[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

Mode for the even filling of the ring with electron

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mode for the even filling of the ring with electron packets, yielding maximum flux; single bunch , used for time-resolved experiments which make use of a single electron bunch with a repetition rate of about 300 kHz but with low-intensity; 16 bunch where the ring is injected with 16 equally spaced electron bunches in order to produce a moderate intensity while allowing certain time-resolved experiments, and hy- brid , which combines a one-third filling of the storage ring with normal electron bunches in opposition to a single bunch. Thus, intensity is maximized while allowing time-resolved experiments requiring a time structure of about 1 µ s. 2.4.3 Generation of SR Bending Magnets Bending magnets (BM) are not only mandatory devices for closing the e -beam orbit but are also reduced heatload devices, capable of producing a smooth continuum spectrum extending over several tens of KeV. For prac- tical purposes the ESRF uses permanent magnets made of SmCo for BMs,
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2 X-Ray Sources 69 with a remanent magnetic field of approximately 0.85 T which emit light in a cone of about Ψ = 1 . 3 (FWHM) or about 0.11 mrad vertical opening. The magnetic field yields a critical energy E c of about 19.2 keV for these devices. The radiation emitted by a BM has a uniform distribution as a function of the photon energy in the horizontal plane. The flux per unit of time per emitted angle in the horizontal and vertical planes, in a 0.1% energy bandwidth, the so-called spectral brightness varies strongly with the vertical viewing angle Ψ as: N (photons / s / mrad θ mrad Ψ ) = 1 . 32 × 10 13 I [A] E 2 [GeV] F ( E ph /E c , Ψ) , (2.13) where E ph is the emitted photon energy, E c the critical energy and F is defined as the sum of the contributions of the horizontal/vertical polarizations: F ( y, Ψ ) = F σ + F π = y 2 (1 + γ 2 Ψ 2 ) K 2 2 / 3 ( y/ 2) + y 2 γ 2 Ψ 2 (1 + γ 2 Ψ 2 ) K 2 1 / 3 ( y/ 2) , (2.14) and K 1(2) / 3 are the modified Bessel functions. Beamlines making use of BM at ESRF employ X-rays in the range 4–100 keV. In a BM the X-rays are collected from a zone where the electrons are in the magnetic field which is approximately 15 cm at ESRF. Insertion Devices Insertion devices (IDs) are the standard top-of-the-line radiation sources at third generation storage rings. Their detailed description represents a subject far out of the scope of the present work. Our purpose here is to give a summary description of their main properties and parameters of interest for the physicist or engineer dealing with these devices for their routine spectroscopy work. For an in-depth account, see the book by Onuki and Elleaume [61] and references therein. IDs are assemblies of alternating magnetic elements (see Fig. 2.29) inserted in the straight sections of the ring. Their advantage is the generation of many undulations in the e -beam which produce photons at each bend, thereby considerably increasing the total emitted flux. Secondly, contrary to BM the magnetic field values of which are fixed, IDs can use variable field values, thus covering a very large spectral range. They are described by their remanent field
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