Since the intensity of synchrotron radiation is on the same order as the intensity of the beam, itself, it would be very harmful to the experimentation if they are scattered at the pole tips of the nearest quadrupole magnet QC1. It is also very difficult to shield them near to the IP. The amount of synchrotron radiation is determined by the maximum size and angular divergence of the beam profile. Both the maximum beam size (x ,y) and the divergence (x' ,y') shall be well defined and controlled by the collimators.
The divergences can be expressed by , where is a (optical) beta function in the final focus system and is an emittance of beams. The beam size and the divergence are related by . If we can not control them by some means, we must change the optics to enlarge so that decreases. We may thus even have to sacrifice the luminosity because of . A similar situation would likely occur at the beginning of operation with a larger emittance than the expected one, as happened in the SLC experiments.
Figure 14.4: Horizontal(6) and vertical(40) beam envelopes through the last bending magnet and five final focus quadrupole magnets(QC1,QC2,QC3,QC4 and QC5). The maximum divergences of the synchrotron radiation are also drawn by arrows.
Figure 14.5: Profiles of the synchrotron radiation at QC1;right figure shows the magnified view around the center of QC1. The profile at the center accompanies the in-coming beam. The two right-hand side ones are passing through QC1 of 2.2m long after a collision with a 8mrad horizontal crossing.
Figure 14.4 shows the development of transverse beam envelops that correspond to . Figure 14.4 covers the region from the IP up to the nearest dipole bend magnet. Here the smearing effect due to collimation is not taken into account, since it was estimated to be very small() as mentioned earlier.
With a mask of 8 mm radial aperture that is located at 30 m from the IP, synchrotron radiation from upstream magnets beyond the last bending magnet can be completely masked. Any synchrotron radiation that passes through the aperture of 8 mm mask would pass through the final quadrupole magnet (QC1) without scattering. The half aperture of QC1 is chosen to be 6.85 mm. As can be clearly seen in Figure 14.4, the radiation from QC3 and QC2 provides the maximum divergence at the IP in the horizontal and vertical directions, respectively. The profiles of the radiation at the QC1 are shown in Figure 14.5. The length and inner aperture of QC1 are 2. 2m and 13.7 mm, respectively. The front face of QC1 is located 2.0 m from the IP. The in-coming radiation passes through the central axis of QC1. After colliding with the opposing beam at a horizontal crossing angle of 8 mrad, the out-going radiation passes off-axis through the QC1 magnet on the other side. The location of the out-going radiation is depicted as two elliptic profiles on the right-hand side of Figure 14.5.
As described above, it is expected that there should be no background problems due to the synchrotron radiation if we carefully optimize the collimation and the optics simultaneously.