Transcript Pushing the LHC nominal luminosity with flat beams • Luminosity with flat beams • Beam aspect ratio at the IP versus triplet aperture • Pushing.

Pushing the LHC nominal luminosity
with flat beams
• 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, 28-10-2005
Luminosity with flat beams
•
Flat beam means
 x   y   but  x*  r * and  y*   * / r with r 1
•
The Xing plane is always the plane where the beam size is
the largest at the IP (i.e. the smallest in the triplet)
1. To gain aperture in the triplet (smaller X-angle requested and
better matching between beam-screen and beam aspect ratio, see next slide)
2. To gain in luminosity (geometric loss factor closer to unity)
L(r, *) 

Luminosity calculated for two head-on colliding round beams
L0
z 

1  
* 
 2 r 
2
r.m.s. bunch length (7.5 cm in collision for the nominal LHC)
 *   x*   y* (55 cm for the nominal LHC) and
r  1 the so - called beam aspect ratio
Full X-angle in  units (9.5  for the nominal LHC)
S. Fartoukh, ABP-RLC meeting, 28-10-2005
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(x,b1 ,x,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(y,b1 ,y,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, 28-10-2005
Beam aspect ratio vs triplet aperture (2/5)
• Nominal LHC (* = 55 cm, round beam r = 1)
ymax~ 6.5 mm
Beam1
in IR1
x,max~ 4.5 km
y,max~ 4.5 km
Beam2
In IR1
Round beam r = 1
x* = y* = 55 cm
S. Fartoukh, ABP-RLC meeting, 28-10-2005
Vertical crossing
 = 285 mrad
n1 ~ 7
(aperture spec. of
Q2 still to be finalised)
Beam aspect ratio vs triplet aperture (3/4)
• Flat beam with r = 2 (i.e. x* = 110 cm and
y* = 27.5 cm)
ymax~ 5 mm
Beam1
in IR1
x,max~ 2.5 km
y,max~ 8.0 km
Beam2
in IR1
Flat beam r = 2
x = 110 cm, y* = 27.5 cm
*
S. Fartoukh, ABP-RLC meeting, 28-10-2005
Horizontal crossing
 = 285/r1/2 = 201 mrad
n1 ~ 7
(even slightly better
than the nominal case)
Beam aspect ratio vs triplet aperture (4/5)
• Optimum reached for r ~ 1.6
ymax~ 5.5 mm
Beam1
in IR1
x,max~ 3.0 km
y,max~ 7.0 km
Beam2
in IR1
Flat beam r = 1.6
x = 88 cm, y* = 34.4 cm
*
S. Fartoukh, ABP-RLC meeting, 28-10-2005
Horizontal crossing
 = 285/r1/2 = 225mrad
n1 ~ 7.5!
Beam aspect ratio vs triplet aperture (5/5)
• Reducing further y* by 15% keeping constant x*
Beam1
in IR1
ymax~ 5.5 mm
x,max~ 3.0 km
y,max~ 8.0 km
Beam2
in IR1
Flat beam x* = 88 cm, y* = 30 cm
(i.e. r  1.71 and β*  β*x  β*y  51 cm)
S. Fartoukh, ABP-RLC meeting, 28-10-2005
Horizontal crossing
 = 225mrad (unchanged
w.r.t. previous case)
n1 ~ 7
Pushing the luminosity by 10-20%
x* [cm]
y* [cm]
 *[mrad]
n1 in the
triplet
Geometric loss
factor [%]
L/Lnom
Nominal
r=1.0, *55cm
55.00
55.00
285
~7
83.9
1.00
Flat
r=2.0, *55cm
110.00
27.50
201
~7
95.1
1.13
Flat
r=1.6, *55cm
88.00
34.37
225
~7.5
92.7
1.10
Flat
r~1.7, *~51cm
88.00
30.00
225
~7
92.7
1.18
Case
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, 28-10-2005
Extrapolating the gain to the LHC upgrade
 Rather than
a)
b)
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
1)
2)
to largely increase the geom. luminosity loss factor for * = 25cm:
57% for a round beam  81/75% for a flat beam with r=2/1.6
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, 28-10-2005
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, 28-10-2005
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:
Qparasitic
  x, y
  x, y
 r 1/ r
x, y
IR1
IR 5
 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:
-on
Qhead
x, y




 x*, y
 x*, y
1
1





 *


1
  * ( *   * ) 
  ( *   * ) 
1 1/ r r 1
y  IR1
y  IR 5
 x, y x
 x, y x
 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, 28-10-2005