A. Friedman

Design of a harmonic generation FEL experiment at BNL

We present design parameters of a harmonic generation FEL experiment to be carried out at the Accelerator Test Facility (ATF) at BNL. This experiment out as a proof-of-principle for the proposed UV-FEL Users Facility at BNL. In the experiment we plan to triple the frequency of a CO{sub 2} seed laser by utilizing two superconducting wigglers and a dispersive section. The first wiggler will be used in conjunction with the CO{sub 2} seed laser to generate a ponderomotive force that will bunch the electron beam. The bunching will then be enhanced by the dispersion section. The second wiggler, tuned to the third harmonic of the seed laser will follow. In the beginning of the second wiggler the bunched beam will produce super-radiant emission (characterized by a quadratic growth of the radiated power), then the radiation will be amplified exponentially. The last part of the wiggler will be tapered. We plan to study the evolution of the various radiation growth mechanisms as well as the coherence of the tripled and exponentially amplified radiation. 12 refs.

Use of eigenvectors in understanding and correcting storage ring orbits

The response matrix A is defined by the equation X = AΘ, where Θ is the kick vector and X is the resulting orbit vector. Since A is not necessarily a symmetric or even a square matrix we symmetrize it by using ATA. Then we find the eigenvalues and eigenvectors of this ATA matrix. The physical interpretation of the eigenvectors for circular machines is discussed. The task of the orbit correction is to find the kick vector Θ for a given measured orbit vector X. We are presenting a method, in which the kick vector is expressed as linear combination of the eigenvectors. An additional advantage of this method is that it yields the smallest possible kick vector to correct the orbit. We will illustrate the application of the method to the NSLS X-ray and UV storage rings and the resulting measurements. It will be evident, that the accuracy of this method allows the combination of the global orbit correction and local optimization of the orbit for beam lines and insertion devices. The eigenvector decomposition can also be used for optimizing kick vectors, taking advantage of the fact that eigenvectors with corresponding small eigenvalues generate negligible orbit changes. Thus, one can reduce a kick vector calculated by any other correction method and still stay within the tolerance for orbit correction. The use of eigenvectors in accurately measuring the response matrixand the use of the eigenvalue decomposition orbit correction algorithm in digital feedback is discussed.

Status of the visible Free-Electron Laser at the Brookhaven Accelerator Test Facility

Abstract The 500 nm free-electron laser (FEL) at the accelerator test facility (ATF) of the Brookhaven National Laboratory is reviewed. We present an overview of the ATF, a high-brightness, 50-MeV, electron accelerator and laser complex which is a users facility for accelerator and beam physics. A number of laser acceleration and FEL experiments are under construction at the ATF. The visible FEL experiment is based on a novel superferric 8.8 mm period undulator. The electron beam parameters, the undulator, the optical resonator, optical and electron beam diagnostics are discussed. The operational status of the experiment is presented.

Three dimensional modelling and numerical analysis of super-radiant harmonic emission in an FEL (optical klystron)

A full 3-D Analysis of super-radiant (bunched electron) free electron harmonic radiation is presented. A generalized form of the FEL pendulum equation was derived and numerically solved. Both spectral and phasor formulation were developed to treat the radiation in the time domain. In space the radiation field is expanded in terms of either a set of free space discrete modes or plane waves. The numerical solutions reveal some new distinctly 3-D effects to which we provide a physical explanation. 12 refs., 9 figs., 5 tabs.

Polarized wiggler for NSLS X-ray ring

We examine the properties of an elliptically polarized wiggler that will generate circularly polarized photons with an energy spectrum of 3-12 keV. The vertical wiggler magnetic field is produced by permanent magnets while the horizontal wiggler field is generated by electric coils capable of AC excitation. The radiation parameters of the wiggler are presented. Numerical values are calculated for radiation from the wiggler. A conceptual design for such a wiggler is discussed. We consider AC excitation of the wiggler to produce the time modulation of the elliptic polarization. The power dissipated in the vacuum chamber due to the eddy current is considered.<<ETX>>

Reduction of FEL gain due to wiggler errors

Abstract For an FEL operating in the exponential regime before saturation, we consider the effect on the gain of longitudinal velocity variations arising from wiggler field errors. The average gain reduction and the width of the output power distribution are expressed in terms of the mean square average of the ponderomotive phase shift per gain length. A scheme for correcting the electron trajectory using position monitors and dipole correctors is analyzed. Analytic results are compared with numerical simulations. Our work is directly applicable to the design of FEL amplifiers and the results are encouraging for the feasibility of such devices.

A digital feedback system for transverse orbit stabilization in the NSLS rings

We report on the design of a prototype digital feedback system for the storage rings at the NSLS. The system will use a nonlinear eigenvector decomposition algorithm. It will have a wide dynamic range and will be able to correct noise in the orbit over a bandwidth in excess of 100 Hz. A Motorola-162 CPU board will be used to sample the PUE`s at a minimum rate of 1 KHz, and HP-742rt board will be used to read the sampled signals ad to generate a correction signal for the orbit correctors and another Motorola-162 will implement that signal.

Eigenvector method for optimized orbit correction

The task of the orbit correction is to find the kick vector Θ for a given measured orbit vector X↘. We are presenting a method, in which the kick vector is expressed as linear combination of the eigenvectors. An additional advantage of this method is that it yields the smallest possible kick vector to correct the orbit. We will illustrate the application of the method to the NSLS X‐ray and UV storage rings and the resulting measurements. It will be evident, that the accuracy of this method allows the combination of the global orbit correction and local optimization of the orbit for beam lines and insertion devices.The eigenvector decomposition can also be used for optimizing kick vectors, taking advantage of the fact that eigenvectors with corresponding small eigenvalues generate negligible orbit changes. Thus, one can reduce a kick vector calculated by any other correction method and still stay within the tolerance for orbit correction.The response matrix A is defined by the equation X↘=AΘ↘, where Θ↘ is ...

Optimization method for orbit correction in accelerators

We present a method to minimize the corrector strengths required to reduce the RMS beam orbit. Any least square correction method will usually lead to undesirably strong corrector settings. The method, we are presenting, minimizes the total kick vector by finding the eigensolutions of the equation X/spl I.oarr/=A/spl thetaspl circ/, where X/spl I.oarr/ is the orbit change vector /spl thetaspl circ/ is the kick vector and A is the response matrix. Since A is not necessarily a symmetric or even square matrix we symmetrize the matrix by using A/sup T/A instead. Eigenvectors with corresponding small eigenvalues generate negligible orbit changes. Hence, in the optimization process the kick vector is made orthogonal to the eigenvectors. The physical interpretation of the eigenvectors will be discussed. We will illustrate the application of the method to the NSLS X-ray and UV storage rings. From this illustration it will be evident, that the accuracy of this method allows the combination of the global orbit correction and local optimization of the orbit for beamlines and insertion devices.<<ETX>>

First results with a nonlinear digital, orbit feedback system at the NSLS

We report on the first experimental results with a nonlinear digital orbit feedback system for the NSLS X-ray ring. The system uses the existing RF receivers and orbit corrector magnets (trims) as well as parts of the NSLS control components. The orbit measurement micro was upgraded to a Motorola 68040 CPU in order to achieve the necessary data rate. Filtering and orbit correction calculations are done in a dedicated HP 742 rt micro. The system operates at a 555 Hz data rate, and achieves a bandwidth of 15-20 Hz.