Gas-loaded Free-electron Laser Research Articles

Small-signal gain in a gas-loaded FEL

Small-signal gain in a gas-loaded FEL

Influence of multiple electron scattering on the gain in a gas-loaded free-electron laser.

We present an analysis of the small-signal gain in the gas-loaded free-electron laser. Multiple scattering of electrons by the atoms of the gas inside the optical cavity is shown to lead to two additional effects, as compared to the case of a vacuum free-electron laser: a loss of coherence between different parts of the electron trajectory and an enhancement of the phase ``jitter.'' Both effects become increasingly important at short wavelengths and significantly reduce the small-signal gain per pass. In one-dimensional approximation, analytical expressions are obtained and numerical calculations are made to estimate the beam and undulator parameters necessary for lasing in the vacuum ultraviolet.

Gain enhancement in a gas-loaded free electron laser

The radiation of electrons in a free electron laser (FEL) below the Cherenkov threshold is investigated. It is shown that the gas-loaded FEL gain is inversely proportional to the fifth power of the electron energy. The presence of the medium allows the generation of photons with a given frequency by lower energy electrons. Using a medium with a certain gas density longitudinal distribution, determined by the electron energy losses along the trajectory, one can provide the synchronism condition along the whole FEL. The gain increases along the trajectory. All these facts result in an essential growth of the intensity of the produced photons. If the technical difficulties can be overcome one can achieve almost complete energy transformation into radiation energy.

Free electron lasers using dielectric media with an internal resonant state

Gas-loaded free electron lasers using a dielectric gas of molecules with two energy levels are investigated theoreticly and numerically. In this case, the amplification of radiation arises in two wavenumber regions. One is the up-shifted ordinary FEL mode, and the other is in the vicinity of the resonance frequency corresponding to the energy difference between two levels of the gas. Furthermore, the latter modes are divided into two kinds, a slow beam mode and a fast beam mode. The fast beam mode grows faster than the slow beam mode in our parameter regime. To analyze the amplification and the saturation of such a gas-loaded FEL, we developed an FEL particle simulation code in the high-gain Compton regime. The results of the simulation code agree well with the results of the dispersion relation derived by the linear analysis, and saturation levels for three kinds of electromagnetic modes are also calculated.

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. >

Electron beam transport in gas-loaded free-electron lasers

The effects of the presence of helical wiggler and axial guide magnetic fields on the quality of the electron beam in a gas-loaded free-electron laser are investigated. The electron velocity space diffusion theory in the free-electron laser is developed and tested in Monte Carlo simulations. The theory is applied in estimating the collisional limitations on the interaction length of the laser. It is shown that two competing effects related to collisions cause the gain loss in gas-loaded free-electron lasers, i.e., (a) the growing phase mismatch between the electrons and the wave and (b) the destruction of the coherent transverse helical beam motion. The second effect dominates in the absence of the guide field, provided the wiggler field strength is sufficiently small.

A gas-loaded free-electron laser at 600 Å

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 Å with λ = 220, is proposed, and detailed design calculations are presented. Near the electronic resonance of helium, which occurs at λ = 584 Å, 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 Å.

An ultraviolet gas-loaded free-electron laser

By loading the wiggler cavity of a free-electron laser (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 Accelerator, 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% over this wavelength interval, comparable to the gain in vacuum. >

Gain in the gas-loaded FEL

In a gas-loaded free electron laser, multiple scattering causes considerable degradation in electron beam quality as the beam propagates through the gas-filled wiggler chamber. To determine the effect on small-signal gain, the conventional gain derivation is revised to include a non-uniform optical mode area, finite electron beam energy spread and finite beam emittance and current density, which both vary with propagation.The results 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 a asymmetic optical mode to enhance the electron-optical field interaction. This design allows substantial gain at wavelengths down to the ultraviolet.

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

The addition of gas to a free electron laser allows oscillation at shorter wavelengths with a given accelerator and wiggler. Stanford's 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.

Novel approaches to FEL operation: The gas-loaded FEL and a high efficiency FEL design

Two novel methods for improving free-electron laser (FEL) oscillator performance are discussed: (a) The gas-loaded FEL (GFEL) allows operation at snorter wavelengths for a given accelerator energy and wiggler. Experimental results of laser operation with a gas retention foil in the electron beam line and with the introduction of gas to the wiggler cavity are presented, (b) An FEL design utilizing a time-ramped microwave field to accelerate electrons as they lose energy to radiation allows for high conversion efficiencies. Parameter constraints for such an FEL are discussed, leading to a structure that integrates a wiggler with a linac. It is shown that conversion efficiencies of 50% at λ = 10 μm with a 2m wiggler length can be achieved for typical FEL parameter values without sacrificing small-signal gain

Propagation of a picosecond-duration, relativistic electron beam through hydrogen at atmospheric pressures

An experiment demonstrating the propagation of a 42-MeV electron beam from an rf linear accelerator through 1 m of H2 at pressures from 10−3 to 1.25 atm is reported. Measurements were made of the transmitted current and the beam’s radius, transverse position, and angular divergence along the path. The beam current was fully transmitted at all pressures, without the often detrimental plasma interactions (space-charge neutralization, magnetic neutralization, and various plasma instabilities) seen in previous studies. The observed beam expansion was consistent with calculations of multiple scattering. The propagation can be attributed to the 4-ps duration of the accelerator’s electron bunches. This time is three to four orders of magnitude shorter than that used in the earlier work and is shorter than the growth times for the beam-plasma interactions. Such a beam should prove suitable for a gas-loaded free-electron laser.

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

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.

Design of a gas-loaded free electron laser experiment

An infrared, vacuum FEL designed to operate at 2.6 μm will be modified to permit the introduction of hydrogen gas into the wiggler section. This gas will be separated from accelerator vacuum using a 9 mm diameter, 1 μm thick BN foil, and tuning will be obtained from 2.6 to 0.40 μm by the addition of up to 1 atm of gas. The Rayleigh range in the optical cavity will be reduced to compensate for the additional emittance produced by multiple scattering. Calculated small-signal gains with the gas are comparable to the gain at the longer wavelength in vacuum. A prior experiment demonstrated propagation of the 43 MeV electron beam through 1 m of H 2 at pressure of 10 −3 to 1.25 atm, with full current transmission and no plasma instabilities.

Picosecond beam propagation for gas-loaded free electron lasers

A gas-loaded FEL requires electron beam propagation through pressures of ≅ 1 atm. Interaction with the resulting plasma can lead to beam pinching, beam expansion, or various plasma instabilities. Here we discuss calculations and an experiment which demonstrate that the 4 ps electron bunches of a linear accelerator are short enough to avoid major disruption from these effects while traversing 1 m of H 2 at 1 atm.

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.