To: Distribution 3 Feb 97

From: Martin Nordby

Subject: IR Engineering and Physics Meeting Minutes of 17 January 97

Hard-Copy Distribution:

Bob Bell	41	Nadine Kurita	18
Gordon Bowden	26	Jim Krebs	41
Pat Burchat	95	Harvey Lynch	41
David Coward	95	Tom Mattison	17
Scott Debarger	17	James Osborn	LBL B71J
Hobey DeStaebler	17	Andy Ringwall	17
Jonathan Dorfan	17	John Seeman	17
Stan Ecklund	17	Mike Sullivan	17
Alex Grillo	95	Uli Wienands	17
John Hodgson	12	Mike Zisman	LBL B71J
Hank Hsieh	LBL B71J
David Humphries	LBL 46-161	Lew Keller	41
Roy Kerth	LBL 50-340
David Kirkby	95

Electronic Distribution:

Curt Belser	Kay Fox	Jeff Richman	Joe Stieber
Lou Bertolini	Fred Goozen	Natalie Roe	Jack Tanabe
Catherine Carr	J. Langton	Ross Schlueter	Rick Wilkins
Al Constable	Georges London	Knut Skarpaas VIII	Fran Younger
David Coupal	Joseph Rasonn	Ben Smith

Q2 Magnet Analysis Update

James Osborn reported on progress in the design and analysis of the Q2 Magnet. The multipole requirement, per the CDR, is <= 1 X 10^-4 for n = 3-15, with a main quad gradient of 85 kG/m. The design gradient is 93.5 kG/m, which includes a 10% extra capacity. The design current is 984 Amps on 9 turns, with a current density of 51 A/mm^2. The expected time to heat by 100 °C with no water present is 7 seconds, which is a figure-of-merit to be used in designing the coil MPS system. There are 9 parallel water circuits per coil, with a velocity of 14.5 ft/sec, dT = 17.5 °C, and a mass flow of 12.4 gpm/magnet.

The magnet is laminated iron, with 0.060" laminations and 2" thick endplates with remove-able chamfers. The magnet will be welded together, using straps which are recessed into slots in the laminations. If welding produces unacceptable warping of the core halves, the straps will be epoxied in place. The lamination design can incorporate either assembly method. Test welding on the lams shows minimal distortion of the cross-section, and plans are to make a full test core to look at bowing distortion (This test core will subsequently be used for mapping stray solenoid fields during BaBar magnetic mapping). The lams were laser cut, which is much less expense than punching for such a small production run. The laser cutting can produce very repeatable dimensions and, with good set-up, can produce accurate shapes to tight tolerance. The cutting produces a slight burr on both sides, which is polished off.

There are three trim windings per magnet: a back-leg dipole trim to cancel the n=3 term in the main quad field, dipole windings in the septum aperture to buck out the solenoid-induced dipole in the HEB passage, and octupole windings in the magnet bore to buck out solenoid-induced skew octupole. The final position of these windings has not been resolved yet, but they will rest somewhere on the pole tips of the magnet.

The magnet was designed with 4 mm of clearance for the vacuum chamber. This should also be sufficient for the vertical offset needed in the magnet position. Nadine Kurita was concerned that this would be tight at the in-board end, where the HEB and LEB chambers came together at the septum, and cooling was needed on the chambers. She will lay out the final chamber geometry based on the Q2 Magnet design presented here.

The coils are held in their coil pockets with NEMA blocks, while the septum coil is held by the septum shield plate, which doubles as a shield for stray fields trying to get into the HEB passage.

2-D 1/8 model Poisson analysis of the magnet core shows that the truncated hyperbolic pole tip shape produces > 10^-3 level harmonics (Rref = 4.23 cm). The 2-D half-symmetric Poisson analysis showed the need for back-leg windings to zero out the n = 3 harmonic. Conformal mapping of the quad shape was used to transform the analysis into dipole space. The pole tip field was preserved in the mapping, to allow direct comparison of the odd harmonics with CDR values. The conformally-mapped model and 2-D mid-plane symmetric model were used to investigate the effects of pole-tip bumps on the harmonics. Bumps had the largest impact on the n=6 harmonic, with less effect on n=10 and 14. Increasing the width of the bump increased the impact on n=6, but actually produced slight increases in the n=10 and 14. James' conclusion is that the bumps have marginal impact on field quality because the magnet is not fully iron-dominated.

Alternatively, moving the coils has a much greater influence on field quality. Moving the single inner winding on the split-plane of the magnet inward by 4 mm reduces n = 6, 10, 14, and 18 to < 2 X 10^-4. Unfortunately, this also interferes with the vacuum chamber at the in-board end of the magnet. James and Nadine will look at other options here, such as shaving the chamber in the region, moving more than one coil by less distance, or sloping the coil back out of the way at the end of the magnet.

3-D Amperes analysis was used to investigate the effect of chamfering the ends of the pole tips. Harmonics in 3-D were generated by integrating radial field along 72 lines running parallel to the magnet axis, distributed on a 3.5 cm radius around 360°. The resulting harmonics were then normalized to the reference radius of 4.23 cm.

While chamfering can eliminate the n = 6 term, it slightly worsens the n = 10 harmonic. A 1 cm X 1 cm chamfer reduces n = 6 to < 1 X 10^-4, but increases n = 10 by 50% to 1.5 X 10^-3. James' plan is to develop a chamfering method which does not affect the n = 10 harmonic. The n = 10 can then be handled by coil positioning, with no concern that chamfering the final magnet will worsen it.

Next Steps:

--Develop working design for displacing coils to reduce n = 6, 10, and 14 harmonics, given the actual design of the Q2 LEB Chamber (James and Nadine).

--Determine placement tolerance for conductors (James).

--Model alternate 3-D chamfer designs to ensure that n = 6 term can be trimmed without adversely affecting N = 10.

Forward Q2 Shielding PEP-II Decisions

Stan Ecklund summarized the current status of the stray solenoid field analysis, and the working decisions of PEP-II. First, past tracking studies have shown that for b4/b2 > 5.5 X 10^-4, the dynamic aperture for the LEB starts to be hurt. This is 3 X the CDR value, and equivalent to 0.16 kG-cm at Rref = 4.5 cm.

According the 3-D model produced by Stefan Mikhailov, with no special shielding or bucking coils, the b4 integrated field is 0.158 kG-cm. Given this result, and those of other models, none of which could produce better harmonics in the space given, PEP-II adopts the following working decisions:

--No bucking coils or other active shunting scheme will be used around Q2.

--Only one mirror plate will be used on the in-board end of Q2, which will have a racetrack shape, per James Osborn's design.

--The Q2 Magnet will include octupole trim windings to correct for any un-expected octupole harmonic above 5.5 X 10^-4 value predicted.

--Out-board of Q2, room will be left for an octupole harmonic correction ring, to correct for as-measured harmonics due to stray field measured in the BaBar solenoid during magnetic mapping, if any.

Q2/4/5 Raft Update

Scott Debarger showed preliminary results of the Raft finite-element analysis. This shows that deflection on the in-board end of the Raft, under gravity load, using the "baseline" design is -0.147 inches. Adding a small gusset under Q2 reduces deflection to -0.095 inches, by making the Raft more equally stiff along its length.

During an earthquake, the in-board support point moves to the in-board end of the Raft, and the max deflection is in the middle of the Raft, under Q4, where it is -0.087 inches with a gusset, and -0.13 inches without. Deflection due to a lateral earthquake reduces the clearance between the Raft and Shielding Plug to 2 mm.

This analysis point to needing a gusset, or to thicker sections to reduce deflection and, hence, increase natural frequency. However, this requires modifying the Plug. Scott will look more at both of these options, and Lew Keller will investigate the magnetic effects.

These minutes, and agenda for future meetings, are available on the Web at:

http://www.slac.stanford.edu/accel/pepii/near-ir/home.html