Transcript Inputs from GG6 to decisions 2,7,8,15,21,27,34 V.Telnov Aug.24, 2005, Snowmass D2. Beam and luminosity parameters For γγ we need beams with the geometric e-e-

Inputs from GG6
to decisions 2,7,8,15,21,27,34
V.Telnov
Aug.24, 2005, Snowmass
D2. Beam and luminosity parameters
For γγ we need beams with the geometric e-e- luminosity as large as possible.
Additional requirements are connected with the disruption angle and
beamstrahlung.
After multiple Compton scattering the minimum energy of particles (which
can give essential contribution to backgrounds) is about E~6 GeV. For such low
energy particles the deflection angle in the field of the opposing beam
  N / ( z E) . For N=2·1010 and σz=0.3 mm the disruption angle is about 10
mrad, which is still acceptable, but for σz=0.15 mm it is too large. Also small σz
leads to coherent e+e- pair production at large ILC energies (in γγ case σx is
smaller). So, the decrease of σz at fixed N is not possible for γγ. One can
simultaneously decrease N, but the geometric luminosity should not decrease
(emittances should decrease simultaneously).
The distance between bunches 337 ns (100 m) is good for the photon collider.
If it is reduced two times, than the distance 50 m is not enough for the loop
around the detector. In this case one should have 2 laser bunches circulating in
100 m cavity. It means 2 times higher average power in the cavity, which is not
desirable.
So, present parameters are almost optimal for γγ, only the decrease of
emittances is desirable.
D7: DR size and shape
For γγ, the DR is preferable which gives smaller product of
horizontal and vertical emittances. Smaller horizontal
emittance allows smaller βx , so the decrease of εnx by a
factor of 2-4 is very desirable. Some decrease of εny will be
also useful.
As we understand, smaller then nominal emittances are
possible by reducing the damping time with the help of
wigglers. We appreciate steps in this direction.
D8: e+ source type conv/undulator/compton
GigaZ needs e+e- at 2E=90 GeV with good polarization of both beams and
small energy spread (0.1%). The scheme with the undulator needs bypasses,
otherwise the energy spread after deceleration is large enough. (See GG6
summary talk with refs to original talks)
Below is response on D27, which is related to D8.
Low energy running is necessary for GigaZ(e+e- at 2E=90 GeV), WW threshold (2E=160
GeV) and γγ→H(120) (electrons with 2E~200 GeV).
All experiments need good emittances (good luminosity) and small energy spreads (for
precision measurement at GigaZ and WW and for smaller
chromo-geometric aberrations for γγ) . In the scheme with the laser positron source,
according to Kubo-san the scheme with acceleration with full gradient and further
deceleration gives smallest emittance dilution, but need more power. It is interesting also
what happens with the energy spread in this scheme? Is such loss of power acceptable or
better to make bypasses ?
In the case of GigaZ and an undulator e+ source, if the beam passes the undulator and
then decelerated, then the energy spread is about 0.3% (desirable 0.1%). It seems that
the scheme with bypasses is better for this case (see D. Scott talk at GG6 or GG6
summary
D15: crossing angle
Minimum crab-crossing angle for γγ is determined by
the disruption angle and the size of the final quads. The horizontal
disruption angle is about 10 mrad, Emin~6 GeV. During the
Snowmass workshop B.Parker found very good design of
the final quad which allows save removal of disrupted
beams at the minimum crab crossing angle about 23-27 mrad for
L*=4.5-3.5 m, respectively. Obtaining of the final number needs some
additional checks, but roughly it is 25 mrad. Note that the dilution of
emittance due to SR in the detector field is small for this angle (see
GG6 summary talk). So, it has sense in the baseline design to fixed
the crossing angle compatible with e+e- and γγ.
D21:gamma-gamma upgrade path
This decision is both political and scientific. My personal opinion is the
following. First of all it is necessary in the near future to make some
political decision on the photon collider. It is absolutely clear that this
option is great and very natural at the linear collider. The incremental
cost is small. The risk is small because the ILC can continue work in
the e+e- mode. The decision is necessary now because the photon
collider influences designs of many ILC system and all requirements
should be taken into account now before beginning of the
construction. Also people will not do any real work when the project is
not supported, has no finances and there is alternative: e+e-.
The optimum pass to the γγ may be the following. The ILC should
have two IP with two detectors, one IP should allow crossing angle
about 25 mrad and all other features necessary for γγ (lower
emittances, special beam dump, place for the laser system, etc.). The
corresponding detector should be specially designed for easy
modification for the γγ mode (replacement of 100 mrad forward
region). Both detectors start simultaneously the work with e+ebeams. People working on the γγ problems participate in e+eexperiments at the IP2 and simultaneously prepare upgrade for the
γγ. After about 4 years of operation one of the detectors is modified
upgrade together with adjustments may take 1-1.5 years. It is not a
problem if there are two IP and the first IP continues e+e- experiments.
If, by some reason the laser system is not ready, then the second IP
continues e+e- experiments. It is better, of course, to avoid such
situation. The development and realization of the required laser
system needs at least ten years. Before installation at the ILC it should
be assembled and fully tested in a separate place.
This program is not easy and needs attention, manpower and
money. It can not be done only on enthusiasm. Therefore the photon
collider should be considered as an integral part of the ILC program,
get sufficient support, all participating HEP people should be members
of the detector-2 collaboration from the start.
Other possible scenarios are the following.
1) One detector, one or two IPs. It is difficult to imaging that e+epeople will agree to finish e+e- experiments, always will be some new
ideas to measure something. Also modification of the detector, test
runs can lead to 1.5-2 years loss of the ILC operation time.
2) Two e+e- detectors with small crossing angle and one free IP with
the large angle for the γγ upgrade. This scenario may be attractive for
e+e-, but will be much more expensive.
D27: have bypass lines for low energy running?
Low energy running is necessary for GigaZ(e+e- at 2E=90 GeV), WW
threshold (2E=160 GeV) and γγ→H(120) (electrons with 2E~200 GeV).
All experiments need good emittances (good luminosity) and small energy
spreads (for precision measurement at GigaZ and WW and for smaller
chromo-geometric aberrations for γγ) . In the scheme with the laser
positron source, according to Kubo-san the scheme with acceleration with
full gradient and further deceleration gives smallest emittance dilution, but
need more power. It is interesting also what happens with the energy
spread in this scheme? Is such loss of power acceptable or better to make
bypasses ?
In the case of GigaZ and an undulator e+ source, if the beam passes the
undulator and further decelerated, then the energy spread is about 0.3%
(desirable 0.1%). It seems that the scheme with bypasses is better for this
case (see D. Scott talk at GG6 or GG6 summary)
D34: L*
In γγ experiment, the forward part of the detector will be
changed therefore L* can be somewhat different.
Smaller L* can give larger luminosity (smaller chromogeometric effects), smaller effect of SR (shifted quad and
the detector field compensate each other). But the crab crossing
angle should be small. The size of the quad has minimum
transverse size, therefore significant decrease of L* is not
possible.
At present the minimum βx is determined by chromogeometric aberrations. This effect restricts the γγ luminosity.
In order to make a final choice it is desirable to see how
the geometric luminosity depends on L*.
Related question. For obtaining zero vertical collision angle
in e-e-, γγ case we plan to shift final quads. Should they be
shifted mechanically or with the help of additional dipole coils?