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Design Considerations for a 60 Meter Pure Permanent MagnetUndulator for the SIAC Linac Coherent Light Source (LCLS)*R. Tatchyn, R. Boyce, K. Halbach, H.-D. Nuhn, J. Seeman,H. WinickStanford Linear Accelerator Center, Stanford, CA 94305, USA
C. PellegriniDepartment of Physics, UCLA, Los Angeles, CA 90024-1547, USAAbstract
In this paper we describe design, fabrication , andmeasurement aspects of a pure permanent magnet (PM)insertion device designed to operate as an FEL at a 1stharmonic energy of 300 eV and an electron energy of 7 GeVin the Self-Amplified SpontaneousEmission (SASE) regime.I. INTRODUCTION
In recent years, progress in the development of short-pulse, low emittance, laser-driven RF photocathode guns [l],and in the modulation and control of high energy particlebeams 2], has made possible the consideration of linac-drivenFree Electron Lasers (FELs) designed or SASE [3] operationat 1st harmonic energies extending well into the soft x-rayrange. In this paper selected design considerations for anundulator optimized for operation in the water window (300-400eV) on a subsection of the Stanford Linear AcceleratorCenter (SLAC) 3km linac are described. Using three-dimensional SASE simulation codes reported on elsewhere[4,5], the basic undulator parameters were derived fromoptimization studies incorporating: 1) the effects of theundulator period hU, 2) the field amplitude BO, and 3) astrong external focussing l3 on both the undulators effectivegain parameter, oeff, and gain length, LG (=hu/4d3oeff).Using the optimization goals of increasing the gain andsimultaneously reducing the gain length (to avoid overly longundulator structures [6]) the following set of basic operatingparameterswas derived:E (electron energy) = 7&V yE(emittance) = 3X1h-mX (1st harmonic) = 4o.A i@eak current )= 2500AX,,(und. period) = 8 cm fi(focussing) = 8.2mBg(field amplitude) = 0.8T LG(gain length) = 2.37mFurther simulation studies investigating the effects of fielderrors on the SASE gain were also conducted [7], and theresults were used to help assess the minimal requiredmechanical and field tolerancesof the undulator components.
II. GENERAL, DESIGN FEATURESGiven the broad base of experience acquired by the
* Supported y DOE Offices of Basic Energy Sciences and HighEnergy andNuclearPhysics ndDepartmentof Energy Contract DE-AC03-76SF001.5.
scientific community in the area of PM undulators [8], thecontinuing improvement of commercially available PMmaterials, and the advantage of relatively straightforwardanalytical investigations, the LCLS group decided to base tsinitial undulator studies on a pure PM design. Uponconsideration of a number of alternatives, the focussinglattice was chosen to consist of current-driven ironquadrupoles n a FODO configuration. As shown in Fig. 1, theconfiguration selected for the PM undulator lattice is of thestandard ype [9], with 8 magnetsper period.
Standard PM Structure
PM blocks
-+ !k$/ PM blockhl+ 543738sA4Figure 1. StandardPM configuration o f the LCLS.The PM blocks dimensional and field parameterswere arrivedat by both analytical [9] and numerical field calculations. Thequadrupole design follows from the computer-studyidentification of an optimal betatron wavelength of 51.4m,which determined the necessary ocal length of the individualquads to be approximately 4.lm. The individual quaddimensions were arrived at by : 1) utilizing 50% of thelongitudinal free spacealong the undulator to help reduce therequired quad gradients, and 2) specifying a minimum quadaperture radius of 6cm to inhibit the perturbation of the PMundulator fields by the quad yoke material. The resultingbasic parametersare given below.
h = 8cm Quad aperture radius = 6cmhmu = 2cm Quad outside diameter=20cmh = 1.9cm Quad length =40cmt = 1.9cm Quad gradient =15T/mW = 4cm FODO period =1.6mg = 1.5cm Phaseadvanceper cell=11.5Br = 1.08T Total Pwr. Budget I 300kW
0-7X03-1203-1/93SO3.001608
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III. MECHANICAL DESIGNAs depicted in Fig. 2, the basic approach to themechanical design of the LCLS undulator is modular. Twobasic reasons for this are: 1) simplification and statisticalcontrol of the fabrication and field measurementprocesses;and 2) facilitation of the installation and alignment of theundulatorin the SLAC FFTB tunnel [lo] prior to operation.
LCLS Permanent Magnet Undulator
1.6 meler Module
6.4 meler Module(thermally stabilized)
SL4C L:nac Laser Alignment PipeFigure 2. LCLS undulator layout showing modular sections.
Given the possibility of tuning the 1st FEL harmonic byvarying E, the conventional use of undulator jaw motion wasdetermined to be dispensable,making possible the design of arelatively simple support system for the PM and quadrupolelattices. At the same time, the small gap necessary for theattainment of the required BO ntroduced troublesome designobstacles to the incorporation of the necessary systemcomponents. In Fig. 3, the basic modular unit of the LCLS, a1.6 meter PM lattice section ntegrated with one period of the
LCLS Undulator(1 6 meter ModUie)
Beam Position Monlto
1.6 meter
Figure 3. Selectedmechanical and electrical details of a 1.6mLCLS module.
FODO lattice, is shown with a minimal repeating set ofsystem components. For clarity, a schematized and enlargedcross section of the LCLS undulator is shown in Fig. 4.
LCLS Cross Sectionl/2 DiameterVacuum PipeVacuum- Pump Port
REC Keeper
I 17&EFigure 4. Selectedcomponent details of the LCLS insertiondevice in cross section.
The field gap is set by spacer blocks with optionalprovisions for limited PM adjustment designed into thekeepers. The computed force/period on each linear PM arrayfor the given parameters s approximately 9Olbs,necessitatingcareful attention to the mechanical and compositional detailsof the keeper assemblies.To allow for longitudinal phasingcontrol and attitude alignment, precision translators areindicated for y-z alignment of each 1.6 meter module. Notexplicitly shown are: 1) short magnet block assemblies orcontinuing the PM lattice in proper phase rom one module tothe next, 2) coarse y-adjustment provisions for each 6.4mmodule, 3) a water-based thermal stabilization system forsuppressing emperaturedeviations in excessof f 0SC alongthe entire length of the LCLS insertion device, and 4) in-vacuum Beam Position Monitors (BPMs).IV. TOLERANCESThe assessment f the effects of random field errors and theircorrection on the SASE gain process was based primarily oncomprehensive3-D simulations [7]. These simulations, whichyield the expected degradation of SASE in the LCLS as afunction of random magnetic errors and the precision ofcompensatingorbit corrections, indicate a reduced sensitivityof the FEL gain to field errors, especially in the high gain
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regime. The results of these studies suggest hat dimensionaland field tolerances ypical of the best currently available 3rdgeneration [B] magnetsshould result in passableperformanceof the LCLS provided: 1) the magnets are optimally sorted;2) sufficiently precise orbit detection is achieved; and 3)equally precise orbit correction is implemented every 3m orless. In practical terms, typical magnet field strengthtolerancesof O.l%-0.2% and easy axis orientation errors of
