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### FREQUENT RECOGGING:EFFECTS ON THE BEAM

Wolfram Fischer

RHIC Spin Collaboration Meeting19 September 2002

Contents

- Introduction
- Run 2001 lifetimes
- Cogging effects
- Effects on integrated luminosity Ldt
- Time lost
- Longitudinal — debunching
- Transverse — luminosity lifetime reduction
- Summary

Introduction

- Assumptions:
- Proton beams at g = 260, 56(112) bunches
- Nb = 1011, eN = 20mm xbeam-beam = 0.0037 / IP
- Store length of 7 hours
- Recogging (worst case)
- Every 2 min
- By 6(3) buckets
- Expect adverse effects on:
- Integrated luminosity Ldt try to estimate DLdt = (Ldt)recogging / (Ldt)
- Polarization (A. Luccio, V. Ptitsyn, V. Ranjbar)

Beam-beam ON

Beam-beam OFF

Moving crossing points if Dfrf0

DX

IP

DX

BPM(x,y)

BPM(x,y)

v=5m·Dfrf

Cogging effects- Cogging moves the collision points longitudinally
- Beyond DX magnets and with crossing angles (intentional or unintentional) transverse beam separation changes Transverse tunes change (beam-beam interaction)

Au after 3h store

Cogged 3 buckets (fully separated longitudinally)

Cogged 2 buckets

Cogged 1 bucket

DQmeas=0.0007

eN=22mm

Beams colliding

Sign of crossing angles

(no tune change if all zero)

P. Cameron

Cogging effects – tune change4th

Cogging effects – working point17th

17th

13th

13th

9th

14th

Out of collisionIn collision

Frequent recogging requires 2 stable working points

Ldt reduction – time lost

- Cogging time:
- Frequency ramp Df/Dt = 10Hz/8s (Dfmax = 10Hz) 4.4 s / 6 buckets
- Overhead 5s (ev-lumi-off, ev-lumi-on, etc.)
- (DLdt)1– 8%
- Experiment’s dead times:
- Are certain detector components switched off during cogging? (DLdt)2reduction
- Fatalities
- Aborted stores,
- Completely debunched beams,
- Lost beam synch clock, …
- (DLdt)3 – 15% (educated guess)

Longitudinal – debunching

- Every cogging step is somewhat non-adiabatic longitudinal emittance growth ultimately debunching
- Run 2001:
- 28 MHz system, 300kV
- Dss / ss 1% / hr (1st hour), almost no debunching
- Run 2003:
- 197 MHz system, 3MV
- Dss / ss ??, debunching ??
- Difficult to estimate debunching effect (DLdt)4 –5% (educated guess)

Transverse – luminosity lifetime

- Run 2001:
- Small tune changes (of order ~x) could result in dramatic changes in beam lifetimewith b*=2(1)m lattice (Yellow)
- Run 2003:
- Expect beam lifetime improvements for Run 2002 with nonlinear IR correction
- Assume 30% beam lifetime reduction in uncogged state I(t) = I0 exp(T1/t1)exp(T2/t2) … (DLdt)5 –10%
- Emittance growth from recogging,difficult to estimate (DLdt)6 –5% (educated guess)

Summary I

(DLdt)tot = P [1 – (DLdt)i]

Not considered:

- Additional experiments dead time for cogging- Loss in polarization

Summary II

- Frequent recogging may reduce the integrated luminosity by 50%
- Risk of total beam loss is increased, but should be acceptable
- Book keeping for colliding spin patterns is not trivial but manageable
- Effect on polarization may need to be studied
- Practical detector operation may be affected

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