Brian D. McVey

Three-dimensional simulations of free electron laser physics☆

Abstract A computer code has been developed to simulate three-dimensional free electron laser physics. A mathematical formulation of the FEL equations is presented, and the numerical solution of the problem is described. Sample results from the computer code are discussed.

Recent results from the Los Alamos free-electron laser

In this paper, we review the most recent experimental results of the Los Alamos free-electron laser program. Three major efforts will be described: lasing at improved efficiency over that previously attained, electron beam improvement, and energy recovery. An extraction efficiency of 2 percent was achieved with a wiggler having a 12 percent wavelength taper. The beam has been improved so that limits to its quality are now caused, not by injector performance, but by wake fields related to the high peak currents achieved. Limits to optical power are set by mirror damage. Experiments are described that demonstrate the successful operation of an energy-recovery system.

Bending and focusing effects in an FEL oscillator I: simple models☆

Abstract The combination of wiggler, electron beam, and optical beam produces not only gain, but also refractive effects. These effects have been modeled for an oscillator configuration with a simple prism and lens. An instability is found to occur whenever the focal strength of the electron beam is sufficiently large. An estimate has been made of the magnitude of these effects and we present a discussion of the likely consequences.

Conceptual designs of a 50 nm FEL oscillator and a 20–40 nm SASE amplifier☆

Abstract This paper consists of two parts; (1) the conceptual design, and optical performance characteristics, of a grazing angle of incidence ring resonator utilizing multifaceted metal mirrors for use with a 50 nm rf linac driven XUV FEL oscillator, and (2) electron beam and wiggler requirements for a self-amplified spontaneous emission (SASE) amplifier to produce high power in the 20–40 nm wavelength range. The basis for these studies is the 3- d FEL simulation code FELEX which, in part (1), is used to derive tolerances on mirror figure and thermal distortion, alignment sensitivity, and alternative output coupling methods. In part (2), the sensitivity of the output characteristics of an XUV FEL SASE amplifier to wiggler field errors is also studied.

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

Abstract 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.

Sideband suppression in free electron lasers using a grating rhomb

The phase shift produced by a grating rhomb is included in free-electron laser (FEL) pulse calculations to investigate whether or not grating rhombs can be used to suppress the sideband instability. The idea is that because the group travel time through a rhomb is an increasing function of the laser wavelength, an FEL oscillator can be designed such that the optical pulse at a chosen central wavelength and the pulse of electrons overlap spatially when they enter the wiggler. Over many passes, light in a small bandwidth about the chosen wavelength receives the greatest amplification because it overlaps the electrons, and light at sideband instability wavelengths that does not overlap the electrons is suppressed by losses in the oscillator cavity. For a 5-m tapered wiggler, the range in rhomb dispersion and cavity loss that yields acceptable FEL performance is defined. At low values of cavity loss, for example 15%, a wide range of rhomb dispersion exists for which both the sideband instability is largely suppressed and the energy extracted from the electrons is high. At larger values of cavity loss, for example 30%, a critical value for rhomb dispersion exists below which the laser pulse is compressed by the rhomb, leading to reduced energy extraction. >

Performance of Variable Selection Methods in Regression Using Variations of the Bayesian Information Criterion

The Bayesian information criterion (BIC) is widely used for variable selection. We focus on the regression setting for which variations of the BIC have been proposed. A version that includes the Fisher Information matrix of the predictor variables performed best in one published study. In this article, we extend the evaluation, introduce a performance measure involving how closely posterior probabilities are approximated, and conclude that the version that includes the Fisher Information often favors regression models having more predictors, depending on the scale and correlation structure of the predictor matrix. In the image analysis application that we describe, we therefore prefer the standard BIC approximation because of its relative simplicity and competitive performance at approximating the true posterior probabilities.

Recent results from the Los Alamos free electron laser

Abstract Accelerator improvements have been made that allow beams to be produced with higher charge, better pulse shape, and better emittance and energy spread. Record levels of efficiency and optical power have resulted. We could achieve higher levels except for problems with mirror damage at these power levels and growth of emittance and energy spread caused by wakefield effects.

Robust controller design of four wheel steering systems using /spl mu/ synthesis techniques

In this paper, a linearized four wheel steering (4WS) system model is deduced and then modified into a form which is appropriate for applying Matlab /spl mu/ Toolbox to design robust controller. Several important topics are discussed in detail, such as: 1) how to make system set-up match Matlab /spl mu/ Toolbox requirement, 2) how to select weights based on plants uncertainty, 3) how to solve controller discretization problem, and 4) how to adjust the system so that the conditions necessary for using a state-space formula to solve H/sub /spl infin// optimal (suboptimal) problem and performing the Matlab /spl mu/ Toolbox D-K iteration procedure are satisfied. Finally simulation results of robust controller and a PID controller are compared.

Integrated numerical modeling of free electron laser oscillators

Abstract The nonideal characteristics of the electron beam have largely determined the performance of all free electron laser (FEL) oscillators that have operated up to the present time. A realistic quantitative theoretical assessment of FEL oscillator performance must therefore include a viable representation of the electron beams characteristics, as well as the properties of the wiggler magnet and the optical resonator. This paper presents results of integrated numerical modeling of the Los Alamos FEL oscillator, using as input to the 3-D FEL simulation code FELEX, a numerically-generated electron pulse obtained from the accelerator code PARAMELA as a solution of a full model of the Los Alamos accelerator system (electron gun, accelerator cavities, and beam transport line).