Working Group 2Closing Summary

T. Sen, W. Fischer,

J.P. Koutchouk,

2- Motivation for the LHC Upgrade

The crossing angle shall be increased due to

the reduction of β* the increased bunch current and number of bunches the possibly increased interaction length (long-range)

The geometric luminosity loss becomes rapidly unacceptable:

Lessons from the SPS experiments• Compensating 1 wire with another

wire at nearly the same phase “works”

• Compensation is tune dependent• Current sensitivity • Alignment sensitivity• Equivalent crossings in the same

plane led to better lifetimes than alternating planes

• Beam lifetime τ ~ d5

d is the beam-wire distance Higher power law expected given

the proximity of high order resonances

Both wires on 1 wire on

Nearly perfect compensation

No wires activated

Quadrupole aperture with BBLR

Wire compensation has thepotential to reduce the aperture required significantly

Dynamic aperture with wire compensation

DC wire compensation increasesthe DA of a nominal bunch by ~2σat most tunes. But it decreases the DA of the extreme PACMAN bunch at most tunes.

The specification of the frequency (439kHZ) needs more study

Lessons from RHIC experiment• Study at injection energy

with 1 bunch and 1 parasitic interaction per beam

• There is an effect to compensate, even with 1 parasitic

• Drop in lifetime seen for beam separations < 7 σ

• Effect is very tune dependent

• How important are machine nonlinearities and other time dependent effects?

• Did they change with the beam-beam separation?

Lifetime versus separationSPS : 5ms (d/)5 [measured 11/09/04]

Tevatron: ~ d3 [reported in F. Zimmermann, LTC 11/24/04]

RHIC : ~ d4 or d2 [measured 04/28/05, scan 4]

4.7 separation

5.54

7.1

Yellow

RHIC Simulation – Ji Qiang, LBNL

Scan 2 – rms emittance vs. time

5 sec real time

Blue

Difference between beams visible for scan 2 parametersLittle effect seen for scan 4 parameters

RHIC BBLR design – locationslong-ranginteraction(vertical)

long-rangcompensation

(up)

long-rangcompensation

(down)

x,y = 6 deg (* = 1m)

RHIC Sector 5 (IR6)[picture mirrored]

RHIC BBLR design – drawing

Main features:

- elliptic copper bar (a/b = 59%)- air cooled heat sinks- on vertically movable stand (60mm movement)

pleasecomment

RHIC BBLR design – parameters

Integrated strength per long-range collision Am 9.6

Integrated strength of compensator IL Am 125Length of wire L m 1.5Major half axis of elliptic bus bar a mm 4.0Minor half axis of elliptic bus bar b mm 2.4Output parametersCurrent in wire I A 83Electric resistance R m 0.87Voltage U mV 72.8Electric power P W 6.1

Max temperature change Tmax K 100

Change in length due to T mm 1.7

~10x single bunch

pleasecomment

For now mechanical design for 125A-m But power up to a max of 30Am.Eases cooling

Proposal - 1 FY06 Plan• Design and construct a wire compensator (BNL)• Beam-beam studies at top energy: beam separation and

tune scan. No wire.• Theoretical studies (analysis and simulations) to test the

compensation and robustness• Install wire compensator on a movable stand in one of the

RHIC rings in 2006 shutdown

FY07 Plan• Beam studies in RHIC with 1 proton bunch in at flat top

and 1 parasitic interaction. • Test tolerances on: beam-wire separation, wire current

accuracy, current ripple, phase advance to the wire.• Simulations to match experiments• Construct and install 2nd wire compensator and current

modulator in 2007 shutdown.

RHIC experimental program proposal

• (d,Qy) scan at 100 GeV

• Single and multiple long-range interactions

Run-6 (2006) w/o BBLR (ask for 2x3hrs)

Run-7 (2007) with 1 or 2 dc BBLR

Run-8 (2008) with ac BBLR

Challenges - 1Sensitivity to alignment errors SPS experiments showed that the tolerance on the wire separation was ~3 sigma. Wire motion can be controlled to ~ 25 microns

Sensitivity to current jitter We could introduce white noise on the wire to induce emittance growth. Tolerance on noise levels and benchmark simulations.

Sensitivity to optics errors Impact of local coupling and spurious dispersion on compensation should be looked at.

Challenges - 2• Sensitivity to phase advance errors between the parasitics and the

wire The phase advance can be changed over a wide range by moving the

location of the parasitic.• Tune dependence of the compensation - RHIC tunes are close to the

LHC tunes Tune scans of the compensation could be done. • Sensitivity to tune spread of the bunch. Do the different rates of emittance growth in RHIC and LHC matter? Perhaps not since the compensation is local

• How important is it to use pulsed wires for compensating the PACMAN bunches, i.e. is it known that average compensation is not good enough for these bunches?

Not known yet - will be studied further with simulations• If pulsed wires are required, what is the right frequency? Does every PACMAN bunch need a different current? Same as above

SimulationsWhat can we expect?• Reproduce the results of the beam-beam

experiment at injection energy Important physics e.g. nonlinear fields including snakes, space charge,

IBS, tune modulation,…?• Simulate 1 parasitic interaction at top energy. Is there a significant impact on the beam? Variation with separation of: dynamic aperture,

emittance change, lifetime,…• Simulate 1 parasitic interaction and wire. Is compensation effective? Tolerances on: alignment, current strength and jitter,

phase advance errors, non-roundness of “strong” beam, …

LHC simulations & wire compensation

Emittance growth

J. Shi

LHC simulations & wire compensation(2)

Predicts that multipole compensation might also work for long-range but at high beam-beam tune shifts

J. Shi

Benchmarking simulations

• Experimental evidence so far• SPS expt: variation of losses with wire

currents, tunes, separations• RHIC experiment: variation of losses with

beam-beam separation, tune variation• What is the common observable in

experiments and simulations?• Hard to simulate lifetimes with good statistical

accuracy, emittances often used• Experiments: hard to measure emittance

changes over the small time scale of simulations

Use of the Electron Lens• Footprint due to head-on collisions can be

efficiently compressed with the electron lens• Requires a location where the beta functions

are equal• Beam-beam interactions are a dominant

source of emittance growth in RHIC. An electron lens in RHIC could help to improve performance.

• Emittance growth is determined by the strength of nonlinearity

• Beam tests in Tevatron (without parasitics) could be a useful first step.

Summary

• For the LHC upgrade, wire compensation has the promise of allowing smaller crossing angles (better use of aperture and higher luminosity) and higher intensities. “More luminosity earlier”

• SPS experiments showed that the compensation principle works for 1 wire compensated by another.

• RHIC experiment showed that there is an effect due to parasitic at 24 GeV. Needs to be repeated at 100GeV.

• Propose installing a wire compensator in RHIC in 2006. Tests of the compensation principle in FY07 and beyond.

• Simulation efforts need to be significantly ramped up in FY06.

• Possibilities of using the electron lens for compensating headon beam-beam interactions in RHIC and perhaps LHC.