1. Pushing the LHC nominal luminosity with flat beams
Beam aspect ratio at the IP versus triplet aperture
Pushing the luminosity by 10-20%
Extrapolating the gain to the LHC upgrade
Open issues and future plans
S. Fartoukh, ABP-RLC meeting,

2. Luminosity with flat beams
Flat beam means
The Xing plane is always the plane where the beam size is the largest at the IP (i.e. the smallest in the triplet)
To gain aperture in the triplet (smaller X-angle requested and better matching between beam-screen and beam aspect ratio, see next slide)
To gain in luminosity (geometric loss factor closer to unity)  
Luminosity calculated for two head-on colliding round beams
r.m.s. bunch length (7.5 cm in collision for the nominal LHC)
Full X-angle in s units (9.5 s for the nominal LHC)
S. Fartoukh, ABP-RLC meeting,

3. Beam aspect ratio vs triplet aperture (1/5)
Beam screen orientation for H/V scheme
In both cases, H-separation of
about 9.5*max(sx,b1 ,sx,b2)
Effect of decreasing the
beam aspect ratio at the IP
(and increasing the vert. X-angle)
In both cases, V-separation of
about 9.5*max(sy,b1 ,sy,b2)
Effect of increasing the
beam aspect ratio at the IP
(and decreasing the vert. X-angle)
Find the optimum matching between
beam-screen and beam aspect ratio
S. Fartoukh, ABP-RLC meeting,

4. Beam aspect ratio vs triplet aperture (2/5)
Nominal LHC (b* = 55 cm, round beam r = 1)
ymax~ 6.5 mm
Beam1
in IR1
bx,max~ 4.5 km
by,max~ 4.5 km
Beam2
In IR1
Vertical crossing
a = 285 mrad
n1 ~ 7
(aperture spec. of
Q2 still to be finalised)
Round beam r = 1
bx* = by* = 55 cm
S. Fartoukh, ABP-RLC meeting,

5. Beam aspect ratio vs triplet aperture (3/4)
Flat beam with r = 2 (i.e. bx* = 110 cm and by* = 27.5 cm)
ymax~ 5 mm
Beam1
in IR1
bx,max~ 2.5 km
by,max~ 8.0 km
Beam2
in IR1
Flat beam r = 2
bx* = 110 cm, by* = 27.5 cm
Horizontal crossing
a = 285/r1/2 = 201 mrad
n1 ~ 7
(even slightly better
than the nominal case)
S. Fartoukh, ABP-RLC meeting,

6. Beam aspect ratio vs triplet aperture (4/5)
Optimum reached for r ~ 1.6
ymax~ 5.5 mm
Beam1
in IR1
bx,max~ 3.0 km
by,max~ 7.0 km
Beam2
in IR1
Flat beam r = 1.6
bx* = 88 cm, by* = 34.4 cm
Horizontal crossing
a = 285/r1/2 = 225mrad
n1 ~ 7.5!
S. Fartoukh, ABP-RLC meeting,

7. Beam aspect ratio vs triplet aperture (5/5)
Reducing further by* by 15% keeping constant bx*
ymax~ 5.5 mm
Beam1
in IR1
bx,max~ 3.0 km
by,max~ 8.0 km
Beam2
in IR1
Flat beam bx* = 88 cm, by* = 30 cm
Horizontal crossing
= 225mrad (unchanged
w.r.t. previous case)
n1 ~ 7
S. Fartoukh, ABP-RLC meeting,

8. Pushing the luminosity by 10-20%
Case
bx* [cm]
by* [cm]
a *[mrad]
n1 in the triplet
Geometric loss factor [%]
L/Lnom
Nominal
r=1.0, b*=55cm
55.00
285 ~7
83.9
1.00
Flat
r=2.0, b*=55cm
110.00
27.50
201
95.1
1.13
r=1.6, b*=55cm
88.00
34.37
225
~7.5
92.7
1.10
r~1.7, b*~51cm
30.00
1.18
All these cases being allowed by the nominal LHC hardware:layout,
power supply, optics antisymmetry, b.s. orientation in the triplets
(only changing the present H/V scheme into V/H scheme)!
S. Fartoukh, ABP-RLC meeting,

9. Extrapolating the gain to the LHC upgrade
Rather than
using crab-cavities or so-called “D0” (orbit corrector very closed to the IP),
or decreasing the bunch length.
Using flat beam is an “hardware-free” alternative
to largely increase the geom. luminosity loss factor for b* = 25cm: 57% for a round beam  81/75% for a flat beam with r=2/1.6
2) to gain about 10% of aperture in the triplet with (almost) no loss of luminosity, i.e. reduce by 10% the peak coil field.
S. Fartoukh, ABP-RLC meeting,

10. Open issues and future plans (1/2)
Chromatic correction:
 Q’ correction is a non-issue with the actual LHC sextupole scheme:
SF, resp. SD, pushed to 36%, resp. 62%, of nominal field even in the worst case of H-H, resp. V-V, scheme (in any case not allowed by actual b.s. orientation in IR1 and IR5)
 Q’’ and off-momentum beta-beat correction
 should be OK (as a first guess based on previous study: S.F., LPR 308)
 But still to be checked (IR phasing can only partially help for the H/V scheme)
S. Fartoukh, ABP-RLC meeting,

11. Open issues and future plans (2/2)
Beam-beam:
 Parasitic b-b compensation is only partially granted by the H/V scheme with flat beam:
for r < 2.4 still better than H-H or V-V scheme with round beams
for r ~ 1.6, similar to the case of only one working low-beta insertion (scenario that has to be envisaged in any case)
 Head-on tune shift independent r for H/V scheme:
Thin lens optics for H/H, V/V, H/V scheme with flat beams still to
be done for robust b-b studies (tune foot-print, DA …)
S. Fartoukh, ABP-RLC meeting,