1. Possible Scenarios for an LHC Upgrade
F. Ruggiero, F. Zimmermann, CERN
Rationale of LHC Upgrade
LHC Performance Limitations
Upgrade Scenarios & Options

2. time scale of an LHC upgrade
courtesy J. Strait
radiation
damage limit
~700 fb-1
time to halve error
integrated L
L at end of year
ultimate
luminosity
design
luminosity
(1) life expectancy of LHC IR quadrupole magnets is estimated to be <10 years due to high radiation doses
(2) the statistical error halving time will exceed 5 years by
(3) therefore, it is reasonable to plan a machine luminosity upgrade based on new low-b IR magnets before ~2014

3. Chronology of LHC Upgrade Studies
Summer 2001: two CERN task forces investigate physics
potential (CERN-TH ) and accelerator aspects
(LHC Project Report 626) of an LHC upgrade
March 2002: LHC IR Upgrade collaboration meeting
October 2002: ICFA Seminar at CERN on
“Future Perspectives in High Energy Physics”
March 2003: LHC Performance Workshop, Chamonix
2004: CARE-HHH European Network on
High Energy
High Intensity
Hadron Beams

4. LHC performance limitations
beam dumping system
compatible with ultimate intensity of 1.7x1011 /bunch, increases
to 2.0x1011/bunch could be tolerated with reduced safety margin
or after moderate upgrade
detector architecture
in their present configurations, the CMS and ATLAS detectors can
accept a maximum luminosity of 3-5x1034 cm-2s-1
collimation & machine protection
machine protection is challenging: beam transverse energy density
is 1000 times that of the Tevatron; simple graphite collimators may
limit maximum transverse energy density to ½ the nominal value in
order to prevent collimator damage; closing collimators to 6s yields
an impedance at the edge of instability; a local fast loss of 2.2x10-6 of
the beam intensity quenches nearby arc magnets
electron cloud
additional heat load on beam screen; its value depends on beam &
surface parameters; at 75-ns spacing no problem anticipated; initial
bunch populations at 25-ns spacing will be limited to ½ nominal value
beam-beam
limits total current; upgrade may be necessary
limits luminosity; detector upgrade in parallel
with accelerator upgrade, which could allow
moving low-b quads closer to the IP
limits total current & b*
may constrain minimum bunch spacing
limits Nb/e, & crossing angle; compensation schemes may help

5. electron cloud
Photo-electrons created at the vacuum pipe are accelerated by proton
bunches up to 200 eV and cross the pipe in about 5ns; slow or reflected
electrons survive until the next bunch; depending on vacuum-pipe surface
conditions (SEY) and bunch spacing, this may lead to an electron cloud
build up with implications for LHC beam stability, emittance growth and
heat load on the cold LHC beam screen.

6. Simulated average arc heat load due to electron cloud and LHC cooling
capacity as a function of bunch population for different values of the maximum
secondary emission yield. Nominal or ultimate LHC intensity and 25 ns spacing are probably ok for well conditioned surfaces.

7. electron cloud blue: e-cloud effect observed red: planned accelerators
experience
at several
storage rings
suggests that
the e-cloud
threshold
scales as
Nb~Lsep;
possible LHC
upgrades
consider
either
smaller Lsep
with constant
Nb, or they
increase Lsep
in proportion
to Nb
longer fewer more
intense bunches
more ‘ultimate’
bunches

8. predicted e-cloud heat load vs. bunch spacing
on a vertical log scale
change in dmax appears as
~constant vertical shift
nominal
LHC
Simulated average arc heat load due to electron cloud for nominal LHC bunch
intensity as a function of the bunch spacing, for two values of the maximum
secondary emission yield dmax. Elastically reflected electrons are included.

9. saturation of e- build up for high bunch intensities
~average
energy of
secondary
electrons
e- line density
109 m-1
the electron
cloud density
saturates
and stays
almost constant
when the
bunch intensity
is doubled from
the beam-beam
limit value for
two IPs of
2.3x1011 to
4.6x1011
Nb=
4.6x1011
2.3x1011
time
10 ms

10. schematic of reduced electron cloud build up for a super-
bunch; most e- do not gain any energy when traversing
the chamber in the quasi-static beam potential
negligible heat load
[after V. Danilov]

11. beam-beam: long-range collisions
LHC: 4 primary interaction points …
… and npar~32 long-range collision points around each primary IP
long-range collisions:
perturb motion at large betatron amplitudes, where particles come close to opposing beam
cause ‘diffusive’ (or dynamic) aperture (Irwin), high background, poor beam lifetime
increasing problem for SPS, Tevatron, LHC, i.e., for operation with larger # of bunches
dynamic aperture due to long-range collisions
minimum
crossing
angle
higher bunch charge, more bunches or
smaller b* all require larger crossing angle
to maintain the same dynamic aperture

12. beam-beam: tune shift
tune shift from head-on
collision (primary IPs)
tune shift from long-range collisions
increases with
reduced bunch spacing
or crossing angle
limit on xHO
limits Nb/(ge)
d: normalized separation,
xHO / IP
no. of IPs
DQbb total
SPS
0.005 3
0.015
Tevatron (pbar) 2
RHIC
0.002 4
~0.008
LHC (nominal)
0.0034
2 (4)
~0.01
conservative value for total tune spread based on SPS
collider experience

13. Schematic of a super-bunch collision, consisting of ‘head-on’ and ‘long-range’ components. The luminosity for super-bunches having flat longitudinal distribution is ~1.4 times higher than for conventional Gaussian bunches with the same beam-beam tune shift and identical bunch population (see LHC Project Report 627)

14. fundamental luminosity equations
below beam-beam
limit, luminosity
is reduced for
long bunches
and large qc
(1)
HV crossing in 2 IPs no linear tune shift due to long-range collisions,
total linear tune shift also reduced by a factor Fbb~F:
(2)
1/F
combine (1) + (2):
at the beam-beam
limit, luminosity
can be increased
by increasing
bunch length or qc
a) higher injection energy
would allow larger (ge) and
hence more intensity & luminosity
b) another possibility to achieve higher luminosity is to operate with large crossing angle (either ‘Piwinski regime’ or ‘superbunches’)
K. Takayama et al., PRL88, 2002
F. Ruggiero, F. Zimmermann, PRST-AB 5, 2002

15. ultimate
Relative increase in LHC luminosity versus bunch length (or crossing angle) for Gaussian and flat (super-)bunches at constant beam-beam tune shift with alternating crossings in IP1 and IP5

16. luminosity upgrade: baseline scheme
1.0
reduce sz
by factor ~2
using higher
frf & lower e||
(larger qc ?)
0.58 A
qc>qmindue to LR-bb
increase Nb
increase F
BBLR
compen-sation
yes bb
limit?
crab cavities
reduce qc
(squeeze b*)
2.3 no
0.86 A
reduce b* by
factor ~2
new IR
magnets
use large qc
& pass each beam
through separate
magnetic channel
4.6
0.86 A
if e-cloud, dump &
impedance ok
increase nb by
factor ~2
simplified IR design
with large qc
luminosity gain
9.2
beam current
1.72 A

17. luminosity upgrade: Piwinski scheme
decrease F
reduce b* by
factor ~2
new IR
magnets
1.0
increase szqc
0.58 A
superbunches?
flatten profile?
increase Nb
reduce #bunches
to limit total
current?
yes no
7.7
15.5
luminosity gain ?
0.86 A
1.72 A
beam current

18. additional considerations
total current limited? (e.g. by e-cloud, machine
protection, dump) fewer bunches with more
charge give higher luminosity, but also increase
the event pile up
minimum b*: depends on IR magnets,
Q’ correction (more critical for larger Dp/prms) &
collimator settings
integrated luminosity ~Tbb/(Tbb+Tturnaround):
reduce Tturnaround by increasing Einj (SuperSPS),
which reduces injection time and snapback
BBLR compensation + SuperSPS larger
intensity at larger en: L L*2
more luminosity with flat (long) bunches
capability of experiments, e.g., bunch structure

19. upgrades to LHC injector complex
possibility being considered also for CNGS beams is to
upgrade the proton linac from 50 to MeV, to
overcome space charge limitations at injection into the
PS booster; then ultimate LHC intensity would be easy to
achieve and a further 30% increase would be possible with
same emittance & filling time
SPS equipped with s.c. magnets (‘SuperSPS’) & upgraded
transfer lines allow LHC injection at 1 TeV instead 0.45 TeV;
this option can increase peak LHC luminosity by nearly a
factor of 2 at constant beam-beam parameter Nb/e, in
conjunction with LR beam-beam compensation schemes;
reduces turnaround time & increases integrated luminosity;
first step in view of LHC energy upgrade (energy swing
reduced by factor of 2)
s.c.linac could replace booster, or FFAG based injector?

20. new IRs goal: reduce b* by factor 2-5
T. Sen et al., PAC2001
T. Taylor, EPAC02
J. Strait et al., PAC2003
F. Ruggiero et al., EPAC04
goal: reduce b* by factor 2-5
options: NbTi ‘cheap’ upgrade, NbTi(Ta), Nb3Sn
new quadrupoles
new separation dipoles
maximize magnet aperture,
minimize distance to IR
factors driving IR design:
minimize b*
minimize effect of LR collisions
large radiation power directed towards the IRs
accommodate crab cavities or beam-beam compensators
compatibility with upgrade path

21. IR ‘baseline’ schemes short bunches & minimum crossing angle & BBLR
crab cavity
triplet magnets
triplet magnets
BBLR
short bunches &
minimum crossing angle &
BBLR
crab cavities &
large crossing angle

22. alternative IR schemes
dipole magnets
dipole
triplet magnets
triplet magnets
dipole first &
small crossing angle
dipole first &
large crossing angle &
long bunches or crab cavities
reduced # LR collisions
collision debris hits D1
N. Mokhov et al.,
PAC2003

23. ‘cheap’ IR upgrade short bunches & minimum crossing angle & BBLR
in case we need to double LHC luminosity earlier than foreseen
triplet magnets
BBLR
short bunches &
minimum crossing angle &
BBLR
each quadrupole individually optimized (length & aperture)
IP-quad distance reduced from 23 to 22 m
NbTi, b*=0.25 m possible

24. bunch structure more (&shorter) bunches nominal & ultimate LHC
plus:
can use crab cavities
event pile up tolerable
more (&shorter) bunches
upgrade
path 1
concerns:
e-cloud
LRBB
impedance
nominal & ultimate LHC
~12.5 ns
25 ns
upgrade
path 2
longer (&fewer) bunches ?
75 ns
plus:
no e-cloud?
less current
concerns:
event pile up
impedance
super-bunch
concerns:
huge event pile up
plus:
no e-cloud
less current
transitions by bunch merging or splitting;
new rf systems required in all cases

25. example parameter sets
baseline
‘Piwinski’
super-bunch

26. LHC upgrade scenarios
LHC phase 0: maximum performance w/o hardware changes
LHC phase 1: maximum performance with arcs unchanged
LHC phase 2: maximum performance with ‘major’ changes
Nominal LHC performance at 7 TeV corresponds to DQbb=0.01
with L=1034 cm-2s-1 in IP1 and IP5 (ATLAS and CMS), halo
collisions in IP2 (ALICE) and low-luminosity in IP8 (LHC-b)
phase 0 baseline
collide beams only in IP1&5 with alternating H-V crossing
increase Nb up to beam-beam limit L=2.3x1034 cm-2s-1
increase dipole field to 9T (ultimate field) Emax=7.54 TeV
phase 0 Piwinski
4. increase longit. emittance & bunch length, e.g., sz=15.2 cm
5. increase crossing angle by ~10%
6. increase Nb up to beam-beam limit L=3.6x1034 cm-2s-1

27. Comparison of tune footprints, corresponding to betatron amplitudes extending from 0
to 6 s, for nominal LHC (red-dotted), ultimate (green-dashed), and large Piwinski
parameter configuration (blue-solid) with alternating H-V crossing only in IP1&5.
(Courtesy H. Grote)

28. modify insertion quadrupoles and/or layout b*=0.25 m
Possible steps to increase luminosity with hardware changes only in the LHC insertions and/or injector complex include:
phase 1 baseline
modify insertion quadrupoles and/or layout b*=0.25 m
increase crossing angle by ~1.4
increase Nb up to ultimate intensity L=3.3x1034 cm-2s-1
halve sz with high harmonic system L=4.6x1034 cm-2s-1
double number of bunches (and increase qc!)
L=9.2x1034 cm-2s-1 (excluded by e-cloud?)
phase 1 Piwinski
modify insertion quadrupoles and/or layout b*=0.25 m
increase crossing angle by ~60%
optionally merge every 3 bunches ns spacing
3. increase Nb up to beam-beam limit L=7.2x1034 cm-2s-1
phase 1 superbunch
2. collide 1-A superbunches with large qc L=9x1034 cm-2s-1

29. phase 2: luminosity & energy upgrade
modify injectors to significantly increase beam
intensity and brilliance beyond ultimate value
(possibly together with beam-beam compensation
schemes)
equip SPS with s.c. magnets, upgrade transfer
lines, and inject at 1 TeV into LHC
install new dipoles with 15-T field and a safety
margin of 2 T, which are considered a reasonable
target for 2015 and could be operated by 2020
beam energy around 12.5 TeV

30. Sketch of the common coil design for a double aperture dipole magnet;
the coils couple the two apertures and can be flat (no difficult ends).
One of the most difficult challenges will be to make the magnets at a
reasonable cost, less than 5kEuro/(double)T.m say, including cryogenics,
to be compared with 4.5 kEuro/(double)T.m for the present LHC.

31. Nb3Sn block-coil dipole reached 16 T field

32. summary & recommendations for future studies and R&D
(1) nominal LHC performance is challenging: learn how to overcome e-cloud effects,
inject, ramp, and collide 3000 high-intensity bunches, protect s.c. magnets, safely
dump the beams, etc. Upgrades in beam intensity are a viable option, require
R&D for cryogenics, vacuum, RF, beam dump, and injectors, and operation with
large crossing angles
(2) radiation limit for IR quads (~700 fb-1) reached by 2013?
new triplet quadrupoles with high gradient and larger aperture (or alternative
IR layouts) are needed for luminosity upgrade; opening the quads has advantage
of letting radiation through
(3) further studies needed to specify field quality of IR magnets, required upgrades
of instrumentation, collimation, and machine protection; to reduce collimator
impedance during b-squeeze and physics conditions, triplet aperture should be large
(4) experimental studies on e-cloud, long-range, and strong-strong beam-beam
effects are important, as well as MDs in existing hadron colliders with large
Piwinski parameter and many (flat) bunches; international collaboration
(US-LARP/CARE) is welcome/needed for LHC machine studies/commissioning
(5) beam-beam compensation schemes with pulsed wires would reduce tune
footprints and loss of dynamic aperture due to long-range collisions; experimental
validation is underway

33. (6) interesting possibilities currently under study to pass each beam through separate
final quadrupoles include: alternative separation schemes with separation dipoles in
front of the triplet quadrupoles, collision of long bunches with large crossing angle,
normal bunches at large crossing angle with crab cavities; luminosities ~1035 cm-2s-1
(7) super-bunch and ‘large Piwinski angle’ options are interesting for large crossing
angles, can potentially avoid electron-cloud effects, and minimize the cryogenic heat
load; one could inject a bunched beam, accelerate it to 7 TeV, and then use a multiple
harmonic rf system to form longer bunches; the larger the number of bunches,
the smaller is the event pile up in the experiment
(8) crab cavities are attractive, likely raise beam-beam limit, and allow for separate
magnet channels; first experience will be gained at KEKB from 2005; viability for hadron
beams (emittance growth due to rf phase noise) should be explored
(9) major & sustained R&D effort on new s.c. materials and magnet design needed
for any LHC performance upgrade; foster & extend collaboration with other labs: new
low-b quadrupoles with high gradient and larger aperture based on Nb3Sn superc-
conductor require 9-10 years for short-model R&D and component development, proto-
typing and final production
(10) increased 1-TeV injection energy into the LHC in conjunction with beam-beam
compensation schemes would yield an integrated luminosity gain >2; a pulsed
SuperSPS (and new s.c. transfer lines) or cheap low-field booster rings in the LHC
tunnel could be the first step for an LHC energy upgrade

34. thank you for your attention!