M. Benedikt & E. Metral
PS-OP shut-down lectures, MCR glassbox, 20/02/2001





Requirements of the LHC on its injectors
What are the nominal & already achieved beams at PS exit?
How is it obtained in the PS complex?
 General aspects



Linac2
PSB
PS
Future work
SPS and LHC filling
E.M, PS-OP shut-down lectures, 20/02/2001 1

 2 main challenges involved in the design of the LHC
 Very high magnetic field to reach the collision energies in the TeV range
 Very high luminosity necessary to provide significant event rates at this energy
L


 norm ,
,
N

1 x y b


 

N b k b e f rev

It is limited by :
- Space-charge effects in the injectors...
- Head-on beam-beam interaction at collision
E.M, PS-OP shut-down lectures, 20/02/2001
Beam current
Brightness = transverse bunch density
It is limited by :
- Collective instabilities
- Cryogenic load (synchrotron radiation and wall current)
- S.C. magnet quench
2

Choice of the nominal LHC parameters
Energy
E [TeV]
Dipole field
B [T]
Luminosity
L [cm -2 s -1 ]
Harmonic number h
Number of bunches k b
Protons / bunch
N b
Bunch spacing
 b.s
[ns] x & y emittances
 x norm,
, y
1
[ 
Long. emittance
 l
2

[eVs]
7
35640
2808
1.1
8.3
10

25
0.5
34
10
3.75

11
1
E.M, PS-OP shut-down lectures, 20/02/2001 3

 LHC project leader  L. Evans
 Major upgrade needed all along the injector chain
 Project leader to prepare the PS complex to be a pre-injector (started in 1995)  K. Schindl (Deputy  M. Benedikt )
E.M, PS-OP shut-down lectures, 20/02/2001 4

Energy
E [GeV]
Harmonic number h
Number of bunches k b
Protons / bunch
N b
Bunch spacing x & y emittances
 norm, x , y
 b.s
[ns]
1
[ 
Achieved
25
84
72
1.1

10
11
25
2.5
Nominal
25
84
72
1.1

10
11
25
3
Long. emittance
 l
2

[eVs]
0.35
0.35
Total bunch length
 b
[ns]
Momentum spread
2  p
/ p

4
2.2

10
-3
4
2.2

10
-3
 The specifications are met in the PS complex
E.M, PS-OP shut-down lectures, 20/02/2001 5

General aspects
 2 main challenges had to be faced
 High brightness production (2  as before) and conservation
 Production of the train of very short bunches with the LHC spacing
 Solutions
 Double-batch filling of the PS (2  1.2 s)

 Lowers the space charge effects at PSB injection ( 50 MeV )
Increase of the PSB ejection kinetic energy (PS injection) : 1  1.4 GeV
 Lowers the space charge effects at PS injection
 1 triple + 2 double splittings to produce the desired number of bunches, longitudinal emittance and bunch spacing
 Bunch rotation to produce the desired bunch length
E.M, PS-OP shut-down lectures, 20/02/2001 6

Linac2
 The initial transverse emittance is given by the duoplasmatron source
Normalised, at 1 
 0 .
4
 m
Depends on extraction aperture, electrode shape and space charge
 The beam is then adiabatically bunched and accelerated in a Radio
Frequency Quadrupole ( RFQ2 ) under high space charge conditions
 0 .
6
 m
 Fine-tuning of the 50 MeV Drift Tube Linac ( DTL ) and of the transfer line to the PSB
 1 .
2
 m
7 E.M, PS-OP shut-down lectures, 20/02/2001

PSB (1/5)
 General aspects
 PSB delivers 2 batches to PS (2 consecutive 1.2 s cycles )
 3 PSB rings per batch ( 3 , 4 and 2 )
 1 bunch per ring ( H1 ) C275, Bdot = 5 Gauss/ms
 Injection at 50 MeV
This is what sets the brightness
 Horizontal plane
 Multi-turn injection : 3 turns exactly  more stability and reproducibility (most homogeneous longitudinal distribution of the unbunched beam)

Adjustment of the horizontal injection steering and injection bump timing to minimise the horizontal emittance  BIX.SKSW2,3,4

Special tune because of large tune shift  Q h
= 4.28

Tiny shaving ~ 30 ms after injection
E.M, PS-OP shut-down lectures, 20/02/2001 8

PSB (2/5)
 Vertical plane

Injection on orbit

Minimisation of vertical oscillations at injection ( 1/2 turn pick-up ) to minimise vertical emittance  BI.DVT 50 and 70


Special tune because of large tune shift  Q v
= 5.44
Shaving vertical  to have a well-defined emittance
 Acceleration from 50 MeV to 1.4 GeV
C275  C765
 Double harmonics operation to increase the bunching factor (bunch flattening) and thus decrease the space charge tune shift at injection
 C02 ( H1 , 1/ring) and C04 ( H2 , 1/ring). C04 voltage slowly reduced to zero at synchronisation/ejection
 Available controlled blow-up  C16 ( H9 , 1/ring)
 No coupling between the transverse planes
 Standard settings of multipoles for resonance compensations
E.M, PS-OP shut-down lectures, 20/02/2001 9

PSB (3/5)
 Synchronisation
Non-standard bunch spacing at ejection to fit the PS H7 RF system
 t
PS h

7

8
7
  t
PS h

8

8

286 ns

327 ns
7
 adjustment with the phase offsets : BA3,4,2.PSYNCOFFSET
( ring 3 is always used as the reference)
 Ejection at 1.4 GeV
C805
 fast extraction towards the PS through the BT/BTP transfer line
E.M, PS-OP shut-down lectures, 20/02/2001 10


10
PSB (4/5)
E.M, PS-OP shut-down lectures, 20/02/2001
Time [ms]
11

Without blow-up
PSB (5/5)
Beam parameters at PSB extraction
1 .
4

10
12
1 .
32

10
12
 x norm, 1


 norm, y
1


 l
2

 b
2
 p
/ p

-3
2 .
2
1 .
8
0 .
9
150
2
2 .
5
2 .
5
1 .
5
195
2 .
45
E.M, PS-OP shut-down lectures, 20/02/2001 12

PS (1/10)
 General aspects
 Double-batch injection : 1 batch of 3 bunches + 1 batch of 3 bunches 1.2 s later  6 bunches out of 7 buckets
 Longitudinal beam slicing  complicated RF gymnastics
PSB exit
PS exit
~ 300 ns
 High brightness conservation  careful control of collective effects, injection oscillations, working point, chromaticities, non-linearities at extraction …
E.M, PS-OP shut-down lectures, 20/02/2001 13

PS (2/10)
 At low energy ( 1.4 GeV kinetic energy)
 1 st injection => 3 bunches ( H7 ) Inj 42 at C170
 Transverse matching between PSB and PS, orbit correction...

Careful control of the working point to avoid blow-up during the long flat-bottom  Q h
~ 6.21 and Q v
~ 6.23

A single-bunch head-tail instability (due to the resistive wallimpedance) develops during the long flat-bottom => it is cured by x-y coupling (skew quadrupoles)
 2 nd injection => 3 bunches ( H7 ) => 3 + 3 = 6 bunches ( H7 )
Inj 42 at C1370
C1380




Momentum adaptation PSB-PS => PSB synchro. made with PS beam
Triple splitting => 6 × 3 = 18 bunches ( H21 )
Acceleration from on H21
1.4
to
C1560
25 GeV
C1450
22 Gauss/ms
3 groups of C10 cavities on H7,14,21
 At transition :  -jump + change of the chromaticities sign
E.M, PS-OP shut-down lectures, 20/02/2001 14

PS (3/10)
Head-Tail resistive-wall instability

10
Beam-Position Monitor
(20 revolutions superimposed)

I skew

0 .
33 A
Time [ms]
m

6
E.M, PS-OP shut-down lectures, 20/02/2001
I skew
 
0 .
4 A
15

C2120

PS (4/10)
Longitudinal coupled-bunch instabilities between 6 and 20 GeV/c cured by controlled longitudinal blow-up Cavities C200 (H420)
 Horizontal orbit correction => PR.GSDHZ15,60-OC
 At high energy ( 26 GeV/c momentum)
It will change this year
=> new high-energy timings
 Synchronisation H1 => the worst


1 st double splitting => 18 × 2 = 36 bunches ( H42 )
2 nd double splitting => 36 × 2 = 72 bunches ( H84 )
1 cavity C20
1 cavity C40
 Bunch compression by a step voltage
=> longitudinal mismatch
1 cavity C40 (H84)
2 cavities C80 (H168)
=> bunch rotation and ejection after 1/4 of synchrotron period
 b

16 ns 
 b

4 ns with
 l

0 .
35 eVs
 Ejection at 26 GeV/c
Ej 16 at C2395
 fast extraction towards the SPS through the TT2/TT10 transfer line
E.M, PS-OP shut-down lectures, 20/02/2001 16

PS (5/10)
Magnetic field and double-batch injection
1000
900
800
700
600
500
400
300
200
100
0
-100
100
I [e10]
B [Gauss] inj1 +10ms
C180
600
E.M, PS-OP shut-down lectures, 20/02/2001 inj2 -10ms
C1360
1100 inj2 +5ms
C1375 ej -6ms
C2389
6000
1600
4000
No 3.5 GeV/c plateau
2000
0
2100
14000
12000
10000
8000
17

PS (6/10)
Longitudinal beam structure in the last turn of the PS
E.M, PS-OP shut-down lectures, 20/02/2001
300 ns/div
30 ns/div
1 ns/div
18

3.5
3
2.5
2
1.5
1
0.5
0
0
PS (7/10)
Normalised transverse emittances at 1 
5 00
BAD
GOOD
20 00
H 54
H 64
V75
V85
25 00 10 00
Ti me [m s ]
15 00
1st batch injection
PSB output
(SEM grids in meas.
line)
2nd batch injection
1.4-25GeV acceleration
PS output
(SEM grids in
TT2 line)
!
without bunch rotation
E.M, PS-OP shut-down lectures, 20/02/2001 19

PS (8/10)
Only 1 measurement is still missing  the transverse emittances in
TT2 in the presence of bunch rotation
H - plane
E.M, PS-OP shut-down lectures, 20/02/2001
V - plane
20

PS (9/10)
=> Electrons are created ...
H - plane
E.M, PS-OP shut-down lectures, 20/02/2001 21

PS (10/10)
Baseline drift on electrostatic pick-ups in TT2
Also observed in the PS
Apparently the beam is not affected  this is only a measurement problem for the PS (contrary to the SPS and LHC)
E.M, PS-OP shut-down lectures, 20/02/2001 22


The nominal beam is within reach, but one item is missing  the quantitative analysis of the non-linear effects due to the stray-field at
PS extraction . It could create an optical mismatch  blow-up
 4 other subjects need to be investigated in the near future
 Consolidation of the nominal beam  improvements in pulse-to-pulse injection mis-steerings, kicker ripples, PSB-PS transverse and energy matching, bunch to bunch intensity fluctuations, instrumentation …
 Multi-gap/multi-spacing beams preparation for SPS MDs (e.g.
50 and
100 ns bunch spacing). In particular, cures for longitudinal instabilities have to be investigated (feedback systems, HOM damping)
 The so-called initial beam should be prepared

1/6 of the intensity, 1/4 of the transverse emittance
 good for collective effects
 bad for injection mis-steerings => damper at injection certainly useful
The so-called ultimate beam
E.M, PS-OP shut-down lectures, 20/02/2001 should also be looked at 1.6  the intensity
23

 The cycle will consist of either 3 or 4 PS injections at 3.6 s intervals
 3 -batch  2.38  10 13 protons in 26% of the SPS circumference
 4 -batch  3.17  10 13 protons in 35% of the SPS circumference
 The injection plateau will therefore lasts up to 10.8 s
 The acceleration phase is about 8.3 s and brings the beam from 26 GeV/c to 450 GeV/c
 A 1 s flat-top is presently assumed. This will be used to prepare the extraction equipment (bumpers, etc...) and perform any RF re-phasing necessary to put the beam on the correct location in the LHC
 SPS issues
Collective instabilities in both longitudinal and transverse planes
 programme for impedance reduction + electron-cloud studies
 Fast extraction towards the LHC through TI2 (via the West extraction channel) or TI8 (via the new East extraction channel)
E.M, PS-OP shut-down lectures, 20/02/2001 24

LHC Proton Injection Cycle (21.6 s)
This cycle is repeated 12 times for each LHC ring . 3 or 4-batch cycles will be interleaved in the form 334 334 334 333 to fill each ring with a total of 2808 bunches. The LHC filling time will be 12  21.6 s = 4.3 minutes per ring
E.M, PS-OP shut-down lectures, 20/02/2001 25

Bunch disposition in the LHC, SPS and PS
E.M, PS-OP shut-down lectures, 20/02/2001 26

 The same M ain T iming G enerator will be used to pilot the PS complex,
SPS and LHC
 Several levels of super-cycles will be introduced
16

2

2

64
16 SPS levels
2 PS levels for each SPS level
Normal + spare
E.M, PS-OP shut-down lectures, 20/02/2001 27