To: Distribution 19 Dec 95
From: Martin Nordby
Subject: Minutes of Near IR Engineering Meeting of 15 Dec 95
Q2 Magnet Mini-Review
Q2 Magnet General Design Issues
Jack Tanabe presented an overview of the Q2 magnet design options. The
CDR conventional Q2 magnet was both offset and rotated with respect to the
LEB orbit. The septum design produced a magnet with high current density
coils and complex cooling circuits.
Various permanent magnet solutions have been investigated. Because the
gradient of the magnet increases with distance as the beam stay-clear tapers,
the effective magnetic center of all of these options is out-board of that for the
conventional design by about 15 cm.
This movement is increased by the need for a dipole component of the quad to
simulate the offset quad. Putting a dipole at the front of the quad PM maximizes
the beam separation at Q4, but also keeps the quad center moved out.
Septum Q2
Fran Younger presented an update of the septum Q2 design. The new beam
stay-clear for the HEB requires a taller channel for the HEB beampipe. This
changes the pole shape, and produces a magnet with marginal harmonics
(around 4 E-4). This should be correctable using shims.
If the magnet is moved 15 cm away from the I.P. (the same as the P.M. Q2
option), then the original pole shape fits. Also, the current density of the coils is
reduced by 15%, as is the power dissipated. The coils can be 8-turn coils,
instead of the CDR 6, and the total power dissipated is 19 kW.
Permanent Magnet Options
The spec's for the P.M. quad designs are:
Quad Gradient: 11.153 T/m
Quad Int Grad: 5.576 T (for 0.5 m effective field length)
Skew Quad Gradient: 8.066 T/m
Skew Quad Int Grad: 0.807 T (for 0.1 m effective length)
Dipole First Integral: 31,400 G-cm
Skew Dipole First Integral: 15700 G-cm
The magnet designs incorporate the skew quad into the quad, allowing 0-10%
quad tuning and independent 0-100% skew quad adjustment. A harmonic
corrector ring (discussed last week by Ross Schlueter) allows tuning of the
quad to reduce as-built harmonics from 1 E-2 (for quad/sxt harmonics) to 1 E-4.
Two design options were discussed. One was a hybrid design, which includes
a 15 cm long dipole magnet on the inboard side of a 44 cm long quad. The
second was an offset quad, 78 cm long, and offset 0.6 cm to produce the
required bending. Both designs start at 277 cm from the I.P., and are followed
by two rotating quad rings to produce the trim and skew capacity.
Comparison of the Halbach formulas with finite-element analysis shows that the
effect of P.M. material with relative permeability >1 is to reduce the overall
strength of the magnet. He has allowed for a 5% overage in magnet strength to
account for this.
Outside the quad, the average field at the HEB centerline is around 50 G. Dave
is looking into adding a thin iron shield around the magnet slices to reduce this.
Uli Weinands said that his back-of-the-envelope calculations show that less
than 10 G is needed.
The physical size of the magnet and support frame is approximately 33.43 cm
wide by 30.8 cm high. Dave and Lou Bertollini are now working on refining the
physical design of the magnet, and the Q2 septum chamber running through it.
Q2 Optics
Mike Sullivan reported on the effects of moving the Q2 magnet out-board by the
needed 15 cm for the P.M. design. This increases the beta-function and beam
stay-clears at Q2 and SCX2, and reduces the nominal strength of Q1 and Q2,
as well as the beam separation at Q2, Q4, and Q5. However, Mike's initial
calc's on this assumed that both the dipole and quad components of Q1 were
weakened the same amount. This gave pessimistic beam separation numbers.
By maintaining the dipole and just reducing the quad field, the changes in beam
separations look reasonable. For a constant dipole, and 5% reduced quad field
in Q1:
dx = -0.44 mm at Q2
dx= -0.30 mm at Q4
dx = -0.88 mm at Q5
This is for the hybrid quad/dipole Q2 design. For the offset quad design, the
changes are slightly less.
One consequence of the change in Q1 strength is that the beam stay-clear
through Q2 increases by 1.6 mm on the radius. This needs to be factored into
the Q2 design, and the design iterated based on the new geometry. Dave
Humphries and Mike Sullivan will work on getting this to converge
The layout of the Q2 septum chamber was discussed, especially the location of
flanges. Ideally, the part of the chamber trapped by the Q2 P.M. magnet would
be flanged, so it could be removed, and the magnet tested. However, there may
not be room for flanges, and they may see unacceptably-high levels of S.R.
Dave Humphries and Lou Bertollini will continue to work on an integrated
mechanical design of this region and report on this in future meetings.
The two key issues which need resolution before a decision can be made
regarding the Q2 magnet design are:
1. Stray fields in the HEB BSC: Uli Weinands is working on refining his
calc's on acceptable field levels, and Dave Humphries is checking on the
effectiveness of a thin iron shield around the P.M. rings.
2. Final design configuration of Q2 slices, with self-consistent BSC numbers
for beam separations produced by that magnet design. Mike Sullivan and Dave
Humphries will iterate on this to converge on a final layout.
These minutes, and agenda for future meetings, are available on the Web at:
http://www.slac.stanford.edu/accel/pepii/near-ir/home.html