CAS 2016 P2 v2 handout€¦ · M.G. Minty and F. Zimmermann: Measurement and Control of Charged...

10/6/2016

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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 1Eva Barbara Holzer 1

Eva Barbara HolzerCERN, Geneva, Switzerland

CAS Introductory level course on Accelerator Physics

Beam Instrumentation and DiagnosticsPart 2

Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 2

Peter Forck: Lecture on Beam Instrumentation and Diagnostics at the Joint University Accelerator School (JUAS), see also the extended Bibliography http://www-bd.gsi.de/conf/juas/juas.html

M.G. Minty and F. Zimmermann: Measurement and Control of Charged Particle Beams, Springer Verlag2003, (book).

Conference series: IBIC (International Beam Instrumentation Conference), IPAC (International Particle Accelerator Conference), historic: DIPAC (Workshop on Beam Diagnostics and Instrumentation for Particle Accelerators), BIW (Beam Instrumentation Workshop)

CERN Accelerator Schools (CAS):http://cas.web.cern.ch/cas/CAS%20Welcome/Previous%20Schools.htm andhttp://cas.web.cern.ch/cas/CAS_Proceedings.html

Rhodri Jones et al.: Introduction to Beam Instrumentation and Diagnostics, CERN-2014-009.

Daniel Brandt (Ed.), 2008 CAS on Beam Diagnostics for Accelerators, Dourdan, CERN-2009-005 (2009).

Heribert Koziol, Beam Diagnostic for Acclerators, Univ. Jyväskylä, Finland, 1992, CERN 94-01, http://cas.web.cern.ch/cas/CAS%20Welcome/Previous%20Schools.htm

Jacques Bosser (Ed.), Beam Instrumentation, CERN-PE-ED 001-92, Rev. 1994

Resources and References

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Intensity Measurement

Derived Quantities:

Trajectory and Orbit

Emittance

Tune

Chromaticity

Betatron phase advance

Beta function

Dispersion

Special challenges for high intensity and high brightness beams

Overview – Part 2


Some depend on beam optics model

Extensive use on algorithms and computer programs, fitting procedures, iterative procedure to derive desired beam quantity from measured values.

Beam optics values and functions typically corrected after measurement to come closer to design / optics model values

Often also feed-forward and/or feed-back system employed:

e.g. for RHIC the orbit, tune and coupling feedbacks were key to higher luminosities, polarization and integrated luminosity/uptime.

Derived Beam Quantities

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Intensity


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4 batches each containing 72 bunches separated by 25 ns

CERN FBCT Readings of LHC Type Beams in the SPS

R. Jones, DIPAC’03

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Trajectory and Orbit


Trajectory: The mean positions of the beam during 1 turn

Trajectories must be controlled in linear machine and transfer lines and in circular machines at injection, ejection, and at transition

Orbit: The mean positions over many turns for each of the BPMs

Closed orbits may change during acceleration, RF “gymnastics” and changes of the transverse optics (e.g. in a collider: bringing beams to collide at physics energy, squeezing of beta function, reducing beam crossing angles)

E.g. four pick-ups per tune value separated by about μ ≃ 90o betatron phase advance

Located at large beta function (e.g. next toquadrupoles)

Definitions

beamtrajectory

Focusing elements (e.g. quadrupoles)

BPM Pickups

s

x, y

M. Wendt

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Threading the first beam through the LHC in 2008

One beam at a time, one hour per beam

Collimators used to stop the beam after each sector

Once the trajectory was corrected open the collimator for the next sector

Example of Orbit Measurement


Beam Screen first two turns of LHC start-up Run2 in 2015

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Example from a measurement of the transfer line optics (beta function)

Transfer Line – Trajectory Response to Dipole Steering Magnet

J. Wenninger, CAS


≈100 collimators and absorbers

Including special dump and injection protection collimators

Three Stage Collimation System

Cleaning insertion Arc(s) IP

Circulating beam

Arc(s)

Deflection:Primary

collimator

Absorption:Secondarycollimators

Tertiary beam halo

+ hadronic showers

Shower absorbers

Triplet Protection:Tertiary

collimators

SCTriplet

Warm aperture Cold aperture

beam

1.2 m

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Find center and relative size of beam at collimator location using BLM signal

Collimator Set-Up with BLM

Beam

Primary Collimator

Secondary Collimator

BLMBLM

Beam

Primary Collimator

Secondary Collimator

BLMBLM

1.

2.

Threshold

BLM Signal

Jaw Positions

Time

G. ValentinoD. Wollmann


Generating losses in H, in V and longitudinal plane separately

Verification of cleaning performance and collimator set-up (jaw position with respect to beam center and size)

Validation of Collimator Set-Up

‘loss map’: losses along the ring normalized to the losses at the primary collimator

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Beam-based setup with BLM signal is time consuming

Tighter tolerances will be required for future LHC operation

BPM integrated in the tapered end of the collimator jaws (10.6mm retraction from jaw surface)

Drastically reduce set-up time

Allow constant monitoring of beam position to jaw position possibility for interlocks

Reduce the margins for orbit changes in the collimator hierarchy allow for smaller β*

Tested in the SPS (D. Wollmann, HB2012) <25 μm difference to BLM setup

- believed to be dominated by the BLM setup method

single pass (transfer line): <90μm rms

no disturbance observed from protons hitting the jaws or from shower particles

New LHC Collimators with Integrated BPMs


Alignment of 16 tertiary collimators

BLM based alignment in 2012

BPM alignment in 2015

No information on beam size –use model value

Experience LHC BPM Collimator Set-Up

G. Valentino, IPAC 2016

1 hour

20 second

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Emittance Measurement


Beam sigma matrix: correspondence of Twiss parameters to second moments of the beam particle distribution ( 0 :

Σ εβα γ

′′ ′

γ ε det Σ

σ

In a storage ring the optical functions (α, β, γ) are periodical. They are completely defined by the machine optical elements (magnets).

In a linear machine (transport line, LINAC) there is no periodic boundary condition. The optical functions depend on the incoming beam distribution as well as on the optical machine elements.

Recap

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Σ εβα γ

′′ ′

Circular machine

In a dispersion free region

In presence of dispersion ∆

Measurement accuracy ≈ 10% (because of uncertainties on optics parameters)

Emittance Measurement – Circular Machine


Σ εβα γ

′′ ′

Method 1:

Slit-grid device (1D) or Pepper pot (2D)

Several slices of transverse profile ( ) and angular distribution ( ′) at one location

Low energy beams, where beam can be stopped in slit / pepperpot mask (for hadrons Ekin < 100 MeV/u)

Method 2:

a) ‘Three grid’ method: transverse profile at different location and linear transformation

b) Quadrupole variation: transverse profile at one location with different setting of quadrupole

Emittance Measurement – Linear Machine

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A mask (slit) cuts out a small slice of phase space with defined x-position.

A drift space converts the angles into position, which is measured with a profile monitor

Reconstruct the angular distribution at the x-position defined by the slit.

Reconstruct the emittance by successive measurements at different slit positions.

Slit-Grid Method

x

z

U. Raich, USPAS

s


Example – Low Energy Ion Beam

P. Forck, JUAS

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Mask has small holes instead of slits

Measures horizontal and vertical in a single shot

Pepperpot Method

Example Pepper-pot GSI-LINAC: 15 × 15 holes with Ø 0.1mm on a 50

× 50 mm2 copper plate Distance: pepper-pot-screen: 25 cm Data acquisition: high resolution

CCD

P. Forck, JUAS


To determine ε, β, α at a reference point in a beamline one needs at least three beam size measurements with different transfer matrices between the reference point and the measurement locations.

Different transfer matrices can be achieved with different profile monitor locations, different focusing magnet settings or combinations of both.

Once β, α at one reference point is determined the values of β, α at every point in the beamline can be calculated.

In practice better results are obtained with more than three measurements.

Method 2 – Based on Beam Size Measurements

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Three Grid Method

Based on V.A. Dimov, HB2016

Rms ellipse

Beam profile

′

?′′ ′


Three Grid Method

′

?

′ ′

position 1 position 2 position 3

Measured rmsbeam size

′ ′

′ ′ But ′ is unknown

′

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Phase space

position 1

Three Grid Method

′


′


Phase space

position 1

Three Grid Method

′ ′

Linear mapping of the measured rms

beam size onto the initial phase space.


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Phase space

position 1

Three Grid Method

′ ′





′

Phase space

position 1

Three Grid Method

′



′ ′

rms emittance ellipseposition 1 position 2 position 3

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One can linearly map the measured profiles onto the initial phase space and use tomography to reconstruct the distribution of particle density in a phase space.

Phase Space Tomography

′

V.A. Dimov, HB2016




Quadrupole Variation Method

H. Braun, CAS

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Quadrupole Variation Emittance Measurement at CTF3

H. Braun, CAS

horizontal vertical


Tune and Chromaticity in a Circular Machine

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The tune Q is the number of betatron oscillations per turn

Measurement gives the non-integer part q; Measure with slightly shifted tune to distinguish q<0.5 from

q>0.5

Caveat: Excitation of hadron beam leads to emittance blow-up

Excite the beam with Single kick (or white noise, or ‘chirp’)

FFT analysis of position reading from one BPM Betatron tune is the frequency with the highest amplitude

response

In the presence of external excitation the method can even work without kick

Example GSI synchrotron

Tune

P. Forck, JUAS


Tune measurement with a network analyzer Beam exited with sinusoidal wave; frequency is stepped over the expected tune range

Response of the beam (amplitude and frequency) is determined

Beam acts like a harmonic oscillator

Measurement of the phase response in general more precise than amplitude measurement

High precision: up to 10-4 but slow (up to minutes)

Beam Transfer Function Measurement

H. Schmickler, CAS

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cos φ - cos(2ωt+φ)

A voltage controlled oscillator (VCO) excites the beam with a sine wave, measures the beam response and locks the excitation frequency to the 90 degree phase difference. Tracks any tune changes

Continuous reading

Phase Locked Loop Tune Tracking

Example of continuous tune tracking while crossing horizontal and vertical tunes

Closest tune approach is a measure for coupling

H. Schmickler, CAS


High Sensitivity Tune Measurement by Direct Diode Det.

Marek Gasior,Faraday Cup Award 2012

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Direct Diode Detection (BBQ - Base Band Tune) CERN

CERN LHC

CERN PS


Example LHC Tune Feed-back System

Hor. spectrum with Tune-FB OFF Hor spectrum with Tune-FB ON

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Chromaticity , or

∆ ∆

ξ ∆

∆

Measure tune for slightly different beam energies (by varying the RF frequency and keeping the magnetic filed constant) and calculate the gradient.

Correct with sextupole magnets

Chromaticity can be tracked continuously by combining RF modulation with PLL tune measurement

Chromaticity Measurement


Beta Function and Phase AdvanceDispersion function

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Twiss parameters: Beta function β , α ⁄ , γ

Phase advance, μ(s): phase change of betatron oscillation

The errors on β and μ are frequently referred to as beta-beating and phase-beating

Dispersion function D(s) is the lateral displacement due to a momentum offset:

∆∆

The actual lattice can deviate from the design lattice due to a variety of errors

In general the measurements are followed a by second step: the correction of the measured lattice errors. This is frequently an iterative process that is repeated until the lattice parameters are judged to be satisfactory

Recap


∆∆

Measure the orbit offset for different beam momenta

beam momentum e.g. by changing the RF frequency, :

∆α

∆γ, α … relativistic gamma, momentum compaction factor

Measure ∆ around the ring (pick-up)

Dispersion function

Example: Horizontal dispersion in the CERN SPS (protons 14 to 450 GeV/c); 6 long straight sections with low horizontal dispersion.

Measurement of Dispersion Function in a Circular Machine

J. Wenninger, CAS

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Can be defined similar to ring. But the dispersion defined in this way depends on the initial condition at the start of the line and also on the location where the energy offset is introduced.

Example: Dispersion measurement in the transfer line from the SPS to the CERN Neutrino to Gran Sasso target (400 GeV/c high intensity protons).

The dispersion is obtained by varying the RF frequency in the SPS ring and measuring the trajectory for different SPS RF frequency settings.

The transfer line bends both horizontally and vertically.

The dispersion is matched to be zero at the target.

Measurement of Dispersion Function in a Transfer Line

J. Wenninger, CAS


Excite coherent betatron oscillation (kicker, or periodic excitation e.g. AC dipole)

Turn-by-turn BPM position measurement:

cos 2

⁄ cos 2

i … BPM number k … turn number 0 … reference location at

Ai and µi … amplitude and phase at i th BPM

With an absolute calibration of the BPM position readings measure the beta function with respect to beta function at the reference location ⁄

The beta function can also be determined from the measured phase advance∆

in combination with the phase advance derived from the optics model

∆

Beta Function and Phase Advance from Turn-by-Turn BPM Measurement

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LEP (45 GeV): The largest step in beta-beating occur near the interaction points (IP) at the low beta insertions.

Example Beta-Beating from Phase Advance Measurement

P. Castro, PhD


A small change, Δk, of the strength of a quadrupole (of length L) in a ring leads to the tune change ΔQ

∆14

∆ ≅∆ ̅

4

Measuring ∆ average beta function at the quadrupole location

Example of the period tune modulation due to the modulation of Δk of a LEP quadrupole (here with a square function).

Beta Function from K-Modulation

O. Berrig et al., DIPAC 2001

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Special Challenges for High Intensity and High Brightness Beams


Damage potential:

Stored beam energy / power

Energy stored in superconducting machine components

High energy / power density

Avoid uncontrolled losses:

All systems which are part of machine protection (e.g. BLM system): high dependability (reliability, availability, safety, maintainability), fast response, full coverage of critical scenarios (e.g. loss scenarios)

Collimation and related monitoring

Halo monitoring

Avoid intercepting measurement devices

Quench magnets

Instrument can be destroyed by the beam

Non-invasive monitoring of all relevant machine parameters!

Small beam sizes

Systematic effects dominate the measurement


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High radiation levels at collimation, interaction points, targets etc.

Can interfere with measurement (e.g. beam loss measurement) and require radiation hard equipment

Monitoring of beam instabilities

Bunch-by-bunch and intra-bunch measurements

High dependability of measurement to be used for feed-back systems

Wakefields and RF heating:

Multi pass machines: strict impedance budget of the instrument, in particular for devices which are numerous (BPMs)

Damage due to RF heating



Thin carbon ribbons (25-100 nm thick, 1-10 μm wide, 2.5 cm long)

Scanned through the p beam to measure beam polarization profiles

Frequent target breakage (also without beam contact, even in park position) installation of cameras RF heating at the wire ends without

touching the beam

Add “fins” to deviate the EM field from the wire ends reduces significantly the heating

RHIC p-Carbon Polarimeter Target

H. Huang et al., IBIC 2014

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http://www.youtube.com/watch?v=hQsOAyQ7Kck

Video 2

Courtesy M. Minty


Overheating pressure rise

or material deformation

Beams induced RF heating – LHC run1

RF contact fingers at magnet interconnectsBeam screen around injection protection jaw

Inje

ctio

n K

icke

r

AT

LA

S A

LF

A

De

tect

or

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Mirror heating correlated to:

Beam intensity

Bunch length

Beam spectrum

Synchrotron Light Extraction Mirror

Failure of mirror holder + blistering of mirror coatingOverheated and broken ferrite absorbers (BSRT)


EM simulations and lab tests are essential for all equipment which is installed on the beam

Mitigation by e.g.:

Design changes to reduce the build-up of wake fields – or deviate from the sensitive location

Adding ferrites to absorb the RF power given there is sufficient cooling for the ferrites

Multi-mode couplers to extract the power and dissipate it outside of the vacuum

RF Heating, cont.

OLD Extraction MirrorNEW Extraction Mirror

Sol

utio

n fo

r R

un I

I w

ith lo

w R

F

‘foot

prin

t’ an

d sh

ield

ed c

aviti

es

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(Quasi) Non-Invasive Beam Size Measurement


Beam imaging with vertex reconstruction of beam gas interactions Reconstruct the tracks coming from inelastic beam-gas interactions

Determine the position of the interaction (vertex)

Accumulate vertices to measure beam position, angle, width and relative bunch populations

Main requirements Sufficient beam-gas rate → controlled pressure bump

Good vertex resolution → precise detectors and optimized geometry

Beam Gas Vertex Monitor

Plamen Hopchev

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Goal: develop a transverse profile monitor for (HL) LHC

Overcome the limitations and complement the existing devices

Demonstrate the potential of this technique by installing a prototype BGV system on one beam at the LHC

Commissioning planned for 2015

BGV Demonstrator

Detector Scintillating

fibres read out with SiPMs Same

technology as for the LHCbupgrade

Plamen Hopchev


Only for electrons & very high energy protons/ions (LHC)

For linear machine: difficult to separate the light from the beam

Difficult to get absolute calibration: Image correction factors typically bigger than the beam size

Dynamic range 200 (105 by changing the attenuation)

Accuracy 30%

Spatial resolution 50μm

Synchrotron Light Monitor

LHC: transverse blow-up of individual bunches

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Residual Gas Ionisation

Dynamic range: up to 103

≈ 10 times more sensitive than luminescence

Image broadening due to space charge

More complicated to build High voltage

Guiding magnetic field

Compensation magnets for the beam

IPM (Ionization Profile Monitors)

M.Schwickert, P.Forck, F.Becker, GSI

T. Giacomini et al., GSI


Beam Induced Fluorescence (BIF)

Insensitive to electric and magnetic fields (e.g. beam space charge)

Sensitive to radiation leading to background

Low signal yield gas injection (e.g. N2, H2)

Dynamic range: ≈ 103

Luminescence Profile Monitor

N2 injection

To signalprocessing

CCD

I [MCP]

Beam

400 l/s 400 l/s

Lens, Image-Intensifierand CCD FireWire-Camera

N2-fluorescent gas

equally distributed

M.Schwickert, P.Forck, F.Becker, GSI

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Profile Collected every 20ms Local Pressure at ≈510-7 Torr

2DSide view

3DImage

Beam Size

Tim

e

Injection

Beam size shrinks asbeam is accelerated

Fast extraction

Slow extraction

Luminescence Profile Monitor – Example CERN SPS


For H- and electrons

Electron is stripped from the H-, deflected and measured, e.g. with a Faraday cup(inverse Compton scattering for electron beams)

Can measure down to µm level

dynamic range: up to 103

Laser wire scanner

A. Alexandrov

1 MW beam powerY. Liu, SNS, PAC’11

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Electron beam scanner (SNS, PAC’11, HB2012, W. Blokland)

Electrons are deflected by proton beam and measured on a fluorescent screen

Electron Beam Scanner


Halo Measurements

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Beam tails (10-2 –10-3) are just within range of standard profile measurements

Halo (< 10-4): need very high dynamic range >105

Not one of the ‘common’ measurements

For high intensity machines it is mandatory to limit beam losses good understanding of the beam tail and halo very helpful

Halo typically not well understood (simulation codes do not reproduce halo populations very well)

Linear machines: Wire scanners shown to work well for halo measurements

Often beam steered to minimize losses: steering on the beam halo rather than on the beam core

Halo can re-populate along the machine after being scraped

Synchrotron: collimator or scraper or wire close to the beam Measure (precise relative measurement) and destroy the halo at the same time

Halo Measurements


Signal: SEM (secondary emission current) for low beam energy or by scintillator for high beam energy or ‘vibrating wire’ method (measured quantity: resonance frequency change due to wire heating – very precise for few interactions)

Use of special techniques to improve S/N ratio and enhance the dynamic range

Wire static in the halo for considerable time suitable for linear machines and transfer lines

A Browman et al. PAC 2003:measurement by SEM current readout at the extraction line of the Los Alamos Proton Storage Ring:

Each position averaging over several beam pulses

dynamic range of 105

Wire Scanner for Halo Measurement

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Counting with the wire at constant position in the halo can be combined with fast wire scan in the beam core, e.g.:

A.P Freyberger, Jefferson Lab, PAC’03 DIPAC’05: coincident counting (for background subtraction); combining multiple wires with different diameter (also using a plate):

huge dynamic range: 108

Wire Scanner cont.

CAS 2016 P2 v2 handout€¦ · M.G. Minty and F. Zimmermann: Measurement and Control of Charged...

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Transcript of CAS 2016 P2 v2 handout€¦ · M.G. Minty and F. Zimmermann: Measurement and Control of Charged...