1. Generation of Higher Brightness Beams for LHC
Needs
Possibilities:
Batch compression in the PS
Linac 4
SPL
Summary
R. Garoby

2. Needs of various LHC luminosity upgrade scenarios*
Luminosity
´1034 cm-2s-1
Comment
Brightness factor**
Protons in 25 ns
(en=3.75 mm)
Protons ejected from the PS*** (en=3 mm)
Nominal
25 ns
1.0
0.58 A
285 mrad 1
1.15 ´1011
1.35 ´1011
Ultimate
2.3
0.86 A
315 mrad
1.5
1.7 ´1011
2.0 ´1011
Short bunches
12.5 ns
9.2
1.72 A
445 mrad 3
3.4 ´1011
4 ´1011
Long bunch
75 ns
8.9
1 A
430 mrad
1.74
2.4 ´1011
1 TeV inj.
3.44 A ?
5.9
6.8 ´1011
8 ´1011
Needs clarification !
* HIF05 paper
** w.r.t. nominal in LHC
*** Transmission PS-> LHC= 0.85
R.G. 2

3. Increasing brightness in the PS Batch compression [1/5]
Proposed procedure:
inject 7 (4+3) or possibly 8 (4+4) bunches from two PSB batches into the PS operating on harmonic 9,
accelerate this beam up to an intermediate energy where space charge is sufficiently reduced,
compress the 7 (8) bunches into 7 (8)/14 of the PS circumference by adiabatically increasing from h=9 to 10,11, 12, 13, 14,
accelerate the beam on harmonic 14 up to 25 GeV,
triple split the bunches using rf on h=14, 28 and 42 (similar process than used at 1.4 GeV for the 25 ns bunch train),
double split bunches, changing the harmonic from 42 to 84, and rotate them before ejection, as in the present 25 ns bunch train scheme.
Þ Finally, a train of 42 or 48 bunches, spaced by 25 ns
is sent to the SPS every 3.6 s.
Best expected performance: assuming that the space-charge limit (with a 1.2 s flat porch at 1.4 GeV) is attained in the PS with 84´1.7´1011 protons over the circumference
Þ 2.6´1011 PS ejection
R.G. 3

4. Batch compression (7 bunches case) [2/5]:
Bucket
Height
(arb. units)
Time
R.G. 4

5. Batch compression (7 bunches case) [3/5]:
Bucket
parameters
during
the process
(arb. units)
Extreme edge
Buckets
(9 bunches)
R.G. 5

6. Batch compression (7 bunches case) [4/5]: “Ultimate” filling scheme for 42 PS bunches
P. Collier, AB/OP
2604 bunches/ring: only 7% fewer than for nominal 72 bunch scheme.
6 injections and 18 s SPS flat bottom: problems with high brightness?
R.G. 6

7. Batch compression in the PS [5/5]: Main characteristics
Stage
Advantages
Limitations & drawbacks
Implementation
Fast & “Low cost”
(low level RF)
Manpower intensive preparation
Need for much machine time
Operation
Delicate operation (manpower intensive & prone to imperfection)
Lower LHC filling factor (~ -7 %)
Longer LHC filling time (~ ´ 1.35)
Reduced availability for other users
1.2 s flat porch in the PS with high space-charge
SPS capability ?
Potential
(ppb at PS ejection)
42 bunches every 3.6 s with 2.6´1011 ppb [DQ ~ 0.25]
R.G. 7

8. Increasing brightness in the PSB with Linac 4 (scheme 1) [1/4]
Procedure 1 (72 bunches):
inject 12 (4´3) bunches from one PSB batch into the PS operating on harmonic 14,
accelerate this beam on h=14 up 25 GeV,
triple split the bunches using rf on h=14, 28 and 42 (similar process than used at 1.4 GeV for the 25 ns bunch train),
double split bunches, changing the harmonic from 42 to 84, and rotate them before ejection, as in the present 25 ns bunch train scheme.
Þ Finally, a train of 72 bunches, spaced by 25 ns
is sent to the SPS every 2.4 s.
Best expected performance: assuming that the PSB can deliver 3.6´1012 protons/ring within 2.5 mm emittances (bg2 x 2 at injection) and that space-charge in the PS at 1.4 GeV can be increased by 1.17 (14/12) (no flat porch)
Þ 2.0´1011 PS ejection
R.G. 8

9. Increasing brightness in the PSB with Linac 4 (scheme 2) [2/4]
Procedure 2 (48 bunches):
inject 4 (4´1) bunches from one PSB batch into the PS operating on harmonic 7,
accelerate this beam on h=7 up to an intermediate energy,
double split bunches, changing the harmonic from 7 to 14,
accelerate on h=14 up to 25 GeV,
triple split the bunches using rf on h=14, 28 and 42 (similar process than used at 1.4 GeV for the 25 ns bunch train),
double split bunches, changing the harmonic from 42 to 84, and rotate them before ejection, as in the present 25 ns bunch train scheme.
Þ Finally, a train of 48 bunches, spaced by 25 ns
is sent to the SPS every 2.4 s.
Best expected performance: assuming that the PSB can deliver 3.6´1012 protons/ring within 2.5 mm emittances (bg2 x 2 at injection) and that space-charge in the PS at 1.4 GeV can be increased by 1.75 (7/4) (no flat porch)
Þ 3.0´1011 PS ejection
R.G. 9

10. Increasing brightness in the PSB with Linac 4 (scheme 3) [3/4]
Procedure 3 (24 bunches):
inject 4 (4´1) bunches from one PSB batch into the PS operating on harmonic 7,
accelerate this beam on h=7 up to an intermediate energy,
compress the 4 bunches into 4/14 of the PS circumference by adiabatically increasing from h=7 to 9,11, 14,
accelerate on h=14 up to 25 GeV,
triple split the bunches using rf on h=14, 28 and 42 (similar process than used at 1.4 GeV for the 25 ns bunch train),
double split bunches, changing the harmonic from 42 to 84, and rotate them before ejection, as in the present 25 ns bunch train scheme.
Þ Finally, a train of 24 bunches, spaced by 25 ns
is sent to the SPS every 2.4 s.
Best expected performance: assuming that the PSB can deliver 3.6´1012 protons/ring within 2.5 mm emittances (bg2 x 2 at injection) and that space-charge in the PS at 1.4 GeV can be increased by 1.75 (7/4) (no flat porch)
Þ 6.0´1011 PS ejection
R.G. 10

11. Limitations & drawbacks
Increasing brightness in the PSB with Linac 4 [4/4]: Main characteristics
Stage
Advantages
(25 ns bunch spacing)
Limitations & drawbacks
Implementation
Cost (P+M ~ 70 MCHF)
Construction time (~ 3 years)
Operation
Reliability (simple & robust operation for the 72 bunches scheme)
Short dwelling time at high space charge
Reduced LHC filling time (~ ´ 0.82 with 72 bunches)
Increased beam availability for other users
Need for similar RF gymnastics than today in the PS
Capability to accept a DQ of 0.44 for a short duration at 1.4 GeV in the PS ?
SPS capability ?
Potential
(ppb at PS ejection)
72 bunches every 2.4 s with 2´1011 ppb [DQ ~ 0.3]
48 bunches every 2.4 s with 3´1011 ppb [DQ ~ 0.44]
24 bunches every 2.4 s with 6´1011 ppb [DQ ~ 0.44]
Safe
Possibly OK
To be demonstrated
R.G. 11

12. Replacing the PSB by an SPL [1/2]: Main features
Injection energy in the PS (SPL CDR-1): 3.5 GeV
Þ bg2 ´ & DQ ´ 0.26
For a given DQ, brightness can be quadrupled.
Þ 8 ´ 1011 ppb at 26 GeV [with DQ ~ 0.31 at injection]
Þ other limitations will be dominant (instabilities)
Longitudinal bunch population can easily be tailored to the needs (number of bunches, distance between bunches)
Acceleration takes place with the harmonic number used at injection (new RF system ?)
No need for RF gymnastics (except “rebucketing” inside 40 or 80 MHz buckets)
Cycling rate is determined by the PS magnetic cycle (~ 1.5 s)
A train of 1 to 80 bunches, spaced by 25 ns (typical) can be sent to the SPS every ~ 1.5 s
R.G. 12

13. Replacing the PSB by an SPL [2/2]: Main characteristics
Stage
Advantages
Limitations & drawbacks
Implementation
Cost (P+M ~ 500 MCHF)
Construction time (~ 5-6 years)
Operation
Renewed & modern PS injector
No need for RF gymnastics in the PS
Reliability (simple & robust operation)
Short dwelling time at high space charge
Reduced LHC filling time (~ ´ 0.82 with 80 bunches)
Increased beam availability for other users
SPS capability ?
Potential
(ppb at PS ejection with 25 ns spacing)
No more space-charge limitation in the PS: train of 1-80 bunches every 1.5 s with up to 8´1011 ppb
Other phenomena will certainly limit intensity at a lower level
R.G. 13

14. Replacing the PSB by an RCS [1/2]: Main features
To improve space charge in the PS, transfer energy has to be significantly above today’s value (1.4 GeV). Typically: 3.5 GeV
Þ bg2 ´ & DQ ´ 0.26
For a given DQ, brightness can be quadrupled.
Þ 8 ´ 1011 ppb at 26 GeV [with DQ ~ 0.31 at injection]
Þ other limitations will be dominant (instabilities)
Space charge in the RCPSB must match the PS capability. Assuming it has the size of the PSB
Þ bg2 ´ 7.8 w.r.t. 50 MeV
Þ Injection energy = 760 MeV
To minimize the duration of the PS flat porch, it must either have 4 rings or cycle fast (typically 50 Hz) and fill 6/7 of the PS in 6 pulses
To match the SPL potential w.r.t. the LHC, it must deliver > 9.6´1012 protons/pulse within en=2.5 mm
A train of 72 bunches (8 ´ 1011 ppb) , spaced by 25 ns (typical) can be sent to the SPS every ~1.5 s
R.G. 14

15. Replacing the PSB by an RCS [2/2]: Main characteristics
Stage
Advantages
Limitations & drawbacks
Implementation
Cost (P+M ~ xy0 MCHF)
Construction time (~ 3-4 years)
Operation
Renewed & modern PS injector
Reliability (simple & robust operation for the 72 bunches scheme)
Medium dwelling time at high space charge
Reduced LHC filling time (~ ´ 0.82 with 80 bunches)
Increased beam availability for other users
Need for similar RF gymnastics than today in the PS
SPS capability ?
Potential
(ppb at PS ejection)
No more space-charge limitation in the PS: train of 72 bunches every 1.5 s with up to 8´1011 ppb
Other phenomena will certainly limit intensity at a lower level
R.G. 15

16. Comparative summary (25 ns bunch spacing)
Batch compression in PS
Linac 4
SPL
RCS
Delay
Fast
3 years
5-6 years
3-4 years
Implementation
MD intensive
Setting-up period during start-up
Operation
Delicate
Limited reliability
Comfort +
Reliability +
Comfort ++
Reliability +++
Reliability ++
Number of bunches / PS pulse 42
72 ( )
1-80 72
Potential intensity per PS bunch
2.6 ´ 1011 ppb
2 ´ 1011 ppb
(3 - 6 ´ 1011)
8 ? ´ 1011 ppb
Repetition period
3.6 s
2.4 s
1.5 s
BEST USE
Exploratory tests
Reliable operation + 50% of upgrades
Reliable operation + all upgrades
R.G. 16