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Brightness and coherence of radiation from undulators and high-gain free electron lasers

Description: The purpose of this paper is to review the radiation characteristics of undulators and high-gain free electron lasers (FELs). The topics covered are: a phase-space method in wave optics and synchrotron radiation, coherence from the phase-space point of view, discussions of undulator performances in next-generation synchrotron radiation facility and the characteristics of the high-gain FELs and their performances. (LSP)
Date: March 1, 1987
Creator: Kim, Kwang-Je
Partner: UNT Libraries Government Documents Department

An interference wiggler for precise diagnostics of electron beam energy

Description: Relativistic electrons passing through two identical magnetic sections generate synchrotron radiation whose spectrum is strongly modulated as the photon energy varies. The modulation is caused by the interference of radiation from each section, and has been observed in the spectrum of spontaneous radiation from transverse optical klystron which utilizes two undulators. In this paper, another device based on two simple wigglers is analyzed. The device, which will be called the interference wiggler, can be used for precise diagnostics of electron beam energy; by analyzing the modulated spectrum with a monochromator, the electron energy can be determined up to an accuracy of 10/sup -3/ or 10/sup -4/. General design criteria for interference wigglers are developed. Several example designs are given for measurement of the electron energy for the planned electron beam facility at CEBAF for the 1 to 2 GeV Light Source at Berkeley.
Date: March 1, 1987
Creator: Kim, Kwang-Je
Partner: UNT Libraries Government Documents Department

RF and space-charge effects in laser-driven rf electron guns

Description: The evolution of the electron-beam phase space distribution in laser-driven rf guns is studied by taking into account both the time variation of the RF field and space-charge effects. In particular, simple formulas are derived for the transverse and longitudinal emittances at the exit of the gun. The results are compared and found to agree well with those from simulation. 10 refs., 7 figs.
Date: August 1, 1988
Creator: Kim, Kwang-Je
Partner: UNT Libraries Government Documents Department

Transition undulator radiation as bright infrared sources

Description: Undulator radiation contains, in addition to the usual component with narrow spectral features, a broad-band component in the low frequency region emitted in the near forward direction, peaked at an angle 1/{gamma}, where {gamma} is the relativistic factor. This component is referred to as the transition undulator radiation, as it is caused by the sudden change in the electron`s longitudinal velocity as it enters and leaves the undulator. The characteristics of the transition undulator radiation are analyzed and compared with the infrared radiation from the usual undulator harmonics and from bending magnets.
Date: December 1, 1994
Creator: Kim, Kwang-Je
Partner: UNT Libraries Government Documents Department

Estimates of SASE power in the short wavelength region

Description: Given a sufficiently bright electron beam, the self-amplified-spontaneous emission (SASE) can provide gigawatts of short wavelength coherent radiation. The advantages of SASE approach are that is requires neither optical cavity nor an imput seed laser. In this note, we estimate the peak power performance of SASE for wavelengths shorter than 1000 {Angstrom}. At each wavelength, we calculate the saturated power from a uniform parameter undulator and the enhanced power from a tapered undulator. The method described here is an adaptation of that discussed by L.H. Yu, who discussed the harmonic generation scheme with seeded laser, to the case of SASE.
Date: March 1, 1992
Creator: Kim, Kwang-Je.
Partner: UNT Libraries Government Documents Department

FEL gain taking into account diffraction and electron beam emittance; generalized Madey's theorem

Description: We derive a formula for the free electron laser gain in the small-signal, low-grain regime which resembles closely the 1-D formula but taking into account the effect of wave diffraction and electron beam divergence and betatron motion. The formula is cast in a form which exhibits clearly the role of the transverse phase space distribution of photons and electrons. 8 refs.
Date: August 1, 1991
Creator: Kim, Kwang-Je.
Partner: UNT Libraries Government Documents Department

Rf and space-charge induced emittances in laser-driven rf guns

Description: Laser-driven rf electron guns are potential sources of high-current, low-emittance, short bunch-length electron beams, which are required for many advanced accelerator applications, such as free-electron lasers and injectors for high-energy machines. In such guns the design of which was pioneered at Los Alamos National Laboratory and which is currently being developed at several other laboratories, a high-power laser beam illuminates a photo-cathode surface placed on an end wall of an rf cavity. The main advantages of this type of gun are that the time structure of the electron beam is controlled by the laser, eliminating the need for bunchers, and that the electric field in rf cavities can be made very strong, so that the effects due to space-charge repulsion can be minimized. In this paper, we present an approximate but simple analysis for the transverse and longitudinal emittances in rf guns that takes into account both the time variation of the rf field and the space-charge effect. The results are compared and found to agree well with those from simulation. 7 refs., 6 figs.
Date: October 1, 1988
Creator: Kim, Kwang-Je & Chen, Yu-Jiuan
Partner: UNT Libraries Government Documents Department

Estimates of SASE power in the short wavelength region

Description: Given a sufficiently bright electron beam, the self-amplified-spontaneous emission (SASE) can provide gigawatts of short wavelength coherent radiation. The advantages of SASE approach are that is requires neither optical cavity nor an imput seed laser. In this note, we estimate the peak power performance of SASE for wavelengths shorter than 1000 {Angstrom}. At each wavelength, we calculate the saturated power from a uniform parameter undulator and the enhanced power from a tapered undulator. The method described here is an adaptation of that discussed by L.H. Yu, who discussed the harmonic generation scheme with seeded laser, to the case of SASE.
Date: March 1, 1992
Creator: Kim, Kwang-Je
Partner: UNT Libraries Government Documents Department

A Review of X-ray Free-Electron Laser Theory

Description: High-gain free-electron lasers (FELs) are being developed as extremely bright sources for a next-generation x-ray facility. In this paper, we review the basic theory of the startup, the exponential growth, and the saturation of the high-gain process, emphasizing the self-amplified spontaneous emission (SASE). The radiation characteristics of an x-ray FEL, including its transverse coherence, temporal characteristics, and harmonic content, are discussed. FEL performance in the presence of machine errors and undulator wakefields is examined. Various enhancement schemes through seeding and beam manipulations are summarized.
Date: December 18, 2006
Creator: Huang, Zhirong & Kim, Kwang-Je
Partner: UNT Libraries Government Documents Department

Critical review of high gain x-ray FEL experiments

Description: There is a renewed interest at the present time to develop x-ray free electron lasers (FELs). The interest is driven by the scientific opportunities with coherent x-rays glimpsed at the third generation light sources. With the recent development in linac technology in producing high-energy, high-brightness electron beams, it is now possible to design intense coherent x-ray source for wavelengths as short as one Angstrom based on the self- amplified spontaneous emission (SASE) principle. Major linac laboratories such as SLAC and DESY are therefore actively pursuing detailed design studies for the x-ray SASE facilities. The x-rays from these facilities will provide a peak brightness more than ten orders of magnitude higher than that of the current synchrotron radiation sources. Short wavelength coherent radiation could also be generated with harmonic generation techniques in linacs or storage rings. However, these schemes are not expected to be effective for 1 {Angstrom} wavelengths. This review will therefore concentrate on the linac based SASE scheme. The critical components of the SASE are: an electron source consisting of an RF photocathode gun with the emittance corrector producing high brightness electron beam; the beam bunching and acceleration; and a long undulator in which the radiation develops from initially incoherent radiation to intense, coherent radiation. We discuss the critical experimental issues in these components highlighting some relevant recent experiments. We also discuss issues related to the SASE experiment which are distinct from the usual free electron lasers. We give a brief survey of the world-wide SASE experiments. We conclude with a summary and outlook.
Date: August 1, 1996
Creator: Kim, Kwang-Je
Partner: UNT Libraries Government Documents Department

Can a free electron laser operate with a broad momentum spread?

Description: The gain in a usual free electron laser (FEL) vanishes when the energy spread of the electron beam is too broad because electrons with positive detuning parameter {ital v} are bunched around a certain phase contributing to a positive gain, while those with negative {ital v} are bunched around another phase contributing to a negative gain. The net gain therefore vanishes for a beam of electrons distributed over a broad range of positive and negative values of {ital v}. To avoid this problem, it was proposed recently to use two undulators and to insert a device between them which separates the electrons of negative v and to displace them in phase so that they contribute to an overall positive gain after passing through the second undulator. The device will be referred to as the ``redistributor``. The scheme, was an effort to extend the idea of the inversionless atomic laser to the FEL, and, if true, would have an important consequence for future FEL development. The purpose of this paper is to show that the scheme does not work as proposed. The reason is simple: consider an initial electron distribution which is uniform extending to a large positive and negative detuning where the FEL interaction vanishes. The distribution, because of Liouville`s theorem, will remain the same after the first undulator. The redistributor would have no effect on this distribution, and the gain of the total system vanishes. The reasoning above is wrong because we did not take into account the contributions of {ital all electrons}. The first group contributes to a positive net gain. However, this is canceled by the negative net gain due to the second group, irrespective of how the phase of electrons in the region of negative detuning is displaced relative to those in the positive detuning.
Date: August 1, 1996
Creator: Kim, Kwang-Je
Partner: UNT Libraries Government Documents Department

Start-up noise in 3-D self-amplified spontaneous emission

Description: The Self-Amplified-Spontaneous Emission (SASE) is receiving a renewed interest recently in connection with the proposal to build x-ray FEL facilities. A consistent theory of how the initial incoherent undulator radiation develops into an exponentially growing coherent signal was derived in 1-D case previously. How the theory could be extended to a 3-D case was also explained previously. In the case of the parallel electron beam, the problem was explicitly solved previously. In particular, the equivalent noise power was identified as the undulator radiation in one gain length. In this paper, we study how the result should be modified when the electrons` angular divergence is taken into account. We find that the equivalent noise power is the portion of the undulator power within the coherent phase space area, a result that should have been expected.
Date: August 1, 1996
Creator: Kim, Kwang-Je
Partner: UNT Libraries Government Documents Department

Emittance in particle and radiation beam techniques

Description: The author discusses the important and diverse role of the phase space area - the emittance - in the advanced techniques involving interaction of particle and radiation beams. For undulator radiation from unbunched beams, the radiation phase space is diluted from the coherent phase space of the single electron radiation. When the undulator radiation is used as a light source, it is important to minimize the dilution by decreasing the beam emittance and matching the phase space distributions of the particle and the radiation beams. For optical stochastic cooling, on the other hand, the phase space should be maximally mismatched for efficient cooling. In the case particles are bunched to a length much shorter than the radiation wavelength, the emittance appears as an intensity enhancement factor. In the operation of free electron lasers, the phase space matching becomes doubly important, once as the dilution factor in the initial stage of energy modulation and then as the radiation efficiency factor at the end where the beam is density modulated. The author then discusses some of the beam cooling techniques producing smaller emittances, especially the recent suggestions for relativistic heavy ions in storage rings or electron beams in linacs. These are based on the radiative cooling that occurs when particle beams backscatter powerful laser beams.
Date: March 1, 1997
Creator: Kim, Kwang-Je
Partner: UNT Libraries Government Documents Department

Gamma ray sources based on resonant backscattering of laser beams with relativistic heavy ion beams

Description: Resonant backscattering of high-power laser beam with non-fully stripped, ultra-relativistic ion beams in storage rings is studied as a source for {gamma}-ray beams for elementary particle physics experiments. The laser frequency is chosen to be resonant with one of the transition frequencies of the moving ions, and the bandwidth is chosen to cover the full Doppler broadening of the ions in the beam. Due to the resonance, the scattering cross section is enhanced by a large factor compared to the Thomson cross section, of the order 10{sup 8} for some examples considered here. The performance of the LHC as a possible {gamma}-generator or a {gamma} {minus} {gamma} collider is estimated. We study the case where hydrogen-like Pb ions with 2.8 TeV per nucleon are scattered by a train of 1100 {Angstrom}, 20 mg laser pulses with the same pulse time format as the ion beam. A free electron laser can be designed satisfying the requirements. It is estimated that {gamma}-rays of maximum quantum energy of 0.4 give at an average rate of 0.67 10{sup 18} are generated in this scheme. The luminosity of the corresponding {gamma} {minus} {gamma} collider will be about 0.9 10{sup 33} cm{sup {minus}2}s{sup {minus}1}.
Date: April 1, 1995
Creator: Bessonov, E.G. & Kim, Kwang-Je
Partner: UNT Libraries Government Documents Department

Performance of hole coupling resonator in the presence of asymmetric modes and FEL gain

Description: We continue the study of the hole coupling resonator for free electron laser (FEL) application. The previous resonator code is further developed to include the effects of the azimutally asymmetric modes and the FEL gain. The implication of the additional higher order modes is that there are more degeneracies to be avoided in tuning the FEL wavelengths. The FEL interaction is modeled by constructing a transfer map in the small signal regime and incorporating it into the resonator code. The FEL gain is found to be very effective in selecting a dominant mode from the azimuthally symmetric class of modes. Schemes for broad wavelength tuning based on passive mode control via adjustable apertures are discussed. 12 refs., 7 figs., 1 tab.
Date: August 1, 1991
Creator: Xie, Ming & Kim, Kwang-Je.
Partner: UNT Libraries Government Documents Department

Finite pulse effects in self-amplified-spontaneous emission

Description: The authors study the effects of the electron density profile on self-amplified-spontaneous-emission (SASE). A general formalism in the linear regime is developed by deriving the coupled Maxwell-Klimontovich equations for an arbitrary density profile and including the effects of the energy spread, diffraction, and the betatron oscillation. An explicit solution is obtained for the one-dimensional (1-D) case. The temporal and the spectral intensity profiles of SASE depend linearly on the initial electron correlation function. The correlation function consists of two terms, a term giving rise to the usual spontaneous radiation and its amplification to SASE, and a term representing the coherent bunched beam effect. The latter term has been neglected so far in the treatments of SASE, but it could be significant when there is a variation in the electron density at a length scale comparable to the wavelength. The theory reproduces the well-known results when the electron density is uniform. It also reproduces a recent theory for a finite top-hat density profile and a vanishing energy spread.
Date: September 1, 1994
Creator: Kim, Kwang-Je & Hahn, Sang June
Partner: UNT Libraries Government Documents Department

Performance of hole coupling resonator in the presence of asymmetric modes and FEL gain

Description: We continue the study of the hole coupling resonator for free electron laser (FEL) application. The previous resonator code is further developed to include the effects of the azimutally asymmetric modes and the FEL gain. The implication of the additional higher order modes is that there are more degeneracies to be avoided in tuning the FEL wavelengths. The FEL interaction is modeled by constructing a transfer map in the small signal regime and incorporating it into the resonator code. The FEL gain is found to be very effective in selecting a dominant mode from the azimuthally symmetric class of modes. Schemes for broad wavelength tuning based on passive mode control via adjustable apertures are discussed. 12 refs., 7 figs., 1 tab.
Date: August 1, 1991
Creator: Xie, Ming & Kim, Kwang-Je
Partner: UNT Libraries Government Documents Department

Gamma-gamma collider based on Compton back-scattering

Description: A {gamma}{gamma} collider would extend and complement the physics capability of a linear collider; e.g. it would be suitable for direct measurement of the partial decay width of a Higgs boson into two gamma quanta. This paper discusses choice of laser parameters, luminosity optimization, electron and laser parameters for a gamma- gamma collider as a second interaction region for the Next Linear Collider, laser path, and the lasers. It is concluded that a gamma- gamma collider is technically feasible; however it will require a significant investment in preparatory R&D.
Date: August 1, 1996
Creator: Kim, Kwang-Je & Group, Gamma-Gamma Working
Partner: UNT Libraries Government Documents Department

Hole coupling resonator for free electron lasers

Description: The performance of two mirror resonators with holes for output coupling is studied for free electron laser application using a Fox-Li type code. The mode profiles inside and outside the cavity, the diffraction losses at the mirror edges and apertures, the amount of useful power coupled through the hole, etc., are calculated for the dominant mode for different hole and mirror dimensions. It is found that resonators in near concentric geometry can develop a mode degeneracy in certain cases, which should be avoided for the stability of the free electron laser gain output. A resonator configuration for a free electron laser at Lawrence Berkeley Laboratory was found which can provide satisfactory performance over a wavelength range from 25 to 50 microns. The possibility of further increasing the tuning range by an adjustable intracavity aperture is discussed. 15 refs., 6 figs.
Date: November 1, 1990
Creator: Xie, Ming & Kim, Kwang-Je.
Partner: UNT Libraries Government Documents Department

Calculation of 3-D free electron laser gain: Comparison with simulation and generalization to elliptical cross section

Description: In the previous paper, we have derived a dispersion relation for the free electron laser (FEL) gain in the exponential regime taking account the diffraction and electron`s betatron oscillation. Here, we compare the growth rates obtained by solving the dispersion relation with those obtained by simulation calculation for the waterbag and the Gaussian models for the electron`s transverse phase space distribution. The agreement is found to be good except for the limiting case where the Rayleigh length is much longer than the gain length (1-D limit). We also generalize the analysis to the case where the electron beam cross section is elliptical as is usually the case in storage rings, and derive the first-order dispersion relation.
Date: August 1, 1992
Creator: Chin, Yong Ho; Kim, Kwang-Je & Xie, Ming
Partner: UNT Libraries Government Documents Department

Tapered undulator for SASE FELs

Description: We discuss the use of tapered undulators to enhance the performance of free-electron lasers (FELs) based upon self-amplified spontaneous emission (SASE), where the radiation tends to have a relatively broad bandwidth, limited temporal phase coherence, and large amplitude fluctuations. Using the polychromatic FEL simulation code GINGER, we numerically demonstrate the effectiveness of a tapered undulator for parameters corresponding to the existing Argonne low-energy undulator test line (LEUTL) FEL. We also study possible tapering options for proposed x-ray FELs such as the Linac Coherent Light Source (LCLS).
Date: September 14, 2001
Creator: Fawley, William M.; Huang, Zhirong; Kim, Kwang-Je & Vinokurov, Nikolai A.
Partner: UNT Libraries Government Documents Department