SPring-8, the large synchrotron radiation facility

Release Date

06 Jan, 2006

January 6, 2006
Japan Synchrotron Radiation Research Institute (JASRI/SPring-8)

 1. Background
     For precise experiments with synchrotron radiation (SR) brilliance of SR is one of the most critical factors to detect a faint signal accurately. To generate highly brilliant SR, circulating electrons, sources of SR must be packed into a narrow phase space as an electron bunch (or beam) with high density. This process is called as "reduction of electron beam emittance". Stored electrons in a light source circulate in the horizontal plane and emit photons when electrons are bent by a magnetic field. By this photon-emission, each electron receives recoil, which excites a horizontal betatron oscillation depending on the energy loss by the photon-emission process. Since this excitation mainly occurs at the inside of bending magnets, suppression of the excitation is principally possible by optimizing electron beam optics to control behavior of the on and off-energy electrons within bending magnets.
     However this kind of low emittance optics has different energy-dependence of the electron orbit from that of the previous optics. This difference caused an operational problem that the electron beam aborted by turning off RF acceleration power damages a part of vacuum chamber at the beam injection section. Furthermore, high-density state enhances coulomb scattering between the electrons in the same bunch and consequently shortens beam lifetime. Due to this short beam lifetime, merits of higher brilliance (calculation: Fig.1, measurement: Fig. 2) are much reduced in various SR experiments.
     To reply the request to make the experiments done in the SPring-8 facility further innovative, we need to immediately realize low emittance operation providing stable and more brilliant x-ray by solving the above problems.

2. Method and Results
[ 2.1. Reduction of electron beam emittance]
     As a method to reduce electron beam emittance, i.e. volume in transverse phase space, it is effective to lower a peak of energy-dependence of the electron orbit, which is called a dispersion function at the inside of bending magnets. This is done by breaking achromatic condition where a dispersion function is localized between only each pair of bending magnets as shown in Fig. 3. The peak reduction can decrease the horizontal oscillation excited by the photon-emission process on average. In this situation the dispersion function is non-zero at insertion devices and the balance between two contributions, the emittance and energy spread of an electron beam determines spatial sharpness of a photon beam emitted from an insertion device. This is because energy spread of an electron beam enlarges the spatial sharpness through the dispersion function.
     Figure 4 shows the relation of phase space volume of the photon beam (vertical axis left) and the dispersion function at each insertion device (vertical axis right) to the electron beam emittance (horizontal axis) in the case of the SPring-8 storage ring. The following explanation might give an easy understanding why phase space volume of the photon beam has a minimum in Fig. 4. As the dispersion localized between each pair of bending magnets decreases, the electron beam emittance decreases. On the other hand, the dispersion leakage to the outside gradually increases the dispersion at each insertion device (see right figure of Fig. 3) and rapidly increases the dispersion when the emittance goes down to or less than 3nmrad (see Fig. 4). Hence, the decrease (decrease in electron beam emittance) and the increase (increase in the dispersion at each insertion device) in the phase space volume occur simultaneously and this competitive actions make the phase space volume has a minimum against the electron beam emittance. To realize the minimum condition for phase space volume of the photon beam, i.e. the electron beam emittance is about 3nmrad, the magnet power-supply system of the SPring-8 storage ring was partially modified.

[ 2.2. Countermeasure against damage of vacuum chamber by aborted electron beam ]
     By the reduction of the electron beam emittance, the dispersion function closed between each pair of bending magnets distributes along whole the ring. Consequently, nonlinear dispersion caused by the dispersion function (linear dispersion) located at sextupole (nonlinear) magnets has a different distribution from that of the previous optics. Owing to this distribution change of the nonlinear dispersion, the operational trouble occurred, in which the aborted electron beam damages a thin part of the vacuum chamber at the beam injection section. As a result of the investigation, it is found that behavior of the aborted electron beam can be simulated accurately by six-dimensional particle tracking code and also found that the aborted electron beam only hits the limited area of the vacuum chamber under possible any condition. To protect the vacuum chamber, the following two main design modifications were carried out:
     (1) Installation of absorber: An absorber made of aluminium was installed just upstream of the collision point. The aborted electron beam is diverged through the absorber by an electro-magnetic shower. Before penetrating into the stainless-steel chamber previously damaged, density of electron beam is reduced to a sufficiently low level.
     (2) Increase in thickness of damaged chamber: Thickness of the damaged stainless-steel chamber was increased from 0.7 to 5 mm.

[ 2.3. Countermeasure against short beam lifetime]
     The low emittance operation reduces the emittance down to 3nmrad, which is one half of the previous value. Although this lower emittance increases brilliance of the photon beam, the beam lifetime is reduced down to the value ranging from one half to one fourth of the previous lifetime and hence, intensity decay of the photon beam becomes large. This large variation of the intensity is inadequate for precise SR experiments. To solve the problem of the shorter beam lifetime, top-up operation to keep the constant stored current by frequent refilling was applied to the low emittance operation.

     By the above modifications, countermeasures and machine tuning, steady top-up operation with low emittance electron beam to provide three-times more brilliant x-ray is firstly achieved.

3. Perspectives deviation
     (1) Brilliance and angular photon flux of monochromatized x-ray are expected to increase by about three times and one and half times. This improvement reduces measurement time and/or improves data accuracy significantly. The reduction of the measurement time not only raises efficiency of the precise experiment but also expands possibility of the micro or submicro beam experiment.
     (2) Smaller micro-x-ray beam by 50% compared with the presently achieved size is available for localized nano- and micro-structure physics. This characteristic expands possibility of localized nano- and micro-structure physics.

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