M. Breidenbach and B. Richter
September 29, 2006
Draft
Push-Pull Interaction Region Issues
M. Breidenbach and B. Richter
Sept. 29, 2006
The ILC GDE is discussing the possibility of a design with a single-beam delivery
system feeding a large hall that houses two detectors in a "push-pull"
configuration. To be effective this requires a fast transition time between the
detectors. In this note we comment on some experience in setting up and aligning
a large detector at the SLAC SLC, and list some the items that have to be
analyzed before implementing a push-pull solution.
The SLD detector weighed ~4,000 tons and was self-shielded in the sense that no
separate, external shield wall was required between the detector and
experimenters. The detector was only rolled onto the beam line once using 8-500
ton Hilman Rollers, which are tank-like assemblies using a tread of hardened steel
rollers traveling on steel plate embedded in the concrete floor.
The massive end doors on the detector were opened and closed frequently, using
100 ton Hilman Rollers traveling on flat steel tracks. Each Hilman had cam
followers on the edge of the steel track, but the actual guidance came from a laser
alignment system. The final focus quadrupole triplet was supported (at its inboard
support point) by a mechanism in the detector door, and alignment of the triplet
was lost as the door opened. It is believed that the triplet was restored to its prior
position to ~1 mm. Beam-based alignment techniques were used to determine
residual offsets, the triplets were moved, and full luminosity was recovered usually
in about a single 8-hour shift.
Although the detector was on jacks that would, in principle, allow adjustment of the
height to compensate for floor settlement, these jacks were never used. The floor
settlement during the time the SLD was in place was probably less than 5 mm.
In the ILC, the desire is that a transition from one detector to the next be made in
a time of less than a few days. The detector designs that are being studied now
range in weight from something similar to SLD's 4,000 tons to up to 25,000 tons.
Self shielding, in the sense that shielding moves with the detector, will be required
and may be more of a problem in the ILC than in the SLC. The ILC beams are 11
MW each while the SLC beams were only 40 KW. The biggest concern is
probably neutrons since the detector calorimeters will likely be thick enough to
shield gamma rays. End caps and shielding of the beam lines between the tunnels
and the detectors also may be issues. Allowed radiation levels in the interaction
region hall and dose rates and doses in a maximal credible accident will have to
be specified.
It will probably not be possible to put these detectors on the beam line with a
tolerance much better than a millimeter. Beam-based alignment will be required
and should be relatively simple if proper preparation is made. The crab crossing
geometry may introduce issues that need analysis and were not things that one
had to deal with at the SLC. We would suggest that the following are a minimum
list of things which need to be analyzed carefully:
1. The floor deformations for the heavier detectors and the floor
recovery time following full and partial unloading;
2. Methods for compensation of the movement of beam-delivery system
elements that are on board the detectors themselves;
3. I&C for the beam-based alignment systems and possible extra beam
line equipment that might be required because of the extremely tight
tolerances on alignment for ILC operations. Beam-based alignment
may have to be done in more than one stage;
4. Shielding requirements beyond those given by what is in the detector
itself need analysis. This will have to include setting the radiation
levels allowed in the collider hall for routine operations and in
disrupted beam scenarios;
5. Power, water, cryogenics, data cable plant, and other services that
link the detector (and detector carried machine instrumentation) and
the “outside world”;
6. Vibration isolation will be an issue, although this is probably no
different in the push-pull scenario than it is in the separate collision
hall scenario;
7. It is assumed that each detector has its own set of final focus
elements, and that a common point can be found to break the beam
lines.
8. It is assumed that the beam line is broken at common points for the
detectors by valve pairs and pump outs. The optics requirements
need to be checked that suitable points are available. Also, HOM
might be an issue for these valves. Vacuum requirements in the final
focus area and at the collision point should be reviewed to see if any
special methods are required to recover a low enough gas pressure
in a timely fashion.
9. Safety may force a run down of the superconducting magnets in the
detectors before a move, possibly both the solenoid and captured
beam line elements. If so, the time required to de-energize and then
re-energize the magnets should be reviewed. Note that it is assumed
that the power supply, dump resistors, and other apparatus
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associated with the superconducting magnets are carried by the
detectors. For the detector solenoid, these times may be noticeable.
We believe that with proper preparation, the push-pull shift of the detectors to and
from the beam line should be able to be accomplished within something around
24 hours. Re-establishing the beams can, in principle, also be done rapidly if
proper preparation is made including the availability of keep-alive beam dumps so
that the accelerator itself can be maintained in the tuned-up state during the actual
downtime for the move.
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