Beam Diagnostics of the SPring-8 Storage Ring using Undulator Radiation(1) Accelerator diagnostics beamline II (BL05SS)
BL05SS has an insertion device light source, two optics hutches, and an experiment hutch. Theinsertion device (ID05) is of out-vacuum type and designed to be able to fit many kinds ofexperiments. 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 76mm long. When the magnet gap
is a minimum value of 20mm generating a peak field of 0.82T, the deflection parameter K is amaximum value of 5.8, which leads to a multi-pole wiggler (MPW) radiation produced on the manyhigher-order harmonics that are useful for electron beam diagnostics. The MPW can emit the total
radiation power of 10.4kW when the electron beam current is 100mA. By widening the magnet gap,
ID05 as an undulator with small K can be also useful for the experiments requiring the higher spectral
photon flux and the 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 a X-slit
and a Y-slit to shape the white X-ray beam of the ID05. The optics hutch I has the graphite filters andthe metal (aluminum) filters, which have moving mechanisms to adjust the intensities and thespectrum of white X-ray beam. The optics hutch II has a cryogenic double crystal monochromator,
which covers the photon energy range of 4 to 38keV by (111) reflection of silicon crystals. Beryllium
windows have been installed downstream of the monochromator. We can measure energy spectra,
angular spectral fluxes and temporal structures of synchrotron radiation (SR) from ID05 inatmosphere, to develop techniques for the beam diagnostics based on observations of the SR. A
transport line of the monochromatic X-ray beam from the optics hutch II to the experiment hutch is
now under construction. The commissioning of the transport line to the experiment hutch will start in
April 2010.

Figure 1: The SPring-8 diagnostics beamline II (BL05SS). The experimental hutch and the SR
transport line from the optics hutch II to the experimental hutch will be commissioned in April 2010.
Currently, Be windows have been installed downstream of the monochromator in the optics hutch II.
(2) Energy spectrum & Angular spectral flux of ID radiation
The energy spectrum produced by a filament electron beam in a given direction consists of a seriesof harmonic peaks, the frequency of which are multiples of the fundamental resonant frequency ω1.
For a planar undulator, ω1 is given byω1 =2cγ 2,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 the observer, respectively. Fora 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 areequal to 1/nN where n is the harmonic number and N is the number of periods. For a given point ofobservation, all electrons of a thick beam do not have the same resonant frequency ω1. This can occur
through the angular divergence of the electron beam, which spreads both the horizontal and verticalangles 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 spread in the angle θxand θy shifts the resonant frequency ω1 to the lower energy side only. As a result, the electron beamemittance 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 σγ/γ of the electron beam isGaussian, it introduces a Gaussian spread in the spectrum of each harmonic:σσω
=2 γ .
ωγeq. (2)2photon flux (photons/s/mrad /0.1%b.w./mA)

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 σγ/γ, and has areduced slope on the low energy side which is dominated by the emittance, beta function andobservation aperture
limited by the finite slit width.
€The energy spectrum is obtained by measuring the spectral photon flux using an ionization chamber
set up in atmosphere, while changing the energy of monochromatic X-ray by scanning Bragg angle of
the Si(111) double-crystal monochromator. A rectangular tantalum slit is placed in front of theionization chamber to measure the on-axis photon flux of ID05. The aperture of the slit was
4.2µrad×4.2µrad. The nominal equilibrium emittance and relative energy spread of 8GeV stored
electron beam are 3.4nmrad and 1.1×10-3, respectively. A typical example of the energy spectrum of
the fundamental harmonic measured at the magnet gap of 80mm, which gives the deflection parameter
K of 0.45, is shown in Fig.2. The tail of the low energy side comes from the horizontal emittance. The
effect of the slit aperture is negligibly small. The slope on the high energy side is dominated by
1/nN~0.02. The energy spread has a negligible effect on it. When the magnet gap is the minimum
value of 20mm, the deflection parameter K of 5.8 leads to the fundamental harmonic of 0.45keV. As aconsequence, many higher-order harmonics are observed in the photon energy range of 4 to 30keV(Fig. 3). For the harmonics higher than the 19th, the energy spread has the non-negligible effect on theslope of the high energy side.
2.5 1015

fundamental harmonic @ID05 magnet gap 80mm

2 1015
1.5 10151 1015
5 1014
06.6

6.8

77.2

7.4

7.6

photon energy (keV)

2photon flux (photons/s/mrad /0.1%b.w./mA)

Figure 2: Measured on-axis energy spectrum of the fundamental harmonic when the magnet gap is
80mm. The deflection parameter K is 0.45.
Higher order harmonics
K=5.8 @ID magnet gap=20mm8 1014

15th

14

7 10

19th

14

6 10

23th

14

27th

5 10

31th

35th

14

39th

4 10

14

3 10142 1014

1 100681012
photon energy (keV)

141618Figure 3: On-axis energy spectrum measured at the minimum magnet gap of 20mm, which gives thedeflection 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. 4 shows anexample of the angular spectral flux observed at the photon energy of 8.3keV, which corresponds tothe peak of the 19th harmonic for the magnet gap of 20mm. The center spot is the spatial profile of the19th harmonic. The outer rings correspond to the off-axis radiations of the 20th and 21th harmonics.
The horizontal width of the center spot is determined by the horizontal angular divergence of electronbeam, while the vertical width is dominated 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 radiationand the total 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 8.3keV corresponding to the peak of
the 19th harmonic for the minimum magnet gap of 20mm. The center spot is the 19th harmonic
radiation, and two outer rings correspond to the off-axis radiations of the 20th and 21th harmonics.

(3) Observation of the temporal structure using an X-ray streak camera
A streak camera is a device to measure the temporal structure of light pulses. The temporal resolution
is typically ~ps. In synchrotron radiation facility, the streak camera is frequently used to measure the SR
pulse duration, which reflects the electron bunch length. The principle of operation of a streak camera isillustrated in Fig. 5. The incident light hits a photocathode through a narrow slit aperture. The
photocathode converts the incident photon pulses into the photoelectron pulses, which have the same
temporal structures as the original photon pulses. After acceleration by an electrode just behind thephotocathode, the photoelectrons go through a streak tube in which sweep electrodes are settled. The
sweep electrode can rotate the photoelectron bunch transversely, namely, the longitudinal (temporal)
structure of the bunch is converted into the transverse profile by the sweep electrode. There are two
sweep electrodes (horizontal and vertical) in the streak tube for fast (vertical) and slow (horizontal)
sweeps. The vertical fast sweep is applied to measure the temporal structure of each light pulse inconsecutive pulse train. The global time structure of the pulse train can be observed by applying thehorizontal slow sweep in addition to the fast sweep. After the sweeps, the photoelectrons amplified by amicro channel plate (MCP) hit a phosphor screen at the end of the streak tube. A CCD camera captures
the images on the phosphor screen, which corresponds to the time structure of the original photon pulses.

Figure 5: Principle of the streak camera with the dual sweeps by the fast and slow electrodes.