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Particle Physics Seminar Tuesday 19 February 2008 Accelerator Physics Challenges in 3 rd Generation Synchrotron Light Sources R. Bartolini John Adams Institute.

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Presentation on theme: "Particle Physics Seminar Tuesday 19 February 2008 Accelerator Physics Challenges in 3 rd Generation Synchrotron Light Sources R. Bartolini John Adams Institute."— Presentation transcript:

1 Particle Physics Seminar Tuesday 19 February 2008 Accelerator Physics Challenges in 3 rd Generation Synchrotron Light Sources R. Bartolini John Adams Institute and Diamond Light Source Ltd

2 Particle Physics Seminar Tuesday 19 February 2008 Summary Introduction: synchrotron radiation storage ring synchrotron radiation sources Accelerator Physics challenges: brightness, flux, stability, time structure Conclusion: future trends 3 rd generation vs 4 th generation

3 Particle Physics Seminar Tuesday 19 February 2008 What 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

4 Particle Physics Seminar Tuesday 19 February 2008 storage ring synchrotron radiation sources (I)

5 Particle Physics Seminar Tuesday 19 February 2008 storage ring synchrotron radiation sources (II) Courtesy Z. Zhao

6 Particle Physics Seminar Tuesday 19 February 2008 storage ring synchrotron radiation sources (III) Courtesy Z. Zhao

7 Particle Physics Seminar Tuesday 19 February 2008 synchrotron 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  mm 2  mrad 2  BW ) synchrotron light

8 Particle Physics Seminar Tuesday 19 February 2008 Brightness Candle X-ray tube 60W bulb X-rays from Diamond will be 10 12 times brighter than from an X-ray tube, 10 5 times brighter than the SRS ! diamond

9 Particle Physics Seminar Tuesday 19 February 2008 Life science examples: DNA and myoglobin Photograph 51 Franklin-Gosling DNA (form B) 1952 Franklin and Gosling used a X-ray tube: Brightness was 10 8 (ph/sec/mm 2 /mrad 2 /0.1BW) Exposure times of 1 day were typical (10 5 sec) e.g. Diamond provides a brightness of 10 20 100 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

10 Particle Physics Seminar Tuesday 19 February 2008 Brightness with IDs

11 Particle Physics Seminar Tuesday 19 February 2008 Time structure Time resolved science requires operating modes with single bunch or hybrid fills to exploit the short radiation pulses of a single isolated bunch 312 ns

12 Particle Physics Seminar Tuesday 19 February 2008 Accelerator 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

13 Particle Physics Seminar Tuesday 19 February 2008 Brightness 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

14 Particle Physics Seminar Tuesday 19 February 2008 Emittance 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

15 Particle Physics Seminar Tuesday 19 February 2008 Small emittance lattices The horizontal emittance is determined by the dispersion generated by the main bending magnets. 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

16 Particle Physics Seminar Tuesday 19 February 2008 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 H V V H

17 Particle Physics Seminar Tuesday 19 February 2008 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 The quadrupole gradients are used in a least square fit to minimize the distance  2

18 Particle Physics Seminar Tuesday 19 February 2008 Quadrupole 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)

19 Particle Physics Seminar Tuesday 19 February 2008 Implementation 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)

20 Particle Physics Seminar Tuesday 19 February 2008 Emittance 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

21 Particle Physics Seminar Tuesday 19 February 2008 Small 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

22 Particle Physics Seminar Tuesday 19 February 2008 Chromatic (energy dependent) effect Optics functions vary with relative energy offset The betatron tunes crosses a wide range of resonances with relative energy offset

23 Particle Physics Seminar Tuesday 19 February 2008 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

24 Particle Physics Seminar Tuesday 19 February 2008 Nonlinear beam dynamics optimisation (II) Frequency Map Analysis allows the identification of dangerous non linear resonances during design and operation ALS measuredALS model The Dynamic Aperture problem Strongly excited resonances can destroy the Dynamic Aperture Vacuum chamber

25 Particle Physics Seminar Tuesday 19 February 2008 Touschek lifetime 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

26 Particle Physics Seminar Tuesday 19 February 2008 How 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 E 2 B 2 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;

27 Particle Physics Seminar Tuesday 19 February 2008 Closed 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:

28 Particle Physics Seminar Tuesday 19 February 2008 Closed 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

29 Particle Physics Seminar Tuesday 19 February 2008 Stability requirements in 3 rd generation light sources 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) but IR beamlines will have tighter requirements Courtesy L. Farvacque

30 Particle Physics Seminar Tuesday 19 February 2008 Beam vibrations induced by ground 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

31 Particle Physics Seminar Tuesday 19 February 2008 At ID source point HorizontalVertical Long Straight Standard Straight Long Straight Standard Straight Position (μm) Target17.812.31.260.64 No FOFB 4.24 (2.4%)3.08 (2.5%)0.70 (5.5%)0.36 (5.6%) Angle (μrad) Target1.652.420.220.42 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) We are within 10% of the beam size and divergence without FOFB

32 Particle Physics Seminar Tuesday 19 February 2008 Performance of Diamond FOFB Significant improvement up to 100 Hz; higher frequencies performance limited mainly by the correctors power supply bandwidth

33 Particle Physics Seminar Tuesday 19 February 2008 At ID source point HorizontalVertical Long Straight Standard Straight Long Straight Standard Straight Position (μm) Target17.812.31.260.64 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) Target1.652.420.220.42 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)

34 Particle Physics Seminar Tuesday 19 February 2008 Long term drifts (30 minutes) SOFB OFF 3  m maximum drift over 30 minutes H rms < 0.7  m V rms < 0.4  m

35 Particle Physics Seminar Tuesday 19 February 2008 Long term drifts (30 minutes) SOFB ON SOFB running at 0.2 Hz H rms < 0.5  m V rms < 0.3  m

36 Particle Physics Seminar Tuesday 19 February 2008 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

37 Particle Physics Seminar Tuesday 19 February 2008 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)

38 Particle Physics Seminar Tuesday 19 February 2008 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 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

39 Particle Physics Seminar Tuesday 19 February 2008 AP 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 312 ns The bunch length and energy spread, increase with current due to the "microwave instability": CurrentBunch lengthLifetime 0.1 mA10 ps92.3 h 0.5 mA12 ps22.7 h 0.8 mA13 ps15.5 h 4 mA18 ps4.3 h 10 mA25 ps2.4 h

40 Particle Physics Seminar Tuesday 19 February 2008 Generation of short radiation pulses in a storage ring Low – alpha Higher Harmonic Cavities RF voltage modulation Femto – slicing 1) shorten the e- bunch2) 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

41 Particle Physics Seminar Tuesday 19 February 2008 Bunch length (low current)  = 1.7  10 –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  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

42 Particle Physics Seminar Tuesday 19 February 2008 Low 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

43 Particle Physics Seminar Tuesday 19 February 2008 Crab Cavities for optical pulse shortening Courtesy M. Borland (APS)

44 Particle Physics Seminar Tuesday 19 February 2008 A possible implementation of crab cavities at Diamond 4  in V 420  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

45 Particle Physics Seminar Tuesday 19 February 2008 Femto-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) electron bunch femtosecond laser pulse W fs pulse “dark” pulse electron-laser interaction in the modulator (a) femtosecond electron 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

46 Particle Physics Seminar Tuesday 19 February 2008 Energy modulation generated by a short laser pulse Natural energy spread 0.1%

47 Particle Physics Seminar Tuesday 19 February 2008 Separation of the radiation from the two modulated 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; modulator radiator

48 Particle Physics Seminar Tuesday 19 February 2008 Comparison of options for short radiation pulses Crab C.

49 Particle Physics Seminar Tuesday 19 February 2008 AP challenges: high current operation The beam and its electromagnetic field travel inside the vacuum chamber while the image charge travels on the chamber itself. Any variation on the chamber profile, on the chamber material, breaks this configuration. 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 wake fields 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) Negative Charged Beam

50 Particle Physics Seminar Tuesday 19 February 2008 AP 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, …)

51 Particle Physics Seminar Tuesday 19 February 2008 Multi-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

52 Particle Physics Seminar Tuesday 19 February 2008 AP 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

53 Particle Physics Seminar Tuesday 19 February 2008 Current 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

54 Particle Physics Seminar Tuesday 19 February 2008 Transverse Multibunch Feedback at Diamond The TMBF system detects coherent betatron oscillation bunch-by-bunch and damps them with a pair of stripline kickers

55 Particle Physics Seminar Tuesday 19 February 2008 Single Bunch Longitudinal collective effects: the microwave instability Bunch length at zero current 17 ps (with 1.9 MV;   = 1.4  10 –4 ) Z || /n ~ 0.4  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

56 Particle Physics Seminar Tuesday 19 February 2008 Transverse 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 Vertical beam blow-up at diamond

57 Particle Physics Seminar Tuesday 19 February 2008 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, …)

58 Particle Physics Seminar Tuesday 19 February 2008 Conclusions (II) It is generally believed that 3 rd generation light sources will not be replaced by SASE-FEL (4 th generation light sources) but rather they can coexist. 3 rd 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, …)

59 Particle Physics Seminar Tuesday 19 February 2008 Conclusions (III) 3 rd generation light source are still fashionable…


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