To: Distribution 16 Jan 96 From: Martin Nordby Subject: Minutes of Near IR Engineering Meeting of 12 Jan 96 Investigations into Effects of Changing the Beam Asymmetry Mike Sullivan reported on the effects of increasing the HEB/LEB asymmetry by a small margin to improve the beam separation through the IR region. By increasing the HEB energy and decreasing the LEB energy, the dipole component of Q2 could be eliminated, both Q1 and Q2 quad magnets and the Q1 dipole could be weakened, and the beam separation through Q1, Q2, and Q4 would increase. This would allow less permanent magnet material for Q1, and more room for trim coils. For Q2, the center of focus could move closer to the CDR value, and radial space in the septum freed for better temperature stabilization and magnetic shielding. Specifically, the changes would look like this: Energy New Value Orig. Value Change HEB energy: 9.175 GeV 9.000 GeV +1.94% LEB energy: 3.05 Gev 3.10 GeV -1.61% Beam Separation Back of Q1: 52.83 mm 51.17 mm +1.66 mm Front of Q2 93.19 mm 90.36 mm +2.83 mm Front of Q4 145.1 mm 142.96 mm +2.14 mm Front of Q5 264.8 mm 265.71 mm -0.91 mm Magnet Strengths Q1 Quad -10.479 T/m -10.651 T/m -1.61% Q1 Dipole 0.2096 T 0.2130 T -1.61% Q2 Quad +10.973 T/m +11.153 T/m -1.61% Q2 Dipole none Clearly, these changes affect many other aspects of the machine dynamics. A first study of some of the key issues for the IR are listed below. Parasitic Crossing --Separation of the beam centroids at the first parasitic crossing (0.63 m) increases from 3.382 mm to 3.500 mm. This amounts to 9.81 of the largest LEB sigma, or 13.85 sigma for a standard LEB beam. Q1 Beam Stay-Clear --Beam separation at the back of Q1 (2.1 m) increases by 1.66 mm. --1.23 mm of this is due to LEB orbit change. This increases minimum B.S.C. aperture for the Q1 magnet. --However, since Q1 would be a weaker magnet, the inner radius may be able to increase to account for this change. Solenoid Orbit Compensation --The Q4 and Q5 offsets were maintained, as a starting point for the new compensation method. --To first-order, scaling the corrector strengths by the change in beam energy re- establishes the solenoid-off configuration. The new corrector strengths are still well within their design limits. --The solenoid-on compensation is also within corrector limits, but the LEB will be slightly harder to compensate for. --Decreasing the solenoid field strength may be another way to help compensate for the decrease in LEB energy. Center-of-Mass Boost --Increasing the asymmetry increases the boost. --The forward angle acceptance of 300 mrads decreases to 290-295 mrads. This may affect solid-angle coverage of some detector components. --To gain back the 10 mrads in the SVT, the I.P. would have to be moved 2 cm up-beam. This is a large number, and is probably not practically attainable. Restoring 3.1-on-9 Gev Asymmetry --To return to the CDR asymmetry, we would not be able to colide head-on, but would need a crossing angle of +/- 0.35 mrads (0.7 mrads included angle). --For comparison, tests at CESR have led them to establish a maximum practical limit to the crossing angle of (0.1)*(sigmaX/sigmaY). This is 1 mrad for PEP-II, which is below the expected angle of 0.7 mrads. --Q1 and Q2 quad magnets would have to be slightly stronger. This may be able to be accomplished with their trims, but would reduce the available trim range at the CDR energies. S.R. Fans --To first-order, all S.R. fans should only move 1-2 mm, since the beam trajectories only move by this amount. Clearly, the S.R. fan strike regions must be re-checked, and all masking re-established. This study was simply a first-look at the effects of changing the asymmetry. If this scheme looks like a viable alternative, more investigation into effects on the LER lattice and solenoid compensation are needed. Also, beam separation and component placement in the IR 10-20 m range must be checked. Other systems which may be hurt by this change are the RF system for the HEB, since most of the beam energy lost is due to SR, and this scales with E^4. Also, the Wigglers may need to provide more damping for the LEB. Q2 Shielding for the H.E.B. Dave Humphries reported on progress in modelling the stray Q2 fields around the H.E.B. The original models were refined and expanded-on to produce results with better numerical accuracy. With a 1 mm thick iron shield, spaced 1 mm away from the apex of the Q2 P.M. block trapezoids, the field in the outside of the iron at the mid-plane is 1 kG. However, at 20 degrees above/below the mid-plane, the field increases to 20 kG. This is essentially at saturation. With concerns of field leakage from the BaBar solenoid, the near-saturation of the HEB shield from Q2 fields shows that it could not tolerate any kG-level field from the solenoid, without saturating. Dave will work on simulating this to establish an upper limit to solenoid field leakage. Using a vanadium permendur shield, the fields in the X-Z plane decrease just outside the Q2 manget, but are actually higher at the HEB centerline (144 G). For a longer 3-D model (20 cm, instead of 5 cm), the shielding actually decreases the stray field to 20 G on the HEB centerline (length-normalized). This suggests that most of the stray field is due to end effects of the shielding. If the shielding could be lengthened beyond the end of the P.M. material, this would further help. However, there is no more axial space to do this. Dave is looking into increasing the segmentation of Q2 from 16 to 24 blocks. This should reduce the stray field, as well as improve the harmonics of the quad field. He will also investigate the effects on Q2 harmonics of an iron shielding collar around the HEB. An issue which was raised during discussion was the impact of harmonics of the solenoid field on the beams. Up to now, the field quality of the solenoid has been assumed to be perfect, but the radial and axial components have been checked, and compensation schemes developed with these values. However, the solenoid field will have its own harmonics, some of which may not be able to be compensated-for. This needs further investigation. Q2 Mechanical Envelope Martin Nordby built a mechanical envelope for the rough Q2 mechanical design presented by Dave Humphries. The model shows there is a small interference between the Q2 base and the cantilever memebers of the Q2/4/5 Raft. It also shows a more significant interfererence between the P.M. support collars on the LEB side on the in-board end of the magnet, and the Q2 Shielding Plug on the door of the detector. The interference does not include the required 1.25 in clearance for EQ motion between the door and the magnet, so the total overlap amounts to 2 inches. Dave will look at refining/trimming the collar design, and Martin will look at modifying the Q2 shielding plug shape with Orrin Fackler. These minutes, and agenda for future meetings, are available on the Web at: http://www.slac.stanford.edu/accel/pepii/near-ir/home.html