Synchrotron beam current

The traditional operation mode of synchrotron light sources is a discontinuous one particles are injected in the storage ring, the beam current is decaying exponentially, and after several hours the synchrotron radiation run is stopped for a new injection. 

Figure 1.8 Example of the spectral flux Ny of the undulator/wiggler radiation measured at the X-Al beam line at NSLS (National Synchrotron Light Source, Brookhaven National Laboratory, LISA) with the undulator parameters K = 1.50, X = 8 cm, N = 35, for a 500 mA beam current, with a 0.1% bandpass and a solid angle of 1 mrad2. The values are corrected for the beamline/monochromator efficiency and the photodiode detector response the dip at 4.4 nm is an artifact due to carbon contamination of the optical elements. From [BRA89], (Reproduced with permission from Review of Scientific Instruments.)...

Figure 1. Spectral distribution of the VUV storage ring at the National Synchrotron Light Source, presently operating at 750 MeV with peak beam currents of 900 mA.

Fig. 4. IR spectra of NO Os in the range 100-2500 cm measured at room temperature for five pressures (indicated in GPa on right hand ordinate). The absorbance has been normalized with respect to the beam current of the synchrotron light source. The sample thickness was about 23 pm. The region 1900-2200 cm is omitted because of interfering absorptions from the Type Ila diamonds used as anvils. Asterisks ( ) indicate lattice modes or combinations, (from Ref. [80])...

The experiments described in this review were performed at the Synchrotron Radiation source (SRS) at the SERC Daresbury Laboratory. The operating conditions were typically 2GeV electron energy and 200 mA beam current. 

Figure 5.12 (a) Schematic drawing of the thermal distortions of a monochromator crystal subject to high heat loads from synchrotron beams. (b) Comparison of the rocking curves for the second crystal in a two crystal monochromator when the first crystal is cooled only at the side, for different CHESS electron beam currents and a six-pole electromagnetic wiggler. From Smither et al (1989) with permission. 

Condition (b) is not always met. At the synchrotron the incident intensity varies with time as the circulating beam current decays or the focal... 

With an ordinary spectrometer, it is very difficult to study optical properties <200 nm, which lie in vacuum ultraviolet. Chen et al. tried a synchrotron radiation instrument to record the spectrum in the range. During their measurements, the electron energy was kept at 800 meV in storage ring, whereas the beam current was 100-250 mA. The pressure of the sample chamber was kept at approximately 1 x 10 Pa. They calibrated the excitation spectra with sodium salicylate. They found the absorptions due to Sq Pi and Sq Pi in addition to host absorption at approximately 170 nm (see Table 14.2) [47, 48]. As Tables 14.2 and 14.3 show, there are few reports on the absorption of So Pi, perhaps due to the limits of instrumentation. 

Soft X-ray absorption measurements are done at low-energy synchrotron X-ray facilities such as the UV ring at NSLS or the Advanced Photon Source (APS) at Lawrence Berkeley National Laboratory (LBNL). The beam size is typically 1 mm in diameter. The electron yield data are usually obtained in the total electron yield (EY) mode, measuring the current from a channel electron multiplier (Channeltron). Sometimes a voltage bias is applied to increase surface sensitivity. This is referred to as the partial electron yield (PEY) mode. Huorescence yield (EY) data are recorded using a windowless energy dispersive Si (Li) detector. The experiments are conducted in vacuum at a pressure of 2 X 10 torr. 

As a result slow variation of the adjustment is observed the intensity of the primary beam will abnormally increase or decrease, the parasitic scattering background will grow, slit scattering will change (cf. Sect. 4.2.3.7). It should be clear that changes of the primary beam intensity which are paralleled by respective changes of the synchrotron current are normal. 

This is most easily done at a laboratory source where the current of the X-ray tube is decreased to the lowest possible value. At a synchrotron beamline this is more complicated, because the measurement of the primary beam requires special adjustment. So, technically this should be done before the final optical adjustment of the device, as long as the slits can be narrowed for the purpose of intensity attenuation and as long as the primary beam stop is not yet mounted. It is not advised to use absorbers that are mounted behind the monochromator, because they change the spectral composition of the X-ray beam. 

Figure 12.8 Schematic plan of a synchrotron. The storage ring at Daresbury is 96 m in diameter, and contains a 250 mA current of 2 GeV electrons. Synchrotron radiation is emitted as a result of acceleration of the beam at each of the 16 magnets, and is tapped off and fed to a number of experimental stations, each of which is equipped to carry out a particular set of experiments.

Synchrotron beamlines are a complex hybrid of hardware and software. Although current designs have achieved a level of robustness inconceivable a decade ago, tight process control is essential. For example, at SGX-CAT the position of the beam is controlled to within 0.5 jxradians (0.000028°). This tolerance corresponds to keeping the X-ray beam centroid within a 25 xm diameter circle at a location 50 m from the undulator source. This performance, reflecting the combined capabilities of the synchrotron and the beamline, is impressive to say the least. 

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