The NLC 2001 Beam Delivery Configuration

 Final Focus System

Requirements:

The final focus system (FF) has to provide focusing of the NLC beam to a desired size in the interaction point (IP). The FF should be tunable, the beam sizes in the IP should be maintainable for this some more or less constantly working feedbacks and orbit correction systems may need to be incorporated. The FF design should be consistent with the detector and interaction region design, in particular the final doublet (FD), the necessary masking, BPMs, etc. must be designed as an integral system. The FF should provide favorable background conditions for the detector, both in terms of synchrotron radiation and in terms of tails of the beam. The NLC Beam Delivery will deliver and focus beams to the high energy IR and to the low energy IR and will have two independent FF systems which should work in sufficiently large energy range without change of hardware.

Technical description:

The NLC final focus system is built accordingly to the recently suggested design which involve local compensation of chromaticity by means of octupoles placed within the final doublet. Such design allows for a reasonably short FF even at high beam energy, moreover, it has advantages of larger bandwidth, better dynamic aperture and higher flexibility.

Overall Layout

The FF consist of the Final Doublet with embedded sextupoles,  section for correction of geometrical aberration, bend which create dispersion

and a beta matching section. See corresponding documents for more details.

Parameter table

Technical issues

 
Integrated design of Interaction Region. The integrated design of the Interaction Region including FD, girders,  masking, BPMs, vertex detector, fast feedback components, optical and inertial sensing components, stabilization and correction systems, etc., requires continues R&D, modeling, iteration on design and realistic prototyping.

Stability of Final Focus components in general and of the Interaction Region elements in particular. Providing the necessary stability requires continuos studies, developments of feedback, fedforward system for stabilization, fast feedback, girder design, etc. Achieving the goal requires ongoing R&D and realistic experience and learning handling the interaction of tuning, stabilization, and other feedbacks with gradual increase if the system integration, approaching parameters of the NLC IR.

Background issues. Experimental experience so far achieved is not very reassuring: neither SLC FF nor FFTB traditional optics would be acceptable for NLC because of the self-generation of background by the traditional systems. And the traditional NLC FF design would be even worse, since L*/beta increased (NLC beta* is ~10 smaller than SLC FF and NLC L* is ~6 larger than FFTB L*). The newly developed optics is designed to be background free (and moreover, an active control of beam tails by means of octupole doublets is possible in the new final focus system). Learning to achieve such properties in a real system would represent invaluable experience for further operation of NLC FF.

Important question, related to background problem, is collimation. For last couple of years the NLC collimation system has been continuously improving and optimized (see the section on collimation). Developments of the new Final Focus has triggered even more changes and further optimization. This optimization is being done now. One foresees that after this optimization is finished, the collimation system would not need to be as tight as it used to be.
 
Pointers to subsystem docs

P.Raimondi, A.Seryi, Phys. Rev. Lett.,  86, 3779  (2001).

For latest developments see, for example, FF talk on the NLC collaboration meeting, March 1, 2001

Pointers to optics decks

See NLC Accelerator Physics web pages, section NLC Optical Lattices.

 

Open issues

There are number of issues that require extensive studies, R&D, modeling and prototyping. They are listed in the "technical issues" section. No one from these issues is thought to belong to the category of "open issues" as there is confidence that an appropriate technical solution exist in each case.
 
Discussion of configuration choices:
 
The new final focus design,  since it has just recently been adopted, gives more flexibility and more ways to optimize the system. This optimization is still going on, however the Final Focus itself now seem to be close to the optimum and the work is concentrated now on integration of the Final Focus with Collimation system and with other components of the Beam Delivery.

Some of the configuration choices in the design is the approach to collimation system design, the choice of strategy of stabilization of final doublet, the choice of technology for the final doublet, etc.
 

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Last modified 02/14/01
Tom Markiewicz