Active behavior of the Cytoskeleton

Free‐electron laser (FEL) theory and experiments are reviewed. The physical mechanism responsible for the generation of coherent radiation in the FEL is described and the fundamental role of the ponderomotive wave in bunching and trapping the beam is emphasized. The relationship of the FEL interaction to the beam–plasma interaction is pointed out. Various FEL operating regimes are discussed. These include the high‐gain Compton and Raman regimes, both with and without an axial guiding magnetic field. The linear and nonlinear regimes are examined in detail, with particular emphasis on techniques for achieving efficiency enhancement. The quality of the electron beam used to drive FEL’s is a critical factor in determining their gain and efficiency. The subject of electron beam quality, for different accelerators, is discussed. Key proof‐of‐principle experiments for FELs in an axial guiding magnetic field, as well as those driven by induction linacs, rf linacs, electrostatic accelerators, and storage rings, are reviewed. Finally, the requirements on wigglers and resonators are discussed.

A review of free‐electron lasers

Acceleration of free electrons by the inverse \ifmmode \check{C}\else \v{C}\fi{}erenkov effect using radially polarized laser light focused through an axicon [J. P. Fontana and R. H. Pantell, J. Appl. Phys. 54, 4285 (1983)] has been studied utilizing a Monte Carlo computer simulation and further theoretical analysis. The model includes effects, such as scattering of the electrons by the gas, and diffraction and interference effects of the axicon laser beam, that were not included in the original analysis of Fontana and Pantell. Its accuracy is validated using available experimental data. The model results show that effective acceleration is possible even with the effects of scattering. Sample results are given. The analysis includes examining the issues of axicon focusing, phase errors, energy gain, phase slippage, focusing of the $e$ beam, and emittance growth.

Modeling of inverse Čerenkov laser acceleration with axicon laser-beam focusing

An experimental demonstration of a strong enhancement of the interaction efficiency in a high-power relativistic backward-wave oscillator when plasma is injected is presented. Controlled plasma injection enhances the interaction efficiency for the vacuum case by a factor of up to eight to a value of about 40%. A linear theory of electromagnetic wave generation in plasma-loaded corrugated wall resonators is reviewed. A number of physical mechanisms are considered to account for the enhanced interaction, including two variations of a three-wave interaction involving the electron-beam slow space-charge wave, the slow electromagnetic waves in the structure, and the quasi-electrostatic waves in the plasma. >

High-power microwave generation by excitation of a plasma-filled rippled boundary resonator

An experimental demonstration of a strong enhancement of the interaction efficiency in a high-power relativistic backward-wave oscillator when plasma is injected is presented. Controlled plasma in- jection enhances the interaction efficiency over the vacuum case by a factor of up to 8 to a value of about 40%. A linear theory of electro- magnetic wave generation in plasma-loaded corrugated wall resonators is reviewed. A number of physical mechanisms are considered to ac- count for the enhanced interaction, including two variations of a three- wave interaction involving the electron-heam slow space-charge wave, the slow electromagnetic waves in the structure, and the quasi-electro- static waves in the plasma.

Microwave Generation by Excitation of a Plasma-Filled Rippled Boundary Resonator

Gas loading of a free-electron laser modifies the phase-matching condition while the small-signal gain expression remains the same when written in appropriate form. This permits a wider parameter space than the vacuum FEL, which is particularly advantageous at shorter wavelength operation. Scattering of the electrons by the gas limits the interaction length, but available gains are still high enough to allow oscillation build-up. For example, a 0.5-μm wavelength helical wiggler FEL utilizing a 41 MeV electron beam is restricted to a length of 14 cm, has a small signal gain of 21 percent, and builds up to saturation in less than 1μs. Tunability of this device over several microns is easily obtained by changing the gas pressure.

Visible and ultraviolet radiation generation using a gas-loaded free-electron laser

Wavelength tuning of 0.4 ..mu..m has been obtained by the addition of 100 Torr of H/sub 2/ gas to a vacuum free-electron laser operating in the near infrared. This experiment demonstrates that a relativistic electron beam from an rf linac can propagate in a partially ionized gas with sufficient quality to achieve free-electron laser action. The picosecond pulse structure of the beam is believed responsible for the avoidance of plasma instabilities. Gain and wavelength measurements are consistent with theoretical expectations.

Experimental results on a gas-loaded, free-electron laser.

A compound refractive lens (CRL), consisting of a series of N closely spaced lens elements each of which contributes a small fraction of the total focusing, can be used to focus x rays or neutrons. The thickness of a CRL can be comparable to its focal length, whereupon a thick-lens analysis must be performed. In contrast with the conventional optical lens, where the ray inside the lens follows a straight line, the ray inside the CRL is continually changing direction because of the multiple refracting surfaces. Thus the matrix representation for the thick CRL is quite different from that for the thick optical lens. Principal planes can be defined such that the thick-lens matrix can be converted to that of a thin lens. For a thick lens the focal length is greater than for a thin lens with the same lens curvature, but this lengthening effect is less for the CRL than for the conventional optical lens.

Characteristics of the thick, compound refractive lens

The performance of a staggered-array wiggler for the far infrared free-electron laser (FIRFEL) project at Stanford has been characterized analytically and experimentally. A 10.8 kG peak wiggler field was measured for a 2.0 mm gap and a 1.0 cm wiggler period at a 7.0 kG solenoid field. The wiggler field uniformity was investigated by the pulsed-wire technique, and measurements showed a 1.2% rms field variation over a 50 cm wiggler section. With fairly simple design schemes, studies indicate the velocity drift in a staggered wiggler can be eliminated.

Performance characterization of a far-infrared, staggered wiggler

A compact size far-infrared free-electron laser (FIR FEL) is currently being built at Stanford. A microwave gun products 1–3.3 MeV electrons, which are sent into a 50 cm long wiggler of 1 cm period through a hole on the upstream mirror to generate radiation at a wavelength of 100 to 1000 μm. A superconducting solenoid along with an array of permeable material is used to generate a 9.6 kG rms wiggler field with a 2.0 mm gap. The electron beam consists of 10 ps micropulses with 10 A peak current, 1% energy spread and unnormalized emittance for 90% of the particles of 2π mm mrad. A 10 dB small signal gain has been calculated with the parameters mentioned above. An overview of the design details as well as a discussion on the uniqueness of our wiggler are presented.

Compact far-IR FEL design

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