LHC

Emittance Preservation

V. Kain, G. Arduini, B. Goddard, B. J. Holzer, J. M. Jowett,

M. Meddahi, T. Mertens, F. Roncarolo, M. Schaumann,

R. Versteegen, J. Wenninger

1

LHC

Introduction

Performance of collider is measured in luminosity

Will present analysis of emittance preservation from SPS extraction to

start of LHC collisions (mainly 50 ns p+)

2

FfkN

L

*

2

4

Small to maximize

luminostiy

Produce small in injectors → need to

keep small in the LHC

LHC

Injector Performance

o Impressive performance of injectors

− From design report (25 ns): PSB 2.5 mm – PS 3 mm – SPS 3.5 mm

3

0.5 mm

0.5 mm

50 ns 2011: blow-up PSB to SPS 0.3 – 0.4 mm

Points to be addressed:

o PSB: poorer

performance ring 1,

ring 2

o PS: more blow-up with

higher intensity

o SPS: larger

measurement spread

at extraction

LHC

Analysed ~ 60 fills between mid July to mid August (50 ns, 1.2 ×1011, *=1.5 m)

Comparison of convoluted emittance from LHC luminosity with SPS wire scan for

144 bunches:

LHC Performance

4

On average ~ 20 - 30 % growth between SPS flattop and

collisions

LHC

Results of analysis based on SPS/LHC wire scanners, LHC synchrotron light

monitor and luminosity of ATLAS and CMS

o Wire scanners in the SPS:

− Measured in the SPS routinely when setting up (intermediate 12 bunches + nominal 144

bunches)

− No synchronous measurement SPS-LHC

− Large spread in short time scales

o Wire scanners in the LHC:

− Measured routinely intermediate batch (12 bunches)

− Sometimes measured first nominal batch (144 bunches) – intensity limit at 2.5 × 1013 p+

− No measurements at 3.5 TeV for physics beam

Preparation of fill 2240

> 50 % spread

Measurement Limitations 2011 (1)

5

LHC

Measurement Limitations 2011 (2)

o Continuous beam size measurement with Synchrotron Light Monitor (BSRT):

− Bunch-by-bunch: 3 s per bunch → 69 min (!) per ring;

− Good for relative measurement under same conditions

Absolute calibration not obvious (wire scanners are used for cross-calibration)

Cannot compare data at different energies – cannot see effect of ramp

Could not compare data for different beams/planes

o Emittance from luminosity

− Single emittance value for different beams and planes

− Not always fully optimized /not publishing the correct value

− Assumes Gaussian beams

6

LHC

Improvements for 2012

o Long list of measurement improvements planned for 2012

o Instrumentation:

− Faster and better calibrated BSRTs

− Commission Beam-gas Ionization Profile Monitor (BGI)

− Pre-prepared wire scanner hardware settings for circulating intensity

− Bunch-by-bunch wire scans in the SPS

o Methods:

− Automatic wire scans during ramp

− Synchronous measurements across the accelerator complex

o Analysis and GUIs:

− Make measured betas available

− More reliable fits

− Write fit results into logging database

7

LHC

CURRENT UNDERSTANDING

OF LHC EMITTANCE BLOW-UP

8

LHC

Used:

o Wire scan data from SPS 12 bunches for fills 1960 - 2025

o Wire scan data from LHC beam 1 12 bunches for fills 1960 - 2025

Mismatch at injection?

9

Emittances conserved at injection within measurement

accuracy

LHC/SPS = 1.07 ± 0.11 LHC/SPS = 0.99 ± 0.12

LHC

o Comparing beam 1 with beam 2 wire scans of 12 bunches

Beam 1 versus Beam 2

10

Using the measured beta functions at wire scanners:

Beam 1 and beam 2 emittances are consistent

LHC1/LHC2 = 0.96 ± 0.08 LHC1/LHC2 = 1.06 ± 0.08

LHC

Growth @ 450 GeV

o Emittances are growing at injection – reasonably consistent with IBS although

slighty faster

11

Fill 1897, beam 1

BSRT gated over 12 bunches Fill 2028, beam 1

BSRT gated over single bunch

Simulations of IBS, uncoupled

T. Mertens

Horizontal emittance: ~ 10 % in 20 minutes

Filling about 30 minutes → D/ 0 – 10% in H

More studies to come in 2012 to understand

faster growth of bunch length and transverse

emittance

LHC

→ Effect on bunch-by-bunch luminosity

12

Specific luminosity 10 %

lower for first bunches

consistent with growth

injection A. Ryd, CMS

LHC

o Cannot use BSRT data, BGI was not commissioned

o Dedicated fills are necessary: low number of bunches to do wire scans through

the ramp

First indication: Abort gap cleaning test fill (12+12+12 bunches, 50 ns)

Used measured at injection and flattop and linear interpolation between

The Ramp

13

Blow-up during the ramp: measurement indicates > 20 % all planes

H0 = 1.6 mm

V0 = 1.4 mm

H0 = 1.6 mm

V0 = 1.3 mm

LHC

The Ramp - continued

Last BI MD in 2011:

Ramp of 4 bunches per ring – different emittances

Unfortunately no useful wire scan data for beam 1

14

> 20 %

emittance D ~ 1 mm emittance D ~ 0.7 mm

Relative growth different,

but…different emittances grow by the same amount

Although larger than for 12 bunch trains

LHC

Possible sources for growth @ ramp o Ramp still needs to be further optimized

− Chromaticity,…

o Effect of reduced damper gain during ramp?

15

Example proton Fill 2254 (1380 bunches per beam)

BBQ hor Beam 1amplitude

damper gain hor beam 1(linear scale) HIGH @450 GeVbefore prepare for ramp

ramp (energy)

drop of dampergain

increase of damper(electronic) gain in rampTo maintain approx.same damping rate

increased BBQ amplitude= more residual beam oscillations=> potentially leading to blow-up;but signal needed for tune feedbackwhich is switched on here

rampprepare forramp

Injection plateau

Reduce damper gain

before ramp for tune

feedback →Increase of BBQ

amplitude

W. Hofle

LHC

Effect of damper gain change

Emittance evolution during fill with single bunch (loss map fill).

Evolution during injection and switch of damper gain during ramp preparation:

16

Effect of damper gain change not clear (data inadequate)!

Required test 2012: change gain and wait at injection

@ 450 GeV

@ 450 GeV

3.5 min

LHC

o Emittance evolution from BSRT

o Looked at the Abort Gap Cleaning Test Fill (12 + 12 + 12 bunches per ring)

− Integration: 3 s per bunch; averaged over 12 bunches → good resolution

o Blow-up during squeeze for beam 1 H

− Took measured beta at 3.5 m and 1 m from optics team into account!!

− Was there anything different in this fill?

Emittance blow-up at 3.5 TeV

17

LHC

Blow-up during the squeeze

o Looked at more fills since Evian workshop

− if emittances of all bunches similar → bunch-by-bunch scan of BSRT gives emittance

evolution in time

18

Looked at several fills for end of 2011 → BSRT data consistently shows blow-up for beam 1 H

between * 5 m and 1.5 m. No obvious source.

Needs to be followed up during squeeze commissioning 2012

Bunch emittances similar

LHC

Dependence on bunch intensity?

19

Approximately constant absolute growth between SPS

extraction and LHC collisions for different bunch intensities

19

LHC

Ions

o Ions also experience blow-up during the ramp

o Wire scans during the ramp not possible with physics beam

Data taken during quench test on 7th of December, single bunch analyzed

20

0

0.5

1

1.5

2

2.5

14:15 14:20 14:25 14:30 14:35 14:40 14:45 14:50

no

rmal

ize

d E

ps.

X (μ

m.r

ad)

Time

Wire Scanner data - B1 - horizontal plane

Ramp

Blow-up of ~ 20 %

Similar to proton trains

LHC

Summary

o Excellent performance of injectors: 1.9 mm for 1.5 × 1011 p+ per bunch

o Emittances grow 20 % - 30 % from SPS extraction to LHC collisions

o LHC injection: emittances preserved within measurement accuracy

o LHC injection plateau: emittance growth apparent (~ 10 % in 20 minutes for

horizontal plane)

− Continue to minimise time at injection

− Dedicated filling cycle? Reduce time in “prepare ramp”…

o LHC ramp: blow-up in H and V > 20 % (50 ns), more for single bunches

o LHC squeeze: blow-up for beam 1 H, > 20 %

o 2012: understand and (hopefully) correct blow-up

− Expect improvements from measurements, tools and analysis

− Need to sort out during commissioning: INSTRUMENTS, blow-up @squeeze, ramp

optimization, minimize time at injection

− MDs: effect of damper gain and working point,…, IBS

21

LHC

EXTRA SLIDES

22

LHC

Challenge for 2012

23

Understand and correct LHC

blow-up

…Make (good) outliers

routine

G. Arduini

LHC

Beta errors for emittance

o Error emittance only taking error on beta function into account.

o Plot 3 sigma error

24

LHC

Dependence on bunch intensity?

25 25

H0 = 2.4 mm

V0 = 2.2 mm

Ib = 2.4 x 1011

beam 2 similar

High pile-up MD, fill 2201 (2.4e+11):

Delta similar to BI MD,

but more than with 12 bunch trains

Delta of ~ 1 mm in H

Delta of ~ 0.6 mm in V