Synchrotron Radiation Sources and Research " F C R ~ - ELI L. C. Teng FR3 16fxJa w;

$*I-/ Argonne National Laboratory

9700 S. Cass Avenue Argonne, Illinois 60439

This is an introduction and a review of Synchrotron Radiation sources and the research performed using synchrotron radiation. I will begin with a brief discussion of the two principal uses of partide storage rings: for colliding beams (Collider) and for synchrotron radiation (Radiator). Then I wil l concentrate on discussions of synchrotron radiation topics, starting with a historical account, followed by descriptions of the features of the storage ring and the features of the radiation fiom the simplest source--tbe bending magnet. I will then discuss the special insertion device sources-wigglers and undulators--and their radiations, and end with a brief general account of the research and other applications of synchrotron radiation.

Storage Rings - Colliders and Radiators

Particle accelerators originated from the need to do research in: Nuclear physics - research is done by shooting accelerated particles at atomic nuclei, Particle physics - research is done by shooting p d c l e s at particles.

For these applications the emphasis is naturally on high energy. The phenomenal success in the quest of high energy can be seen from the following: - Year Particle enerm Accelerator - 1930 - 100 keV Electrostatic generators - 2000 - 10 TeV SSC, LHC, etc. This is a 108-fold increase in - 70 years. Not bad!!

A storage ring is a ring of d.c. magnets which can stably store a circulating beam of particles for a long time-from one hour to one month-depending on the design and the particle

* type and energy. Storage rings are used as Colliders (colliding beams storage ring) or Radiators* (synchrotron radiation storage ring).

In a Collider two oppositely circulating beams are stored in two storage rings. The rings are designed to crisscross, thus bring the circulating beams together to collide at designated locations around the circumference. Particles and their antiparticles of the same energy can be stored in one and the same storage ring to circulate in opposite directions and collide. Colliding beams axe used to achieve high center-of-mass collision energies, overcoming the great reduction due to the relativistic transformation in collisions of high energy accelerated particles with particles at rest (beam on stationary target). For two identical particles of energy ycm (in rest energy units) colliding head-on, the center-of-mass energy is 2r,. To get this, the equivalent energy y, of a projectile particle colliding with a stationary particle is y, = 2ym2 - 1. Thus, for the head-on collision of two 10 TeV protons (ym = 10660) the equivalent beam-on-stationary- target energy is 2.13 x 16-TeV = 0.213 EkV (exa-electron Volt = 10" eV). In this sense the energy quest has resulted in a 2 x 10'*-fold increase in 70 years or about a factor 10 every 5.7 years. This will be discussed in more detail in the next talk by Alex Chao.

To get high luminosity (event rate per unit reaction cross-section) one needs to collide very high intensity beams. The intensity that can be used in colliding beams is limited by the

"Icoined this term myself. You will not find it used this way anywhere else.

DETRIBUTION OF MIS DOCUMENT IS UNLIMITED P A

electromagnetic interaction between the beams at the point of collision. The available luminosity has increased only by two or three orders of magnitude since the very first colliders.

A single storage ring can also be used as a Radiator. For a stored circulating e-beam (electron or positron) the centripetal acceleration causes the particles to radiate "synchrotron radiation" which generally comes in short pulses for a bunched beam, is very bright, covers a broad frequency spectrum, and has unique polarization properties. This synchrotron radiation can be used to study a l l types of electromagnetic structures such as atoms, molecules, condensed matters, crystalline structures, and surface structures. The standard way to study electromagnetic structures is to scatter neutral particles-neutrons or photons-off of the structure. The approximate energy range of interest and the available sources of these particles are:

Particle Desired Enernv Ranne Source Twes Neutron (n) meV to eV Reactor, Spallation source Photon (hv) 0.1 eV to 10 keV Laser, FEL, U V lamp, X-ray tube,

Synchrotron radiation source

Figure 1 shows that the energy range covered by synchrotron radiation coincides exactly with what is required, thereby making synchrotron radiation the ideal source of photons for studying electromagnetic structures.

For radiation sources used as research tools the quest is naturally for high brightness which is defined as follows: Brightness = photon flux per unit frequency bandwidth per unit source area per unit emission solid-angle. The commonly used unit of brightness* is

Bu = No. of ph~tondsec/lO-~ k!/mm2/mrad2 . 0 --

T T T band- source solid width area angle

T flux

For synclJotron radiation the source is an e-beam for which the produc (source area) x (solid angle) is minimized when the dispersion widths are zero and the product of emittances is at the lowest value. Over the years the quest for brightness resulted in the curve shown in Fig. 2. We see that in the 40 years from 1960 to 2000 the highest brightness attained increased roughly from lo8 to 1020 Buy mainly due to the advent of synchrotron radiation sources. Figure 3 gives the spectral (energy) distribution of the brightness of the synchrotron radiation from various sources in relation to various atomic absorption edges.

Historical Accounts of Synchrotron Radiation

The electromagnetic radiation created by accelerating charges has been studied

A. Lienard (1898) - a general investigation made shortly after the Lienard- mathematically for nearly a century. The most noteworthy of these studies are:

Wiechert potentials were formulated.

*I also coined the designation Bu for the brightness unit. It seems that such a long and complicated unit deserves a name. To give an idea of the magnitude of this unit, the brightness of the filament of a 100-W light bulb is about 108 Bu.

G. A. Schott (1912) - for charges in uniform circular motion. D. Kerst (1941), L. Schiff (1942), D. Ivanenko, and I. Pomeranchuk (1944) -

various refinements and discussions of the computation. All these works were performed essentially as mathematical exercises on an interesting consequence of the Maxwell equations. The most complete and most often quoted papers published after the first experimental observation were written by J. Schwinger [l] [2].

Synchrotron radiation was first observed in 1945 by J. P. Blewett [3] on the General Electric Company 70-MeV electron synchrotron and was first used for research in 1968 on the 240-MeV Tantalus electron storage ring in Stoughton, Wisconsin at the instigation of E. Rowe. Since then synchrotron radiation facilities have mushroomed all over the world. A recent tabulation is given in Chapter 1 of reference [4].

Features of Storage Rings for E-beams (electrons or positrons1 used as Synchrotron Radiation Sources

Radiators are classified as first, second, and third generation. These are defined as

First generation: The synchrotron or storage ring was built for high-energy physics but used either parasitically or later, after high energy physics utilization is exhausted, converted to be used exclusively for synchrotron radiation research.

Second generation: Designed and built for dedicated use as synchrotron radiation sources, but without the special tailored features.

Third generation: Dedicated synchrotron radiation storage rings designed with special features of high brightness and long beam life, and with many straight sections to accommodate wiggler and undulator sources.

The difference between a second- and a third-generation Radiator is most clearly evident in the magnet lattices of the rings. Figure 4 shows the magnet lattice of the second-generation

, ring at the NSRL, Hefei and Fig. 5 gives that of the third-generation ring at the SRRC, Hsinchu. In the following we give a discussion of the special features of a third-generation Radiator.

follows:

Magnet lattice a. Long zero-dispersion straight sections - The ring is composed of these straight

sections joined by achromatic bends. The high-brightness wiggler and undulator sources are accommodated in these straight sections where the beam widths and divergences are small.

Low emittance - In addition to producing achromatic bends, the quadrupole lattice is arranged in order to yield the minimum practical emittance. This requirement always results in a ring with very strong focusing.

. b.

Beam chamber and vacuum system Chamber geometry, material and surface treatment are designed to yield high

vacuum (c generally either aluminum or stainless steel. Midplane slots are sometimes provided on the large radius side to remove the synchrotron radiation outgassing sites away from the beam. Distributed pumping along the whole length of the vacuum chamber is provided by non-evaporative getter pumping strips next to the outgassing sites.

Conductive rf skins are installed inside the beam pipe to shield flanges, bellows, and other discontinuities so that the beam sees only a continuous smooth pipe. This continuous smooth surface minimizes the voltage induced in the beam pipe by the beam current. (Induced

a. Torr) under the rather heavy synchrotron radiation outgassing. The material is '

b.

'. ,

voltage acts back on the beam and causes instability which limits the maximum beam current that can be stored.) In this manner one can obtain high current, high stability, and long life for the stored beam.

Orbit vibration The orbit of the very thin beam must be extremely stable so that the synchrotron radiation

spot on the experiment target many tens of meters away does not jitter more than some 10% of the spot size which can be less than 0.1 millimeter. The orbit jitter can be caused by mechanical vibrations and power supply noises. In addition to minimizing these causes one can also use electronic feedback systems to reduce the orbit jitter.

Iniection energy In principle the storage ring should best be operated d.c. and the beam should be injected

at the storage energy, but some cost savings can be incurred by injecting at a lower energy and . ramping the storage ring up to the storage energy.

Radiation sources Bending magnet - The 360" of bending magnets in the storage ring constitute the

unavoidable source of synchrotron radiation. The dipole radiation has rather broad spectral and angular (horizontally fan-out) distribution and was the first to be used for doing research. The brightness of the dipole radiation is comparatively low, but the total radiated power is generally the largest of all sources in the ring.

Since the critical energy of the dipole radiation is proportional to the dipole field strength, one can obtain radiation of a desired energy (frequency) from the dipole of a chicane* with the appropriate field strength inserted in a straight section. This is called a frequency shifter.

Insertion devices: wigglers and undulators - These are specially designed sources I which are inserted in the straight sections of the ring. They are designed to give specific desired radiation spectrum and brightness and will be discussed in detail later.

a.

b.

c.

Choice of beam enerm One can get a wide range of critical energy E, (= ,E2B with E E e-beam energy and B E

dipole field strength) from the frequency shifter but since the radiative power P has a different dependence on By namely P = E2 B2, the useful range of the frequency shifter is limited. For . example, to lower E, by a factor 10 using B alone will lower P by a factor 100 to a value too low to be useful. Beyond a factor 10, one needs a different e-beam energy - dl0 = 3.2 times lower. Thus, the e-beam energies in dedicated synchrotron radiation sources generally fall into two groups: 1-2 GeV for VUV lights and soft X-rays, and 6-8 GeV for hard X-rays. This grouping is clearly evident from the e-beam energies of existing synchrotron radiation facilities.

Choice of beam particle Electrons and positrons give identical synchrotron radiation. The only consideration is

that an electron beam, being negative, will attract and trap positive ions from the residual gas in ~~ ~ ~

*A chicane is a series of short dipoles, some with reverse field to form a local bypass (detour) in the orbit. The dipoles can be of arbitrary field strength.

the vacuum. This neutralizes the beam and sometime causes it to be unstable. This problem can be avoided by leaving a chumferencial gap in the beam to let the trapped ions escape. A positron beam does not have this difficulty, but is somewhat more difficult to produce.

Features of DiDole Radiation

The synchrotron radiation from a charged particle in uniform circular motion is the simplest to analyze and is the subject of most of the early papers. We wil l only exhibit the results and refer to these papers for derivation.

S w t r a l and anpular distributions

The spectral (0) and angular (Q) distribution of the radiated power P for a single electron is

d2P 3e2 f q2 X2

m o 4n2 2np 1 + x 2 1 + x 2 -= - -

where I& and K,, are the modified Bessel functions, + x 2 y n ,

and = critical frequency,

where shows in the

x E w, y = vertical angle. The fact that only do, and x = w appear in the formula that (1) the spectrum shifts up or down together with o, and (2) the radiation is a fan-out horizontal orbit plane and has a vertical angular spread of the order of lly radian. Integrating over o we get the total angular distribution

1

7 1 + x 2 integrating over Q we get the total spectral distribution

dP - 8n e2 d o 9 2np

- - where

and integrating over both o and Q we get the total radiated power

which gives, in turn, the radiated energy per revolution

2np - - - 4n e2 - E4 = (8.85~10-~ L] - E4 . C 3 (mc2)4 P GeV3 P

Us-

. .

For a circulating beam of current I in a Radiator we only have to replace 2np in these formulas by ecL. The normalized spectrum S is plotted in Fig. 6 and the angular distribution is illustrated in Fig. 7.

It is interesting to note that the emission of photons by a stored electron is really rather sparse. In a 3-m-long dipole of the Argonne Advance Photon Source (APS) an electron emits only about three photons on the average. This amounts to about 245 photons per revolution of the 1104-m-circumference storage ring which contains 80 dipoles.

Polarization Also shown in Fig. 7 is the fact that in the midplane the radiation is linearly polarized

with the electric field lying in the midplane. Going away from the midplane the radiation becomes elliptically polarized. At x P l’or w 2 l/y the ellipticity is -0.6.

Time structure The e-beam is generally bunched with bunch length of the order of tens of picoseconds.

Thus the time structure of the synchrotron radiation is also in pulses tens of picoseconds long with flexibly adjustable pulse repeating patterns.

Spin-flip radiation Given above is the principal part of the dipole radiation which is produced by a classical - -

point-charge. If the ProperDirac-particle electronis treated, one finds in addition to the radiation described above, a much weaker magnetic dipole radiation emitted by the magnetic moment of the particle. This radiation, however, causes spin-flips, and thereby polarizes the beam. This was fist discovered and studied by A. A. Sokolov and I. M. Ternov in 1963. The polarization time constant z is given by

- e2 mc h

mc

where ro - - = classical electron radius

h = - = electron Compton wavelength;

For the CERN e*-Collider LEP, E = 55 GeV, p = 3096 m and we have z = 97 min. The final . equilibrium polarization value is Pq = 8/(543) = 92.4%. .

Wigglers and Undulators (Insertion Devices, ID)

In its elemental form an insertion device is an array of short dipoles alternating in polarity which is placed in a straight section. The alternating dipole field produces a wavy orbit over the length of the ID but leaves untouched the orbit outside the ID. The dipoles are excited either electromagnetically by conventional or superconducting coils or by permanent magnets. Conventional copper coils are bullq and are generally used only when the period length & is long. Superconducting coils are capable of producing high fields at shorter period lengths. For k,, 10 cm strong rare-earth permanent magnets such as neodymium-iron-boron (Nd-Fe-B) are widely used (Figs. 8 and 9).

History Concept first proposed by H. Motz First wiggler built and used for research

1953 1979

Permanent magnet IDS proposed and

(Undulators with up to 100 periods are now widely used) developed by N. Vinokurov and K. Halbach 1980s

Radiation We first define the deflection parameter K (assuming sinusoidal deflection field)

max. orbit deflection angle - 1 B k 1 K = - radiation spread angle 27t mc2P = 0.934 [Bo] [hu(cm)] for P G 1

where B is the peak field and (Bp) = & mc2/e is the magnetic rigidity of the beam. If the orbit deflection angle is much larger than the radiation spread K >> 1, the radiations

from different periods of the orbit wiggle are not coherent, each having the same broad spectrum as a dipole radiation. The radiation from all N periods (2N dipoles) will be incoherently superposed to give 2N times the flux. Such a device is called a wiggler. The wiggler radiation has the same spectral and vertical angular distributions as the dipole radiation. However, the horizontal angular distribution, instead of being a fan-out, has a spread of 2Kly radian. In addition to increasing the flux one can shift the spectrum of the radiation over a wide range by varying the field strength of the wiggler dipoles. Thus, a wiggler can also be used as a frequency shifter.

If, on the other hand, the maximum orbit deflection angle is of the order of or less than the radiation spread K 3 1, the radiations from different orbit wiggles are coherent. Such a device is called an undulator. The radiation from an undulator has a line spectrum (Fig. lo).* The wavelengths of the lines can be derived simply by first Lorentz transforming the undulator , field to the rest-frame of the beam. The radiation by the beam in its own rest-frame induced by the undulator field is then Doppler shifted back into the laboratory-frame. This gives

where n is the harmonic order and 0 is the deviation angle of the direction of observation from the forward direction. The widths of these lines are simply = l/nN and the peak .brightnesses of the lines are proportional to p. With symmetric longitudinal pole contour the radiation of a single electron will have only odd harmonics, but the radiation from a beam of particles will also have even harmonics, although they are relatively much weaker.

The expression for & indicates that the wavelength of the undulator radiation can be "tuned" through the deflection parameter K by varying the gap, hence the peak field B. Table 1 gives the major parameters of the standard wiggler and undulator used at the APS.

We shall not reproduce the formulas for the spectral and angular distributions of the undulator radiation here. For special cases and under some simplifying approximations the radiation can be expressed in analytical forms which can be found in standard literature. In general, for realistic undulator field shapes and for bearps with finte emittances and energy spreads extensive computer calculation is required. In Fig. 11 we show the comparison between the measured and the calculated spectra of the radiation from the APS standard undulator. The agreement is excellent.

*The dotted undulator curves in Fig. 3 are the envelopes of the line spectra.

___ - - - _ _ __

Period length, & Number of periods, N Minimum gap, g,, Maximum deflection parameter, Total length, L Permanent magnet material Pole material

Table 1 Standard Insertion Devices Used at the APS Undulator A Wiggler A 3.3 cm 8.5 cm 72 28 10.5 mm 21.0 mm 2.57 7.94 2.4 m 2.4 m

Nd-Fe-B Vanadium-permendur

The undulator wiggles the b e k at the period length %,, (generally - cm) and radiations from different periods are coherent. In a free-electron laser, however, the strong radiation- particle interaction bunches, or at least modulates, the beam at the radiation wavelength & (generally < p). Thus, one gets particle coherence (at least partially) and a much higher brightness.

n u x and brightness limits The radiation flux incresses with the e-beam current which is generally limited

by beam instabilities. The voltage induced in the beam pipe acts back on the beam particles and causes instabilities at high beam current: The induced voltage is given proportional to a coupling impedance Un of the beam pipe. In practice very careful provisions in the design and construction of the beam pipe can reduce 2% to a fraction of an ohm but not much further. This generally limits the maximum attainable beam current to not much more than one ampere.

The flux also increases with the total length or the period number N of the undulator. The "useful length" & limitkd by the focal depth of the e-beam, diffraction of the radiation, and the construction imperfections of the undulator. Beyond the "useful length" one runs into a situation of diminished return. At present, however, this limit is rather far beyond the practice. The lengths of undulators are now determined by more mundane considerations such as need, cost, ease of handling, etc.

The brightness increases with decreasing emittance of the e-beam. The "apparent emittance" is limited by diffraction of the emitted radiation. If the transverse dimension of the source is 6x, diffraction will enlarge the angular spread of the radiation by 6x' = 112 (h16x). Again, one runs into diminished return at emittances smaller than &d = 6 x 6 ~ ' = h12. For a '10- keV X-ray this is &d = 0.62 x lo-'' m-rad which is about 1% of the horizontal emittance now attained in most third-generation Radiators or about the same as the vertical emittance with 1% verticalhorizontal coupling. Thus, the diffraction limit allows only - 100-fold increase in brightness by further reducing the e-beam emittance.

a.

b.

c.

Exotic insertion devices Many different exotic ID designs have been proposed, some more realistic than others.

We will lis; some of them here without discussion. Most of these proposals are aimed at extending ID capabilities along two directions.

Different polarization states - Configurations have been proposed which yield radiations with turnable elliptical and hybrid polarization states. These structures include the horizontaVvertical superposed ID, the helical ID, the planar helical ID, the asymmetric-pole ID, the crossed undulator, etc. ,Ultra-short period length - These include pulsed air-core structure, enhanced-high- harmonic structure, miniaturization using LIGA technology, microwave-in cavity or waveguide undulator, plasma-wave undulator, etc.

a.

b.

Svnchrotron Radiation Research and Amlication

We have already mentioned that photon scattering is one of the most powerful and generally applicable methods of studying electromagnetic structures. The different types of physical processes that can be used for these studies are:

Total absorption - Photoexcitation, Extended X-ray Absorption Fine Structure (EXAFS): hv + (excitation) Elastic scattering - Thomson scattering, Diffraction scattering: hv + hv Inelastic scattering - Raman effect, Fluorescence: hv + hv’ + (excitation)

Photoemission: hv + e + (excitation) Photoionization - photoelectric effect: hv + e + (ion) Photodissociation: hv + (dissociated species)

a.

b. c. d. e. f. g.

’ Compton scattering: hv + hv’ + e

In addition to its very high brightness, synchrotron radiation is energy tunable, and angle and . time resolved. These features make the initiation and the observation of the above reactions more

precise and more tractable, thus producing data that are more critical and more informative. For medical and industrial application we list here the most widely sought-after

possibilities: a. b. Micromachining (LIGA technology) c.

d. Microscopy, Topography, Holography

APS Photos

Lithography - for sub-micron X-ray etching of computer chips

Angiography - Using subtraction-absorption technique to image the heart and blood vessels in real-time motion

Finally, I show two photographs of the newest Radiator coming into operation-the 7- ’GeV Advanced Photon Source (APS) at the Argonne National Laboratory. Figure 12 shows a section of the storage ring tunnel with an APS standard undulator installed in one of the 40 straight sections.. Figure 13 gives an aerial view of the entire APS facility.

Referknces

J. Schwinger, Phys. Rev, 70,798 (1946). J. Schwinger, Phys. Rev, 75, 1912 (1949). J. P. Blewett, Phys. Rev, 69, 87 (1946). H. Winick, ed., Synchrotron Radiation S o u r c e s 4 Primer, World Scientific Publishing Co. (1994).

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof. nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, procws, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not. necessarily state or reflect those of the United States Government or any agency thereof.

ENERGY WAVELENGTH (eleclron- (centi-

volts) meters)

10-6- 10-5

10-3 10-2 10-1

100 10'

FM I Tubes I I - 102

10-~-waves f a Bee - 100 - 101

- Magnetrons I - 10-1

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Figure 1. The electromagnetic spectrum and the range covered by synchrotron radiation.

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- - X-Ray Tubos b - -

I 1 I 1 I I .

106 lo7 io8 109

1900 1020 1940 1880 1980 zoo0 2020 Ymr

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I Accelerators I Proton - 10-13

I Isotopes - -Gamma I I - Rays I - I

Figure 2. Spectral brightness of x-ray sources as a function of time.

10'

c

f 10'

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f 7' 101

f

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7

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a 101; 0 - 2 d 10"

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Wavelength, MI 124.0 1 2 4 t.24 blN

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Figure 3. Spectral brightness as a function of photon energy for various synchrotron radiation sources.

E c

-1 0.8

0.2 - 0.1 -

0 1 I 1 I - 0 Q5 LO 15 20 25

€ = W / u c - u/u,

Figure 6. Normalized power spectrum S(o/o,) of the synchrotron radiation from a bending magnet source.

Figure 7. Emission pattern of radiation from relativistic electrons in circular motion.

Figure 8. Magnet and pole arrangement of a hybrid permanent magnet undulator.

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Figure 9. Drawing of a 30-period adjustable-gap undulator showing the support structure.

1019

10'8

10"

10'6

IO'S

10" 0 1 2 3 4

PhatonEnergy (kev) .

Figure 10. Spectrum of radiation from an Advanced Light Source undulator D.

1

0 10 20

kSV

30 40

Figure 11. Comparison of calculated and measured radiation fluxes from APS undulator A.

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