Direct spectral measurements of a quasi-cw free-electron laser oscillator

Danly, B.G.; Evangelides, S.G.; Chu, T.S.; Temkin, Richard J.; Ramian, G.; Hu, James

doi:10.1103/physrevlett.65.2251

Cited by 19 publications

(14 citation statements)

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“…For optical FELs, the time required for the cavity fields to reach equilibrium is of order r Q/we 2 . This time may be quite long compared to the beam pulse lengths found in most FEL experiments [8], because the slippage is generally quite small.…”

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Single-mode operation of a Bragg free-electron maser oscillator

et al. 1994

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Detailed studies of the spectral characteristics and spatial mode structure Three other modes are also present in the theoretical spectrum of the resonator; modes at 27.1 GHz, 28.1 GHz, and 28.3 GHz. For the three lower frequency modes observed in the experiment, all three had starting currents lower than the beam current at the nominal operating beam energy of 310-320 keV. For this reason, mode competition is a significant issue for the present experiment.The overall experimental setup is illustrated by Fig. 1. Apart from the Bragg resonator, the other main components of the device are the high-voltage modulator, the thermionic electron gun, the solenoids which produce the axial magnetic field, and the helical permanent magnet wiggler. The system operates with a flattop pulse length 1-= 1 As. Beam compression to a radius of 4 mm was achieved with minimal scalloping, in good agreement with adiabatic theory. The beam axial energy spread, before injection into the wiggler interaction region, is inferred from experimental data to be Ayli/yli < 0.5 %. The beam is transported through the interaction region by an axial magnetic field of magnitude 2.35 kG. A permanent magnet helical wiggler with 3 cm period and 500 G amplitude is used to provide the 4 perpendicular momentum of the interacting electrons. To ensure stable high-quality group I helical orbits in the interaction region, the wiggler has a 10-period long linearly tapered introduction. The resulting equilibrium orbits at 320 keV beam energy have /.3 = 0.18,

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“…This paper reports on the successful single mode operation of a high power FEM oscillator. Mode competition results are discussed in the context of multi-mode oscillator theory [7] and previous experiments [8].…”

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Single-mode operation of a Bragg free-electron maser oscillator

et al. 1994

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“…We then apply the model to two different FEL devices-one at the University of California at Santa Barbara (UCSB) [1,2] and the other at the Dutch FOM-Institute for Plasma Physics [3]. Whether singlemode operation was truly realized within the lifetime of the electron pulse in the UCSB experiment has been a source of theoretical controversy in the past [4,5].…”

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confidence: 99%

“…These devices exhibit a strong tendency to evolve into a single-mode state [1][2][3]. The linewidths realized can be very small, making them ideal for demanding applications such as spectroscopy or isotope separation.…”

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Ginzburg-Landau Model and Single-Mode Operation of a Free-Electron Laser Oscillator

Bhattacharjee

1999

Phys. Rev. Lett.

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It is shown that the radiation field in a long-pulse, low-gain free-electron laser oscillator obeys the complex Ginzburg-Landau equation. The question of single-mode operation is investigated by analysis and simulation, and the results are compared with experiments at the University of California at Santa Barbara, as well as the Dutch fusion free-electron-maser experiment. It is shown that the intervention of a frequency-dependent reflection coefficient can facilitate the realization of single-mode operation.[S0031-9007(99)08757-8] PACS numbers: 41.60.Cr, 42.65.Tg, 52.35.Mw In a free-electron laser (FEL) oscillator, the intensity of the radiation field is built up over many passes of the radiation in a resonator cavity at the expense of the energy of a relativistic electron beam. Several cavity modes are usually excited due to the interaction of the electron and the optical beam. This paper is motivated by two principal questions: (i) What is the equation governing the nonlinear dynamics of the radiation field when multiple modes are excited. (ii) What are the conditions under which a single-mode state emerges spontaneously from the nonlinear evolution of a broad spectrum of unstable modes competing for the energy of the electron beam?The theory developed in this paper is applicable to lowgain FEL oscillators driven by long-pulse electron beams. These devices exhibit a strong tendency to evolve into a single-mode state [1-3]. The linewidths realized can be very small, making them ideal for demanding applications such as spectroscopy or isotope separation. For such devices, we demonstrate that the radiation field amplitude A͑z, t͒ can be modeled by the complex Ginzburg-Landau equation (GLE),where z is the coordinate along the axis of the undulator, t is time, and c 1 and c 2 are real parameters calculated by the theory. We then apply the model to two different FEL devices-one at the University of California at Santa Barbara (UCSB) [1,2] and the other at the Dutch FOM-Institute for Plasma Physics [3]. Whether singlemode operation was truly realized within the lifetime of the electron pulse in the UCSB experiment has been a source of theoretical controversy in the past [4,5]. Based on a large number of high-resolution simulations of the GLE with random initial conditions, we conclude that, in the absence of frequency selective feedback, a single-mode state was most probably not realized in the UCSB experiment. We also apply the GLE, with some modifications, to the Dutch fusion free-electron maser (FEM) experiment which has recently reported single-mode operation [3]. We show that, although it takes a very long time to realize a single mode in the absence of any frequency discrimination, the intervention of a frequency-dependent reflection coefficient may have facilitated the realization of a single-mode state within the lifetime of the electron beam in the Dutch experiment.In earlier work, we have derived the GLE [6,7] from the FEL amplifier equations in the high-gain Compton regime. In this paper, we derive the ...

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Electrostatic-Accelerator Free-Electron Lasers

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Pinhasi

Electrostatic Accelerators

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Direct spectral measurements of a quasi-cw free-electron laser oscillator

Cited by 19 publications

References 10 publications

Single-mode operation of a Bragg free-electron maser oscillator

Single-mode operation of a Bragg free-electron maser oscillator

Ginzburg-Landau Model and Single-Mode Operation of a Free-Electron Laser Oscillator

Electrostatic-Accelerator Free-Electron Lasers

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