Bunch compression at theSPring-8 linac for successive
generation of THz pulse train
in the isochronous ring
Y. Shoji,
T. Matsubara,
Y. Hisaoka,T. Mitsui,
NewSUBARU, LASTI,

T. Asaka, H. Hanaki,
H. Dewa, T. Kobayashi,
A. Mizuno, T. Taniuchi,
S. Suzuki, H. Tomizawa,
K. Yanagida

University of Hyogo

SPring-8, JASRI

Contents
・ Introduction to CSR・ Basic Idea
・ Demonstration Experiment・ Upgrading plan

Introduction
Applications of Short Electron Bunch
・Short Pulsed X-Ray
For time resolving experiments→ Sub-ps (femto-second) pulse→ Intense ps pulse is still valuable

・Coheremt synchrotron radiation (CSR)→ extremely strong THz radiation→ stable radiation

Coherent Synchrotron Radiation (CSR)
beamBunch lengthzRadiation power from N electrons in a bunch[Ptot (ω ) = p(ω ) N + ( N − N ) f (ω )22]

p(ω) : power from an electronf(ω) : form factor

Form factor

f (ω ) = ∫ ρ (z )exp(iωz / c)dz

( ρ(z) : charge density, ∫ ρ(z)dz) = 1

The Concept
1. Make short and intense bunch in a linac
2. Let the bunch circulate in an isochronous ring
3. Use short pulsed X-ray train or THz CSR

Pulsed X‑ray train
THz CSRNo storage !!

Merits(i) Short and intense pulse is obtained at ring BL.
--> Light for many BLs at the same time(ii) Short pulse train with fixed period--> It helps to confirm the synchronization.
(iii) Existing acceleratiors are enough for a few ps pulse.
--> No special expense is required

Demonstration Experiment
1. ECS cavity --> energy gradient
2. Linac-NS BT--> magnetic compression
3. Streak camera --> measure bunch length
NewSUBARU
Energy Compression System (ECS)

Linac-NS BTElectron Gun0SPring-8 Linac

Booster synchrotron

20(m)
40

Simulation of bunch compression
Magnetic compression along the Li-NS transport
255(a) normal; ∆E/E=+0.3%(b) our choice; ∆E/E=+0.5%32015102(c) too muchcompression; ∆E/E=+1.2%

1500-1-5020

40

6080

100120Location in Linac-NS transport line (m)Bunch length (ps)Dispersion η(m)4

Bunch Length Measurement
0Time (ps)Normal Operation
10ParametersData
Gaussian fitTime profile at theinitial turn in the ring2030

σ = 5.1 ps

40

1σ = 5.1 ps

50

Intensity (arb. unit)

Time (ps)ECS parameters
10Optimized

Data
Gaussian fit

2030

σ = 2.2 ps

40

BW < 10ps50

1σ = 2.2 ps

50

100

150

200

Intensity (arb. unit)

250

300

Intensity(arb. unit)

0-0.4

BW = 0.75%

-0.2

00.2

∆E/E (%)

0.4

Multi-turn Circulation
Energy dependence of path-length∆L /L = α1δ + α 2δ 2 + α 3δ 3 + ...

(here δ = ∆E) Eαn : n-th momentum compaction factor
NewSUBARU storage ring
Invert Bend →control α1・α1 =1.3×10-3 → ≈ 0・α2 = 0 (setting accuracy ≈ 10-3)・α3 no control knob (α3 ≈ 0.5 )BMIB

ModifiedModifiedDBA
DBA

Tracking simulation in the non-linear rf bucket
Initial state; just after injection∆E/E = ±0.5% ∆τ = ± 4ps

∆E/E

(%)

∆τ (ps)Tracking simulation in the non-linear rf bucket
Initial state; just after injection∆E/E = ±0.5% ∆τ = ± 4ps
α1 = 0 After 100 turns∆E/Eα1 = -1 X10-5 After 100 turns(%)

α2 = 0α3 = 0.5
α4 = -20
∆τ (ps)Bunch length : σ (ps)Quasi-isochronous ring702550α 1=1.3×10
α 1=06

Turn number-33 ps
injection-35

α 1=-0.02×104

α 1=-0.06×10-3320

α 1=-0.06×10-32030

102 psinjectionTime(µs)

Optimum α1 was larger than the expected
Bunch elongation was faster than the expected

50 turns with σ < 3ps

CSR detection
Signal waveform

Set-up of micro-wave detector (90-140GHz)

0.1

Voltage (V)

0.075
0.05
0.025
0-0.025-0.0500.05

0.10.15

Time (µs)

Revolution period=0.4µs

Turn by turn CSR power
0.325075 Turn number

50Power (mW)

0.25
0.2
0.15

Compressed,Single bunch,
QI ring

0.1

NormalOperation

0.05
0051024 pC/bunch
20 pC X 3 bunches

15202530 Time from injection (µs)

Stronger CSR at the injection
It lasted longer than the normal condition

Bunch length : σ (ps)Bunch length & CSR power
725050Turn number-3

α 1=1.3×10
α 1=063 psinjection-35

α 1=-0.02×10

4

α 1=-0.06×10-33-320.3

α 1=-0.06×1001020025 Time(µs) 502 ps
injection

3075 Turn number

Power (mW)

0.25

24 pC/bunch

0.2
0.15

Compressed,Single bunch,
QI ring

0.1

NormalOperation

0.05
0051015202520 pC X 3 bunches

30 Time from injection (µs)

Reduction of CSR
Sensitive to asmall change of the bunch length

contribution of form factor f(ω)-1Wave number (cm )Evolution of time profile after injection
101

Data
Gaussian fit

13 cm-1

2022530

2.4ps
(1σ)

2.3ps

2.9ps

4.1ps

5.3psf (ω)

Time (ps)15100.1

Triangle L = 7.6ps
140GHzGaussian σ = 2.9ps
90GHz

3501st10203040
Time from injection point ( µs)

50th50

0.01

10100

Frequency (GHz)Summary of Experiment

Short Bunch Circulation・succeeded to keep 1σ < 3ps for 50 turns
・larger CSR lasted longer

Of course, still there are many problems …

Future Improvement

Ring (magnet system)
・improve stability; temperature control; magnetic field; better tuning= improvements for the daily operation
Linac (electron gun)・thermionic gun --> photo-cathode RF gunPhoto-cathode RF gun
RF gun R&D room
Rf gunAcceleration
structure0Thermionic Gun

20(m)
40

Photo-cathode rf gun
Electron Gun
Energy Spread
Bunch Length
Bunch Charge

Thermionic+ 0.5%
2.2 ps< 0.1 nC

Photo-Cath+ 0.1%
< 1 ps> 1 nC

Initial state∆E/E = ±0.5%; ∆τ = ± 2.2ps
α1 = -1.5 X10-6; after 500 turns∆E/E(%)

Initial state∆E/E = ±0.1%; ∆τ = ± 1ps
α1 = -0.5 X10-6; after 500 turns∆τ (ps)Comparison with other methods
Short Bunch Circulation(SPring-8 Linac & NewSUBARU)
Demonstration
Photo-cathodegun<1.03 (1σ)

Beam Parameters

QI operation(BESSY-II)

Laser Slicing(ALS)

bunch length (ps)1.0 (1σ)0.16 (1σ)

charge (pC/bunch)~1~ 10241000

Pulses per shot

quasi-dc

1~ 50>100 ?

stableshortstrong

Beam physics study
1. Stable operation of quasi-isochronous ring
2. Circulation of an extreme beam
Similarity with ERL’s arc
Problem would be enhanced with multi-turn circulation
Ring parameter can be tuned using stored beam

3. Future project at Tohoku Univ.
Circulation of sub-mm pulse[ H. Hama, 27th International FEL Conference (2005)].