To: Distribution 23 Oct 96

From: Martin Nordby

Subject: Minutes of the IR Engineering and Physics Meeting of 18 Oct 96

Hard-Copy Distribution:

Bob Bell	41	David Kirkby	95
Lou Bertolini	LLNL L-287	Jim Krebs	41
Gordon Bowden	26	Harvey Lynch	41
Pat Burchat	95	Tom Mattison	17
David Coward	95	James Osborn	LBL B71J
Scott Debarger	17	Andy Ringwall	17
Hobey DeStaebler	17	John Seeman	17
Jonathan Dorfan	17	Knut Skarpaas VIII	18
Stan Ecklund	17	Mike Sullivan	17
Alex Grillo	95	Uli Wienands	17
John Hodgson	12	Mike Zisman	LBL B71J
Hank Hsieh	LBL B71J	Bob Holmes	LLNL L-287
David Humphries	LBL 46-161	Orrin Fackler	LLNL L-291
Roy Kerth	LBL 50-340	Lew Keller	41

Electronic Distribution:

Curt Belser	Rick Iverson	Jeff Richman	Jack Tanabe
Catherine Carr	Nadine Kurita	Natalie Roe	Rick Wilkins
David Coupal	Georges London	Ross Schlueter	Fran Younger
Fred Goozen	Joseph Rasonn	Joe Stieber

Q2 Shielding Plug Fabrication

Lew Keller reported on the beginnings of progress on the actual mechanical design for the Shielding Plug, including how it would be fabricated and put together. The three fingers will be fabricated from separate forgings, and turned to final shape. They will then be bolted together, using non-magnetic spacers. The socket-head cap screws would both position the fingers relative to each other, and keep the fingers from separating due to the large magnetic loads on them. The first finger is subjected to 70 tons of magnetic load, which tends to separate it from the middle finger. Just the tip of the middle finger see 40 tons of magnetic load, which bends the finger inward, and tries to separate it from the third finger.

Raft Layout Update

Scott Debarger reported on the latest modifications to the Raft design. Last week, Scott presented changes to the Raft which made it left/right symmetric. This was in response to the tentative decision by BaBar to go with the "Big Bore" axisymmetric Plug design. Since the Big Bore cutout was much larger than the PEP-II stay-clear required, Scott got the OK to expand the stay-clear (mostly on the HEB side) to use up the extra space. This will make the Raft much easier to fabricate, and somewhat increase its stiffness.

However, one big problem with last week's design was that the "Small-Finger" bucking solenoid was inside the Raft structure. Some head-scratching by Scott and Martin showed that this makes the Raft nearly impossible to load with components, since the solenoid traps cables, plumbing, and the Q2 Chamber, but the cables go in before the Q2 Magnet, and the Chamber must go in after the Magnet. Bottom line is that this assembly would only work in a universe of Mobius-strip solenoids and Klein-bottle vacuum chambers.

Scott and Catherine laid out three options to put the solenoid outside the support cylinder surrounding the Q2 Chamber:

Tapered solenoid: this maximized room for the Raft transition cone from under the Q2 Magnet to around the Q2 Chamber. A 12.3 cm^2 solenoid would be wound on a conical mandrel, then fit against the conical transition cone. This all stays inside the new PEP-II symmetric stay-clear presented by Scott last week.

Stepped cone: at the opposite extreme, the transition cone can be turned into a flat plate just in front of the Q2 mirror plate. This essentially shrink-wraps the support profile around the machine elements, and maximizes room for the solenoid and/or Shielding Plug. This has two big problems. First, it leaves no room to route any cables or water under Q2, so all services from inside BaBar would have to exit across the F-Cal. Second, this makes the transition region very flimsy.

Compromise: the middle ground between these options is to use last week's stay-clear, but put a slight kink in the support transition cone. This makes room for the solenoid at the tip of the third finger on the Plug, while preserving most of the stiffness of the transition cone. There appears to be adequate room for 15 cm^2 of coil, which should be enough for the coil.

Editor's Note:

After the meeting, Martin and Scott worked up three coil designs which easily fit into this region. The preferred design uses 0.34 inch square conductor with a 0.183 inch diameter hole. 12 turns at 492 Amps produces the needed 5900 A-Turns, with a current density of 8.61 W/mm^2 (5552 A/in^2). Total power dissipated is 3950 Watts, which is handled by one cooling circuit, running at 0.72 gpm, and 8.8 ft/sec, with a 150 psi pressure drop.

This wedge-shaped coil package should fit neatly into the kink in the transition cone, with only one set of water fittings and coil leads. The coil has a large enough inside radius to slip over the 10 inch flange on the end of the Q2 Chamber, and would be installed after all machine services are mounted in the Raft, and the lid put on. See the hardcopy for coil spec's and a rough layout.

Q2 Magnet Analysis Update

James Osborn reported on the status of 3-D analysis of the Q2 Magnet. He investigated the benefit of adding an iron "Septum Bracket" to separate the HEB channel from the septum coil. Looking at int(Br*dz) for a cylinder running the length of the HEB channel, James found that the septum bracket must extend beyond the end of the steel. Integrated field with and without the bracket changes from 2200 G-cm without, to 1600 G-cm with the bracket, when it stopped at the end of the steel core. This shows that the bracket mostly shields the HEB from stray 3-D fields due to the coil turn-arounds.

To check the effect of the septum bracket on the LEB, James looked at harmonics in the LEB with and without the bracket. All harmonics are the same to 10^-4, showing that the bracket has no effect on the field quality in the LEB passage.

To check these integrated 3-D harmonics, James also looked at the expected harmonics at the symmetry plane of his 3-D model, half-way through the magnet. The n = 3, 4 harmonics agreed at the 2-3 x 10^-4 level, but the magnitudes were 2 x 10^-3 of the main quad field, 20 times higher than the specification, and higher than Fran Younger's initial 2-D analysis of last spring.

James will check on this, but feels that the problem lies with the 3-D Amperes analysis, not with the design of the magnet. His past experience has shown that the absolute accuracy of the results from Amperes can not always be trusted, so he will look into the discrepancy.

Regarding the mirror plate, James has used the new BSC values from Mike Sullivan, and added 1 mm for alignment tolerance, and 3 mm for the chamber wall thickness. This "shrink-wrapped" chamber then defines the bore diameter of the LEB and HEB channels for the magnet, which in turn sets the minimum inner radius for the mirror plate hole. The pole tip radius is 47.8 mm, while the hole in the mirror plate is 58.5 mm, with the center of the hole on the LEB bore center.

The "shrink-wrapped" chamber is smaller than the LLNL chamber design, however, since it exactly follows the BSC contour. To make fabrication easier, Bob Holmes hopes to make the both the LEB and HEB chambers straight tapered cones, even though the LEB BSC bulges slightly in the middle of the magnet. This requires that the chamber is bigger than its absolute minimum. Bob Holmes will work on generating a new chamber design to feed to James.

Also, James is looking at the effect on LEB channel harmonics if the hole in the mirror plate is moved off-center. This would allow the hole to be much smaller, but may not be magnetically feasible.

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

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