Beam Instabilities at the ISIS Neutron & Muon Source - PowerPoint Presentation

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Beam Instabilities at the ISIS Neutron & Muon Source

Rob Williamson

Supervisors: Chris Warsop (ISIS), Ivan

Konoplev

(JAI)

Contents

Introduction

ISIS Synchrotron

Loss Mechanisms at ISIS

Head-tail R&D

Damping Systems

Summary and Plans

Introduction

800 MeV

Synchrotron (50 Hz)

H

-

ion source, 35

keV

H

-

RFQ,

665

keV

H

-

Linac, 70 MeV

Target Station 1 (40 pps)

Target Station 2 (10 pps)

Circumference: 163 mEnergy:

70–800 MeVRepetition Rate: 50 HzIntensity:

~3x1013 pppPower: ~190 kWInjection: 220 µs, 130

turn, charge exchangeExtraction: single turn, verticalBetatron Tunes:

(

Q

x

,

Q

y

) = (4.31

, 3.83

), programmableBeam Losses: Injection: 2%, Trapping: <3%,

Acceleration/Extraction: <0.5%RF system: h=2, 1.3-3.1 MHz, 160 kV/turn h=4, 2.6-6.2 MHz, 80 KV/turn

ISIS Synchrotron

Extraction

Injection

Time (

ms)

Tune shift ≥0.5 over 0 - 0.5 msHalf integer limitWhat is the mechanism for beam loss?

Non-linear driving termsMeasurements show non-linear resonance linesConformal vacuum vessel, however beam envelope varies with tuneNon-negligible image termsNon-linear magnet fields

Loss Mechanisms at ISIS - Transverse Space Charge

BG Pine

CM Warsop

Current intensity limitDual harmonic operationSymmetric bunches unstableNo longer able to cure with tune ramp

Driven by impedancesResistive wallNarrowband cavity structuresLoss Mechanisms at ISIS

- Head-tail Instability

Normal beam +

Θ

shift

Large loss!

Normal beam

Low loss

ISIS Beam Bunches at ~ 2

ms

Sum signal

Difference signal

Beam Loss vs Time 0-5

ms

Loss!

Measurements with V Kornilov (GSI)

Transverse instability Wakefield excited by head of bunch interacts with particles in the same bunch

Head-Tail Instability

Position monitor signal

I(t)*y(t)

I(t)

Time

Time

Characterised by mode structure in position monitor difference signal

Variation with

chromaticity

Tune varies with energy offset

Phase shift between head and tail

Increasing Chromaticity

ExperimentsSimplify to single harmonic RF operationLow intensity => ignore space chargeTheory and simulations with resistive wall predict

mode number m=2 (two nodes)Observations show m=1 (one node)What is the driving impedance?

Head-Tail R&D* Modified CERN code

Measurement Vs HEADTAIL* Simulation

:

Difference Signal

Beam based impedance measurementsCoasting beam at injection energyGrowth rate from BPM difference signalVary tune to scan frequencyGrowth rate proportional to impedance

ObservationsResistive wallLow frequency narrowband impedance

Head-Tail R&D

Effective Impedance Vs Frequency

BPM Difference Signal, FFT & Sideband

Use existing components: BPM, ferrite loaded kickerLLRF electronics for processing signalsFPGA for ADC/DAC, digital filter, delays and gainDynamically updated 3-tap FIR filter allows for correct phase for kick through acceleration

Head-Tail Damping System

Vertical pickup

(R4VM1)

Vertical

Q-kicker

Phase Advance Correction

Programmable

Fine

D

elay

Power

Amplifier

G

ain

Vertical head-tail motion 1 - 2.5 ms through acceleration cycleSuppressed byRamping vertical tune

Asymmetric longitudinal distributionControl of longitudinal and vertical paintingHead-tail effectively damped and beam losses reducedFurther commissioning required

Experimental Results

Improve simulation model & compare with experimentImprove machine model to aid in impedance diagnosis and damping operationAutomatic tune measurement

Summary

High intensity R&D is an important subject of study for ISIS and future high intensity proton machines

Half integer studies

Image effects

Non-linear magnet

terms

Plans

Head-Tail Studies

Experiments

Measurements of the instability at ISIS

Impedance measurements

Simulations

Development/implementation of in-house code

Impedance modelling

Damping System