The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Insertion Devices for Third-Generation Light Sources
Hideo Kitamura
JAERI-RIKEN SPring-8 Project TeamThe Institute of Physical and Chemical Research (RIKEN)
SPring-8, Kamigori, Ako, Hyogo 678-12, Japan

Abstract
By the introduction of the concepts of short-period
undulators or mini pole undulators, we can raise the upper
limit of the fundamental photon energy of undulator
radiation up to 400 keV region without any unreasonable
heat load, so that the utilization of synchrotron radiation
without any wiggler may be possible. Furthermore, we
can bring down the lower limit of the fundamental without
either unreasonable heat load or unnecessary higher
harmonics by adopting helical undulators.

considered. In case of 3rd-generation light sources,
however, the above solution is thought to be undesirablefrom the view point of heat load problems as well as cost
performance, because most of construction cost or man
power has been shared for obtaining undulator radiation
with high brilliance in 3rd-generation light sources, wherehigh brilliance or high coherent flux should be pursued byundulators without any unreasonable heat load. From thispoint of view, this paper describes what kind of insertion
devices should be installed in 3rd generation light sources.

2. UNDULATORS FOR HARD X-RAY

1. INTRODUCTION
A. Short-period undulators
Synchrotron radiation (SR) from bending magnets has
wide-band characteristics ranging from far infrared to xray region, so that one can obtain a monochromatic lightat any wavelength by using an appropriate monochromator. Although it is obvious that such characteristics
have initiated the advance of synchrotron radiation science,
it should be noted that the spectral band is too widecompared to that of the resolution of the monochromator.
As a result, the monochromatized light may be
accompanied with unwanted stray or higher order lights
and the optics may be damaged by heat load derived fromthe source itself. Such heat load may be called as
unreasonable heat load because it is brought mainly byunnecessary spectral part of the source. Therefore, wecan say that the spectral property desired for synchrotron
radiation source is not wide-band, but narrow-bandcharacteristics with wide tunability, which is one of themotivations for the development of insertion devices,particularly undulators.
The undulator, a single-mode source in thetransverse phase space, generates a single cone of radiation.
The angular spread, a,.= .fi1i [1] where Lis the lengthof the undulator, is so narrow that the property of theradiation may be greatly improved by lowering beamemittance. On the other hand, the bend source is regarded
as a multi-mode source since the radiation is composed ofmany small unit sources being distributed in the orbital
plane. The angular radius of the unit source is given
approximately by a,.= 0.3JYpy-l [2] where p is theradius of the bending magnet. The wiggler, another kindof insertion device, is regarded as a superposition of many
bend sources. Therefore, the essential nature of theradiation is the same as that of the bend source. The
efficiency of the improvement by lowering emittance is
not so high compared to the case of undulator radiation.
Furthermore, the optics may be damaged by unreasonable
heat load.
Nevertheless, for the utilization of very high energy
radiation, the use of high field wigglers is generally

Since the minimum vertical vacuum gap is allowed as ......,
25 mm in case of ordinary storage rings, the minimum
magnetic gap of undulators may be "-'30 mm taking thethickness of the vacuum chamber into account. To obtainthe tunable range of one octave, the maximum deflection
parameter, Kmax, is required as 1.5, which may be satisfied
by the undulator with the period length of 50 mm. In thenear future, the SR community can use the beam energy
up to 8 GeV (SPring-8[3]) except for the large high-energy
machines, Tristan-MR[4] and PETRA[5J. Therefore, thefundamental of undulator radiation covers the photon
energy range up to 10 keV with the conventional undulator
described above. Taking the use of higher harmonics up
to 5th into account, the range can be extended up to 40
keV. However, it should be noted that unreasonable heatload may increase. For example, the first and third
harmonics may contribute to unreasonable heat load when
one uses the 5th harmonic. We hope that such an energy
range can be covered by the fundamental. Therefore, wehave to develop short-period undulators with the period
length below 30 mm, which requires magnetic gap shorter
than 10 mm. There are three approaches for realizing such
short-period undulators, thin-wall chamber, flexible

Out-of-Vacuum

Flexible

In-Vacuum

(a)

(b)

(c)

Fig.l. Examples for short-period undulators, (a)thin-wall-32-chamber[6], (b) flexible chamber and (c)in-vacuum
types.

The_]] th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997Table 1. Spectral properties of the minipole undulator, MPU, compared to those of the wiggler, W.
The calculation is made from the beam parameters of SPring-S. The unit of brilliance isphotons/s/mmA2/mradA2 in 0.1% bandwidth.

AUNBorKmmw

90

MPU

350
166

B=1.25 T
K=0.17l:yJ.UI1~·l:.-J.UI1

llrad

llrad

Brillianceat 200keV

kW810
131

51
2.6

360
53

30
27

1.3E17
1.7E18

28.2
0.72

~

chamber and in-vacuum types.
At Advanced Photon Source, APS, the vacuum
chamber with very thin wall of 1 mm has been developedfor realizing a short magnetic gap of 14 mm[6] as shownin Fig.1-a. In the Top-off operation proposed atAPS[7],the beam loss originated in narrow aperture may be
negligible since the loss can be easily compensated byvery short interval of injection. So, much shorter gap will
be realized.
The vertical aperture required for the stored beam
are different among the various operating modes of thering. In case of the conventional undulator with a vacuum
chamber of fixed type, the inner aperture of the chambershould be designed taking the maximum case into accountSo, the minimum gap is generally limited to be aconservative value. On the other hand, in the undulator
with a flexible vacuum chamber as shown in Fig.1-b, theinner aperture can be varied according to the operating
modes of the ring. Therefore, the very short magnetic
gap can be realized according to the beam lifetime required.
The first undulator of flexible type has been developed by
the joint work with LBL and SSRL[8]. Recently, thesimpler one has been developed at HASYLAB[9].
In case of in-vacuum type as shown in Fig.1-c, thepermanent magnets are arranged in the ultrahigh vacuum.
Therefore, the magnetic gap can be made much shorter
than the two types mentioned above because the device
has no vacuum chamber in the gap. Since it is easy toobtain a short gap below 10 mm, undulators with the shortperiod below 30 mm can be realized. However, there are
two serious problems to be overcome. One is outgassing
from the surfaces of the rare earth magnets due to their
pourus structure. The other is irreversible demagnetizationof the magnets during ultrahigh vacuum (UHV) bakeout

Ptota~

p....,.'YkW/mradA227041

at the temperature higher than 100"C. The former can besolved by the coating on the magnet surface by nickel
electroplating or titanium nitride ionplating. To solve thelatter, we have to adoptpermanent magnets having very
high coircivity. In addition to that, it is necessary to apply
aging process to the magnets at the temperature some"!"hathigher than that ofUHV bakeout[lO]. The frrst in-vacuum
undulator has been developed at KEK[11]. The minimum
gap is achieved as 10 mm without any beam loss.
Thus, short-period undulators with a period length
below 30 mm can be realized. With a beam energy of 8
GeV for example, the fundamental of undulator radiation
can cover the energy range up to 40 keV. Taking the useof higher harmonics up to 5th into account, the available
photon energy can be extended up to 80 keV.

B.Minipole undulators
To obtain very high energy photons above 100 keVwith the beam energy of 8 GeV, the use of wigglers may
be generally considered. As mentioned above, however,this solution may bring unreasonable heat load in theoptics. To avoid such a problem, the Ininipole undulator
concept has been proposed(Fig.2). This concept, derived
from that of Inicropole undulators with the period lengthof subrnilimeters[12], aims at the compatibility with thestorage ring operation. Therefore, the period length will
be chosen as several milimeters with the magnetic gap
above 1 m:m. As a matter of course, such a device should
be of in-vacuum type without any vacuum chamber in thegap. To obtain stable orbit with a very narrow aperture,
the betatron function should be designed to be very small,typically 0.1 m, with additional quadrupole magnets
located at the both ends of the device. Such a strong

i..u=1-8mm

phasetype)

Revolver type

50cm

Fig.2

Schematic illustratiobn of a minipole undulator.

Fig.3-33-(helical type)

Machin•.gun type

Schematic illustration of Multiundulator systems forminipole undulatrors, revolver and machine-gun types.

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997
4 10,. ,------.--~S~P-.-r~in~g;_-8'-.-",'--rE_=~8:,_Ge-.-e--,Vr-',~l~=-10-'--0.-m~A~-.----,

1stHelical Undulator"-u=1 Ocm, N=45, K=3.34

2nd~

4

10~ '--"---<-~~~~-'--~~--'--~~L.-..._~__,__J~

Photon Energy (eV)Fig.4

Planar Undulator

0.E"-u=10cm, N=45, K=4.72

heat load!

Calculated spectral brilliance from an example of arevolver composed of 8 mini pole undulators withdifferent periods from 2 mm to 8 mm. The spectrum
from the wiggler is also shown for comparison.

focusing means that the device length should not be solong, typically 0.5 m. However, the number of periods isvery large because of very short-period, so that highbrilliance of the radiation can be expected.
Table 1 shows spectral properties of the mini poleundulator with the period length: 3 mm, the number ofperiods: 166, and the K value: 0.17, compared to those of
the high field wiggler with the period length: 120 mm, thenumber of periods: 50, and the magnetic field: 1.25 T. As
shown in the table, the calculated brilliance of the mini poleundulator is higher by one order than that of the wiggler,and their total fluxes are not so different. In addition tothat, the total radiated power or power density of themini pole undulator is much lower than that of the wiggler.
Although the ring operation may be difficult with minipole
undulators, they are very practical for the x-ray optics.
However, mini pole undulators have a disadvantage;
the K value is much lower than unity, which means thatthe tunable energy range is not so wide. To solve thisproblem, the multi-undulator system, revolver or machine
gun, should be adopted (Fig.3). Figure 4 shows thecalculated spectral brilliance from an example of a revolver
composed of 8 mini pole undulators with different periods
from 2 mm to 8 mm. In the figure, the spectrum from thewiggler is shown for comparison. The spectral
performances of the mini pole undulators is found to bemuch better than that of the wiggler in the photon energy
region from 70 keV to 300 keV.

Photon Energy (eV)Fig.5

Spectral brilliances obtained from the planer and helicalundulators having the same period length of 10 em andproducing the fundamental with the same photon energy.

3. UNDULATORS FOR VACUUMULTRA VIOLET RADIATION
Since the SR community can use the beam energy up to 8GeV, it is easy to obtain the fundamental by conventional
undulators in the vacuum ultraviolet region below 4 keV.
However, another heat problem may appear. As an
extreme case, we suppose that one desires 500 eV photons
with the beam energy of 8 GeV. Table 2 shows the spectral
parameters of 3 possible undulators, UlOO, USOO andHU100, with the same device length of 4.5 m. Although
the brilliance of the planer undulator, UlOO, is as high as1.5 X 1018 , the power density is so high due to the high K
value that the optics may be damaged. On the contrary,
the power density of the other planer undulator, USOO, isvery low. However, the brilliance is only 2 X 1017 due tothe small number of periods, which is a nonsense result
since this value can be obtained easily in 2nd generation
light sources. The last undulator, HUIOO, is of helicaltype with the same period length as that of UlOO. TheTable 2. Spectral parameters of three possible undulators, U100,U500 and HU100 for producing thefundamental with the photon energy of 500 eV. The calculation is made from the beam parametersof SPring-8. The unit of brilliance is photons/s/mmA2/mradA2 in 0.1% bandwidth.
NameType

Polarization

AUNK

Brilliance
at 500eV

Pdensity
Ptota!
kW kW/mradA2

9

1.7
4.7
3.34

2.4E17
1.5E183.0E180.023
4.64.6mm
U500
UlOO
UlOO

planarplanarhelicallinearlinearcircular

500
100
100

4545-34-

1.3
1000.6

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 199'1SPring-8, E=8GeV, 1=100mA
Helical Undulator, A.u=10cm, N=45, K=3.34.....

1 o" r-o--r~--,-~.-~.---.-;.~--,-~.-~,---.---,-~--.

10==

10fl.

1014.ci>.
~ii:

Cl.._CD

10°cw
100

200

300

400

500

600

700

800

900c

1000

00

Photon Energy (eV}

.r:.

a.
Fig.6

Spectral flux obtained from HUlOO for various angular
acceptances. The.horizontal or vertical angulardivergence of the source are denoted as ~ x',y'.

brilliance is 3 X 1018, twice higher than that of UlOO.
Nevertheless, the power density is much lower by 2 orders.
To reveal the above inconsistency, the comparison is madein Fig.5 between the spectral brilliances obtained fromboth undulators. As shown in the figure, the spectrum ofUlOO has many higher harmonics while in the spectrumof HUlOO most of the higher harmonics disappear, whichis the reason why helical undulators have very low power
density. It is obvious that the higher harmonics appearing
in the UlOO spectrum contribute to unreasonable heat load.
In case of helical undulators with high K value,
most of the radiated power derived from the higher
harmonics is existing off axis. Therefore, an appropriate
slit system is required to eliminate not only the heat load
but the higher harmonics. The wide acceptance dose notconfirm the increase of the fundamental flux, but brings
unnecessary higher harmonics as shown in Fig.6, wherethe flux spectra obtained from HUlOO are shown forvarious angular acceptances.

23456 7 810Beam Energy (GeV)
Fig.7 Available photon energy range of various kinds ofundulators for the beam energies from 1 GeV to 10 GeV.

light source is aiming at the production of laser light inthe VUV or x-ray region.

References

4.SUMMARY
We can raise up the upper limit of the fundamental photon
energy to adopt short-period undulators or minipole
undulators as shown in Fig.7 where the available photon
energy range of various kinds of undulators are shown forthe beam energies from 1 GeV to 10 GeV. For example,
40 keV or 400 keV photons can be obtained by shortperiod or minipole undulators, respectively, with the beamenergy of 8 GeV (SPring-8). Furthermore, we can bring
down the lower limit of the fundamental without either
unreasonable heat load or unnecessary higher harmonics
by adopting helical undulators. Although at present sucha merit cannot expected in the utilization of linearlypolarized radiation, the novel undulators, which cangenerate linearly polarized VUV radiation with low onaxis power density, are under development at SPring-8.
The pursuit for pure radiation (monochromaticradiationwithout any higher harmonics) is very importantfor 3rd generation light sources because the next generation

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