[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Question 4 - Straight or curved tunnel?




Dear colleagues,

I was notified by Peter Tenenbaum that my question below was not distributed to the WG1 mailing list. Now it should reach you.

Sincerely yours,
Philip Bambade.

---------------------------------------------------
Philip Bambade       E-mail: bambade@xxxxxxxxxxxx
http://www.lal.in2p3.fr/recherche/delphi/bambade/
Home Institute:                At CERN:
LAL, Campus Universitaire Delphi Collaboration pice 129,Bt.208,B.P. 34 EP Division F-91898 ORSAY CEDEX France F-01631 CERN CEDEX
Tel. 0033-(0)1-64468370        Tel. 0041-22-7676012
Fax. 0033-(0)1-69079404        Fax. 0041-22-7823084
---------------------------------------------------

On Sun, 11 Sep 2005, Philip Bambade wrote:


Dear colleagues,

Concerning evaluating straight/curved linac tunnels for the ILC, is it established that there is no significant impact on vertical emittance growth up to multi-TeV CMS energies (in case one would like to make the ILC site compatible with such an upgrade in the future) from:

- synchrotron radiation emitted in the required bends,

- chromatic filamentation of the phase-space in case of mismatched betatron or dispersion functions at the input or along the linac ?

Is there a reference which can be consulted describing the results of calculations or simulations, taking into account, for the latter effect, the larger correlated energy spread needed e.g. in CLIC to mitigate wake-field effects ?

I think it is an important question and we should have quantitative answers to be able to feed any political debate on the articulation of ILC and CLIC with proper technical arguments on the compatibility of the sites.

I have asked these questions to our CLIC colleagues who attended Snowmass, and they promised to make a list of all aspects relevant to this and invest some work in this direction. But maybe there are papers with computations of this already available in the literature ?

Yours sincerely,
Philip Bambade.

---------------------------------------------------
Philip Bambade       E-mail: bambade@xxxxxxxxxxxx
http://www.lal.in2p3.fr/recherche/delphi/bambade/
Home Institute:                At CERN:
LAL, Campus Universitaire Delphi Collaboration pice 129,Bt.208,B.P. 34 EP Division F-91898 ORSAY CEDEX France F-01631 CERN CEDEX
Tel. 0033-(0)1-64468370        Tel. 0041-22-7676012
Fax. 0033-(0)1-69079404        Fax. 0041-22-7823084
---------------------------------------------------

On Sat, 10 Sep 2005, Peter Tenenbaum wrote:

It's not as clear cut as this because the magnitude of the dipoles used as guide fields in the continually curved case is comparable to the magnitude of the dipoles used as steering correctors in simulation studies of the laser-straight case. This means that:

1. We may well find that the MPS implications of dipole failure in the stright tunnel are just as dire as the implications in the curved tunnel,

or

2. That since the guide field in the curved case is equivalent to a few hundred um of systematic misalignments of the quads, the placement of the quads along the curve of the tunnel produces enough guide field to prevent disaster even in the absence of the guide field from the dipoles.

I realize that this is not the way a storage ring works -- there, if the guide fields fail, the beam leaves the vacuum chamber. The difference here is that we are talking, essentially, of a "storage ring" with a radius of curvature which is orders of magnitude larger than any existing facility; the rules are different.

Having said that: I agree with Marc's judgement that it is desirable to have places in the linac where a runaway beam can be safely dumped. The dumps can probably be relatively simple (at least compared to the main dumps) because of the low average power: assuming that a dump may have to take a few hundred bunches, if we dump that many bunches at a given location once per hour the average power is still only 65 watts. We would want to be sure that the abort kicker cannot accidentally put the beam into an aperture in the linac other than the runaway beam dump. One possibility would be to put a short warm section into the linac, and use a circular collimator with an aperture much smaller than the cavity iris as the dump. The strength of the kicker could be such that it could put the beam on the dump aperture but not onto a cavity iris. It would probably be necessary to sweep the beam on the collimator as well (say with a vertical kicker to get the beam pointed at the collimator, and a swept horizontal kicker to spread out the train on the face of the collimator).

The reason I want this is the following: there are two proposals for the MPS interlock in ILC's LET: the pilot bunch and a more conventional system which looks at magnet currents and fault statuses, RF fields, etc, and which also limits the amount that devices can be changed by operators during luminosity running (the former system catches faults but not cockpit errors; the latter system prevents cockpit errors). In the case of BC or BDS, both pilot bunch and hardware permits can provide equal levels of protection, and so there is an added level of safety. In the linac, if there are not a series of runaway beam ramps, then the pilot bunch provides no protection and only hardware permits can prevent a disaster. This means that each device's MPS thresholds have to be set correctly because if they are set wrong there is no additional protection from a pilot bunch. I am worried that this would be an error-prone and failure-intolerant protection system.

-PT


1) Does a kinked-tunnel linac require additional protection
collimators? Does a non-straight linac require abort kicker / dump
systems. In totalling the failure modes, effects, time-scales and
likelihood, we find that a laser straight tunnel is relatively easy
to protect. Because of this, the proposed pilot bunch scheme does not
contribute to the linac MPS, and is really intended only for the
bunch compressor / beam delivery MPS. With a curved or kinked tunnel,
the potential failure of the linac dipoles becomes one of the main
failure modes which the MPS design must be effective against. It may
become necessary to implement several abort kicker/dump stubs along
the length of the linac. The number required should be roughly equal
to the number of kinks or one per kilometer (the spacing appropriate
for a 10-15 us pilot to main train gap). In the kinked scheme
protection collimators may be effective. Both of these have
significant cost impact of perhaps a few 10's of M$.