Electron Micropulse Research Articles

Study of hot electron transport in foil, wedge, and cone targets irradiated with ultraintense laser pulses

We investigate hot electron transport in foil, wedge, and cone targets irradiated with ultraintense femtosecond laser pulses by observing the transition radiation emitted from the targets rear side. Two-dimensional images and spectra of coherent transition radiation along with x-ray detection have revealed the spatial, temporal, and temperature characteristics of hot electron micropulses. Various patterns from different target and laser configurations suggest that hot electrons were guided by the strong static electromagnetic fields at the target boundary.

Bunching phase evolution of short-pulse FEL oscillator system

We studied numerically the short-pulse FEL oscillator system using properly defined bunching phase θ B and Ψ B . In stable operation, we have found that the optical field “locks” the phase to π/2 at the trailing edge, which gives the maximum gain. Moreover, electrons can be detrapped from ponderomotive bucket due to the spatial variation of the optical field, and this detrapping effect is a major cause of the limit cycle oscillation of the system. The `bump’ of the output power during the amplification usually exists at the near-perfect cavity synchronism regime, which can be explained as the change of the matching condition between electron micropulse and optical pulse.

Simulation of radiation build-up in an FEL oscillator: effect of electron energy spread and micro pulse width

The characteristics of radiation build-up in an FEL oscillator and the effect of electron beam energy spread on the radiation build-up including its effect on electron micropulse width, radiation wavelength and slippage distance is investigated. By the numerical simulation, it is clarified that the effect of electron beam energy spread on radiation build-up for the case using the coherent start up oscillator scheme is smaller than that for the case of the normal FEL (long electron micropulse case), while the saturation power of radiation for the normal FEL is larger than that for the coherent start up FEL. So in order to make the best use of these characteristics, at first the short electron micropulses are driven to the wiggler to attain the fast radiation build-up in the oscillator, then long electron micropulse is driven causing bunching after a few interactions, then reaching saturation. This oscillation scheme may be effective in the millimeter to sub-millimeter regime of radiation wavelength.

Simulation of a waveguide FEL oscillator using an RF linac

Abstract A one-dimensional, multi-frequency simulation code has been developed for FELs which use a waveguide to confine the radiation field. Using this simulation code, we analyzed the spontaneous emission from electron micropulses produced by an RF linac. It is found that for some parameters both high and low frequency waveguide modes are growing simultaneously and the resulting spectrum thus contains two peaks. Finally, the experimentally obtained dependence of the radiation power on cavity length detuning is analyzed.

NEW PERFORMANCE OF RADIO FREQUENCY FAR INFRARED WAVEGUIDE FREE ELECTRON LASER

Idealizing the ultrashort pulse electron beam modulated by radio-frequency as a delta function on time scale, we calculate the coherent synchrotron radiation emitted by the electron micropulse in a rectangular waveguide with the method of eigenmode expansion. We find that when the frequency of radiation is the integral times of the radio frequency ωrf, the radiation field pattern will appear as "pure" eigenmodes of waveguide. In addition, we analyse the frequency performance of the coherent synchrotron radiation and the slippage effect. And finally, the gain of a rectangular electron pulse amplifying the radiation is deduced.

Simulataneous operation of a free-electron laser on two harmonically related wavelengths

The interaction of light at the fundamental and the third harmonic frequencies in a free-electron laser (FEL) oscillator is explored using the 1-D finite-pulse-mode code HFELP. The code, which assumes that only the TEM 00 transverse mode is present at both harmonic frequencies, tracks the temporally-finite pulse electric field amplitudes of the fundamental and the third harmonic as they interact with an rf-linac-generated electron micropulse inside a wiggler. The evolution of the pulse profiles is tracked over many passes through the wiggler and optical resonator, for various initial conditions including frequency-dependent mirror reflectivities. Results include pulse-dependent third-harmonic coherent-spontaneous emission (CSE) with, and without, multiple-pass interference effects; the effects of fundamental sidebands on third-harmonic CSE; and lasing competition between the fundamental and third harmonic in overlapping spatial regions of the electron micropulse.

Limit cycle behaviour in FELIX

The free electron laser for infrared experiments (FELIX) operates at wavelengths up to λ = 110 μm. A radio-frequency linear accelerator is used to produce electron micropulses with a duration of about 3 ps. With N = 38 undulator periods, this puts FELIX well into the regime where the slippage length, Nλ, exceeds the electron micropulse length, and prominent short pulse effects are expected. One of these effects, stable limit cycle oscillations of the pulse energy, has not been detected experimentally before. Such oscillations occur when the saturated optical pulses move away from the electron pulses, due to the changing balance between lethargy and desynchronism, while new subpulses grow periodically. In FELIX, limit cycle behaviour is clearly demonstrated. The observations are in agreement with numerical simulations of the pulse propagation, and the oscillation period is given by a simple formula containing the slippage length and the desynchronism between optical and electron pulses. We also show how lethargic behaviour can be used to reduce the optical bandwidth of the FEL and to store optical energy in the optical cavity without saturation limiting the energy stored.

INEX simulations of the optical performance of the AFEL

Abstract The AFEL (advanced free-electron laser) Project at Los Alamos National Laboratory is presently under construction. The project's goal is to produce a very high brightness electron beam which will be generated by a photocathode injector and a 20 MeV rf linac. Initial laser experiments will be performed with a 1 cm period permanent magnet wiggler which will generate intense optical radiation near a wavelength of 3.7 μm. Future experiments will operate with “slotted-tube” electromagnetic wigglers (formerly called “pulsed-wire” wigglers). Experiments at both fundamental and higher harmonic wavelengths are planned. This paper presents results of INEX (integrated numerical experiment) simulations of the optical performance of the AFEL. These simulations use the electron micropulse produced by the accelerator/beam transport code PARMELA in the 3-D FEL simulation code FELEX.

Numerical studies of the spectral evolution of a narrow-bandwidth FEL oscillator

Abstract The design study of an FEL oscillator with a bandwidth of Δω/ω 0 −5 has been carried out. In this conceptual design, a grating rhomb is used to suppress the sideband instability. The spectral evolution of the FEL has been simulated using a one-dimensional, time-dependent FEL code. The system parameters for the simulations are: electron beam energy 120 MeV, current I=50 A , electron micropulse length δt p =330 ps , optical wavelength λ s =0.5 μ m , and wiggler period λ w =2 cm . The detailed dependence of the spectral bandwidth on the frequency chirp introduced by the grating rhomb was investigated. A signal with FWHM bandwidth Δω/ω 0 =4×10 −6 was obtained in a simulation using a large chirp parameter τ s /δt p =80 , where τ s is the time lag of the sideband (with Δω/ω 0 =−0.01 ) behind the central frequency.

Simulation of an rf thermionic gun

An rf thermionic gun is simulated using Superfish and Parmela. A strong front-end compression for the bunch is demonstrated. The energy spread, phase spread and emittance of a single electron micropulse are examined subtly by cutting the bunch into slices corresponding to different initial emission phases of the particles.

Pulse compression on the Mark III FEL using energy chirping

We have performed preliminary simulations of the optical-pulse formation in the Mark III FEL using electron micropulses which exhibit a linear energy dependence on time, and have demonstrated optical pulses whose frequency chirps agree fairly well with those obtained analytically by assuming that the resonance condition determines the lasing wavelength during the pulse. In a typical case, we project pulse compression by a factor of 13.3, from an initial pulse width of 3.13 ps to a final pulse width of 236 fs, at a wavelength of 3.35 μm and an electron energy chirp of +2% (energy increasing towards the back of the pulse). This represents an optical pulse less than half as short as the slippage length of 47 magnet periods for this wavelength.

INEX simulations of the Los Alamos HIBAF free-electron laser MOPA experiment

We present results of Integrated Numerical Experiment (INEX) simulations of the performance of a 1 m untapered wiggler FEL oscillator driving a 2 m wiggler FEL amplifier for the new HIBAF (high-brightness accelerator free-electron laser) facility at Los Alamos. INEX simulations utilize a numerically generated electron micropulse, from ISIS/PARMELA calculations of the photoinjector/ linac/beam-transport system, in the 3D FEL simulation code FELEX.

Numerical simulations of free electron laser oscillators

A numerical simulation capability has been developed to model the physics and realistic design constraints of free electron laser oscillators driven by rf linear accelerators. Two computer codes have been written, FELEX and FELP. The code FELP is a one spatial dimension code with essentially unlimited time or spectral resolution. The codes are complementary and their use is dependent upon the problem being addressed. The code FELP is used to model optical and electron micropulse structure, broadband noise, and the sideband instability. The code FELEX models accelerator generated electron beam distributions, the transport of these distributions through wigglers with misalignments and field errors, self-consistent interaction with the optical field, and propagation of the optical field through resonators with realistically modelled components. FELEX is routinely used to match resonator designs to the optical parameters of the electron beam, and used to investigate the physics of 3-D micropulse effects. Some details of the codes will be presented along with various examples of simulation results.

The generation and suppression of synchrotron sidebands

Computer simulations of FEL lasing differ in the degree to which they approximate real experiments. One of the FEL codes used extensively at Los Alamos takes account of the features of each electron micropulse and follows the growth and saturation of the optical micropulse. With no additional adjustments, this code displays the development of sidebands and demonstrates their control when optical filters of various kinds are used. Other codes that do not include a description of the micropulse do not automatically display sidebands but need to have artificial noise of some kind added. This is not unexpected because sidebands are generated by an FEL instability; instabilities, in general, need some kind of initiating disturbance.In this paper we have 1.(1)|identified the disturbance that triggers the instability in the pulse code,2.(2)|discovered a practical way to suppress the instability without using filters,3.(3)|compared these results with experiments, and4.(4)|discussed these findings.

A prebunched free electron laser

The narrow width of an rf-accelerated electron micropulse can be exploited to yield gain significantly higher than in the conventional FEL. If the micropulses are much shorter than the wavelength of the generated wave and the electrons are resonant with the FEL beat wave, then a low order prebunched FEL interaction results. All electrons lose energy to the wave

Diagnostics of the Los Alamos free electron laser using streak systems

The applications of “streak systems” that provide time-resolved diagnostic data for the Los Alamos free electron laser energy recovery experiments have been extended in the last year. We have used these systems with time resolutions of 10 μs, ∼ 20 ps, and 2–8 ps to address both macropulse and micropulse issues. As one example, the time-dependent extraction efficiency behavior during the macropulse is presented. In addition, the effects on the electron micropulse temporal shape of several accelerator parameters have been studied. These results include the evidence of electron beam peak currents that approach 200 A.

The effects of linear-accelerator noise on the Los Alamos free-electron laser

The optical power produced by the Los Alamos free-electron laser has been flawed by a peristent instability. This instability appears as fluctuations in output power in a frequency band centered around 100 kHz. We have found that the fluctuations are dependent on the strength of the lasing and on variations in the electron micropulse's arrival time. When the lasing is strong, the fluctuations are small; when the lasing is weak, the fluctuations are large. The variations in the electron micropulse's arrival time primarily are due to fluctuations in the accelerator gradients. The primary noise sources in the accelerator are the electron gun and the klystron amplifier chain. In addition, this noise is uncontrolled because of a lack of bandwidth in the feedback controls for the RF system. Appropriate improvements are being made to eliminate these fluctuations in optical power.

Time-Resolved Electron Beam Diagnostics for the los Alamos Free-Electron Laser Oscillator Experiment

Measurement of the time-dependent electron beam energy distributions in the Los Alamos Free-Electron Laser (FEL) Oscillator Experiment will be accomplished by employing gated proximity-focussed microchannel-plate intensified vidicon cameras, an electron spectrometer employing a fluorescent target, and synchronized electron beam deflection techniques. General diagnostic system design features are described that provide information in the 30-ps, 50-ns, 10 ?s, and 100-?s time regimes. These techniques provide a real-time electron micropulse optimization diagnostic and an on-line, time resolved measurement of the energy lost by the electrons within the optical cavity.