Bunch compression at the SPring-8 linac for successive generation - PowerPoint PPT Presentation

Bunch compression at the SPring-8 linac for successive generation of THz pulse train in the isochronous ring T. Asaka, H. Hanaki, Y. Shoji, H. Dewa, T. Kobayashi, T. Matsubara, A. Mizuno, T. Taniuchi, Y. Hisaoka, S. Suzuki, H. Tomizawa, T. Mitsui, K. Yanagida NewSUBARU, LASTI, 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) beam Bunch length z Radiation power from N electrons in a bunch p ( ω ) : power from an electron tot ( ω ) = p ( ω ) N + ( N 2 − N ) f ( ω ) [ ] 2 P f ( ω ) : form factor Form factor ∫ ∫ ρ ( z ) dz = 1 f ( ω ) = ρ ( z )exp( i ω z / c ) dz ( ρ ( z ) : charge density, )

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 P u l s e d X - r a y t r a i n T H z C S R N o s t o r a g e ! !

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 Linac-NS BT Energy Compression System (ECS) Electron Gun (m) 20 40 0 SPring-8 Linac Booster synchrotron

Simulation of bunch compression Magnetic compression along the Li-NS transport 5 25 (a) normal; ∆ E/E =+0.3% 4 20 Bunch length (ps) Dispersion η (m) 3 (b) our choice; ∆ E/E =+0.5% 15 2 10 (c) too much 1 5 compression; ∆ E/E =+1.2% 0 0 -1 -5 0 20 40 60 80 100 120 Location in Linac-NS transport line (m)

Bunch Length Measurement 0 Time profile at the Normal Operation Data initial turn in the ring Gaussian fit 10 Parameters Time (ps) 20 30 σ = 5.1 ps 40 1 σ = 5.1 ps 50 Intensity (arb. unit) 0 ECS parameters Data Gaussian fit 10 Intensity(arb. unit) Optimized BW = 0.75% Time (ps) 20 σ = 2.2 ps 30 1 σ = 2.2 ps BW < 10ps 40 50 -0.4 -0.2 0 0.2 0.4 50 100 150 200 250 300 Intensity (arb. unit) ∆ E / E (%)

Multi-turn Circulation Energy dependence of path-length ∆ L / L = α 1 δ + α 2 δ 2 + α 3 δ 3 + ... (here ) δ = ∆ E E α n : n -th momentum compaction factor IB BM NewSUBARU storage ring Invert Bend → control α 1 ・ α 1 =1.3 × 10 -3 → ≈ 0 ・ α 2 = 0 (setting accuracy ≈ 10 -3 ) ・ α 3 no control knob ( α 3 ≈ 0.5 ) Modified DBA Modified 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  α 1 = 0 After 100 turns ∆ E/E  α 1 = -1 X10 -5 After 100 turns (%)  α 2 = 0  α 3 = 0.5  α 4 = -20 ∆τ (ps)

Quasi-isochronous ring Turn number 25 50 0 7 Bunch length : σ (ps) α 1 = 1.3 × 10 -3 6 α 1 = 0 3 ps -3 5 α 1 =- 0.02 × 10 injection α 1 =- 0.06 × 10 -3 4 3 2 ps α 1 =- 0.06 × 10 -3 injection 2 0 10 20 30 Time( µ s) Optimum α 1 was larger than the expected 50 turns with σ < 3ps Bunch elongation was faster than the expected

CSR detection Signal waveform Set-up of micro-wave detector (90-140GHz) 0.1 0.075 0.05 Voltage (V) 0.025 0 -0.025 -0.05 0 0.05 0.1 0.15 Time ( µ s) Revolution period=0.4 µ s

Turn by turn CSR power Turn number 75 25 50 0 0.3 0.25 Power (mW) 0.2 0.15 Compressed, 24 pC/bunch Single bunch, 0.1 QI ring Normal 20 pC X 3 bunches 0.05 Operation 0 0 5 10 15 20 25 30 Time from injection ( µ s) Stronger CSR at the injection It lasted longer than the normal condition

Bunch length & CSR power Turn number 25 50 0 7 Bunch length : σ (ps) α 1 = 1.3 × 10 -3 6 α 1 = 0 3 ps -3 5 α 1 =- 0.02 × 10 injection α 1 =- 0.06 × 10 -3 4 3 2 ps α 1 =- 0.06 × 10 -3 injection 2 0 10 20 30 Turn number Time( µ s) 25 50 75 0 0.3 0.25 Power (mW) 24 pC/bunch 0.2 0.15 Compressed, 20 pC X 3 bunches Single bunch, 0.1 QI ring Normal 0.05 Operation 0 0 5 10 15 20 25 30 Time from injection ( µ s)

Reduction of CSR Sensitive to a contribution of form factor f ( ω ) small change of the bunch length -1 ) Wave number (cm Evolution of time profile after injection 1 10 10 Data Gaussian fit 3 cm -1 1 15 20 Time (ps) 25 2 f ( ω ) 0.1 4.1ps 5.3ps 2.9ps 2.3ps 30 2.4ps (1 σ ) Triangle L = 7.6ps 35 140GHz Gaussian σ = 2.9ps 0 10 20 30 40 50 90GHz Time from injection point ( µ s) 0.01 50th 10 100 1st Frequency (GHz)

Summary of Experiment Short Bunch Circulation ・ succeeded to keep 1 σ < 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 gun Acceleration structure (m) 20 40 0 Thermionic Gun

Photo-cathode rf gun Electron Gun Thermionic Photo-Cath Energy Spread + 0.5% + 0.1% Bunch Length 2.2 ps < 1 ps Bunch Charge < 0.1 nC > 1 nC Initial state ∆ E/E ∆ E / E = ± 0.5%; ∆τ = ± 2.2ps (%)  α 1 = -1.5 X10 -6 ; after 500 turns Initial state ∆ E / E = ± 0.1%; ∆τ = ± 1ps  α 1 = -0.5 X10 -6 ; after 500 turns ∆τ (ps)

Comparison with other methods Beam Parameters QI operation Laser Slicing Short Bunch Circulation (BESSY-II) (ALS) (SPring-8 Linac & NewSUBARU) Demonstration Photo-cathode gun bunch length (ps) 1.0 (1 σ ) 0.16 (1 σ ) 3 (1 σ ) <1.0 charge (pC/bunch) ~ 1 ~ 10 24 1000 Pulses per shot quasi-dc 1 ~ 50 >100 ? stable short strong

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)].