Advanced laser technology for 3D-shaping ~ toward to the highest brightness of electron beam source ~

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1 Advanced laser technology for 3D-shaping ~ toward to the highest brightness of electron beam source ~ Hiromistu Tomizawa Accelerator Division, Japan Synchrotron Radiation Research Institute (SPring-8) 1. Motivation for 3D-laser pulse shaping 2. Strategy of 3D-laser pulse shaping 3. Optimization system of 3D-Laser pulse - Automation with DM + Genetic Algorithms - UV-Pulse Stacker 4. Emittance measurements 5. Summary and future plan (Appendix)

2 1. Motivation for 3D-laser pulse shaping 1-1. Ideal 3D-laser profile: Cylindrical or ellipsoidal? σ = σ 2 + σ 2 + SC RF σ 2 Th Space charge effect: Nonlinear term should be suppressed. 1. Cylindrical If suppress non-linear term of space charge effect, the aspect ratio of the Laser Profile is important! 2. Ellipsoidal If the slice density is kept constant during acceleration, non-linear term will be suppressed!

3 2. Strategy of 3D-laser pulse shaping D-Laser pulse System 3D-Laser shaper: 1. Combination of Spatial shaper (2D) + temporal shaper(1d) 1-a. Fixed shaping systems: MLA, pulse stacker 1-b. Adaptive shaping systems: DM, SLM It should be no influence between both shaping technique! 2. Directory 3D shaping 2-a. Fixed shaping systems: Fiber bundle, DOE 2-b. Adaptive shaping systems: 2D-SLM MLA : Micro Lens Array DOE : Diffractive Optical Elements SLM : Spatial Light Modulator DM : Deformable Mirror

4 2. Strategy of 3D-laser pulse shaping 2-2. Ti:Sa Laser System Configuration ~ fs- TW- Ti:Sa Laser System with 3D-pulse shaper ~

5 2. Strategy of 3D-laser pulse shaping D- Laser Beam Shaping system ~ present status at SPring-8 ~ UV- Laser source (total stability!) Spatial Profile: Laser Pulse Energy : Pointing Stability & Reproducible Timing Jitter < 1 ps Temporal Profile: Pulse duration: 2.5 ~ 20 ps UV- Pulse Stacker Pulse duration: 2.5 ps Pulse duration: 10 ps Diameter:1 mm Distribution: Flattop Deformable Mirror Gaussian Pulse Stacker 10 pps Deformable Mirror Flattop Streak Image of stacked pulses

6 2. Strategy of 3D-laser pulse shaping 2-4. Automatic Laser Beam Quality control system ~ Arbitrary Laser Pulse Shaping ~ A) Computer-aided SLM (Spatial Light Modulator) Rectangular Pulse shaping (Arbitrary Shape) B) Computer-aided DM (Deformable mirror) Flattop spatial profile (Arbitrary Shape) SLM Automatic Control Optics Spatial shaping (DM) Pulse shaping (SLM) Wave front Control (DM) DM ))) 2 ~ 12 ps Fundamental 2 ~ 5 ps THG (263 nm)

7 2. Strategy of 3D-laser pulse shaping 2-3. Directly 3D-shaping System Fiber Bundle with computer-aided Deformable mirror PC Profile Data PC for control Deformable mirror and Evaluate resulting Laser Profile Electron Beam Profile OR Video Signal Control DM Fiber Bundle (0.5~1m) Lens CCD sensor (LBA-PC) Laser Light source (THG: 263nm) Deformable mirror

8 2. Strategy of 3D-laser pulse shaping 2-4. Automatic Laser Beam Quality control system ~ Adaptive Diffraction lens in stretched chirped pulse for 3D-shaping ~ 2D-SLM

3. Optimization system of 3D-Laser pulse 3-1.

9 3. Optimization system of 3D-Laser pulse D- Laser Beam Shaping system ~ present status at SPring-8 ~ UV- Laser source (total stability!) Spatial Profile: Laser Pulse Energy : Pointing Stability & Reproducible Timing Jitter < 1 ps Temporal Profile: Pulse duration: 2.5 ~ 20 ps UV- Pulse Stacker Pulse duration: 2.5 ps Pulse duration: 10 ps Diameter:1 mm Distribution: Flattop Deformable Mirror Gaussian Deformable Mirror Pulse Stacker 10 pps Deformable Mirror Flattop Streak Image of stacked pulses Pulse Stacker (3 stages)

! Demerit: too many Possibility: 256 59 Al-coated SiN-Membrane

week) Hexagonal elements (59 channels) DM Note that: Membrane is very

10 3-2. Spatial profile shaping with DM Deformable Mirror Merit: adjustable and actively controllable!! Demerit: too many Possibility: Al-coated SiN-Membrane Necessity of special algorithm to optimize (R > 70% in UV after 1 week) Hexagonal elements (59 channels) DM Note that: Membrane is very delicate!! We build dry N2-Housing for DM. ~ Deformation Steps: 256 ( 0 ~ 255 V ) ~ Genetic + Neuron model Algorithm

11 3-2. Spatial profile shaping with DM Deformable Mirror Actuator (ex. 37ch) Voltage: 0 ~ 255 V Actuator: Initial State (All: 0V) All: 125V All: 255V (Max. Voltage) Random Voltage

12 Spatial profile shaping with DM Automation of optimization Genetic Algorithm (GA) ~ Idea of Evolution ~ Example: Search the Maximum Value!! Note that, many local maximum points!!! Searching points near by max. survive. Searching point corresponds to individual life. Iterations The survivors make new generation (Searching point).

13 Fitting Function to evaluate Flattop profiles Fitting Function to evaluate Flattop profiles Laser profile during optimization (Laser beam profiler :LBA-PC) 2 Effective Diameter & 4 Set circle(aperture Fraction) Fitting function: weight (a, b, c, d, e, f, g, h, i) f( profiles ) = a1 + b2 + c3 +d4 + e5 + f6 + g7 + h8 + i9 9 Beam Diameter 8 Beam Center 1.Top Hat Factor: Maximize the Top Hat Factor (0 ~ 1) 2.Effective Diameter: Minimize the difference between the diameters of set circle and measured 3.Flatness (Std Dev/mean): Minimize the standard deviation divided by the average in a flattop area 4.Aperture Fraction: Maximize the integrated energy within the set circle area 5.Peak-to-peak: Minimize the difference between the max. and min. in a flattop area 6.Hot Spot(max.): Minimize the max. in a flattop area 3 Flatness (Std Dev/mean) 6 Hot spot 5 Peak to Peak 7 Dark spot 1 THF Intensity distribution (cross section) 7.Dark Spot(min.): Maximize the min. in a flattop area 8.Beam Center: Minimize the difference from the initial center position (x, y) 9.Beam Diameter: Minimize the difference from the set diameter

14 Weight of each term of fitting function for Flattop ~ decided by comparing convergence status ~ Term Meaning Absolute convergence value with 500step System Weight 1 Top Hat Factor Maximize the Top Hat Factor (0-1) (Flattop: THF = 1.0) Effective Diameter Minimize the difference from the diameter of set circle Flatness (SD/mean) Minimize the standard deviation divided by the average in a flattop area Aperture Fraction Maximize the integrated energy within the set circle area Peak-to-peak Minimize the difference between the max. and min in a flattop area 60 1 (norm) 6 Hot Spot (max.) Minimize the max. in a flattop area (60) same as Peakto-peak 1 7 Dark Spot (min.) Maximize the min. in a flattop area (60) same as Peakto-peak 1 8 Beam Center Minimize the difference from the initial center position (x, y) Beam Diameter Minimize the difference from the set diameter

15 To the photocathode (RF electron gun) Beam splitter Main laser beam to illuminate cathode Monitor laser beam for feedback CCD Laser Beam profiler (LBA-PC CCD sensor) 3D Video signal with beam parameter 2D PC (LBA) Power PCI ISA Control Laser Beam ( 263nm) resulting profile 105mm 22mm Supply Box PC for controlling DM and evaluating resulting laser profile DM Control Signal concave f = -300mm concave f = -100mm convex f = 51.5mm Deformable mirror (DM) Laser beam Video signal with beam parameter DM Control Signal Closed Loop Line From pulse stacker

16 Spatial profile shaping with DM Results of the combination DM+GA This shaping with computer-aided DM was Flattop shaping OK! Computer-aided DM for UV (THG) No problem for FHG (197 nm) Auto-Shaping (1000 steps)

17 Temporal profile shaping (Pulse stacking) UV-Pulse Stacker S 2~3 ps λ/2 plate S P P S P S P S P S P Entrance window should be double AR-coated! Not utilize Brewster Window! The polarization of the input UV-laser is rotated 45 degrees by the half lambda waveplate. UV-laser is split into two equal portions by the each cubic polarizer. But, consider QE deference between S and P!! S P S P S P S P 16~24 ps

18 3. Optimization system of temporal profile Chirped pulse with deferent compressor length Changing compressor length, 2.5-ps original pulse is generated! Not that, laser pulse will be positively chirped & stretched through the silica material! S P S P To avoid interferences on the plateau of stacked macro pulse, S- and P- polarized pulses are alternatively positioned! S P S P Positive 20 ps Negative

19 Time chart of pulse stacking: 3 stages for generation of 20 ps square pulse 1/2 waveplate +45 time 10 ps 1 st Stage: S P 1/2 waveplate P S 2 nd Stage: 1/2 waveplate 3 rd Stage: 5 ps S P ps 2.5 ps 2.5 ps 2.5 ps S P S P S S 5 ps P If we mask the P-pulse at each stage; Easily shift to 10 ps, P, 5 ps! S P P

20 3. Optimization system of temporal profile Developed UV-Pulse Stacker

21 Two 6.5-ps stacked Pulses (14 ps between them) Shifting optical delay 1 All stacked together (15.5 ps FWHM)

22 Usage Photocathode with energy analyzer as a streak camera Temporal shift between S & P Spatial misalignment on the photocathode S P P S

23 Laser Optical Transport & Monitors AR- Entrance Window Bending Magnet Electron Energy Analyzer Laser Powermeter Electron Beam Profiler Variable Down Collimator Laser Profiler UV-Laser after 3D-shaped with DM &Pulse Stacker

24 ps between P-pol & S-pol ( in Measurements) P wave (MeV) S wave (MeV) Measured time deference between two beamlets:15ps Energy (MeV) RF phase (degree) Shifted optical delay with a length of 4.5 mm Energy (MeV) S-Pol was shifted to P-Pol! (In caliculation, shifted with phase of +15 degrees!) P wave (MeV) S wave (MeV) RF phase (degree)

25 Usage Photocathode with energy analyzer as a streak camera Original pulse is too short! Beamlets are not equivalently optimized! Pulse duration and intervals are optimized! Stacked Pulse duration: 20 ps ( Original pulse duration: 2.5 ps )

26 4. Emittance measurements D- Laser Beam Shape for experiment ~ present status at SPring-8 ~ Flattop : φ0.8 mm Square pulse:20 ps Q-scan emittance measurement Laser normal incidence

27 4. Emittance measurements 4-2. low emittance electron beam generation ~ we can provide low emittance beam for a month~ Accelerator system:26 MeV Result of emittance measurement: 2.0πmm nc Pulse duration:20 ps We are preparing experiment with further fine optimization of 3D-laser pulse for 2 month long.

28 4. Emittance measurements 4-3. low emittance electron beam generation ~ we are testing with different 3D-parameter ~ Result of emittance measurement: 2.0πmm nc Pulse duration:20 ps

5. Summary and future plan ~ stable & reliable beam ~ A.

: 24 hours, 1.5 months, non-stop THG: 1.4% rms stability for 1-day mesurments B.

(Gaussian or Flattop) ~ Arbitrary Laser Shaping ~ ~ However, it takes 1 hour to optimize.

29 5. Summary and future plan ~ stable & reliable beam ~ A. We realized stable laser system Oscillator : 24 hours, 4.5 months, non-stop TW- Amp. : 24 hours, 1.5 months, non-stop THG: 1.4% rms stability for 1-day mesurments B. Automatically shaping Spatial Profile with DM + GA was successful! (Gaussian or Flattop) ~ Arbitrary Laser Shaping ~ ~ However, it takes 1 hour to optimize. C. Square pulse generation with UV-pulse stacker was successful at THG (263 nm)! Square Pulse: ~2-20 ps; Rising-time: ~ 700 fs D. Emittance:2.0πmmmm nc

30 A. Next shaping optics? ~ Holographic optics can 3D-shape in UV!? ~ DOE & HOE can be the goal of Laser shaping. However, we have to fix the structure of optics through steadying optimal pulse shape with adaptive optics!! Chromatic aberration is stronger than conventional optics. But, this characteristics can be used for temporal pulse shaping!?

31 How to get more efficiency?

32 A. Both profiles shaping with Fiber Bundle ~ FB can 3D-shape the UV-pulse & make easy to transport ~ Cable Strand Microlens Array Silica Fiber Bundle

33 A. Both profiles shaping with Fiber Bundle ~ Transparent Cathode with Fiber Bundle ~ Pulse Stacking with 2,000 different Optical Passes Transparent Cathode for backward illumination UV-Laser (266nm) Fiber Bundle: Length: 2.0 m Bundle size:12 mm No. of Fibers:1967

34 A. Both profiles shaping with Fiber Bundle 1. Results of spatial profiles with shaping Spatially homogenizing is very strong with FB Any kind of bad profile can be corrected! Pulse shaping & stretching with FB is pulse-stacking Depend on the length and mapping of FB Fiber-Shaping (1m long)

result due to mainly Pulse Staking effect ~ Width (FWHM): 16

35 A. Both profiles shaping with Fiber Bundle 2. Results of temporal profiles with shaping ~ Pulse shaping result due to mainly Pulse Staking effect ~ Width (FWHM): 16 ps Fiber Bindle Length: 1 m Mapping: Random Input UV-pulse energy: down to 60 nj

36 Closed Control System for Fiber Bundle with computer-aided Deformable mirror PC Profile Data PC for control Deformable mirror and Evaluate resulting Laser Profile Electron Beam Profile OR Video Signal Control DM Fiber Bundle (0.5~1m) Lens CCD sensor (LBA-PC) Laser Light source (THG: 263nm) Deformable mirror

37 A. Summary of Fiber Bundle Shaping Shaping with computer-aided deformable mirror could generate Flattop. It is very flexible to optimize the spatial profile (electron bunch) with genetic algorithm. Fiber Bundle is ideal as a 3D-shaper It is very simple to shape : You have to optimize the length of the Bundle for aimed pulse duration: 15 ps ~ 1-m long 3D-laser profile: It can generate ellipsoidal from any profile. Short working distance: It needs to develop back illumination. Laser fluence limit: Laser 100 fs <1.5 mj/cm2 It is possible to use as 3D-shaper down to 60 nj/pulse. Transparent cathode for shaping complex system with fixed fiber bundle & adjustable deformable mirror might have a lot of possibilities with fine tuning.

3D- Laser pulse shaping System to Minimize Emittance for Photocathode RF gun ~ toward to the highest brightness of electron beam source ~

Adaptive 3D- Laser pulse shaping System to Minimize Emittance for Photocathode RF gun ~ toward to the highest brightness of electron beam source ~ Hiromistu Tomizawa Accelerator Division, Japan Synchrotron