Question 2: Luminosity Parameters
contact: Nick Walker
13.09.2005
Baseline Parameters
Four possible and self-consistent luminosity parameter sets were initially proposed by
T. Raubenheimer after the first ILC workshop in November 2004. Each parameter set
achieved a luminosity of ~2×1034 cm-2s-1.
The four sets represent a parameter space, in which key parameters are traded off
against each other in order to achieve the desired luminosity goal. Each set tends to
push the achievable limits of one or more specific sub-systems:




Nominal set
Low bunch charge
(low Q)
large *y
(large Y)
Low power
(low P)
used as a reference to scale to the other sets
reduction in bunch charge by a factor of two
increased vertical beam size due to factor two larger
emittance growth in LET
reduced number of bunches (nb) by factor of two.
In addition to these, three additional parameter sets exist:



TESLA TDR parameter set with L  3.4 1034 cm-2s-1
US LC options study ‘cold’ set with L  2.7 1034 cm -2s -1
High luminosity set with L  5 1034 cm-2s-1
The latter represents the peak luminosity achievable if all parameters from
L  2 1034 cm -2s -1 are taken at their most demanding value. While L  5 1034 cm-2s-1
is almost certainly unrealistic, it indicates the potential ‘safety margin’ (factor 2.5)
available, if all sub-systems achieve their design goals.
The parameter sets are not intended to be considered as fixed sets, but as an indication
of the degree of flexibility which we would like to build into the machine. Hence each
sub-system should accommodate – where possible – the most demanding parameters.
The final ‘operating point’ is almost certainly going to be within the space defined by
these parameter sets, but will not necessarily correspond to any one of them.
The alternative of designing to a single ‘default’ parameter set – while appealing from
the point of view of the sub-system designers – would effectively remove the margins
and flexibility outlined above. Since we do not as yet know where the bottlenecks in
the machine will be, it is extremely desirable to maintain all such options in the
parameter plane. In addition, allowing for the overhead also keeps open the option of
achieving a luminosity greater than the nominal 2 1034 cm-2s -1 .
Clearly designing to the most aggressive parameters for each sub-system is only
possible if it is (i) technically feasible, and (ii) not a major cost impact.
Feedback from WGs
WG1: LET beam dynamics, bunch compressor design
WG1 had discussed several designs for single- and two-stage bunch compressors. For
a final bunch length of 150 m (low Q, High L), a two-stage compressor is mandatory.
For the nominal length of 300 m a single-stage system is still an option, although a
two stage is still desirable.
It was noted that the quoted emittance dilution budget of 50-100% for the entire LET
had yet to be demonstrated (in simulation).
WG2: Main linac
While there was no direct feedback from WG2, a proposal to increase the pulse length
to 2ms was presented (Padamsee, Foster). Increasing the pulse length by a factor of
two while maintaining the number of bunches reduces the peak beam power by a
factor of two, and (assuming the same klystron peak power of 10MW), would halve
the number of klystrons required – a possible cost saving. This saving is
counterbalanced to a certain degree by the increased cyroplant capacity required (and
assumes no significant cost increase in the RF power source). Other implications still
need consideration (for example for the warm RF and flux concentrator in the
sources). As the baseline hardware for the RF cannot currently support 2ms pulses,
this option cannot be considered baseline. However, the possibility of longer pulses
should be considered in more detail as ‘alternative’ solutions (note that any such
solution requires significant hardware R&D).
WG3a: Sources
The sources group reported that all parameter ranges could be achieved, both for the
electron source, and any of the positron productions schemes under consideration.
WG3b: Damping Rings
One of the most challenging parameter for the damping rings is the ~6000 bunches
required for the low Q option. J. Gao proposed a ‘very low Q’ parameter set with
0.6×1010 particles/bunch (40% lower than the ‘low Q’ option). However, the
‘compensation’ to recover L  2 1034 cm -2s -1 requires stronger demagnification at the
IP, and a corresponding short bunch length of 120m. The result is to increase the
demands on both the bunch compressor and the final focus (tighter tolerances). In
addition, an increase in the repetition rate to 8 Hz and 6000 bunches is required.
S. Guiducci suggested reducing the number of bunches by 2 (as in the low Pav set),
but maintaining the 1ms pulse. Luminosity is recovered by increasing the repetition
rate to 10 Hz, requiring a factor of two in the damping time (possible with a factor
two shorter ring, assuming the same kicker rise time). The resulting factor two drop in
linac beam current reduces the peak beam power by a factor of 2 and could potentially
save half the 10MW klystrons; but the fill time also increases, thus reducing the
efficiency and increases the dynamic cryoload by a factor of two. Increasing the
vertical emittance by a factor of two, while reducing the horizontal emittance by the
same factor was also discussed (Guiducci); this option increases the beamstrahlung by
a factor of four, however, assuming that no modifications to the demagnification in
the final focus are made.
WG4: Beam Delivery
A critical constraint on the parameter range performance of the BDS is the energy
acceptance of the extraction line, which sets limits on the amount of beamstrahlung
which can be tolerated. A. Seryi reported that all proposed 500 GeV parameter sets
could be accommodated for the 20mrad crossing angle. There is some difficulty with
the 2mrad crossing angle, especially for the high luminosity set. For the 1TeV
parameter sets, Seryi reported that the large beamstrahlung for the high luminosity
parameter set could not be accommodated, and a reduced beamstrahlung set was
suggested. This is achieved by increasing the bunch length from 150 m to 300m, at
the same time as increasing the beam cross section slightly. One demanding aspect of
this set is the reduction of the damping ring vertical emittance from 20 nm to 18 nm,
and requiring 5 nm LET emittance growth, at an increase single-bunch charge
( 2.4  1010 ). A more realistic solution may be to reduce the luminosity for the 1TeV
high-L set (that is, effectively reduce the ‘safety margin’).
Recommendations
Many of the luminosity parameter sets discussed already reflect values which are
contained in the original proposed parameter plane. At this time, there are no
compelling arguments to modify the originally proposed 500 GeV values, and the
sub-systems should still design to the most demanding parameters. This will be again
reviewed as and when new information from the WGs is made available (particularly
from WG3b).
As alternatives, 10 Hz and long-pulse operation should be considered, although the
impact (feasibility) on the linac hardware must clearly be first evaluated.