Synchrotron Radiation Effects in the IR Solenoid Flux Excluder

We examine the emittance dilution due to synchrotron radiation in the fringing fields at the end of the “flux excluder ” solenoid which protects the final doublet quadrupoles from the main detector solenoid field, and also the effect of SR in the main solenoid field itself. Because the deflection due to the excluder fringe field is opposite in polarity from that of the main solenoid, the resulting dispersive rays cancel at the IP; as a result the synchrotron radiation from the two magnetic fields produces only a small dilution of the vertical spot size. The contribution to the spot size from the finite opening angle of the synchrotron flux is found to be comparable to the contribution from solenoidal dispersion, and both are acceptable. We conclude that SR considerations do not rule out use of a flux excluder, and that the range of crossing angles and solenoidal fields available is large. 1 introduction One of the more controversial aspects of the NLC IR design is the configuration of the detector solenoid and the corresponding flux exclusion solenoids: the main detector solenoid produces a large field (presently expected to be 4 Tesla) in the entire volume of the IR, while a pair of smaller solenoids with the same number of windings per centimeter, powered in parallel with the main solenoid but in opposite polarity, cancel the resulting Bz in the area surrounding the final magnets. The purpose of the flux excluder is to have a zero-field region around the permanent magnets and to prevent depolarization effects in the crucial last lenses of the collider. Figure 1 shows a cartoon of the layout of the system. The main solenoid and the flux excluders are all aligned with their axes along the IP axis, as shown in Figure 1, while the beam emerges from the flux excluder with an angle relative to the solenoidal field (presently assumed to be 10 mrad). Consequently the beam passes off-center through the fringe field of the flux excluder, potentially exposing the beam to a strong transverse field next to the detector. In this Note we study only the synchrotron radiation effects of the flux excluder/solenoid system. A further study of optical effects, while worthwhile, has not been done at this time. 2 Magneti