Particle Physics Seminar Tuesday 19 February 2008 Accelerator Physics Challenges in 3 rd Generation...

Particle Physics SeminarTuesday 19 February 2008

Accelerator Physics Challenges Accelerator Physics Challenges in 3in 3rdrd Generation Synchrotron Light Sources Generation Synchrotron Light Sources

R. Bartolini

John Adams Institute and Diamond Light Source Ltd


SummarySummary

Introduction:

synchrotron radiation

storage ring synchrotron radiation sources

Accelerator Physics challenges:

brightness, flux, stability, time structure

Conclusion:

future trends

3rd generation vs 4th generation


What is synchrotron radiationWhat is synchrotron radiation

Electromagnetic radiation is emitted by charged particles when accelerated

The electromagnetic radiation emitted when the charged particles accelerated

radially (v a) is called synchrotron radiation

. It is produced in the synchrotron radiation sources using bending magnets undulators and wigglers


storage ring synchrotron radiation sources (I)storage ring synchrotron radiation sources (I)


storage ring synchrotron radiation sources (II)storage ring synchrotron radiation sources (II)

Courtesy Z. Zhao


storage ring synchrotron radiation sources (III)storage ring synchrotron radiation sources (III)

Courtesy Z. Zhao


synchrotron radiation sources propertiessynchrotron radiation sources properties

Broad Spectrum which covers from microwaves to hard X-rays: the user can select the wavelength required for experiment

High Flux and High Brightness: highly collimated photon beam generated by a small divergence and small size source (partial coherence)

High Stability: submicron source stability

Polarisation: both linear and circular (with IDs)

Pulsed Time Structure: pulsed length down to tens of picoseconds allows the resolution of processes on the same time scale

Flux = Photons / ( s BW)

Brightness = Photons / ( s mm2 mrad2 BW )

synchrotron light


BrightnessBrightness

1.E+02

1.E+04

1.E+06

1.E+08

1.E+10

1.E+12

1.E+14

1.E+16

1.E+18

1.E+20

Bri

gh

tnes

s (P

ho

ton

s/se

c/m

m2 /m

rad

2 /0.1

%)

Candle

X-ray tube

60W bulb

X-rays from Diamond will be 1012 times

brighter than from an X-ray tube,

105 times brighter than the SRS !

diamond


Life science examples: DNA and myoglobin Life science examples: DNA and myoglobin

Photograph 51 Franklin-Gosling

DNA (form B)

1952

Franklin and Gosling used a X-ray tube:

Brightness was 108 (ph/sec/mm2/mrad2/0.1BW)

Exposure times of 1 day were typical (105 sec)

e.g. Diamond provides a brightness of 1020

100 ns exposure would be sufficient

Nowadays pump probe experiment in life science are performed using 100 ps pulses from storage ring light

sources: e.g. ESRF myoglobin in action


Brightness with IDs Brightness with IDs


Time structureTime structure

rms bunch length (ps)

0

5

10

15

20

25

30

35

40

45

ESRFAPS

SPRING8

ALSTL

S

ELETTRA

PLSLN

LS

MAX-II

BESSY-II

NewSUBARU

SLSANKA

CLS

SPEAR3

SAGA-LS

ASP

Indus

-II

Diamon

dSoleil

Petra-II

I

SSRFALBA

SESAME

CANDLE

MAX-IV

NSLS-II

TPS

rms

(ps)

Time resolved science requires operating modes with single bunch or hybrid fills to exploit the short radiation pulses of a single isolated bunch

2/3 filling pattern

0

0.2

0.4

0.6

0.8

1

1.2

0 200 400 600 800

bunch number

a.u.

312 ns

312 ns


Accelerator Physics challengesAccelerator Physics challenges

Brightness, Flux

Stability

Time structure

Small Emittance

Insertion Devices (low gaps)

High Current; Control Impedance; Feedbacks

Control Vibrations; Orbit Feedbacks; Top-Up

Short Pulses; Short Bunches


Brightness and emittanceBrightness and emittance

The electron beam emittance is a parameter of the storage ring that defines the source size and divergence

brightness 1 / emittance

NSLS-II


Emittance in an electron storage ringEmittance in an electron storage ring

The quantum nature of the synchrotron radiation emission is responsible for the finite beam size, emittance and energy spread of the electron beam.

Transverse electron oscillations are excited by the emission of a photon and are damped on average when the electron travels through the RF cavities

Oscillation damping and excitation counterbalance and an equilibrium emittance is reached


Small emittance latticesSmall emittance lattices

The horizontal emittance is determined by the dispersion generated by the main bending magnets.

dipolex

x HJ

2

2222 ''2 xxxxxxxx DDDDH

Low emittance and adequate space in straight section to accommodate long

Insertion Devices are obtained in the so called

DBA and TBA lattices

Theoretical Minimum Emittance

33

2

min,

10

bb

xx NJ


Commissioning of small emittance optics (I)Commissioning of small emittance optics (I)

During commissioning the Accelerator Physicists have to ensure that storage ring operates successfully in the nominal linear optics.

Linear optics studies are based on the analysis of the closed orbit response matrix (LOCO-like approach)

Using the Singular Value Decomposition (SVD) of the Response Matrix R we can invert R and correct

the closed orbit distortion

The orbit response matrix R is the change in the orbit at the BPMs as a function of

changes in the steering magnets strength

y

xRy

x

measuredmodel

H

V V

H


Commissioning of small emittance optics (II)Commissioning of small emittance optics (II)

The response matrix R is defined by the linear lattice of the machine, (dipoles and quadrupoles), therefore it can be used to calibrate the linear optics of the machine

ji

BPMsBPMsijBPMsBPMs GQRRGQ,

2modelmeasuredij

2 ,...),,(,...),,(

The quadrupole gradients are used in a least square fit to minimize the distance 2


Quadrupole gradient correctionQuadrupole gradient correction

LOCO varies the quadurpoles individually to fit the measured RM;

Initially the quadrupole variations generated by LOCO could reach 4%;

Quads variation reduced with better closed orbit correction, BBA and SVD threshold for LOCO;

Within each family quads variations are less than 2 % with respect to the mean for each quad family. (Up to 5 % with respect to the nominal calibration)


Implementation of small emittance opticsImplementation of small emittance optics

Residual beta-beating can be reduced to 1% or less

The optic functions measured at the BPMs location (circles) agree very well with the measured one

(crosses)


Emittance measurements with two pinhole cameraEmittance measurements with two pinhole camera

Emittance

2.78 (2.75) nm

Energy spread

1.1e-3 (1.0e-3)

Emittance coupling

0.5%

Measured emittance very close to the theoretical values confirms the optics


Small emittance and nonlinear beam dynamicsSmall emittance and nonlinear beam dynamics

Small emittance Strong quadrupoles Large (natural) chromaticity

strong sextupoles (sextupoles guarantee the focussing of off-energy particles)

strong sextupoles reduce the dynamic aperture and the Touschek lifetime

additional sextupoles are required to correct nonlinear aberrations

[Consider the effect of realistic errors (and define magnetic error tolerances)]

Courtesy A. Streun


Chromatic (energy dependent) effectChromatic (energy dependent) effect

Optics functions vary with relative energy offset

The betatron tunes crosses a wide range of resonances with

relative energy offset


Nonlinear beam dynamics optimisation (I)Nonlinear beam dynamics optimisation (I)

It is a complex process where the Accelerator Physicist is guided by

• (semi-)analytical formulae for the computation of nonlinear maps, detuning with amplitude and off-momentum, resonance driving terms

• numerical tracking: direct calculation of non linear tuneshifts with amplitude and off-momentum, 6D dynamics aperture and the frequency analysis of the betatron oscillations

Many iterations are required to achieve a good solution that guarantees a good dynamic aperture for injection and a good Touschek lifetime


Nonlinear beam dynamics optimisation (II)Nonlinear beam dynamics optimisation (II)

Frequency Map Analysis

allows the identification of dangerous non linear resonances during design and

operation

ALS measured ALS model

The Dynamic Aperture problem

Strongly excited resonances can destroy the Dynamic Aperture

Vacuum chamber


Touschek lifetimeTouschek lifetime

ringC

accxyxrings

be

Touschek

dsssss

C

C

cNr

023

2

)()()()(

1

8

1

Synchrotron radiation light sources require a large off-momentum aperture

The full 6D dynamic aperture has to be optimised

Electrons performing betatron oscillations may scatter and be lost outside the momentum aperture available from RF voltage and the 6D dynamic aperture


How to achieve and even smaller emittanceHow to achieve and even smaller emittance

Reduce the emission of radiation in bending magnets with either lower energy or weaker magnetic field → larger circumference (NSLS-II, Petra-III, PEP, Tristan). The radiated energy is proportional to E2B2

Damping wiggler in the storage ring (NSLS-II, PETRA-III): beam dynamics still manageable; sub-nm emittance looks feasible !

Tailor the magnetic field in the dipole – azimuthal dependence - in order to reduce the integral of the dispersion invariant in the dipole (studies ongoing at ESRF, SLS, Soleil): Dynamic Aperture correction quite complicated;


Closed Orbit correction and orbit stabilityClosed Orbit correction and orbit stability

The beam orbit is corrected to the BPMs zeros by means of a set of dipole corrector magnets: the BPMs can achieve submicron precision and the orbit rms is corrected to below 1 m:


Closed orbit disturbancesClosed orbit disturbances

• ground settling

• tidal motion

• day/night (thermal variations)

• re-injection

• thermal drifts of the electronics

• insertion device gap movements

• ground vibrations

• air conditioning units

• refrigerators, compressor (cooling systems)

• power supplies

• cooling water flow

• high current instabilities

Courtesy C. Bocchetta


Stability requirements in 3Stability requirements in 3rdrd generation light generation light sourcessources

mmx 3.121231.0 radradx 4.2241.0'

mmy 6.04.61.0

xx 1.0 '1.0' xx yy 1.0 '1.0' yy

Beam stability should be better than 10% of the beam size and divergence

For Diamond nominal optics (at the centre of the short straight sections)

radrady 4.041.0'

but IR beamlines will have tighter requirements

Courtesy L. Farvacque


Beam vibrations induced by Beam vibrations induced by ground and girder vibrationsground and girder vibrations

Integrated H Ground PSD 0.018 um Integrated H Beam PSD 2.41 um

Integrated H Girder 1 PSD 0.090 um Integrated H Girder 2 PSD 0.088 um Integrated H Girder 3 PSD 0.072 um


At ID source point

Horizontal Vertical

Long StraightStandard Straight


Position (μm)

Target 17.8 12.3 1.26 0.64

No FOFB 4.24 (2.4%) 3.08 (2.5%) 0.70 (5.5%) 0.36 (5.6%)

Angle (μrad)

Target 1.65 2.42 0.22 0.42

No FOFB 0.49 (2.9%) 0.61 (2.5%) 0.14 (6.2%) 0.24 (5.8%)

Beam stability at the source points (1-100Hz bandwidth) Beam stability at the source points (1-100Hz bandwidth)

We are within 10% of the beam size and divergence without FOFB


Performance of Diamond FOFBPerformance of Diamond FOFB

Significant improvement up to 100 Hz; higher frequencies performance limited mainly by the correctors power supply bandwidth


At ID source point

Horizontal Vertical



Position (μm)

Target 17.8 12.3 1.26 0.64

No FOFB 4.24 (2.4%) 3.08 (2.5%) 0.70 (5.5%) 0.36 (5.6%)

FOFB On

0.89 (0.5%) 0.63 (0.5%) 0.19 (1.5%) 0.11 (1.7%)

Angle (μrad)

Target 1.65 2.42 0.22 0.42

No FOFB 0.49 (2.9%) 0.61 (2.5%) 0.14 (6.2%) 0.24 (5.8%)

FOFB On

0.10 (0.6%) 0.13 (0.5%) 0.04 (1.7%) 0.07 (1.7%)

Beam stability at the source points (1-100Hz bandwidth) Beam stability at the source points (1-100Hz bandwidth)


Long term drifts (30 minutes) SOFB OFFLong term drifts (30 minutes) SOFB OFF

3 m maximum drift over 30 minutes

H rms < 0.7 m

V rms < 0.4 m


Long term drifts (30 minutes) SOFB ONLong term drifts (30 minutes) SOFB ON

SOFB running at 0.2 Hz

H rms < 0.5 m

V rms < 0.3 m


Improving stability: Top-Up operation (I)Improving stability: Top-Up operation (I)

Top-Up operation consists in the continuous (very frequent) injection to keep the stored current constant

Top-Up improves stability

• constant photon flux

• constant thermal load on components

provides more flexibility

• Lifetime less important

• Smaller ID gaps

• Lower coupling

Already in operation at APS and SLS


• Total current stable at 128.4mA to 0.1%

• Hybrid bunch stable at 0.43mA to 3.2%

Pk-pk ~ 0.2mA

σ ~ 0.06mA

Improving stability: Top-Up operation (II)Improving stability: Top-Up operation (II)


Beam-line safe for top-up if:

1. Electrons travelling forwards from straight section cannot pass down beam-line

2. Electrons travelling backwards from beam-line cannot pass through to straight section

3. Electrons travelling in either direction do not have same trajectory at any intermediate point

Safety case for Top-Up operationSafety case for Top-Up operation

Machine Interlocks have to be defined to prevent a top-up accident under faulty

conditions:

BTS energy ILK and stored beam ILK are adequate for Diamond


AP challenges: Time structureAP challenges: Time structure

Diamond present layout:

Injector and timing allow a very flexible fill pattern control (single bunch – camshaft, etc)

but bunch length limited to 10 ps

Rep rate higher than 533 kHz

2/3 filling pattern

0

0.2

0.4

0.6

0.8

1

1.2

0 200 400 600 800

bunch number

a.u

.

0

0.5

1

1.5

2

2.5

3

0 2 4 6 8 10

I (mA)

z

/

z0 a

nd

/

0

312 ns 312 ns

The bunch length and energy spread, increase with current due to the

"microwave instability":

Current Bunch length Lifetime

0.1 mA 10 ps 92.3 h

0.5 mA 12 ps 22.7 h

0.8 mA 13 ps 15.5 h

4 mA 18 ps 4.3 h

10 mA 25 ps 2.4 h


Generation of short radiation pulses Generation of short radiation pulses in a storage ringin a storage ring

Low – alpha

Higher Harmonic Cavities

RF voltage modulation

Femto – slicing

1) shorten the e- bunch 2) chirp the e-bunch + slit or optical compression

3) local energy-density modulation

e– bunch

Crab Cavities

Synchro-betatron kicks

There are three main approaches to generate short radiation pulses in storage rings


dzVdf

c

RFsz /2

3

Bunch length (low current)Bunch length (low current)

= 1.710–4; V = 3.3 MV; = 9.6 10–4 z = 2.8 mm (9.4 ps)

z depends on the magnetic lattice (quadrupole magnets) via

(low_alpha_optics) 10–6

6101 ds

D

Lx

z 0.3 mm (1 ps)

We can modify the electron optics to reduce

The equilibrium bunch length is due to the quantum nature of the emission of synchrotron radiation and is the result of the competition between quantum excitation

and radiation damping. If high current effects are negligible the bunch length is


Low alpha opticsLow alpha optics

When the bunch is too short Coherent Synchrotron Radiation generates further instabilities

Microbunch instability (Stupakov-Heifets)

for Diamond the Microbunching threshold is about 10 A per bunch at 1 ps rms length

Single bunch: 10 A; 1 ps; rep. rate 530 kHz

Full fill: 10 A * 936 bunches; 1 ps; rep. rate 500 MHz

Bessy-II data

Bessy-II, ALS and SPEAR3 have successfully demonstrated low-alpha operation with few ps bunches for Coherent THz radiation

Courtesy P. Kuske


Crab Cavities for optical pulse shorteningCrab Cavities for optical pulse shortening

Courtesy M. Borland (APS)


A possible implementation of crab cavities at A possible implementation of crab cavities at DiamondDiamond

4 in V420 rad V kick

Crab cavities are located at 1.1 m from the centre of the long

straight

Looks feasible to get sub-ps x-ray pulses with very good transmission (80%)

Emittance degradation is modest

Impedance issues have still to be addressed (machine impedance,

LOM and HOM in crab cavity)

This scheme is yet unproven


Femto-second slicingFemto-second slicing

A.A. Zholents and M.S. Zolotorev, Phys. Rev. Lett. 76 (1996) 912.

wiggler

spatial or angular separation in a dispersive section

(b)

electronbunch

femtosecondlaser pulse

W

fs pulse

fs pulse

“dark” pulse

electron-laser interaction in the modulator

(a)

femtosecondelectron bunch

fs radiation pulses from a radiator

(c)

BESSY-II, ALS and SLS have successfully demonstrated the generation of X-ray pulses with few 100 fs pulse length


Energy modulation generated by a short laser pulseEnergy modulation generated by a short laser pulse

Natural energy spread 0.1%


Separation of the radiation from the two Separation of the radiation from the two modulated bunchletsmodulated bunchlets

max 1.5 % energy modulation

Pulse stretching at radiator 35 fs

separation x = 1.8 mm (w.r.t. 200 um beam size rms)

separation x’ = 0.6 mrad (w.r.t. 0.3 mrad opening angle of radiation)

Radiation pulses of 35 fs can be generated;

modulatorradiator


Comparison of options for short radiation pulsesComparison of options for short radiation pulses

Crab C.


AP challenges: high current operationAP challenges: high current operation

The beam and its electromagnetic field travel inside the vacuum chamber while the The beam and its electromagnetic field travel inside the vacuum chamber while the image charge travels image charge travels onon the chamber itself. the chamber itself.

Any variation on the chamber profile, on the chamber material, breaks this Any variation on the chamber profile, on the chamber material, breaks this configuration.configuration.

The beam loses a (usually small) part of it is energy that feeds the electromagnetic The beam loses a (usually small) part of it is energy that feeds the electromagnetic fields that remain after the passage of the beam. Such fields are referred as fields that remain after the passage of the beam. Such fields are referred as wake wake

fieldsfields

Wake fields generated by beam particles, mainly affect trailing particles in previous Wake fields generated by beam particles, mainly affect trailing particles in previous bunches (long range wakes) or in the tail of the same bunch (short range wakes)bunches (long range wakes) or in the tail of the same bunch (short range wakes)

NegativeNegativeCharged BeamCharged Beam


AP challenges: High currentAP challenges: High current

Collective effect are usually categorised as

Multi-bunch and Single-bunch;

Transverse and Longitudinal;

Main causes in synchrotron light sources are

Resistive Wall impedance (narrow gap chambers, SS vacuum chambers)

Ion related instabilities (Ion Trapping; Fast Ion Instability)

Poor design of vacuum chamber elements (tapers, bellows, BPMs, …)

RF cavities High Order Modes (HOMs)

Main cures are

Operation with high positive chromaticity

Bunch lengthening (low voltage RF voltage, Harmonic cavities)

Feedback systems (TMBF, LMBF)

better design of vacuum chamber elements (SCRF, HOM damping, …)


Multi-bunch modesMulti-bunch modes

V instability visible at 17 mA for zero chromaticity

Onset of sidebands not too far from predicted RW threshold (40 mA)

Increasing chromaticity counteract the instability

Beam is stable up to 110 mA with chromaticity +2 in both planes

60 mA

2/3 fill


AP challenges: High currentAP challenges: High current

Measurements at 160 mA (Chromaticity +2/+2)

Vertical betatron lines appears at about 12 MhZ

Some evidence of ion related instabilities

Fill pattern and chromaticity dependence are under investigations.

Tracking shows that chromaticity > 5 impacts injection efficiency (95% to 65%)

TMBF is required to damp these instabilities

160 mA

2/3 fill


Current thresholdsCurrent thresholds

Ion related instabilities are clearly visible in the initial stage of commissioning. They become less important with vacuum improvement due to synchrotron radiation

cleaning of the vacuum chamber, but a TMBF is required.

Diamond Soleil


Transverse Multibunch Feedback at DiamondTransverse Multibunch Feedback at Diamond

The TMBF system detects coherent betatron oscillation bunch-by-bunch and damps them with a pair of stripline kickers


Single Bunch Longitudinal collective effects: Single Bunch Longitudinal collective effects: the microwave instabilitythe microwave instability

Bunch length at zero current 17 ps

(with 1.9 MV; = 1.410–4)

Z||/n ~ 0.4

nZe

EEI ECpeak

//

2002

When the current per bunch is larger than the instability threshold:

the single particles get excited by the wakes on exponentially growing

longitudinal oscillations. Because of non-linearities, the oscillation frequency changes with amplitude limiting the

maximum amplitude and in most of the cases no particle loss happens.

The net effect on the bunch is an increase of the energy spread above

threshold with a consequent increase of the bunch length


Transverse mode coupling instabilityTransverse mode coupling instability

The transverse impedance of the machine can generate an instability of internal

modes of oscillation of a bunch (head-tail instability real part of the impedance)

or shift the frequency of the modes until they coalesce

(transverse mode coupling instability imaginary part of the impedance)

It can be cured with increasing chromaticity and the voltage

It cannot be cured simply by the TMBF system

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 0.5 1 1.5 2

current (mA)

Re

( -

ws)/w

s

Vertical beam blow-up at diamond


Conclusions (I)Conclusions (I)

Third generation light sources provide a very reliable source of high brightness, very stable X-rays

Medium energy machines (~ 3 GeV) performances are now covering the needs of a wide user’s community and the number of beamlines per facility is increasing;

Future developments will target

even lower emittance < 1 nm

higher stability tens of nm over few hundreds Hz

short pulses < 1 ps

higher current > 500 mA

more undulator per straights (canted undulators)

Technological progress is expected to further improve brightness and stability (IDs, BPMs, …)


Conclusions (II)Conclusions (II)

It is generally believed that 3rd generation light sources will not be replaced by SASE-FEL (4th generation light sources) but rather they can coexist.

3rd generation will remain unrivalled in terms of stability and cost effectiveness, and will still be competitive in terms of average brightness, tunability, reliability.

4-th generation light sources will be superior in their peak brightness and time structure, providing fs and sub-fs radiation pulses.

Although it is a mature technology and one cannot expect many order of magnitude improvements in the coming years, upgrades and new ideas are continuously proposed and new light sources are under commissioning (SSRF) or under constructions (ALBA) or planned (NSLS-II, …)


Conclusions (III)Conclusions (III)

3rd generation light source are still fashionable…

Particle Physics Seminar Tuesday 19 February 2008 Accelerator Physics Challenges in 3 rd Generation...

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