# Presentation: 550625

## Beam-beam compensation at RHICLARP Proposal

- Tanaji Sen, Wolfram Fischer
- Thanks to
- Jean-Pierre Koutchouk, Frank Zimmermann

## Topics

- Motivation for LARP
- Lessons from SPS experiments
- Lessons from RHIC experiment
- Proposal
- Challenges specific to the LHC
- Need for simulations

- LHC IR Upgrades: Oct 3-4, 2005

- Tanaji Sen

## Motivation

- “Provide more luminosity earlier”
-      Space is reserved in the LHC for the wires. An early test in RHIC will determine the effectiveness of the compensation and possibly address the challenges to the compensation. If effective, will allow a smaller crossing angle and more beam intensity.
- Provide direction to an IR upgrade path
-       If compensation is proven to be effective, then the quadrupole first option seems to be a more natural path for the IR upgrade. Otherwise, the dipole first option will be more attractive.

- LHC IR Upgrades: Oct 3-4, 2005

- Tanaji Sen

## 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 ~ d3
-       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

- LHC IR Upgrades: Oct 3-4, 2005

- Tanaji Sen

## 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?

- LHC IR Upgrades: Oct 3-4, 2005

- Tanaji Sen

## Wire compensation at RHIC

- Compensation of 1 wire by another wire worked well in the SPS under LHC conditions.
- Real test of the compensation principle requires 2 beams
- Beam studies in RHIC show that parasitic interactions have strong influence on beam loss
- Favorable location for wire has been found in IR6, phase advance to parasitic ~6 degrees at top energy

**Proposed wire location**

**Location of parasitic**

- LHC IR Upgrades: Oct 3-4, 2005

- Tanaji Sen

## 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.

- LHC IR Upgrades: Oct 3-4, 2005

- Tanaji Sen

## Proposal - 2

- FY08 Plan
- Compensation of both beams with the 2 wires
- Elliptical beams at the parasitic interaction – test robustness to changes in the aspect ratio
- Compensation of multiple bunches in RHIC with pulsed wire current.

- LHC IR Upgrades: Oct 3-4, 2005

- Tanaji Sen

## RHIC proton parameters

|  |  |
| --- | --- |
| Beam energy [GeV]
Bunch intensity
Transverse emittance [mm-mrad]
Beta* [m]
RMS energy spread
RMS bunch length [m]
Beam-beam tune shift |  100
  2 x 1011
  15
   1.0
  0.3 x 10-3
   0.7
 -9.8 x 10-3 |

- LHC IR Upgrades: Oct 3-4, 2005

- Tanaji Sen

## Beam studies with 1 wire

- Choose tunes so that only 1 beam is affected by the beam-beam interactions
- Wire strength: IWL = ec Npara Np
-     Beam-wire distance equal to beam-beam separation.
-     Beam-wire distance > 3 σ
- Minimum separation: Study the compensation over a range of separations, the minimum separation at which compensation is effective will be a measure of the smallest crossing angle allowed
- Alignment tolerance: At fixed beam-beam separation, find the range over which the beam-wire separation can be varied with compensation still effective.
- Tolerance on current: Vary the current until compensation is lost
- Tolerance on current jitter: Introduce current jitter, measure emittance growth. Compare with theory.
- Tolerance on phase advance: Move the location of the parasitic interaction by changing the rf phase.

- LHC IR Upgrades: Oct 3-4, 2005

- Tanaji Sen

## Issues 

- Is cooling required with the currents to be used? If yes, how will the cooling be done?
- Tune and orbit correction for each beam-wire separation
-  What is the accuracy of the tune measuring instrument and the BPMs – relative to the size of the expected tune and orbit shifts
- Over what range is the wire movable?
-  Does the wire pose an aperture restriction at small distances from the beam?
- In the case of a magnet failure, can the beam hit the wire? Is the wire adequately protected?
- How should the effectiveness of the compensation be monitored?
- How much machine study time should be requested for wire compensation of 1 bunch in one ring? For compensation of 1 bunch in both rings? For compensation of several bunches?
- Time scale of these experiments ?
- What do simulations predict for the compensation?

- LHC IR Upgrades: Oct 3-4, 2005

- Tanaji Sen

## Challenges - 1

*Sensitivity to alignment errors*
-     Question: How do we quantify the sensitivity of the compensation to alignment and the precision with which the beam-wire separation should be set ?
*Sensitivity to current jitter*
-     Question: How large an inductance is necessary to damp the high frequency noise in the wire (close to the betatron frequency) ? Should we introduce measurable jitter to induce emittance growth -> benchmark simulations?
*Sensitivity to optics errors*
-      Spurious dispersion and local sources of coupling close to the wire may change the efficiency of the compensation.
-     Question: Should we attempt to create sources of optics errors in RHIC? If yes, a) what are the important optics errors that can spoil the compensation and b) can these optics errors be created in RHIC ?

- LHC IR Upgrades: Oct 3-4, 2005

- Tanaji Sen

## Challenges - 2

*Sensitivity to phase advance errors between the parasitics and the wire*
-      Question: Is there some means by which the phase advance to the wire can be changed ? By how much?
*Tune dependence of the compensation - RHIC tunes are close to the LHC tunes*
-     Question: Is it necessary to investigate the tune dependence in RHIC - over what range?
*Sensitivity to tune spread of the bunch.*
-      Beams in RHIC and LHC will have different tune footprints. Is the compensation sensitive to this footprint?  e.g does space charge induced tune dependence on amplitude matter?
-     Do the different rates of emittance growth in RHIC and LHC matter?

- LHC IR Upgrades: Oct 3-4, 2005

- Tanaji Sen

## Challenges - 3

*Sensitivity to non-round beams*
-     Question: Can the aspect ratio at the parasitic be changed, either by changing the ratio of β_y*/ β _x* or by changing the ratio of transverse emittances e.g by scraping ?

- At aspect ratio = 1.8, need a beam-beam separation of ~12σ to match the field of a round wire. Do these differences matter?

- LHC IR Upgrades: Oct 3-4, 2005

- Tanaji Sen

## Challenges - 4

- 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?
- If pulsed wires are required, what is the right frequency?
-     Does every PACMAN bunch need a different current?
-  Will such modulators and power supplies be available in FY07?

- Wire current proportional to Npara

- LHC IR Upgrades: Oct 3-4, 2005

- Tanaji Sen

## Simulations - 1

-  Capabilities
- BBSIM (FNAL) – wire module included, used for predicting alignment tolerance in SPS studies
- Frank Zimmermann & student (CERN) – initial results on RHIC beam-beam experiment
- Beambeam3D – J. Qiang (LBL)
- J. Shi (University of Kansas)

- LHC IR Upgrades: Oct 3-4, 2005

- Tanaji Sen

## Simulations - 2

- What 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 IR Upgrades: Oct 3-4, 2005

- Tanaji Sen

## Summary

- Propose installing a wire compensator in RHIC in 2006
- Tests of the compensation principle in FY07 and beyond.
- Several issues and challenges to be addressed.
- Simulation efforts need to be significantly ramped up in FY06
- Successful tests of wire compensation expected to improve the performance of the LHC and help direct the upgrade path.

- LHC IR Upgrades: Oct 3-4, 2005

- Tanaji Sen