states of the emitting electron are involved in the dynamics, which is governed by equa- As a re- tions of the Raman-Nath or Mathieu type. ' FEL radiation is, in principle, not sult, short-wavelength non- coherent (in the Glauber sense), ' exhibiting Poissonian photon statistics. In the quantum regime of operation, quantum recoil (which is responsible for gain) hampers coherence and, during the start-up stage, many- particle effects lead to thermal statistics. ' In the classical steady-state regime, too, small deviations from coherence are predicted. ' (TCE's), i. e. , emitters confinement (b) Transverse emitters whose motion is bound in the direction(s) trans- verse to the direction of propagation. These include rela- tivistic electrons and positrons channeled in crystals' or other hollow structures, ' as well as fast ions wherein a bound electron is perturbed by the crystal potential' or laser light. The emission wavelengths are determined by the nearly discrete spectrum of their transverse-energy states and by the Doppler shift due to their longitudinal motion. Planar channeled positrons in crystals are the only TOE's whose nearly harmonic transverse confining potential requires the inclusion of many states in the dynamical description. All other emitters in this category are confined by strongly anharmonic potentials and there- fore their dynamics involves directly only the upper and lower nearly discrete states of a particular spectral (nonde- generate) transition. Hence, these systems can, in princi- ple, exhibit true lasing (coherent stimulated emission). The American Physical Society

The Hughes low-voltage free electron laser program

The Hughes Free Electron Laser Program is based upon a previous low voltage oscillator experiment in which a 225-kV electron beam was used to generate 60 kW of 30-GHz radiation in a test model FEL [1]. FEL kinetics, beam propagation and the 4.6% gain require a pulsewidth greater than 2 μs with a pulse flatness greater than 1% for oscillation in the test model FEL to occur. Propagation of the beam through the wiggler is a significant problem. A special “square” wiggler was designed to accomodate beam confinement in both the x and the y direction. The low gain of the system required that a high Q cavity must also be present for oscillation to occur. To this end a distributed-Bragg-reflector resonator was installed, which had a Q greater than 10 000. The results of these tests and previous experiments have led to the design of a second generation low voltage FEL which will be driven at 400 kV and operate between 60 and 100 Ghz. This FEL will be designed to facilitate operation with a second stage 6-MeV electron beam at UCSB to generate 7-μm radiation from a two-stage, two-beam FEL.

Applications of a tunable VUV laser in atomic and molecular physics

The vacuum ultraviolet wavelength region from, say, 200 to 50 nm, is of basic interest and importance for many areas of physics and chemistry. Experimental work in this region has gained a dramatic increase by the synchrotron radiation. In recent years, the region is rapidly also becoming accessible to tunable lasers. In fact, the whole range down to about 100 nm can now in principle be covered by tunable, narrow-band, pulsed laser radiation of relatively high intensity, and projections are that soon the range will be further extended to ∼50 nm. FEL radiation would mean, of course, another breakthrough in this spectral region with the potential of a wide spectrum of experimental applications. In this paper we wish to indicate the state-of-art of tunable laser radiation in the vacuum ultraviolet by discussing some cases of applications in various parts of physics and chemistry. Examples will be given where tunable VUV laser light is employed both for state selective pumping and probing of atomic and molecular elementary processes, for instance, photoionization, photodissociation and chemical reactions.