Status from the collimator impedance MD in the LHC

Collimation team: R. Assmann, R. Bruce, A. Rossi. Operation team: G.H. Hemelsoet, W. Venturini, V. Kain , G. Crockford.

Impedance team (and friends): H. Burkhardt, W. Hofle, E. Métral, N. Mounet, B. Salvant.

Many thanks for their help to:G. Arduini, M. Gasior, B. Goddard, S. Redaelli, F. Roncarolo,

G. Rumolo, R. Steinhagen, D. Wollmann.

LCU meeting – June 10th 2010

This is a work in progress!!!

Agenda

• Context

• Objectives

• Methods

• MD results

• Conclusions and next steps

Context:LHC impedance and collimators

• LHC transverse impedance is predicted to be one of the potential limitations to reach nominal beam parameters for collisions at top energy (14 TeV/c).

• LHC collimators are predicted to be the major contributors to the LHC transverse impedance at top energy. An upgrade of the collimation system is under study to reduce the impedance and improve the cleaning efficiency (Phase 2 collimation).

• Impedance theories, EM simulations, RF bench measurements and MDs in the SPS with a prototype collimator were all consistent and showed that we seem to understand the impedance of a single collimator.

• Now that beam is in the LHC, it is important to compare the LHC beam-based observations with predictions, in order to take decisions for the Phase 2.

Context:previous LHC observations and predictions with different intensities

• Several measurements performed at injection by Stefano Redaelli et al and Brennan Goddard et al.

• They monitored the tunes for bunches of different intensities (see here):

Qx~ -1.5 10-3 and Qy~ -2 10-3 10/04/2010 (B2)

Qx~ -5 10-4 and Qy~ -2 10-3 16/04/2010 (B1)

• Our predictions of tune shift with HeadTail macroparticle simulations for nominal collimator settings and nominal beam parameters at 450 GeV/c were

Qx~ 3.5 10-4 and Qy~ 4.4 10-4

these measurements seem to indicate that the impedance may be a factor 5 higher than predicted by the model…

need for dedicated measurements, to try to record and control the beam parameters

for Nb~1.1011 p/b

for Nb~1.1011 p/b

Agenda

• Context

• Objectives

• Methods

• MD results

• Conclusions and next steps

MD objectives

1) Assess the collimator impedance by (a) moving the jaws of a chosen set of LHC collimators

(b) monitoring the transverse coherent tune shifts, and other beam parameters.

2) Assess the impedance of the whole LHC machine by

(a) changing the beam intensity (through scraping)

(b) monitoring the transverse coherent tune shifts, and other beam parameters.

We got 4 hours of MD time (May 28th, 2010 - from 13:00 to 17:00).

Agenda

• Context

• Objectives

• Methods

• MD results

• Conclusions and next steps

Methods

• LHC impedance model is calculated through ZBASE– Includes theoretical models of phase 1 collimators at desired settings,

beam screens, warm pipe, MQW, MBW and a BB impedance (from design report).

– Significant contributors could be missing (kickers, PIMS, etc.).

• Tune shift predictions with LHC model– Impedance Sacherer formulae for single bunch transverse tune shifts

– Wake Headtail macroparticle simulations SUSSIX transverse tune shifts

• MD– Record relevant beam parameters, in particular:

intensity Nb, transverse tune shifts Q, bunch length L, as we expect

L

NZQ beff

Agenda

• Summary

• Context

• Objectives

• Methods

• MD results– Moving IR 7 collimators from 5 sig to 15 sig– Moving injection protection collimators from nominal to retracted (TDIs+TCLIs)– Scraping in IR3

• Conclusions and next steps

B2: Moving IR7 collimators

Summary for beam 2 (moving IR7)

Collimator gap open at 15 sigma

Collimator at 5 sig

Bunchlength=1.4 ns

Qx<7 10-4

Nb=9.3 1010

Qy~-2.4 10-4

B2: effect on vertical tune shift of moving IR7 collimators

Std(sussix)=0.6 10-4

Std(fft)=0.7 10-4

The tune shift is correlated to the collimator position. Qy (meas.) ~ -2.4 10-4

Vertical tune shift prediction when moving IR7 from 15 to 5 (ZBASE model with measured collimator settings and Sacherer formula with measured beam settings): Qy (theory) ~ -2.0 10-4

out (15 )

in (5 )

out

in

B2: effect on horizontal tune shift of moving IR7 collimators

the H tune seems to jump from one peak to the next difficult to estimate the tune shift, but we can write Qx<7e-4.

Sideband -1

Tune peaks?

B2: effect on horizontal tune shift of moving IR7 collimators

- Very large vertical tune signal… Coupling?- Many peaks around the tune make it difficult to analyze the horizontal tune.- During the MD, Wolfgang may have had a cleaner signal from the feedback pickups. To be checked.

IR7 in out in

B2: Moving injection protection collimators

B2: effect on horizontal tune shift of moving injection protection collimators (TDI+TCLIs)

Tune shift due to injection protection collimators from B2 measurements: Qy~ -3 10-4 and Qx~0 Coarse extrapolation from nominal model (only TDI): Qy~ -1.2 10-4 and Qx~0

Correlation between the collimator gap and the vertical tune shift The horizontal tune switches to another peak when collimators are in. To be investigated in more detail.

TDI Collimator gap Qx~0 Qy~-3 10-4

B2: Scraping the beam in IR3

B2: effect of scraping in IR3

Bunch length Qx QyIntensity

• Tune shift with intensity:– Scraping was performed with one collimator in IR3, resulting in a large bunch length decrease.– Accounting for this bunch length decrease and comparing with Sacherer tune shift from the nominal

450 GeV/c impedance model (collimators at nominal settings in the model instead of in the measurements):

From 9.3 1010 p to 1 1010 p, tune shifts are less than 10-3 and look similar in H and V

B2: effect of scraping in IR3

Vertical tune shift (beam 2) Horizontal tune shift (beam 2)

At Nb=9.3 1010 p/b, nominal model predicts Qy~-5.3 10-4 (5.9) and Qx~-5.7 10-4 (6.3) measurements Qy~-7.3 10-4 and Qx~-7.5 10-4

Warning!!! Preliminary results obtained with nominal collimator settings!!!The model is being refined now to account for exact collimator positions during the MD.

-1.E-03

-9.E-04

-8.E-04

-7.E-04

-6.E-04

-5.E-04

-4.E-04

-3.E-04

-2.E-04

-1.E-04

0.E+00

0.E+00 2.E+10 4.E+10 6.E+10 8.E+10 1.E+11

intensity

Ho

rizo

nta

l tu

ne

sh

ift

measurements

ZBASE model

ZBASE model (sum)

-1.2E-03

-1.0E-03

-8.0E-04

-6.0E-04

-4.0E-04

-2.0E-04

0.0E+00

2.0E-04

0.E+00 2.E+10 4.E+10 6.E+10 8.E+10 1.E+11

Bunch population

Vert

ical tu

ne s

hif

t

measurements

ZBASE model

ZBASE model (sum)

B2: Overinjection

Overinjection tune shift for beam 2

Overinjection tune shift is Qx~ -10-3 and Qy~ -1.5 10-3

(bunch length of 1.1 ns for high intensity, but not well defined for the pilot bunch)

Differences also in collimator gaps (injection protection retracted, IR7 at 5 sigma)

Conclusions and next steps

• The predictions with the impedance model from ZBASE and the measurements on 28th May 2010 seem reasonably consistent.

• More detailed simulations with Headtail should be performed.

• We need to work on getting a cleaner tune measurement

• The injection protection collimators may have a slightly larger impedance than expected, and this has to be investigated in more details.

• The ZBASE impedance model should improved (e.g. adding new kickers and other suspected sources of impedance).

Thank you for your attention!

B1: Moving IR7 collimators

B1: effect on tune shift of moving IR7 collimators

Moving IR7 collimators from 5 sigma to 15 sigma leads to a tune increase of: 1 or 2 e-4 in horizontal plane (the tune jump to -1/+1 sideband shadows the graph... to be filtered)

3e-4 in vertical plane (first guess)

B1: Moving injection protection collimators

B1: Moving injection protection collimators

Moving TDI collimators from 5 sigma to 15 sigma leads to a tune increase of:

2e-4 in vertical plane (first guess)? in horizontal plane (the tune jump to +1 sideband shadows the graph... to be filtered)

B1: scraping in IR3

Scraping beam 1 from 0.91e11 p to 1e10p lead to a tune shift of 8e-4 in vertical planehorizontal plane is too jumpy to tell. Deeper analysis is needed. (Tune shift range between 3e-4 and

10 e-4)

B1: overinjection

Overinjection tune shift for beam 1

Overinjection tune shift is Qx~ -10-3 and Qy~ -1.3 10-3

(bunch length of 1.1 ns for high intensity, but not well defined for the pilot bunch)