M. Özcan

Effects of introducing a gas into the free-electron laser

Many interesting applications of the free-electron laser (FEL) require the extension of the operating wavelength into the ultraviolet region of the spectrum. The introduction of a gas into the wiggler section of a 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, and 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 μm. 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% 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 ≈1200 A, and with helium the wavelength can be reduced to 600 A.

Observations of gain and pressure tuning in a gas-loaded FEL

Abstract The addition of gas to a free electron laser allows oscillation at shorter wavelengths with a given accelerator and wiggler. Stanfords Mark III near-infrared (vacuum) FEL has been modified to permit operation as a gas-loaded FEL (GFEL). By adding up to 100 Torr of H2, the emission wavelength was tuned over a range of 0.4 μm, in agreement with the GFEL synchronism condition incorporating the index of refraction of the gas. The variation of gain with pressure was consistent with calculations including multiple small-angle scattering of the electron beam by the gas. Extension to higher pressures and shorter wavelengths is discussed.

Gas-loaded free-electron laser experiments on the Stanford superconducting accelerator

The modification of the free-electron laser (FEL) on the Stanford superconducting accelerator to operate as a gas-loaded FEL (GFEL) is described. The addition of a gas to the wiggler chamber of a FEL changes the phase velocity of the electromagnetic wave, and so provides a simple method for wavelength tuning without changing beam energy or wiggler parameters. A wavelength shift of over 950 AA has been achieved from a vacuum wavelength of 1.6 mu m by using 15 torr of hydrogen gas, in agreement with the GFEL synchronism condition incorporating the index of refraction of the gas. >

Free-electron laser oscillator in a symmetrical, confocal resonator

A tapered wiggler is used in a free-electron laser (FEL) oscillator to improve the saturation efficiency. During signal buildup the tapered wiggler does not provide optimum phase synchronism between the electron beam and the electromagnetic wave, resulting in an appreciable loss in small-signal gain. If the taper is too large, the decrease in gain during buildup may preclude the onset of oscillation. This problem can be ameliorated with a multicomponent wiggler, which is a combination of a uniform wiggler and a tapered section. During buildup gain is primarily contributed by the linear element, and at high power levels the gain and efficiency are enhanced by the taper. Ideally, one would like to have a uniform wiggler at small-signal levels and then be able to substitute a taper at saturation. Placing the FEL in a symmetrical confocal resonator approaches this desired effect automatically.

High efficiency free-electron laser operation in a symmetrical confocal resonator

Abstract A tapered wiggler is used in a free-electron laser (FEL) oscillator to improve the saturation efficiency. During signal buildup the tapered wiggler does not provide optimum phase synchronism between the electron beam and the electromagnetic wave, resulting in an appreciable loss in small-signal gain. This problem can be ameliorated by using a multicomponent wiggler, which is a combination of a uniform wiggler and a tapered section. During buildup, gain is primarily contributed by the linear element, and at high power levels the gain and efficiency are enhanced by the taper. Ideally, one would like to have the optical waist location near the linear section at small-signal levels and near the tapered section at high power levels. Placing the FEL in a symmetrical confocal resonator approaches this desired effect automatically since it has the unique characteristic that a stable mode exists for all locations of the waist of a Gaussian beam along the axis of the interferometer.

Gas-loaded free-electron lasers

Gas‐loaded free‐electron lasers offer the prospect of broad wavelength tuning and short wavelength operation without the need for high electron beam energy. Experiments that were performed using hydrogen gas in the infrared confirm the theory, and new experiments are described for the extreme ultraviolet (XUV) region using helium gas.

A gas-loaded free-electron laser at 600 Å

Abstract The addition of a gas to an FEL wiggler changes the phase velocity of the electromagnetic wave, and so provides a fairly simple way of wavelength tuning. This effect has been demonstrated by reducing the wavelength of an infrared FEL by 0.73 μm with the introduction of 200 Torr of hydrogen gas. In this paper, a helium-loaded FEL experiment, which is to operate at λ = 600 A with λ = 220, is proposed, and detailed design calculations are presented. Near the electronic resonance of helium, which occurs at λ = 584 A , the index of refraction becomes large, so it is possible to use low gas pressures. This is advantageous because at lower pressures there is less degradation of the beam quality due to multiple scattering. Using typical electron beam parameters, in 115 cm of wiggler length, the power in the wave is increased by a factor of ∼ 9 at λ = 600 A .