J. Feinstein

Coherent emission and gain from a bunched electron beam

The radiation from a modulated electron beam, such as that produced in a radio-frequency accelerator, passing through a magnetic undulator is analyzed. The authors show that in a waveguide free electron laser (FEL), this may lead to an emission of a significant amount of coherent radiation in the far infrared to millimeter wave range. A simple and powerful method of calculating the spectral distribution of the radiated power is presented along with an analysis of the gain and the saturated power. The experimental results of spontaneous emission measurements on an RF driven FEL, are presented and compared to theoretical predictions. >

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

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.

Characteristics of the thick, compound refractive lens

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.

The Effect of Unit Lens Alignment and Surface Roughness on X-ray Compound Lens Performance

The required alignment tolerances and surface roughness for unit lens elements in a compound refractive lens (CRL) for x rays are discussed. Contrary to what one might expect and what has been stated in the patent literature, alignment tolerances are large and for typical parameter values the effect of misalignment is minor. For a parabolic lens the focusing properties of the CRL are unaltered by misalignment and there is a small increase in absorption. For a lens with spherical aberration, there is a slight change in focal length, a minor translation of the image, and a small increase in absorption. This article also shows that lens gain is not appreciably reduced if the phase shift that is introduced by the roughness is limited to ±π/4 or if the transverse period of the roughness exceeds a specified value. The CRL can benefit from a managed misalignment of the elements to reduce the phase error introduced by surface imperfections of the lens.

Interferometer mirrors with holes on-axis

Abstract Placing holes on-axis in the mirrors of an optical interferometer used for a free-electron laser (FEL) oscillator has several desirable effects. For a given amount of power generated at saturation, the peak power density on the mirrors is decreased, thereby ameliorating the problem of mirror damage; the electron beam can be introduced into the cavity in a collinear fashion; and the small-signal gain can be increased. Surprisingly, these holes may be quite large without a significant increase in cavity loss. This is a consequence of the fact that higher-order modes are generated which sum to a null of the field at the position of the holes.

Performance characterization of a far-infrared, staggered wiggler

Abstract 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.

Compact far-IR FEL design

Abstract 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.

The free-electron laser as a laboratory instrument

A free-electron laser (FEL) with a component cost, including the accelerator, of approximately

Gain in spatially varying optical fields: Applications to high emittance beams and gas dielectric FEL's

300k, has lased at a wavelength of 85 /spl mu/m with /spl ap/12 ps micropulse duration, achieving a power growth four orders of magnitude greater than the coherent spontaneous emission, and with a small-signal, single-pass gain of 21%. The price is about an order of magnitude less than other FELs for the far infrared, and transforms the device from the role of a national facility to that of a laboratory instrument. Cost reduction was achieved by employing several novel features: a microwave cavity gun for the accelerator, a staggered-array wiggler, and an on-axis hole in the upstream cavity mirror for electron ingress and radiation egress. >

Benefits and costs of the gas-loaded, free electron laser

The conventional free-electron laser small-signal linear gain equation is revised to include the effects of nonuniform optical mode area, finite electron beam emittance, and electron beam energy spread. It is shown for a typical case that the conventional equation overestimates gain by more than 50 percent. The results further show that gain can result from FEL designs with interaction lengths longer than the conventional maximum given by the criterion of accumulated phase shift equal to π. This makes possible a gas-loaded FEL which utilizes a long wiggler and an asymmetric mode to enhance the electron-optical field interaction. Although substantial emittance is induced by scattering, the gas-loaded FEL should have modest gain at wavelengths down to the ultraviolet.