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.

Not be too exciting and hot ! Let’s start to talk on this !

• 2. Definition of Beam

Emittance(1)

One Dimensional Harmonic Oscillator (X” + kX = 0 ) In conservative system, emittance ε (invariant) for a single particle is 2J, where J is: Phase space area / 2π

Beam Motion Governed by 1D Harmonic Potential

Px= 2Jsinφ X = 2Jcosφ J(Action Variable) φ(Angle Variable) dφ/ds 2J

• 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 σx=sqrt(βε βε), σx’=sqrt(ε/β ε/β).

• 2. Definition of Beam

Emittance(3)

Real beam has 3D-structue, which means we need 3 emittances, εx, εy and εz to describe beam profile.

X Y Z

εx, εy --> photon beam transverse profile εz --> photon beam temporal structure

Electron Beam Profile

In light sources, 3D emittances are generated by random fluctuation of photon emission process.

• 3. Emittance v.s.Property
• f 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
• f Light (2)

Undulator radiation case:

• Peak reduction
• Spectrum broadening

Frequency Hz

S S S S S N N N N S S S S N N N N N

e- e- e- Observation Point

• 3. Emittance v.s.Property
• f Light (3)
• εx, εy << photon beam emittance
• εz =< pulse width

Required emittance Presently,

• εx>>, εy ~ photon beam emittance
• εz --> ~40psec

• 4. Practical Emittance for Users(1)

In beam dynamics, r.m.s. emittance is determined by the distribution of betatron

• scillation amplitudes.

However, for users the above emittance is not adequate.

• 4. Practical Emittance for Users(2)

Fluctuations faster than detector measuring period can increase “effective emittance” for user experiments.

Px X X Probability

Practical emittance ~ Emittance+Orbit stability

• 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

0.1 1 10 20 40 60 80 100 PDS [micron_rms/sqrt(Hz)] Frequency [Hz]

Horizontal

2000/9/16 2002/11/27 after chamber

• vib. suppr. II

• 5. Control of Beam Emittance

5.1 Emittance given by the balance between radiation excitation and damping processes

Px > Px (b) Damping (a) Excitation E E-dE E

5.2 Horizontal emittance reduction by breaking the achromat (1)

• Key: smaller perturbation and good matching

to suppress radiation excitation

Max. ( ηx2, η'x2)

>

(1) Achromat (2) Non-achromat ( ηx1, η'x1) Max.

• Achromat has larger perturbation vector

5.2 Horizontal emittance reduction by breaking the achromat (2)

• Achromat makes matching difficult
• By breaking the achromat, better

matching is obtainable X' X' (1) Poor Matching Case (2) Excellent Matching Case ( ηx , η'x ) ( ηx , η'x ) X X

5.2 Horizontal emittance reduction by breaking the achromat (3)

6.6 --> 3.4nmrad @Calc & GFO

• 10

10 20 30 40 50 60

• 0.2

0.2 0.4 0.6 0.8 1 1.2 1.4 50 100 150 Betatron Function [m] Dispersion Function η

x [m]

Path Length from Injection Point s [m] LSS Matching Section Normal CG Cells Normal CG Cells

βx βy ηx

Phase-II Optics Low Emittance Version 3.4 nm.rad

5.2 Horizontal emittance reduction by breaking the achromat (4)

• 1σdp/p

+1σdp/p 200µm

• 1σβx

+1σβx ~600µm Equilibrium Orbit shift Due to δ within±1σ

Dispersion of 10cm@Source point

Amplitude of betatron

• scillation within±1σ

“Drawback by finite dispersion”

Not serious impact to sizes How serious to distribution?

5.2 Horizontal emittance reduction by breaking the achromat (5)

ID extra radiation still works as a damping force @10cm Disp in STs

“How is the ID radiation effect, excitation or damping”

0.8 0.85 0.9 0.95 1 1.05 5.2 10-9 5.4 10-9 5.6 10-9 5.8 10-9 6 10-9 6.2 10-9 6.4 10-9 6.6 10-9 5 10 15 20 25 εx/εx0 εx [nmrad] ID Number Installed ηx=0cm ηx=10cm Blue: Red:

5.2 Horizontal emittance reduction by breaking the achromat (6)

“Measured emittance”

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

• 6. Further Emittance Reduction and

Its Limit 6.1 Lower limit of emittance(1)

εx: ~1nmrad by εy: ~0.1pmrad by

εx_min = Cqγ2 1 +4Jx JE 12 15Jx 1 γ

σz: ~0.3µm by

σz = (α0 Ωs )

2 + (T0αs)2 σδ

εx 3nmrad factor 3

6.1 Lower limit of emittance(2)

Emittance Achieved Gain εy a few pmrad factor 10 σz(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)

Energy spread enhanced E E-dE Radiation_in > Radiation_out Out In In Out Field Gradient Field gradient is gen - erated by off-energy + dispersion + QMs Out Field Gradient In E1 E2 E1(Out) > E2(In)

“Concept of Damping Partition Control”

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 S S S N N N N N S S S S S N N N N

Crab Cavity-1 Crab Cavity-3 Crab Cavity-2 Crab Cavity-4

proposed by N. Kumagai “Concept of Bunch Slicing@SPring-8”

• 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.