BL05SS: Undulator Radiation and its Application to Beam Diagnostics(1) Accelerator diagnostics beamline II (BL05SS)
BL05SS has an insertion device light source, two optics hutches, and an experiment hutch.
The insertion device (ID05) is out-of-vacuum type and designed to be able to fit many kindsof experiments. The mounted pure-permanent magnet array, which is made of the ternary
alloy Ne-Fe-B (NEOMAX-44H), is of planar Halbach type with the 51 periods of 76 mmlong. When the magnet gap is a minimum value of 20 mm generating a peak field of 0.82 T,
the deflection parameter K is a maximum value of 5.8, which leads to a multi-pole wiggler(MPW) radiation produced on the many higher-order harmonics that are useful for electron
beam diagnostics. The MPW can emit the total radiation power of 10.4 kW when theelectron beam current is 100 mA. By widening the magnet gap, ID05 as an undulator withsmall K can be also useful for the experiments requiring the higher spectral photon flux andthe lower heat load to avoid the thermal drift of a frontend slit or the monochromator.
The layout of the X-ray transport channel of BL05SS is shown in Fig.1. The front end has aX-slit and a Y-slit to shape the white X-ray beam of the ID05. The optics hutch I has thegraphite filters and the metal (aluminum) filters, which have moving mechanisms to adjust
the intensities and the spectrum of white X-ray beam. The optics hutch II has a cryogenic
double crystal monochromator, which covers the photon energy range of 4 to 38 keV by(111) reflection of silicon crystals. Beryllium windows installed in the experimental hutch
allow us to handle the monochromatic X-ray beam in atmosphere. To develop techniques forthe beam diagnostics based on observations of synchrotron radiations (SR), in theexperimental hatch, we can measure energy spectra, angular spectral fluxes and temporal
structures of undulator radiations from ID05.

Figure 1: Schematic view of the SPring-8 diagnostics beamline II (BL05SS).
(2) Energy spectrum & Angular spectral flux of ID radiation
The energy spectrum produced by a filament electron beam in a given direction consists ofa series of harmonic peaks, the frequency of which are multiples of the fundamental resonant
frequency ω1. For a planar undulator, ω1 is given by2cγ 2
ω1 =,λu (1+ K 2 /2 + γ 2θ x2 + γ 2θ y2 )eq. (1)

where λu, K, γ, c, and (θx, θy) are the period length, the deflection parameter, the electron
beam energy divided by mc2, the light speed, and the angle between the electron and theobserver, respectively.
For a single electron and an observation point, the lobe of on-axis€emission is zero for all even harmonics 2, 4, 6, while it reaches a maximum for the odd
harmonics 1, 3, 5, the spectrum widths of which are equal to 1/nN where n is the harmonic
number and N is the number of periods. For a given point of observation, all electrons of a
thick beam do not have the same resonant frequency ω1. This can occur through the angular1

divergence of the electron beam, which spreads both the horizontal and vertical angles of (θx,θy). A spread of the transverse position of the electron beam also induces a spread of ω1
because an observer located sufficiently close to the source point sees the radiation emitted
by the various electrons under a different angle (θx, θy). Equation (1) indicates that the spreadin the angle θx and θy shifts the resonant frequency ω1 to the lower energy side only. As aresult, the electron beam emittance leads to the low energy tail of the spectrum through a slit.
The energy spread of electron beam also has an effect on a spread of ω1. If the relative
energy spread σ /γ of the electron beam is Gaussian, it introduces a Gaussian spread in thespectrum of each harmonic:γσσω=2 γ .
ωγeq. (2)To be summarized as follows, the peak of the harmonics observed on-axis of the electron
beam always has a steep slope on the high energy side which is dominated by nN and by σ /γ,
and has a reduced
slope on the low energy side which is dominated by the emittance, beta€
function and observation aperture limited by the finite slit width.
The energy spectrum is obtained by measuring the spectral photon flux using an ionizationchamber set up in atmosphere, while changing the energy of monochromatic X-ray byscanning Bragg angle of the Si(111) double-crystal monochromator. A rectangular tantalum
slit is placed in front of the ionization chamber to measure the on-axis photon flux of ID05.
The aperture of the slit was 4.2 µrad × 4.2 µrad. The nominal equilibrium emittance andrelative energy spread of 8 GeV stored electron beam are 2.4 nm.rad and 1.1×10-3,
respectively. A typical example of the energy spectrum of the fundamental harmonicmeasured at the magnet gap of 80 mm, which gives the deflection parameter K of 0.45, isshown in Fig.2. The tail of the low energy side comes from the horizontal emittance. Theeffect of the slit aperture is negligibly small. The slope on the high-energy side is dominatedby 1/nN~0.02. The energy spread has a negligible effect on it. When the magnet gap is theminimum value of 20 mm, the deflection parameter K of 5.8 leads to the fundamental
harmonic of 0.45 keV. As a consequence, many higher-order harmonics are observed in thephoton energy range of 4 to 30 keV (Fig. 3). For the harmonics higher than the 19th, theenergy spread has the non-negligible effect on the slope of the high-energy side.

2

photon flux (photons/s/mrad /0.1%b.w./mA)

γ

2.5 1015

fundamental harmonic @ID05 magnet gap 80mm

152 101.5 1015
1 1015145 10

0
6.6

6.8

77.2

7.4

7.6

photon energy (keV)

Figure 2: Measured on-axis energy spectrum of the fundamental harmonic when the magnet
gap is 80 mm. The deflection parameter K is 0.45.

22

photon flux (photons/s/mrad /0.1%b.w./mA)Higher order harmonics
K=5.8 @ID magnet gap=20mm

8 101415th147 1019th14

6 10

23th

27th

14

5 10

31th35th14

39th4 1014

3 10142 1014

1 1006810

12
photon energy (keV)

141618Figure 3: On-axis energy spectrum measured at the minimum magnet gap of 20 mm, which
gives the deflection parameter K of 5.8. We can see many higher-order harmonics.
Angular spectral photon flux can be also observed using an X-ray CCD camera. Fig. 4shows an example of the angular spectral flux observed at the photon energy of 8.3 keV,which corresponds to the peak of the 19th harmonic for the magnet gap of 20 mm. The
center spot is the spatial profile of the 19th harmonic. The outer rings correspond to theoff-axis radiations of the 20th and 21st harmonics. The horizontal width of the center spot isdetermined by the horizontal angular divergence of electron beam, while the vertical width isdominated by the energy spread of electron beam and the intrinsic angular divergence(λ/2L)1/2 of undulator radiation, where λ and L are the wavelength of the radiation and thetotal length of ID, respectively. The vertical angular divergence of electron beam has anegligible effect due to small emittance coupling ratio.	 

Figure 4: Observed angular spectral flux at the photon energy of 8.3 keV corresponding to
the peak of the 19th harmonic for the minimum magnet gap of 20 mm. The center spot is the19th harmonic radiation, and two outer rings correspond to the off-axis radiations of the 20thand 21st harmonics.

(3) Application: Emittance diagnostic technique using single-slit X-ray Fresnel diffraction
A new emittance diagnostics technique based on X-ray Fresnel Diffractometry (XFD) [1] hasbeen developed for measuring micron-order electron beam size at a source point of an insertion
device (ID). The XFD uses monochromatic X-ray Fresnel diffraction by a single slit with
optimized width to create a double-lobed diffraction pattern as shown in Fig. 5. The principle isbased on the correlation between the depth of a median dip in the double-lobed pattern andsource size at the ID, namely the valley of the dip shallows with increase of the source size. An3example of the vertical source size measurement by the XFD at the beam diagnostics beamline
II (BL05SS) with ID05 is shown. The undulator radiation of 7.2 keV was diffracted by afront-end 4-jaw slit with the optimized vertical width of 150 µm and horizontal width of 200µm. The slit is located at distance L=26.8 m from the source point. The Fresnel diffraction
pattern was observed by using a high-resolution X-ray imaging system (HAMAMATSU) which
consists of a P43 fluorescent screen, an imaging optics with lenses and a CCD camera. Theimaging system is placed at distance R=65.4 m from the slit. Figure 6 shows the observed
double-lobed diffraction images for some different vertical emittances. The double-lobedstructures are clearly observed in the vertical direction, though in the horizontal direction, thediffraction patterns are smeared out due to large horizontal emittance. The degradation ofcontrast of the double-lobed patterns indicates growth of the vertical electron beam size at theID05 source point due to increase of the XY betatron coupling. The smallest among the beam
sizes evaluated from the double-lobed patterns is 8.1 µm (r.m.s.) for the 2D image in Fig. 6(a)with the highest contrast. A light source size at the ID smaller than 10 µm is successfully
resolved by the XFD.
[1] M. Masaki et al., "X-ray Fresnel diffractometry for ultralow Emittance diagnostics ofnext generation synchrotron light sources", Phys. Rev. ST-AB 18, 042802 (2015).

Figure 5:

Layout of ID source size measurement using the XFD.

Figure 6: Double-lobed diffraction images observed at four different vertical emittances,changing operation points by moving horizontal betatron tunes νx, turning on/off skew
quadrupole magnets for XY betatron coupling correction.

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