beam diagnostics lecture 1.ppt - cern · beam diagnostics lecture 1 ulrich raich cern ab - bi (b i...

Beam DiagnosticsgLecture 1

Ulrich RaichCERN AB - BI

(B I t t ti )(Beam Instrumentation)

U. Raich CAS Frascati 2008Beam Diagnostics

1

Overview

• First hour: Introduction– Introduction

– Overview of measurement instruments• Faraday Cupy p• Beam Current Transformer• Beam Position Monitor• Profile Detectors• Profile Detectors

– SEMGrids– Wire Scanners

• Beam Loss Monitors

• Second hour– Some depicted examples of beam parameter measurements


2

– Some depicted examples of beam parameter measurements

Introduction

An accelerator can never be better than the instruments measuring its performance!performance!


3

Different uses of beam diagnostics

Regular crude checks of accelerator performanceBeam Intensity– Beam Intensity

– Radiation levelsStandard regular measurementsStandard regular measurements– Emittance measurement– Trajectories– Tune

Sophisticated measurements e.g. during machine d l t idevelopment sessions– May require offline evaluation– May be less comfortable


4

– May be less comfortable

Diagnostic devices and quantity measured

Instrument Physical Effect Measured Quantity Effect on beamFaraday Cup Charge collection Intensity DestructiveCurrent Transformer

Magnetic field Intensity Non destructive

Wall current monitor

Image Current IntensityLongitudinal beam shape

Non destructive

Pick-up Electric/magnetic field

Position Non destructive

Secondary emission monitor

Secondary electron emission

Transverse size/shape, emittance

Disturbing, can be destructive at lowemission monitor emission emittance destructive at low energies

Wire Scanner Secondary particle creation

Transverse size/shape Slightly disturbing

Scintillator screen Atomic excitation Transverse size/shape DestructiveScintillator screen Atomic excitation with light emission

Transverse size/shape (position)

Destructive

Residual Gas monitor

Ionization Transverse size/shape Non destructive


5

A beam parameter measurement


6

Required Competence in a beam diagnostics groupg g p

• Some beam physics in order to understand the beam parameters to be measured and to distinguish beam effects p gfrom sensor effects

• Detector physics to understand the interaction of the beam with the sensorwith the sensor

• Mechanics• Analogue signal treatment

– Low noise amplifiers– High frequency analogue electronics

• Digital signal processing• Digital signal processing• Digital electronics for data readout• Front-end and Application Software


7

Layout of a Faraday Cup

• Electrode: 1 mm stainless steel• Only low energy particles can be y gy p

measured• Very low intensities (down to 1

pA) can be measuredp )• Creation of secondary electrons of

low energy (below 20 eV) • Repelling electrode with someRepelling electrode with some

100 V polarisation voltage pushes secondary electrons back onto the electrode


8

Schema: V. Prieto

Faraday Cup


9

Electro-static Field in Faraday Cup

-92V

In order to keep secondary Collect eur

-2.3

V

-9V

-24.

5V

-31.

5V

-40V

-17V

-47V

-62VIn order to keep secondary electrons with the cup a repelling voltage is applied to the polarization electrode

Since the electrons have energies of less than 20 eV some 100Vrepelling voltage is sufficient

Elect r ode de polar isat ion-100V


10

Energy of secondary emission electrons

• With increasing repelling voltage the electrons do not escape the Itotal vs. eV

90keV50keV30keV

the electrons do not escape the Faraday Cup any more and the current measured stays stable.

• At 40V and above no decrease in 1 2

1.4

1.6total

the Cup current is observed any more

0.8

1

1.2

I(µA)

0.4

0.6

0

0.2

0.1 1 10 100 1000


11

V

Faraday Cup with water cooling

For higher intensitiesFor higher intensities water cooling may be needed


12

Current Transformers

Magnetic fieldFields are very low

ri

ro

y

Capture magnetic field lines with cores of highrelative permeability

(CoFe based amorphous alloy Vitrovac: μ = 105)

Beam currentNN β r020μμ

alloy Vitrovac: μr= 105)

lcqeN

tqeNIbeam

β==i

r

rrlNL 020 ln

2πμμ=


13

The ideal transformerThe AC transformerThe active AC transformer

dIL beamU droop

t

eRtItU τ−

= )()(

RF

RLs

Rdt

L beam=U pbeam e

NtItU = )()( RL

C

A

RL

C

Inductance L of the winding

CSCS R

Beam signalTransformer output signalssrise CL=τ

Ldroop RR

L+

=τL

Lf

droop RL

RA

RL ≈+

=τ


14

A

Principle of a fast current transformer

ImageC tCurrent

Ceramic Gapp

BEAM

Calibration winding

80nm Ti Coating⇒20Ω to improve

impedance


15

Diagram by H. Jakob

• 500MHz Bandwidth• Low droop (< 0.2%/μs)

Fast current transformers for the LHC


16

The transformer installed in the machine

NeedsMagnetic Shielding


17

Magnetic shielding

Shield should extend along the vacuum chamber length > diameter of openinglength > diameter of openingShield should be symmetrical to the beam axisAir gaps must be avoided especially along the beam axisg p p y gShield should have highest μ possible but should not saturate

monitor

S ft i ( 1) Transformer steel (μ2)

Permalloy (μ3)


18

Soft iron (μ1) Transformer steel (μ2)

Calibration of AC current transformers

The transformer is calibrated with a very precise current sourceprecise current sourceThe calibration signal is injected into a separate

lib ti i dicalibration windingA calibration procedure executed before the running periodA calibration pulse before the beam pulsethe beam pulse measured with the beam signal


19

Current transformer and its electronics


20

Display of transformer readings

Transformers ina transfer lineCalculated lossestrigger a watchdogDisplay distributed via video signal


21

The DC current transformer

AC current transformer can be extended to very long droop times but not to DCMeasuring DC currents is needed in storage ringsMust provide a modulation frequencyTakes advantage of non/linear magnetisation curve

B

H


22

Principle of DCCTV V

Synchronousdetector

Va-Vb

VVa

modulatorPower supply

Vb

beam

pp y

Compensationcurrent Ifeedback=-IbeamV=RIbeam

R


23

Modulation of a DCCT without beamB f(t)B=f(t)

B

dtdBNAU =

H

dt

0BNA

UdtB += ∫

Modulation current

NA

has only odd harmonic frequencies since the signal is

1 2 53 4

the signal is symmetric


24

1 2 53 4

Modulation of a DCCT with beamB=f(t)

B

H

Sum signal becomes non-zeroE h iEven harmonics appear

11 2 53 4


25

11 2 53 4

Modulation current difference signal with beam

• Difference signal has 2ωm

• ωm typically 200 Hz – 10 kHz• Use low pass filter with

ωc<<ωm

Provide a 3rd core normal• Provide a 3rd core, normal AC transformer to extend to higher frequencies


26

Photo of DCCT internals


27

Measuring Beam Position – The Principle

-- - - -+

+ + ++- +

+ +- -+ -++-

- +-- +

+- + -

- -- - - -+

+ + ++- +

+ +- -+ -++-

- +-- +

+- + -

- -- - - -+

+ ++- +

+ +- -+ -++-

- +-- +

+-


28Slide by R. Jones

Wall Current Monitor – The Principle

V

-- - - -+

+ + ++- +

+ +- -+ -++-

- +-- +

+- + -

- +-

- -- - - -+

+ ++- +

+ +- -+ -++-

- +-- +

+-Ceramic Insert


29

Wall Current Monitor – Beam Response

V

R CRfH π2

1=R

Res

pons

e CRπ2

LRfL π2

=

IB

C

Frequency

R

00

L

IB Frequency0

IB


30

Electrostatic Monitor – The Principle

V

-- - - -+

+ + ++- +

+ +- -+ -++-

- +-- +

+- + -

-- - - - -+ +

++- ++ +- -+ -+ +- -+ -+ -

- -- - - -+

+ ++- +

+ +- -+ -++-

- +-- +

+-+-- - - - - - -


31

Electrostatic Monitor – Beam Response

C V)

1

espo

nse

(V

CRf L π2

1=

VB V Frequency (Hz)

Re

00

VB VRFrequency (Hz)0


32

Position measurements

wU

U

Ud

dd

If the beam is much smaller than w, all field lines are captured andU is a linear function with replacementelse: Linear cut (projection to measurement plane must be linear)


33

(p j p )

Shoebox pick-up

UL UR

Δ=∝ RL U-UxLinear cut through a shoebox Σ=

+∝

RL UUx


34

Doubly cut shoebox

• Can measure horizontal and vertical position at once• Has 4 electrodes• Has 4 electrodes

ab

cdd


35

Simulatenous horizontal and vertical measurement

verticalhorizontal vertical

ab a

b

c dc

d

UUUU dbc

Σ+−+= )()(UX a

UUUUY dcb

Σ+−+= )()(Ua


36

Interaction of particles with matter

Coulomb interactionAverage force in s-direction=0Average force in s-direction=0Average force in transverse direction <> 0

s

b

Beam particle

Mostly large impact parameter => low energy of ejected electron

Atomic shell electronF

electronElectron mostly ejection transversely to the particle motion


37

Bethe Bloch formula

]2

[ln4 2222

2

222 ββγ

βρπ −=−

IcmZ

AZ

cmrNdxdE epT

eeA

• with the following constants:NA: Avogadro’s number

β IAdx T

me and re: electron rest mass and classical electron radiusc: speed of light

• the following target material properties:ρ: material densityρ yAT and ZT: the atomic mass and nuclear charge

• and the particle properties:Zp: particle chargeβ: the particles velocity and 21 βγ =β: the particles velocity and

Dependance on

1 βγ −=

2pZ


38

High energy loss a low energies

50 ][ MeVdE

10

20

]/

[ 2cmgdx

2

5

10

gas

solid

1 0.1 0.01 1

2

Ekin[GeV]10 100 1000

Heavy ions at low energy are stopped within a few micro-metersAll energy is deposited in a very small volume


39

All energy is deposited in a very small volume

Scintillating Screens

Method already applied in cosmic ray experiments

• Very simple• Very convincingNeeded:Needed: • Scintillating Material • TV camera

I / t h i• In/out mechanismProblems:• Radiation resistance of TV camera• Heating of screen (absorption of

beam energy)• Evacuation of electric charges


40

g

Frame grabber

25000

30000

35000

10000

15000

20000

25000

Inte

nsity

(u.a

.)

-10 -5 0 5 10 15 20 25 30 35

0

5000

Y (mm)

20000

• For further evaluation the video signal is

10000

15000

ensi

ty (u

.a.)

the video signal is digitized, read-out and treated by program

-10 0 10 20 30 40 50 60 70

0

5000Inte

X (mm)


41

X (mm)

Test for resistance against heat-shock

Materialρ cp at 20ºC

J/gK

k at 100ºC

W/mK

Tmax R at 400 ºC

Ωg/cm3 ºC Ω.cm

Al2O3 3.9 0.9 30 1600 1012

ZrO2 6 0.4 2 1200 103 Better for electrical conductivity (>400ºC)

BN 2 1.6 35 2400 1014 Better for thermal properties(higher conductivity, higher heat capacity)

100

1000

10000

sity

(u.a

.)30000

40000

50000

Start 90μC (30min) 450μC (2h30min) 585μC (3h15min) 720μC (4h)

ensi

ty (u

.a.)

-10 0 10 20 30 40

0.1

1

10 Test start +2min +3min +1h30 +3h

Ligh

t int

en

-10 0 10 20 30

10000

20000

Ligh

t int

e

Y (mm)


42

Y (mm)

Degradation of screen

Degradation clearly visibleHowever sensitivity stays essentially the same


43

Screen mechanism

• Screen with graticule


44

Profile measurements

• Secondary emission grids (SEMgrids)

When the beam passessecondary electrons areejected from the ribbons

The current flowing back onto the ribbons is measuredmeasured

Electrons are taken awayby polarization voltageby po a a o o age

One amplifier/ADC chainchannel per ribbon


45

SEMgrids with wires


46

Profiles from SEMgrids

Projection of charge densityprojected to x or y axis is Measured

O lifi /ADC iOne amplifier/ADC per wireLarge dynamic range

Resolution is given by wireResolution is given by wire distance

Used only in transfer linesUsed only in transfer lines


47

Wire Scanners

A thin wire is quickly moved across the beamSecondary particle shower is detected outside the vacuum chamberSecondary particle shower is detected outside the vacuum chamberon a scintillator/photo-multiplier assembly Position and photo-multiplier signal are recorded simultaneously


48

Wire scanner profile

High speed neededbecause of heating.because of heating.

Adiabatic damping

Current increase due toSpeed increase

Speeds of up to 20m/s=> 200g acceleration


49

Beam power in the LHC

shotshot

The Linac beam (160 mA, 200μs, 50 MeV, 1Hz) is enough to burn a hole intothe vacuum chamberWhat about the LHC beam: 2808 bunches of 15*1011 particles at 7 TeV?1 bunch corresponds to a 5 kg bullet at 800 km/h


50

Stored Beam Energies(Based on graph from R Schmidt)

100.00

1000.00

[MJ]

LHC topenergy

LHC injection(12 SPS b t h ) Factor

(Based on graph from R. Schmidt)

10.00

d in

the

beam

[ (12 SPS batches)

ISR SPS fixed

targetHERA

Factor~200

0.10

1.00

Ener

gy s

tore

d

LEP2

TEVATRON

SPSppbar

SPS batch to LHC

RHIC proton

0.011 10 100 1000 10000

Momentum [GeV/c]

E

SNSLEP2 p

Momentum [GeV/c]

Quench Levels Units Tevatron RHIC HERA LHCInstant loss (0.01 - 10 ms) [J/cm3] 4.5 10-03 1.8 10-02 2.1 10-03 - 6.6 10-03 8.7 10-04

Steady loss (> 100 s) [W/cm3] 7.5 10-02 7.5 10-02 5.3 10-03


51

y ( ) [ ]

Beam Dammage

primary collimatorprimary collimator

Fermi Lab‘sTevatron has 200 times less beam power than LHC!


52

p

Beam Loss Monitor Types

• Design criteria: Signal speed and robustness• Dynamic range (> 109) limited by leakage current through insulator

ceramics (lower) and saturation due to space charge (upper limit)ceramics (lower) and saturation due to space charge (upper limit).Ionization chamber:

– N2 gas filling at 100 mbar over-pressure

Secondary Emission Monitor(SEM):– Length 10 cm over pressure

– Length 50 cm– Sensitive volume 1.5 l– Ion collection time 85 μs

Length 10 cm– P < 10-7 bar– ~ 30000 times smaller

gain Ion collection time 85 μs

• Both monitors:– Parallel electrodes (Al SEM:

gain

– Parallel electrodes (Al, SEM: Ti) separated by 0.5 cm

– Low pass filter at the HV input


53

p– Voltage 1.5 kV

Quench levels


54

Industrial production of chambers

Beam loss must bed ll dmeasured all around

the ring=> 4000 sensors!


55

System layout


56

Successive running sums


57

beam diagnostics lecture 1.ppt - cern · beam diagnostics lecture 1 ulrich raich cern ab - bi (b i...

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