Current sources radiation modes

In the previous chapter we examined the excitation of modes of a fiber by illumination of the endface with beams and diffuse sources, i.e. by sources external to the fiber. Here we investigate the power of bound modes and the power radiated due to current sources distributed within the fiber, as shown in Fig. 21-1. Our interest in such problems is mainly motivated by the following chapter, where we show that fiber nonuniformities can be modelled by current sources radiating within the uniform fiber. Thus, isolated nonuniformities radiate like current dipoles and surface roughness, which occurs at the core-cladding interface, can be modelled by a tubular current source. 

Another remarkable feature of IRSR in comparison with a thermal source, whose intensity varies at random, is that the power delivered is intrinsically stable in time, and, as outlined above, for an energy E of the electrons >0.5 GeV is directly proportional to the current circulating in the beam. The circulating current can be monitored in real time and there is a trend to operate SR dedicated storage rings with a constant current (top up mode) so that IRSR provides the spectroscopists with an absolute radiation source. 

The tubular current source was described in Section 21-6, where we showed that it is ineffective in exciting bound modes unless either of the resonance conditions of Eq. (21-15) is satisfied. A similar conclusion holds for the radiation fields. If the tube length 2L is large compared to the spatial period 2n/Sl, where 2 is the frequency in Eq. (21-13), it is intuitive that power will be radiated essentially at a fixed angle to the fiber axis. This is also a consequence of Floquets theorem [7]. However, unlike the current dipole, radiation now depends on the orientation of the currents on the tube. 

When light propagates along a fiber and impinges on nonuniformities due to imperfections in the fiber, some of its power is scattered, as shown schematically in Fig. 22-1 (a). Part of the scattered power is distributed into forward-and backward-propagating modes, while the remainder is radiated. For multimode fibers, the distribution of scattered power is best treated by the ray methods of Chapter 5. Here we are primarily interested in fibers that propagate only one or a few modes. We treat the nonuniformities of the perturbed fiber as induced current sources within the unperturbed fiber. The results of the previous chapter can then be used to describe excitation of bound modes and the radiation field [1-3]. 

If current sources with density J are present within the waveguide, the radiation field is given by the second summation in Eq. (31-32), since the amplitudes of the radiation and evanescent modes are z-dependent within the region occupied by currents. Outside of this region the modal amplitudes are constant. We deduce from Eq. (31-37) that... 

Another major source of noise is the loop consisting of the output rectifiers, the output filter capacitor, and the transformer secondary windings. Once again, high-peak valued trapezoidal current waveforms flow between these components. The output Alter capacitor and rectifier also want to be located as physically close to the transformer as possible to minimize the radiated noise. This source also generates common-mode conducted noise mainly on the output lines of the power supply. 

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. 

More and more radiation sources are switching from discontinuous mode to top-up mode. This means that the user is continuously supplied with synchrotron radiation of almost constant intensity. The loss of the electron current is either compensated continuously or in intervals of several hours (at the ESRF 6 h). 

Common mode noise is present equally and in phase in each current carrying wire with respect to a ground plane or circuit. Common mode noise can be caused by radiated emission from a source of EMI. Common mode noise can also couple from one circuit to another by inductive or capacitive means. Lightning discharges may also produce common mode noise in power wiring,... 

EXAFS data (Rh K-edge ((23220 eV) or Ir Lm-edge (13419 eV)) were collected in transmission mode on station 9.2 of the Daresbury Synchrotron Radiation Source, operating at 2 GeV with an average current of 150 mA. A water-cooled Si(220) double crystal monochromator was used, with its angle calibrated by running an edge scan of a 5 pm Rh or Ir foil. For each sample 2-10 scans were recorded at room temperature in the... 

Here X = E,M denotes the type of radiation, either electric or magnetic, index j takes the values 1, 2,..., and index m = —j,..., j. The complex field amplitudes are defined in terms of the source functions, describing the local distribution of current and intrinsic magnetization [25], The mode functions in (17) can be represented in the following form [2,26,27] ... 

---