@52q#EB‘@2~/g@(?The Magnetic and Diagnostics Syste’ &Q~jhe Advanced Photon Source

Self-Amplified Spontaneously Emitting FEL7

E. Gluskin, C. Benson, R.J. Dejus, P.K. Den Hartog, B.N. Deriy, O.A. Makarov,

S.V. Milton, E.R. Moog, V.I. Ogurtsov, E.M. Trakhtenberg, K.E. Robinson*,

I.B. Vasserman, N.A. Vinokurov$, and S. Xu

Advanced Photon Source, Argonne National Laboratory, Argonne IL 60439

*STI Optronics, 2755 Northup Way, Bellevue WA 98004

# Budker Institute of Nuclear Physics, 11Ac. Lavrentyev Prosp.,

630090 Novosibirsk, Russia

----- ----- ----- ----- ----— ----- ----- ----- ----- ----- ----- ---

Abstract

A self-amplified spontaneously emitting (SASE) flee-electron laser (FEL) for the

visible-to-ultraviolet spectral range is under construction at the Advanced Photon

Source at Argonne National Laboratory. The ampIifier part of the FEL consists of

twelve identical 2.7-meter-long sections. Each section includes a 2.4-meter-long,

33-mm-period hybrid undulator, a quadruple lens, and a set of electron beam

and radiation diagnostics equipment. The undulatory will operate at a iixed

magnetic gap (approx. 9.3 mm) with K=3.1. The electron beam position will be

monitored using capacitive beam position monitors, YAG scintillators with

imaging optics, and secondary emission detectors. The spatial distribution of the

photon beam will be monitored by position sensitive detectors equipped withThe submitted manuscript has been createcby the University of Chloago as Operator oArgonne Nallonal Laboratory ~Argonne.’under Contract No. W-31-109-ENG-3S witl-the U.S. Department of Energy. The U.S

1 Government retains for itself, and others actIng on Its behalf, a paid-up, nonexclusive

,? irrevocable worldvdde license in aald artickto reproduce, prepare derivative works, distrfbute coplesto the public, and perform publicly and display publicly, by or on behalf othe Govamment.

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DISCLAIMER. .

This report was prepared as an account of work sponsoredby an agency of the United States Government. Neither theUnited States Government nor any agency thereof, nor anyof their employees, make any warranty, express or implied,or assumes any legal liability or responsibility for theaccuracy, completeness, or usefulness of any information,apparatus, product, or process disclosed, or represents thatits use would not infringe privately owned rights. Referenceherein to any specific commercial product, process, orservice by trade name, trademark, manufacturer, orotherwise does not necessarily constitute or imply itsendorsement, recommendation, or favoring by the UnitedStates Government or any agency thereof. The views andopinions of authors expressed herein do not necessarilystate or reflect those of the United States Government orany agency thereof.

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DISCLAIMER

Portions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocument.

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narrow-band filters. A high-resolution spectrograph will be used to observe the

spectral distribution of the FEL radiation.

T Supported by the U.S. Dept. of Energy, BES-Material Sciences, under

Contract No. W-31-109-Eng-38.

----- ----- ----- ----- ----- ----- ----- ----- ----- ----- --------

Keywords: Free-electron laser

Please send proofs to:

Elizabeth R. Moog

XFD-401

Argonne National Laboratory

9700 S. Cass Ave.

Argonne, IL 60439

phone 630-252-5926

Fax 630-252-9303

e-mail: moog@ANL.GOV

2

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1. Introduction

The SASE FEL now under construction [1] at the Advanced Photon Source

(=S) will consist of two major parts: the APS injection system 600-MeV linac

coupled to a small-emittance electron gun, and a set of twelve undulatory. Initial

operations will be at a lower energy in order to produce visible 532-rim light. The

parameters for initial operations are given in Table I. Further information about

the particle beam production and characteristics are given in references 1. After

experience is gained with visible light, the linac energy will be increased, and the

FEL will produce 120-nm ultraviolet light. ~

Simulations of the beam bunching in an FEL were pefiormed [2], showing that

the undulator line can have separated undulatory without significant deleterious

effects on the bunching. The drift spaces between undulatory can then

accommodate diagnostics and quadruples for horizontal beam focusing. Since

the horizontal focusing can be separate horn the undulatory, the design for the

undulatory becomes much simpler, and the approach used for the design and

construction of the undulatory on the APS storage ring can be directly applied to

the FEL. Also, the Insertion Device Magnetic Measurement Facility of the APS

and the magnetic tuning techniques developed for the APS undulatory are directly

applicable to the measurement and tuning of the undulatory for the FEL. A drift

space also allows much greater flexibility in the choice of diagnostics for both the

particle and light beams. These advantages have led to the choice of a separated

undulator design for the APS FEL.

3

—

The FEL line will consist of a series of twelve identical cells, where each ceil

includes an undulator, a diagnostics section, and a quadruple singlet. The

choice of the lattice was made with the help of the FEL simulation codes [2].

Configurations with a single quadruple, a doublet, or a triplet placed in the drift

section were considered. The singlet was found to give the best particle beam

bunching. The codes have also been used to optimize the quadruple strength.

The beta fimction for an undulator cell is shown in Fig. 1 for a quadruple

strength of 7.22 kG (for a focal length of 1 m) and an energy of 220 MeV. Vertical

beam focusing is intrinsic to the undulator field; no additional vertical focusing

between the undulatory will be added. The horizontal focusing will match the

vertical focusing for. equal two-plane focusing.

The length of the drift space between successive unduIators must be carefdy

chosen so as to maintain the proper phasing between undu.lators [3]. The phasing

is affected by the strength of the undulator magnetic field and by the end fields.

Measurements of the end field on existing magnetic structures and a calculation

of their effect on the phasing have led to choosing a drift length of 326.5 mm born

the end of one-undulator to the beginning of the next undulator, for an overall cell

length of 2.7265 m.

The functions of beam steering and horizontal beam focusing have been .

combined in the quadruple magnet. Separate windings on the quadruple poles

allow it to also serve as a dipole correction magnet, steering the beam vertically

and horizontally.

2. Characteristicsof the FEL Undtdator

The period length of the undulatory to be used for the FEL is 33 mm.

Simulations of the expected gain have been performed using period lengths as

short as 27 mm, and the results showed very little sensitivity to changes in the

period. (Changing the period length born 27 mm to 33 resulted in a gain length

change between +20% and -7% depending on whether the undulator K parameter

was kept constant or allowed to increase as it normally would.) Therefore, since

the well-understood and standard APS Undulator A is a 33-mm-period device, the

decision was made to proceed with that period length, and, in fact, with the same

design for the magnetic structure. Since horizontal focusing is separate from the

undulator, there is no need to cant the undulator poles [3], and the standard

storage-ring undulator design can be used. However, the criteria for magnetic

tuning for the single-pass, fixed-gap FEL are somewhat different than for a

variable-gap Undulator A in the storage ring, as described below. STI Optronics,*

of Bellevue, WA, designed, built and tuned the Undulatory A, they are ako

building and tuning the FEL magnetic structures. The final tuning of the ends to

match the phasing [3] to the drift length will be done at APS.

The magnetic structures will be held at a fixed magnetic gap when they are

installed in the FEL tunnel. For convenience in tuning, however, a variable-gap

support and drive system will be used in the measurement room. Once the

5.,.

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magnetic structure is tuned, it will be mounted on the fixed-gap system and its

magnetic characteristics will be confirmed. In both the fixed-gap and variable-

gap systems, the supports holding the magnetic structures apart are located at

the same longitudinal positions along the undulator to minimize differences in

the strongback deflections.

Some of the magnetic tuning requirements for FEL undulatory are more

demanding than for a storage ring undulator. For an FEL, it is critically

important that the particle beam trajectory coincide with the axis of the emitted

radiation, and that the coincidence extend over many gain lengths. The most”

convenient means of achieving this is to keep the trajectory of the particle beam

straight through the undulator end regions as well as through the full-field

regions. This leads to the requirement that the second field integral (averaged

over each period) remain less than 3300 G-cm2 through the entire length of the

undulatory, including the end sections. (Note that this is much less than the

second integral of the vertical component of the earth’s field over the length of an

undrdator, so that differences in the ambient field between the measurement

room and the installation site need to be taken into account.) For a beam energy of

220 MeV, the requirement corresponds to a trajectory displacement of 45 p.

(The corresponding requirement for a storage ring undulator is that the second

field integral through the full-field region be below 105Gcm2 for all gaps, with no

special requirement for the ends.) The requirement that we be able to confirm the

trajectory straightness in the vertical direction to this accuracy means that the

6,

horizontal field component must be measured accurately, despite the planar Hall

effect. The work done to develop this capability is reported elsewhere [4].

The effective magnetic field strengths for each undulator must be matched so

that the light produced by one undulator is at the resonant wavelength for the

next. Simulations were carried out in which a K of 3.10 was assigned to some

undulatory and a K of 3.11 to others, corresponding to a change in undulator field

of 32 G. The electron bunch peak current density dropped by -35% compared to

the ideal case at one position along the FEL, but began to recover further

downstream. This change in field strength is larger than the 8 G dMerence that

would make the wavelengths from diHerent undulatory different by 5% of the

width of the first harmonic peak horn one gain length of undulator. The

undulatory will be tuned to place all the effective fields within a 15-Gwide range.

A field strength change of 15 G could be caused by a change in magnetic gap of 16

pm. The undulator gap will be near 9.3 mm, but the gap of each undulator will be

adjusted individually to compensate for undulator-to-undulator variations.

The requirement that the field strengths for the undulatory be nearly identical

leads to a temperature uniformity requirement for the FEL line. The permanent

magnets are made of Nd-Fe-B, which loses strength reversibly with a

temperature coefficient of 0.09%/”C. There is also a change in gap, since the

spacer blocks that hold the magnetic structures apart will expand. For a l°C

increase in temperature, the loss in magnet strength will result in a field

decrease of 9 Gauss, and the thermal expansion of brass spacers will result in a

7.

field decrease of about 0.2 Gauss. To keep a <15 Gauss variation requirement, the

temperature of any undulator must be the same as the temperature of any other

undulator to within 1.50C (or AO.75”C).

The vertical focusing of the particle beam by the undulator is the result of

variation in the undulator field with vertical position, so variations in the

midplane horn pole to pole can afl’ect the focusing. We sought to determine the

vertical center under each pole with an accuracy better than the 50 pm tolerance

on the overall vertical position [5]. The preferred technique, given the drift in the

Hall probe zero, was to measure BY(y)under each pole (y is vertical position). The

minimum value of BYis the center height. The results of the measurements are

shown in Fig. 2. Mo@ of the scatter in the magnetic center heights is not due to

measurement error. Instead, it arises horn a variety of factors inherent in the

assembly of undulatory. The permanent magnet blocks vary in strength; for this

device the rms error in the magnet strength was 0.43%. There can be errors in

the mechanical height of the poles. The shims, especially the skew shims, used

for tuning the device can affect the magnetic center height, as can other sources

of magnetic errors such as the presence of ambient magnetic material. This

scatter is inconsequential, however, since it is on a much shorter scale than the fi-

function of the particle beam.

Other FEL requirements are less demanding than the corresponding

requirements for storage-ring undulatory. Since the FEL undulatory will operate

at a single fixed gap, magnetic tuning only needs to be done at that one gap. Also,

8

a small phase error is important for a storage ring undulator to ensure high

brilliance in higher-order harmonics, whereas FEL operation only relies on a

brilliant first harmonic. Because the brilliance of the first harmonic is much less

affected by phase errors than the brilliance of higher harmonics, the rms phase

error requirement is less demanding for an FEL undulator. The criterion used

for the FEL is that the first harmonic intensity should not decrease by more than

5% due to phase errors, which leads to the requirement that therms phase error

be less than 10 deg.

Simulations [5] have also been used to determine tolerances for the alignment

of the undulatory to the beam and to adjacent undulatory. The calculated

tolerances given in Table II are based on requiring that the power output not

change more than approximately 10% for a given parameter. The focal length of

the quadruple assumed for these calcrdations was 2.4 m. The simulations of the.

effect of the quadruple strength on the beam bunching found that while a focal

length of 1 m gave the best beam bunching, it also resulted in tighter tolerances.

This somewhat longer focal length may be a better compromise.

3. Beam diagnostics

The diagnostics serve two purposes: one is to monitor and maintain the

coincidence between the particle beam and the undulator radiation, the second is

to evaluate the characteristics of the light that is produced by the FEL.

.

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A schematic of the diagnostics section that will be located between the

undulatory is shown in Fig. 3. Since it is critical that the particle beam and the

axis of the emitted light beam coincide through the entire series of undulatory,

three different

been included.

and complementary monitors of the particle beam position have

The capacitive button BPM, or beam position monitor, is the same

as the BPMs used at the ends of the insertion device straight sections in the APS

storage ring. The relative positions of the buttons are different than in the storage

ring, however — since the FEL vacuum chamber has a smaller vertical aperture

than the usual storage ring ID vacuum chamber, the buttons will be vertically

closer. They will also be closer transversely in order to improve their sensitivity

[61. The secondary-emissionwire BPM is an absolute position monitor that

consists of two perpendicular sets of four parallel wires. The spacing between the

15-pm wires is, in order, 0.5 mm, 1 mm, and 0.5 mm. The current to individual

wires is monitored as the particle beam is steered to strike the wires. The beam

can be centered vertically and horizontally by steering it to determine where it hits

the wires on opposite sides of the beam centerline, then splitting the difference.

During normal operation the beam will not strike the wires because the spacing

between the central wires will be a few times the beam size. The third beam

position monitor is the CCD image of the YAG scintillator crystal. The opticaI

system wiIl be designed to make the size of a CCD pixel comparable to the 10-pm

resolution reported [7] for the YAG crystal itself.

10,.-’

Upstream of the undulatory, there will be a chicane for the particle beam [8].

The synchrotrons radiation produced at its bends will be monitored as a means of

characterizing the particle beam, and it will also provide a place for an alignment

laser to be inserted. The alignment laser wiII be directed down the inside of the

vacuum chamber and will be used to define the desired straight-line beam path.

Since the alignment laser light will travel through the same optical systems as

the FEL light and the light fioxn the YAG crystal, the desired position of the light

on the CCD arrays can be defined.

The lens and CCD in the upper left of Fig. 3 will be used to check the

distribution in angle of the incoming light as well as its position, by varying the

focus born ini?mity to the downstream end of the nearest undrdator. When the

focus is at ifinity, i.e., the distance between the CCD and the lens is the focal

distance, all the light incident on the lens parallel to a particular angle will be

imaged to the same p,oint on the CCD. In this configuration, all position

information about the incoming light is lost and the image on the CCD will reflect

the distribution in angle of the incoming light. A deflection between undulatory

or a trajectory kick within an undulator will appear as a displacement in the CCD

image. A bandpass filter will limit the angular spread of red-shifted light that

reaches the lens. With the focus adjusted to lie at closer distances, such as within

the undulator, the positions of the emitted light along the length of the undulator

will be monitored.

u.

One of the filter wheels in the upper left of Fig. 3 will carry bandpass filters;

the other will have neutral-density filters so that the light levels can be adjusted to

suit the CCD. Of the bandpass filters that have been selected, one will pass the on-

axis first-harmonic FEL light. Another will pass red-shifted light, which will be

off-axis and in the shape of a cone around the axis, with the angle between the

cone and the axis depending on the wavelength transmitted. Using the red-

shifted light to guide adjustments of the relative trajectories through two

consecutive undulatory may allow more accurate adjustments. The red-shifted

light appears as a ring, and two rings are easier to align than two spots. Also, the

width of the ammlus of red-shifted light is smaller than the size of the on-axis

spot, so the difference is between aIigning two sharp rings as opposed to two

broader spots.

As shown in Fig. 3, a mirror is inserted into the particle (and light) beam path

in order to reflect the FEL or alignment laser light into the optics at the upper left

of the figure. This mirror will have three positions: one where the mirror is

removed born the beam path, one where the mirror completely blocks the beam

and reflects all the light, and one with a small hole to allow the particle beam to

pass, unperturbed, while still reflecting much of the light into the optical system.

Demanding requirements have been placed on the motion of this mirror so that ~.

the position of the light on the CCD is repeatable to within a pixel despite the

approximately l-m-long distance between this mirror and the next mirror in the

light path. In order to more readily achieve this repeatability, the direction of

I!2

motion of the mirror between its different positions is parallel to the plane of the

mirror face.

Another use for the optics in the upper left of Fig. 3 is as a diagnostic for the

light produced by the FEL. Each set of these optics will be calibrated to the same

intensity standard. They will then be used to measure the intensity fkom each

undulator individually, as follows. The mirror in the particle beam path after the

fist undulator will be positioned so that the hole allows the particle beam to pass

unperturbed. A small fraction of the undulator light will also pass through the

hole, but most of it will be reflected into the optics where the absolute intensity of

the light from the first undulator will be measured. The small amount of light

that passes through the hole is still all the light from the first undulator that -

would interact with the particle beam in the second undulator to induce

bunching. When the Iight is viewed after the second undulator, the contribution

flom the first undulator will be a small portion of the total intensity. Since the

undulatory are longer than a gain length, almost all the intensity will be from the

second undulator. The light intensity horn the different undulatory can then be

compared. If no beam bunching is occurring, then the absolute intensity seen

after each undulator will be the same. Intensity measurements can be made

after every one of the undulatory for a single incident beam pulse, so that the

intensity growth can be studied without variations introduced by changes in the

incident beam.

13/’

—— .—. — -.——— . . .. . ___ .. . ..

A second diagnostic of the l?EL light will be located in an end station. A

Paschen-Runge-type spectrograph will be placed on the low-radiation side of a

shielding wall at the downstream end of the undulator line. A schematic of the

spectrograph is shown in Fig. 4. It will be used for high-resolution spectral

measurements near the first harmonic, and since the goal is to measure the

spectral structure in the SASE light, each pulse born the linac will be individually

measurable. Light sent to this station will have been picked off after any one of the

undulatory (including after the last undulatory), using the removable mirror

shown in the upper-left: of Fig. 3vand then passed through a hole in the shiekling

wall. It will go through a bandpass filter and, if necesssxy, a neutral density

filter before being reflected and focused onto the slit by a concave mirror. The slit,

the spherical grating, and the detector all Iie on a Rowland circle. The CCD wilI

be cooled to reduce the dark current and improve the signal-to-noise ratio. It is

expected that the light” &om a single unduIator with no FEL amplification and

from a single incident bunch will be readiIy measurable.

14/’

.-. ..—. ___ ———.. .. . -— ,—. - -

References

[1] S.V. Milton, E. Gluskin, N.D. Arnold, S. Berg, W. Berg, Y.-C. Chae, E.A.

Crosbie, R.J. Dejus, P. Den Hartog, H. Friedsam, J. N. Galayda, A. Grelick, J.

Jones, Y. Kang, S. Kim, J.W.. Lewellen, A.H. Lumpkin, J.R. Maines, G.M.

Markovich, E.R. Moog, A. Nassiri, E. Trakhtenberg, I. Vasserman, N.

Vinokurov, D.R. Walters, J. Wang, and B. Yang, Nucl. Instrum. Meth. Phys.

Res. A 407 (1998) 210, and S. V. Milton, J.N. Galayda, and E. Gluskin, “The

Advanced Photon Source Low-Energy Undulator Test Line”, proceedings of-.

PAC97, held 12-16 May 1997, Vancouver, B.C., Canada.

[2] R.J. Dejus, O.A. Shevchenko, and N.A. Vinokurov, “An Integral-Equation-

Based Computer Code for High Gain Free-Electron Lasers”, these proceedings.

[3] ICE. Robinson, D.C. Quimby, J.M. Slater, IEEE J. Quantum Electron. QE-

23 (1987), 1497.

[4] I. Vasserman, Argonne National Laboratory Report No. ANLJAPWTB-32,

1998.

[5] R.J. Dejus and I.B. Vasserman, Argonne Natio&l Laboratory Report No.

APS/IN/LEUTL/98-1, 1998.

[61 Glenn Decker, Argonne National Laborato~, private communication.

[7] J. Sfianek and P.M. Stefan, proceedings of EPAC ’96, the Fifth European

Particle Accelerator Conference, p. 1573.

[81 B. Yang, Argonne National Laboratory, private communication.

M,

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TABLE I. FEL parameters

*

if

Wavelength I 532 run

Beam energy 220 MeV

Normalized emittance 5X mm-mrad

Peak current 150 A

Energy spread 0.1 %

Focusing separate quadruples

Undulator period 33 mm

Undulator parameter K 3.1

IUndulator effective field I 10.061 kG

INomin& magnetic gap I 9.3 mm (tIxed)

IUndulator length I 2.4 m

I Cell length I 2.7265 m

! Number of cells 11.2 IGain length -0.8 m I

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TABLE II. Acceptable tolerances

1Parameter tolerance ILongitudinal undulator displacement 1 mm

Vertical undulator displacement 150pm I

ILateral undulator displacement lmm

Horizontal alpha function, (XX 0.20

Vertical alpha function, a, 0.20

Horizontal beta fimction, ~x 0.50 m

Vertical beta ftmction, & 0.20 mi

Horizontal incident beam coord., ~ 200 pm

Vertical incident beam coordinate, yO 150pmI

Horizontal incident beam angle, x: 100 prad

Vertical incident beam angle, y: 50 prad I

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Figure captions

Fig. 1. The beta fimction for a cell of the FEL lattice. The cell consists of a

7.785-cm drift length, followed by a 5-cm quadruple, then a 22.665-cm drift length

for optical diagnostics, and finally a 233.7-cm undulator (not including the end

poles).

Fig. 2. The height of the magnetic center for each pole along an undulator at

gaps of 9.3 mm and 10.5 mm. Therms error is 23 p.

Fig. 3. Schematic of the diagnostics section (not to scale)

Fig. 4. A top view of the Paschen-Runge-type spectrometer that will analyze

the light horn the S~E FEL.

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— SphericalGrating

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,,..,, ,,, -,,.7,.- r<~, .= —-+WC, .—v--i . . . .. . ,. . -. m,,. .’ e . . . .. ‘.-T--T-T -. .,-.< . . . . . . . . . . . . . . ,.. . . . .. .. ,.