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All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

A free-electron laser in a uniform magnetic field

The study shows that a free-electron laser can operate in a uniform, longitudinal magnetic field. The fully relativistic Lorentz force equations are examined and solved order by order in a radiation field strength to obtain analytic expressions for the electron trajectory and energy as functions of initial electron position within a wavelength of light. Analytic expressions for the longitudinal and transverse bunching and for laser gain are found. The bunching of this laser process is compared to the bunching processes involved in (1) a Stanford free-electron laser and (2) a cyclotron maser. The results received can be useful in exploring light amplification in astrophysical magnetic fields, the magnetosphere, and in laboratory devices.

Ride, S. K.↗
NASA NTRS1981

The nonlinear wave equation for higher harmonics in free-electron lasers

The nonlinear wave equation and self-consistent pendulum equation are generalized to describe free-electron laser operation in higher harmonics; this can significantly extend their tunable range to shorter wavelengths. The dynamics of the laser field's amplitude and phase are explored for a wide range of parameters using families of normalized gain curves applicable to both the fundamental and harmonics. The electron phase-space displays the fundamental physics driving the wave, and this picture is used to distinguish between the effects of high gain and Coulomb forces.

Colson, W. B.↗
NASA NTRS1981

Equations of motion for a free-electron laser with an electromagnetic pump field and an axial electrostatic field

The equations of motion for a free-electron laser (FEL) with an electromagnetic pump field and a static axial electric field are derived using a Hamiltonian formalism. Equations governing the energy transfer between the electron beam and each of the electromagnetic fields are given, and the phase shift for each of the electromagnetic fields is derived from a linearized Maxwell wave equation. The relation between the static axial electric field and the resonant phase is given. Laser gain and the fraction of the electron energy converted to photon energy are determined using a simplified resonant particle model. These results are compared to those of a more exact particle simulation code.

Hiddleston, H. R.↗
NASA NTRS1980

The free-electron laser - Maxwell's equations driven by single-particle currents

It is shown that if single particle currents are coupled to Maxwell's equations, the resulting set of self-consistent nonlinear equations describes the evolution of the electron beam and the amplitude and phase of the free-electron-laser field. The formulation is based on the slowly varying amplitude and phase approximation, and the distinction between microscopic and macroscopic scales, which distinguishes the microscopic bunching from the macroscopic pulse propagation. The capabilities of this new theoretical approach become apparent when its predictions for the ultrashort pulse free-electron laser are compared to experimental data; the optical pulse evolution, determined simply and accurately, agrees well with observations.

Colson, W. B.↗
NASA NTRS1982

Free-electron laser from wave-mechanical beats of 2 electron beams

It is possible, though technically difficult, to produce beams of free electrons that exhibit beats of a quantum mechanical nature. (1) the generation of electromagnetic radiation, e.g., light, based on the fact that the beats give rise to alternating charge and current densities; and a frequency shifter, based on the fact that a beam with beats constitutes a moving grating. When such a grating is exposed to external radiation of suitable frequency and direction, the reflected rediation will be shifted in frequency, since the grating is moving. A twofold increase of the frequency is readily attainable. It is shown that it is impossible to generate radiation, because the alternating electromagnetic fields that accompany the beats cannot reform themselves into freely propagating waves. The frequency shifter is useless as a practical device, because its reflectance is extremely low for realizable beams.

Lichtenstein, R. M.↗
NASA NTRS1982

Optical pulse evolution in the Stanford free-electron laser and in a tapered wiggler

The Stanford free electron laser (FEL) oscillator is driven by a series of electron pulses from a high-quality superconducting linear accelerator (LINAC). The electrons pass through a transverse and nearly periodic magnetic field, a 'wiggler', to oscillate and amplify a superimposed optical pulse. The rebounding optical pulse must be closely synchronized with the succession of electron pulses from the accelerator, and can take on a range of structures depending on the precise degree of synchronism. Small adjustments in desynchronism can make the optical pulse either much shorter or longer than the electron pulse, and can cause significant subpulse structure. The oscillator start-up from low level incoherent fields is discussed. The effects of desynchronism on coherent pulse propagation are presented and compared with recent Stanford experiments. The same pulse propagation effects are studied for a magnet design with a tapered wavelength in which electrons are trapped in the ponderomotive potential.

Colson, W. B.↗
NASA NTRS1982

Axial expansion of the electron pulse in a free-electron laser

The necessity of including longitudinal electrostatic fields in free electron laser theory is investigated. A simple model is proposed which examines the axial expansion of a free electron beam of finite length, and criteria are obtained for including macroscopic fields. It is shown that longitudinal Coulomb forces do not pose a problem in current free electron laser experiments. Transverse space-charge spreading is neglected, and negligible emittance is assumed.

Speziale, T.↗
NASA NTRS1982

The effect of space charge fields due to finite length electron beams in the free-electron laser

The space charge electric field of a finite length electron beam in the free electron laser amplifier with a tapered wiggler is analyzed. In the free drift region between the accelerator and laser, expressions for the increase of energy spread due to the self field are presented. In the FEL interaction region, the general conditions on the importance of the self electric field in the equations of motion is obtained. A numerical example of the FEL experiment at 10.6 microns is given.

Tang, C.-M.↗
NASA NTRS1982

Gain-enhanced free-electron laser with an electromagnetic pump field

The feasibility of enhancing the gain for the free electron laser (FEL) with an electromagnetic (EM) pump field has been considered. The enhancement is provided by reacceleration of the electrons in the interaction region by means of a static, axial electric field. A FEL utilizing a low energy electron beam and a wiggler magnet with a periodicity on the order of 1 cm would produce far infared (FIR) radiation with wavelengths on the order of a few hundred microns. The use of the FIR radiation as the pump field in a two-stage FEL is envisioned to obtain visible radiation with a low energy electron beam. A summary is provided regarding the theory and equations of motion for the EM-pumped FEL. The derived relations are applied to the second stage of such a two-stage FEL. The obtained equations have been incorporated into a computer code which has been used to calculate laser gain and energy conversion efficiency.

Hiddleston, H. R.↗
NASA NTRS1984

Orbital stability in combined uniform axial and three-dimensional wiggler magnetic fields for free-electron lasers

Zachary Phys. Rev. A 29 (6), 3224 (1984) recently analyzed the instability of relativistic-electron helical trajectories in combined uniform axial and helical wiggler magnetic fields when the radial variation of the wiggler field is taken into account. It is shown here that the type 2 instability comprised of secular terms growing linearly in time, identified by Zachary and earlier by Diament Phys. Rev. A 23 (5), 2537 (1981), is an artifact of simple perturbation theory. A multiple-time-scale perturbation analysis reveals a nonsecular evolution on a slower time scale which accommodates an arbitrary initial perturbation. It is shown that, in the absence of exponential instability, the electron seeks a modified helical orbit more appropriate to its perturbed state and oscillates stably about it. Thus, the perturbed motion is oscillatory but nonsecular, and hence the helical orbits are stable.

Johnston, S.↗
NASA NTRS1987

Free-electron laser simulations on the MPP

Free electron lasers (FELs) are of interest because they provide high power, high efficiency, and broad tunability. FEL simulations can make efficient use of computers of the Massively Parallel Processor (MPP) class because most of the processing consists of applying a simple equation to a set of identical particles. A test version of the KMS Fusion FEL simulation, which resides mainly in the MPPs host computer and only partially in the MPP, has run successfully.

Vonlaven, Scott A.↗
NASA NTRS1989

An ultraviolet gas-loaded free-electron laser

By loading the wiggler cavity of an FEL with hydrogen gas, it is possible to tune the FEL to below 130 nm, just above the electronic resonance of molecular hydrogen. The change in wavelength results from the refractive index of the gas, which alters the phase velocity of the wave and so modifies the synchronism condition. Using the parameters of the wiggler on the beamline of the Stanford Superconducting Accelerataor, this FEL can be tuned from 500 to 130 nm with a pressure ranging from 0 to 55 torr. The calculated electronic gain varies between 5.6 and 11 percent over this wavelength interval, comparable to the gain in vacuum.

Reid, Max B.↗
NASA NTRS1989

Effects of introducing a gas into the free-electron laser

The introduction of a gas into the wiggler section of a free electron laser (FEL) alters the phase velocity of the electromagnetic wave, and so changes the synchronism condition relating wavelength to wiggler parameters and beam energy. This provides a means for tuning the frequency of an oscillator, with the addition of 200 torr of hydrogen gas, the wavelength of a FEL operating in the near infrared without gas was reduced by 0.73 microns. The plasma generated from ionization of the hydrogen molecules by collisions with the electron beam diminished the oscillator gain, but this effect was eliminated by the addition of less than 0.1 percent of an electron attachment gas. Gain is also reduced by multiple scattering of the beam electrons, but this effect is not severe for a 1-m wiggler length. When hydrogen is used, a FEL with fixed wiggler parameters and electron energy can be tuned from the near infrared to about 1200 A, and, with helium, the wavelength can be reduced to 600 A.

Pantell, R. H.↗
NASA NTRS1979

Field resonance propulsion concept

A propulsion concept was developed based on a proposed resonance between coherent, pulsed electromagnetic wave forms, and gravitational wave forms (or space-time metrics). Using this concept a spacecraft propulsion system potentially capable of galactic and intergalactic travel without prohibitive travel times was designed. The propulsion system utilizes recent research associated with magnetic field line merging, hydromagnetic wave effects, free-electron lasers, laser generation of megagauss fields, and special structural and containment metals. The research required to determine potential, field resonance characteristics and to evaluate various aspects of the spacecraft propulsion design is described.

Holt, A. C.↗
NASA NTRS1980

Hydromagnetics and future propulsion systems

Futuristic hydromagnetic propulsion systems for spacecraft are examined with emphasis on systems that use regular coherent patterns of magnetic and electric fields of very high strength to interact with the structure of space-time to effect a quick translation from one space-time point to another. A particular type of this system is discussed: namely, the field resonance propulsion concept which will utilize superconducting magnets and a configuration of tunable free-electron lasers.

Holt, A. C.↗
NASA NTRS1984

End effects of a bifilar magnetic wiggler

The magnetic field at the entrance of a wiggler magnet in a free-electron laser (FEL) determines the character of the electron orbits and influences the FEL gain. To obtain the desired adiabatic entrance conditions, it is necessary to investigate different methods of connecting leads to the wiggler as well as methods of gradually reducing the wiggler field. The field of a current-driven, finite-length bifilar helical wiggler magnet is derived and is used to investigate the magnetic field end perturbations. For a limited range of wiggler parameters, flaring the wiggler end produces an adiabatic entrance region. Alternatively, gradually terminating the wiggler current at the wiggler entrance is shown to produce an adiabatic entrance for a large range of parameters.

Fajans, J.↗
NASA NTRS1985

Molybdenum-silicon multilayer mirrors for the extreme ultraviolet

Multilayer structures of molybdenum and silicon have been synthesized by sputter deposition onto flat silicon single-crystal silicon substrates and spherically ground (0.5and 22.0-m radii) fused silica substrates; and the reflectivities for 170.4-A (72.8-eV), 160.1-A (77.4-eV), and 228-A (54.4-eV) light measured at near normal incidence. Observed peak values ranged from 26.2 to 78 percent, the highest reflectivities occurring closest to normal incidence. Energy resolutions were about 10 in all cases. Model calculations were performed using optical constants and experimentally determined multilayer structural parameters. In all cases the measured reflectivities were equal to or larger (by up to a factor of 2) than the calculated values, a result attributed to uncertainty in the optical constants used in the calculations. Experimental and calculated angular-peak positions and energy resolutions were in good agreement. The high reflectivities of these molybdenum-silicon structures will make possible application of traditional optics approaches in the EUV and support new developments including free-electron lasers.

Barbee, T. W., Jr.↗