Simple Talk on Beam Emittance Related Matter JASRI Accel.Div. - PowerPoint PPT Presentation

Simple Talk on Beam Emittance Related Matter JASRI Accel.Div. Hitoshi TANAKA

Outline 1. Introduction 2. Definition of Beam Emittance 3. Emittance v.s. Property of Light 4. Practical Emittance for Users 5. Control of Beam Emittance (SPring- 8’s Approach) 6. Further Emittance Reduction and Its Limit 7. Ideas for Breakthrough -- What is the Future of SPring-8 ? --

1.Introduction(1) Since the middle of November this year, the low emittance optics has being used for the regular user operation. The natural emittance was reduced from ~6 to 3 nmrad. After this, we heard from the users following positive and negative comments: • Brighter light achieved • Smaller micro-beam obtained • Inconvenient Shorter lifetime • Obvious photon axis jump by the COD correction • Stupid beam profile, twin peaks appeared • ….

1.Introduction(2) It looks timely to discuss on this “beam emittance problem” with the users, because just now they are interested in this and faced to real problems induced by this modification in the routine user operation. Let’s start to talk on this ! Not be too exciting and hot !

2. Definition of Beam Emittance(1) Px= 2Jsin φ One Dimensional J(Action Harmonic Oscillator Variable) 2J (X” + kX = 0 ) φ (Angle d φ/ ds Variable) In conservative system, X emittance ε (invariant) = 2Jcos φ for a single particle is 2J, where J is: Phase space area / 2 π Beam Motion Governed by 1D Harmonic Potential

2. Definition of Beam Emittance(2) To treat multi-particle system of which distribution is pseudo Gaussian, we introduce ensemble-averaged emittance as ε = ( ε = ( 1/Np ) sum(Ji) = sqrt( <X**2><X’**2> - <XX’ > **2 ) At elected position s, a transverse beam size and an angular divergence are given by σ x=sqrt( βε βε ), σ x’=sqrt( ε/β ε/β ).

2. Definition of Beam Emittance(3) Real beam has 3D-structue, which means we need 3 emittances, ε x, ε y and ε z to describe beam profile. ε x, ε y --> photon beam transverse profile Y ε z --> photon beam temporal structure In light sources, 3D emittances are generated X by random fluctuation of photon emission process. Z Electron Beam Profile

3. Emittance v.s.Property of Light (1) Axiom: Ideal light source is a point source with zero 3D emittances. • Perfect spatial coherence • Perfect temporal coherence Infinite emittances degrade the above properties of light.

3. Emittance v.s.Property of Light (2) Undulator radiation case: S N S N S N S N S e- Observation Point e- e- N S N S N S N N S • Peak reduction • Spectrum broadening Frequency Hz

3. Emittance v.s.Property of Light (3) Required emittance • ε x, ε y << photon beam emittance • ε z =< pulse width Presently, • ε x>>, ε y ~ photon beam emittance • ε z --> ~40psec

4. Practical Emittance for Users(1) In beam dynamics, r.m.s. emittance is determined by the distribution of betatron oscillation amplitudes. However, for users the above emittance is not adequate.

4. Practical Emittance for Users(2) Px Fluctuations faster than detector measuring X period can increase “effective emittance” for user experiments. Probability Practical emittance ~ Emittance+Orbit stability X

4. Practical Emittance for Users(3) Fluctuations sources: • Mag.PS current ripple • Vacuum chamber vibration • Mag. & girder vibration • ID error fields • Periodic COD correction • Naughty users’ actions in experimental hall • …..

4. Practical Emittance for Users(4) Beam orbit stabilization is also crucial in this sense 10 Horizontal PDS [micron_rms/sqrt(Hz)] 2000/9/16 1 2002/11/27 after chamber vib. suppr. II 0.1 0 20 40 60 80 100 Frequency [Hz]

5. Control of Beam Emittance 5.1 Emittance given by the balance between radiation excitation and damping processes (a) Excitation (b) Damping E E Px > Px E-dE

5.2 Horizontal emittance reduction by breaking the achromat (1) • Key: smaller perturbation and good matching to suppress radiation excitation • Achromat has larger perturbation vector (1) Achromat (2) Non-achromat > Max. ( η x1, η 'x1) Max. ( η x2, η 'x2 )

5.2 Horizontal emittance reduction by breaking the achromat (2) • Achromat makes matching difficult • By breaking the achromat, better matching is obtainable (2) Excellent Matching Case (1) Poor Matching Case X' X' ( ) η x , η ' x ( ) η x , η ' x X X

5.2 Horizontal emittance reduction by breaking the achromat (3) 6.6 --> 3.4nmrad @Calc & GFO Phase-II Optics Low Emittance Version 3.4 nm.rad 60 1.4 β y 50 1.2 Normal CG Cells Normal CG Cells Dispersion Function η Matching Section Betatron Function [m] 40 1 β x 0.8 30 0.6 20 0.4 η x 10 0.2 x [m] LSS 0 0 -0.2 -10 0 50 100 150 Path Length from Injection Point s [m]

5.2 Horizontal emittance reduction by breaking the achromat (4) “Drawback by finite dispersion” Equilibrium Orbit shift 200 µ m Due to δ within ± 1 σ Amplitude of betatron ~600 µ m oscillation within ± 1 σ -1 σβ x +1 σβ x -1 σ dp/p +1 σ dp/p Not serious impact to sizes How serious to distribution? Dispersion of 10cm@Source point

5.2 Horizontal emittance reduction by breaking the achromat (5) “How is the ID radiation effect, excitation or damping” 6.6 10 -9 1.05 η x =0cm Red: 6.4 10 -9 η x =10cm 1 Blue: 6.2 10 -9 ε x [nmrad] 0.95 6 10 -9 ε x / ε x0 5.8 10 -9 0.9 5.6 10 -9 0.85 5.4 10 -9 5.2 10 -9 0.8 0 5 10 15 20 25 ID Number Installed ID extra radiation still works as a damping force @10cm Disp in STs

5.2 Horizontal emittance reduction by breaking the achromat (6) “Measured emittance” Ex/Ex0_calc 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 7 2.5 Measured Horizontal Emittance (nm rad) ID Full Open against Ex ID Full Close Measured BL29_Flux_ratio**2 Against Ex/Ex0 6 B**2~1/Ex 2 5 4 1.5 3 1 2 2 3 4 5 6 7 2 3 4 5 6 7 Ex_calc[nmrad] Calculated Horizontal Emittance (nm rad)

6. Further Emittance Reduction and Its Limit 6.1 Lower limit of emittance(1) 1 +4 J x C q γ 2 ε x: ~1nmrad by J E ε x _ min = 12 15 J x ε y: ~0.1pmrad by 1 γ ( α 0 2 + ( T 0 α s ) 2 σ δ σ z: ~0.3 µ m by σ z = ) Ω s

6.1 Lower limit of emittance(2) Emittance Achieved Gain ε x 3nmrad factor 3 ε y a few pmrad factor 10 σ z(FWHM) 1cm factor 3~10

6.2 Further emittance reduction(1) Hori. Emitt. reduction: • Damping Partition Control (studied by T. Nakamura) • Optics optimization

6.2 Further emittance reduction(2) Field Gradient Out Out E In In E-dE Radiation_in > Radiation_out Field Gradient Field gradient is gen - Out erated by off-energy E1 + dispersion + QMs In E2 “Concept of Damping E1(Out) > E2(In) Partition Control” Energy spread enhanced

6.2 Further emittance reduction(3) Vert. Emitt. reduction: • Resonance suppression improvement Bunch Length shortening: • Momentum compaction control • Bunch slicing with crab cavities

6.2 Further emittance reduction(4) S N S N S N S N S N S N S N S N S N Crab Cavity-1 Crab Cavity-2 Crab Cavity-4 Crab Cavity-3 “Concept of Bunch Slicing@SPring-8” proposed by N. Kumagai

7. Ideas for Breakthrough(1) -- What is the Future of SPring-8 ? -- • Suppression of stochastic fluctuation in photo-emission process • Introduction of strong transverse momentum loss without strong radiation excitation

7. Ideas for Breakthrough(2) • ERL based storage ring having full transverse spatial coherency and short bunch It can switch two operation modes, classical SR and ERL operation modes.