Waveguide Free-electron Laser Research Articles

Solving the spectral gap problem with a four-mirror bow-tie cavity in waveguide free-electron laser oscillators

The generation of intense coherent radiation pulses in the far-infrared and terahertz regimes is of considerable interest to the free-electron laser (FEL) radiation user community. At long wavelengths, the diffraction effect can be quite severe, therefore, an optical waveguide is required to confine the radiation field. However, it will also bring about some new phenomena, and the most noteworthy one is the spectral gap phenomenon: at some particular wavelengths, regardless of electron beam adjustments, the coupling efficiency and output power of waveguide FEL oscillators drop significantly. Such spectral gap has an adverse effect on experimental results since numerous experiments require continuous spectral scanning. In this paper, we propose to utilize a bow-tie cavity instead of conventional cavities to the waveguide FEL to solve the spectral gap problem. The simulation was carried out based on the parameters of FELiChEM, a newly built user facility in China. Numerical simulation code OPC combining with modified GENESIS is used to enable the modelling, for the first time, of a bow-tie cavity based FEL in the far-infrared wavelength regime. The simulation results indicate that this novel structure can effectively eliminate the spectral gaps and substantially enhance long-wavelength laser performance.

Theoretical study of a waveguide THz free electron laser and comparisons with simulations

This paper fills a gap in the waveguide free electron laser theory by considering transverse distribution and energy spread of the electron beam.

Laser mode complexity analysis in infrared waveguide free-electron lasers

We analyze an optical phenomenon taking place in waveguide free-electron lasers, which disturbs, or forbids, operation in far infrared range. Waveguides in the optical cavity are used in far-infrared and THz ranges in order to avoid diffraction optical losses, and a hole coupling on output mirror is used for laser extraction. We show that, when the length of the waveguide exceeds a given limit, a phenomenon of ``mode disorder'' appears in the cavity, which makes the laser difficult, or impossible, to work properly. This phenomenon is even more important when the waveguide covers the whole length of the cavity. A numerical simulation describes this effect, which creates discontinuities of the laser power in the spectral domain. We show an example with an existing infrared Free-Electron Laser, which exhibits such discontinuities of the power, and where no convincing explanation was proposed until now.

Effect of bunch periodicity on coherent spontaneous emission in a waveguide free-electron laser

Investigated is coherent spontaneous emission of periodic electron bunches in a closed waveguide free-electron laser when the bunch period is equal to the round-trip time of the radiation pulse. In addition to the main resonance, auxiliary resonances due to the bunch periodicity are observed.

Mode and gain analysis for symmetric and staggered grating-waveguide free-electron laser

A grating-waveguide free-electron laser is driven by an axial electron beam that propagates between two planar gratings. This investigation addresses the mode analysis, gain calculation, and frequency tuning for such a device with arbitrary longitudinal and transverse displacements between the two gratings. It demonstrates that a grating waveguide free-electron laser is a promising coherent THz emitter when it is adopted as a backward-wave oscillator or a distributed-feedback oscillator.

Effect of dispersion on the operation of free-electron lasers driven by short electron bunches

The effect of dispersion on the lasing of waveguide free-electron lasers driven by short periodic electron bunches is considered. In the high-Q cavity approximation, the generation of electromagnetic pulses is described in terms of a parabolic equation. For the linear stage of interaction, starting lasing conditions are found analytically and the spatio-temporal structure of supermodes, which represent a set of phase-locked eigenmodes of the cold cavity, is determined. Dispersion is shown to allow the free-electron laser to operate at both positive and negative mismatches between the period of electron bunch injection and the time taken for the electromagnetic pulse to circulate over the cavity. The simulation of the nonlinear lasing conditions with allowance for dispersion spread makes it possible to find the stationary profile of radiated pulses and the optimum values of the group and time detunings that provide a maximum efficiency of the waveguide free-electron laser. It is also shown that, when the length of interaction is much longer than the starting value, the laser operates under the conditions of periodic or chaotic pulse profile self-modulation.

Effect of betatron oscillations on waveguide free electron laser gain

In this paper we analyse the spontaneous emission and gain spectrum of a waveguide free electron laser with betatron motion included in the analysis. The zero slippage condition and the small signal gain in the waveguide free electron laser is discussed.

Analysis of angular gain of a waveguide free-electron laser

In this paper we analyse the spontaneous emission and gain spectrum of a waveguide free-electron laser with an imperfect injection angle. As a consequence it is seen that both odd and even harmonics are emitted into TE modes. The zero-slip condition and the small signal gain in the waveguide free-electron laser is studied and presented for a comparison with a waveguide laser with perfect injection.

A millimetre-wave second harmonic waveguide FEL

Second harmonic waveguide free electron lasers (FELs) have been shown to be capable of both effective electron energy reduction and large interaction gain in the X-band region. In this paper, we consider the extension of this unconventional waveguide FEL scheme into the millimetre-wave region. Through assessment of their resonant conditions, small-signal gain, energy spread effects, and amplification saturation, second harmonic waveguide FELs are studied for their operation around 300 GHz and compared to their operation in the X-band. Against the same set of system parameters, further comparisons are also made to comparable conventional waveguide FELs. These studies suggest that second harmonic waveguide FELs are as effective at 300 GHz as at 8 GHz as a generic technique to reduce the necessary electron energy for a strong FEL radiation at a given frequency.

Nonlinear amplification in a second-harmonic waveguide free-electron laser

This paper describes the results of numerical simulation of second-harmonic waveguide free-electron lasers (FELs) from the small-signal regime to the large-signal regime. Aimed at reducing the size and hence the cost of compact waveguide FELs operated from the microwave to the far infrared, these unconventional waveguide FELs can substantially decrease the minimum electron energy required for strong FEL radiation at a given frequency while increasing the small-signal gain. This contribution focuses on their saturation behaviors, taking into consideration variation in wiggler field and electron-energy spread. Depending on the roundtrip power loss within the FEL cavity and the initial electron-energy spread, the computed relationship between interaction gain and in-cavity power can be used to maximize the output power at a given electron current. Furthermore, it is found that gain degradation due to electron-energy spread remains relatively unchanged regardless of radiation power and wiggler field.

Energy spread effects in second harmonic waveguide FELs

It has been recently established that second harmonic waveguide free electron lasers (FELs) can significantly reduce electron energy required for strong laser amplification at a given frequency. Compared to conventional waveguide FELs, their interaction gain has a stronger dependence upon the wiggler field and as such it is preferable to operate them with large wiggler field to achieve high interaction gain. Given that the energy dependence of the electron velocity and hence the phase of the ponderomotive potential is likely to be amplified at large wiggler field, it is possible that the high gain anticipated with large wiggler field may be compromised with more severe energy spread effects. To this end, we use a recently developed numerical code for waveguide FELs to study energy spread effects in second harmonic waveguide FELs. Numerical examples are used to assess both the small and large wiggler cases. It is shown that the energy spread induced gain degradation remains relatively unchanged as the wiggler field increases. Instead the bandwidth of positive interaction is found as a dominant factor to control the gain degradation. Comparison with conventional waveguide FELs is also presented to reinforce the above finding. Therefore the wiggler field strength can be treated as a free design parameter and used to achieve large interaction gain without contributing to the energy spread induced gain degradation.

Strong wiggler field assisted amplification in a second-harmonic waveguide free electron laser

As a technique to reduce the size of compact waveguide free electron lasers (FELs) operated from microwave to the far infrared, a longitudinal interaction mechanism was recently proposed to operate waveguide FELs at the second harmonic. With a gain formulation based on Madey's theorem in the limit of small wiggler field, it was shown analytically that second harmonic waveguide FELs can reduce significantly the electron energy required for radiation at a given frequency. As it is advantageous to operate second harmonic waveguide FELs with strong wiggler field, Madey's theorem is used here to reformulate their interaction gain for strong wiggler fields up to a/sub /spl omega///sup 2///spl gamma//sub 0//sup 2//spl beta//sub z0//sup 2/=1with the axial electron velocity Taylor expanded to the eighth order of the wiggler field. Given that Madey's theorem has not been established for second harmonic waveguide FELs, their interaction gain is also formulated independently by solving their pendulum equation without recourse to Madey's theorem. These two gain formulas are not analytically identical, but numerically they lead to an excellent agreement over a wide range of system parameters, thus confirming the applicability of Madey's theorem. The interaction analyses presented form a thorough and detailed description of second harmonic waveguide FELs in the small-signal regime and for wiggler field that is both practical and beneficial.

Gain enhancement in a two-frequency high-gain waveguide free-electron laser

In a waveguide monomode free-electron laser (FEL), two resonant frequencies can be amplified by the electron beam. At the CEA/CESTA facility, single-pass high-gain FEL experiments have been performed over the last five years using relativistic electron beams provided by induction linacs. Most of the work was done in the amplifier regime (at the higher frequency) with the aim of producing a 35 GHz bunched beam. However, super-radiant measurements were also made and have shown that the FEL gain at the upper frequency is higher than in the amplifier regime and may be driven by the lower frequency FEL interaction.

Electron bunching at large wiggler field in a second harmonic waveguide free electron laser

Second harmonic waveguide free electron lasers (FELs) have recently been considered as an electron energy reduction technique for compact waveguide FELs operated from microwave to the far-infrared. As their gain mechanism is based on a strong coupling between the oscillatory component of the axial electron velocity and the axial electric field of TM modes in a cylindrical waveguide, second harmonic waveguide FELs are preferably operated with large wiggler field to obtain sufficiently high interaction gain. Under these high wiggler field conditions, the electron trajectory becomes a highly nonlinear and complex function of the wiggler field and as such electron bunching may be significantly altered from previously approximated analytically. In this paper, we employ a recently developed computer code for compact waveguide FELs to compare to our previously obtained analytical gain formulation in the small-signal regime. By modifying our computer code to mimic the mathematical treatment of our analytical analysis, we demonstrate that detailed features of the axial electron velocity, excluded in our analytical analysis, result in an enhanced electron bunching and thus an increased gain. Simple compensation measures are then suggested such that the original analytical gain formulation can still be used as a simple and reasonably accurate design tool for second harmonic waveguide FELs.

Improved optical cavity quality of a waveguide free-electron laser with hole coupling by using a grating

A method is proposed to improve the quality of a hole- coupled resonator for a free-electron laser (FEL) oscillator. By using a mesh or grating with a moderate reflectivity to cover the hole, the optical cavity Q can be effectively improved, as is confirmed by three-dimension numerical simulations. The potentiality of the mesh as an electron- transparent mirror to make the FEL device more compact and less expensive is discussed.

Low-voltage interaction of electrons with TM modes in waveguide free electron lasers

One important issue in the development of compact waveguide free electron lasers (FELs) is to reduce the necessary electron energy for radiation at a given frequency. Recently, an electron energy reduction technique was suggested by exploiting a second harmonic coupling between the longitudinal electron velocity and the longitudinal electric field of TM modes of the waveguide electromagnetic wave. It was shown that these second harmonic waveguide FELs have a greater small signal gain than comparable conventional waveguide FELs. In this paper, the gain dependence on wiggler field in second harmonic waveguide FELs is studied numerically to assess how high-gain operation may implicate their bandwidth and saturation power. It is shown that a large small signal gain can be achieved very efficiently by increasing the wiggler field without sacrificing bandwidth while it does lead to a low saturation power. Nevertheless, the rate of gain increment is significantly greater than that of power reduction.

Effects of non-resonant electron velocity components in a second harmonic waveguide free electron laser

As a technique to reduce the size and hence the cost of compact waveguide free electron lasers (FELs) operated in the spectrum from microwave to the far infrared, a longitudinal interaction mechanism was recently proposed to operate waveguide free electron lasers at the second harmonic. Unlike conventional waveguide FELs based on the electron interaction with the transverse electric field component of the waveguide electromagnetic wave, these second harmonic waveguide FELs have electron velocity components that are not in resonance with the waveguide electromagnetic wave. These non-resonant components were considered to be negligible. In this contribution, we consider the effects of these non-resonant velocity components on the interaction gain. With a new gain formulation in the small signal regime, it is shown that the non-resonant velocity components affect the interaction gain significantly. Energy spread effects are then considered and compared to that in conventional waveguide free electron lasers.

Saturation behaviour of a waveguide optical klystron in the presence of electron energy spread

A waveguide optical klystron configuration has recently been suggested to enhance the small signal gain of compact waveguide free electron lasers (FELs) driven by low current electron beams. Based on an analytical formulation, it has been demonstrated that the small signal gain of waveguide free electron lasers can be increased significantly when incorporated into the waveguide optical klystron configuration. However, this enhanced small signal gain may be realised at the expense of achievable saturation power when the device is operated as an oscillator. In this study, a computer package was developed to investigate numerically saturation behaviours of waveguide optical klystrons taking into account the energy spread effects. It is shown that when the small signal gain of a waveguide FEL is only marginally greater than the round-trip loss of the radiation power, the waveguide optical klystron arrangement can provide not only a much larger small signal gain but also a greater saturation power.

Mode-selective amplification in a waveguide free electron laser with a two-sectioned wiggler

One important issue in waveguide free electron lasers (FELs) involves an interaction of the electron beam with one waveguide mode at two different resonant frequencies. Since the low-frequency mode often has a higher pain, the usually preferred high-frequency mode is suppressed as a result of mode competition. In this paper, possible control of this mode competition is considered using a nonstandard wiggler magnet consisting of two cascaded wiggler sections with different periods and field strengths. It is demonstrated that with an appropriate differentiation between the two wiggler sections the high-frequency mode may be amplified preferentially. This mode-selective amplification may be used to suppress the low-frequency mode. A small signal gain formulation is developed for a waveguide FEL with such a two-sectioned wiggler arrangement and numerical examples are used to demonstrate its applicability to mode control in waveguide FELs. Effects of wiggler field errors and electron energy spread are also considered. It is shown that the requirement for wiggler field errors and electron energy spread in the two-sectioned wiggler arrangement is similar to that in the usual straight wiggler configuration.

A low voltage waveguide free electron laser based on a longitudinal interaction mechanism

Low current waveguide free electron lasers (FELs) operating in the spectrum from microwave to the infrared have a distinct advantage of small size and low cost, important for realistic applications in medicine and industry. One of the long-term objectives in the development of these compact FELs is to reduce the necessary electron beam voltage for a strong FEL interaction at a given frequency. In this paper, we consider a longitudinal interaction between the longitudinal component of the electron beam's undulating motion and the longitudinal electric field of TM modes of the waveguide electromagnetic wave. A small signal analysis is presented to study the longitudinal interaction mechanism. It is shown that the electron beam voltage can be reduced considerably from what is needed in the conventional transverse interaction of the electron beam with TE modes at the same frequency. In a design example of an X-band FEL amplifier, this reduction is found to be more than half. Gain calculation suggests that with a reasonable wiggler field such a longitudinal interaction leads to a sizeable FEL amplification of TM modes.