Magnetic guiding fields

Another important feature of e e machines is that the magnetic guide field together with the synchrotron radiation lead to transverse polarizations of the beams, with electron (positron) polarization antiparallel (parallel) to the magnetic field. The polarization arises because the synchrotron radiation induces up-down spin transitions which, for the positron, say, are larger for down—>up than for up-+down, where up means along the guide field B. 

After exiting the moderator, the slow positrons have to be accelerated to their final energy and transported to the experimental site. The simplest system to perform this consists of an acceleration gap and a magnetic guiding field. Figure 3.3 shows schematically the magnetic beam line at the University of... 

The measured time distribution of the prebunched positrons at the entrance slit of the chopper is about 2 ns (FWHM). This width is mainly caused by the energy spread(A sa 0.4eVFWHM) of the moderated positrons [133,136]. The positrons, which travel in a longitudinal magnetic guiding field of about 7 mT, are deflected by 90° and enter the new chopper device with an energy of 200 eV (see Figure 4.39). This chopper is a double-plate system with an external resonator, positioned outside the vacuum system. 

The deflection plate Di is coupled to a 50 MHz sine wave of amplitude C/q superimposed on a dc voltage of about C/q. At the minimum (0 V), no transverse field is acting on the positrons. However, due to the magnetic guiding field, the positrons... 

Hahn echo known from NMR. After the rr/2-flipper, the polarization in z-direction is again rotated in the direaion of the magnetic guide field and then probed by the analyzer which transmits neutrons to the detector with a probability that ideally is T = [1 + cos( I )]/2, with P the resulting precession angle at the rr/2 flipper. 

Handling a polarized beam is not a trivial matter either, since in zero magnetic field the beam becomes depolarized quickly the application of a guide field over the entire flight path is the most common solution to this particular problem. 

Thus, the discrete values of P for the bound inodes of Eq. (33-1) are replaced by a continuum of values for P(Q). We explained in Chapter 25 why it is more convenient to work with the radiation mode parameter Q, which is defined inside the back cover. We are also reminded that both the electric and magnetic transverse fields, e, and h, of the vector bound modes of weakly guiding waveguides are solutions of the scalar wave equation. However, only e Q) of the vector radiation modes satisfies the scalar wave equation, as we showed in Chapter 25. 

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