Phenomena related to the laser incidence on the cathode surface

Transcription

1 Phenomena related to the laser incidence on the cathode surface Hiromistu Tomizawa Accelerator Division, Japan Synchrotron Radiation Research Institute (SPring-8) These topics have been discussed in LAAA since Feb LAAA (Laser-aided Accelerator Association) is a Vatual Laboratry in Japan. ( Hiromitsu Tomizawa is the initiator and has been the leader of this vatual lab. to discuss about general common topics over photoinjector. )

2 Near by cathode surface Cathode surface issue: what is the important to characterize the cathode? Response time? Cu~ 10fs, CeTe~ <1ps, GaAs (Bulk)~ 100 ps QE distributions? Laser on the cathode: Surface structures on the cathode works as sub-wavelength optics? Several effects on the cathode: polarizations, wavefront distortion, etc Simulation Issue: Most difficult part to make simulations! Image Charge, the methods to generate electron on the cathode (random generation vs. quiet start ), Wake field, etc take into account.

3 Pulse stacker ~ good tool to study sliced electron bunch! ~ S IN : ~3ps λ/ plate S P 4~6 ps P S P S P S P S P 8~1 ps OUT S P S P S P S P 16~4 ps

4 Accuracy of zero positions (Pulse stacker) SSS: Reference (No delay) Find zero positions of PSS,SPS,and SSP Beam position of each micro pulse beam on screen. avarage(1000pulsex5sets) σ(5sets) SSS.8mm ± mm PSS.7mm ± mm SPS.4mm ± mm SSP.14mm ± mm (0.11mm/ps) accuracy less than 0.5 ps is possible!

5 Examples of stacked pulse duration Optical delays for 0 ps pulse length 1st stage Zero + 10 x 0.15 (mm) nd stage Zero + 5 x 0.15 (mm) 3rd stage Zero +.5 x 0.15 (mm) 8 pulses Optical delays for 10 ps pulse length 1st stage Zero + 5 x 0.15 (mm) nd stage Zero +.5 x 0.15 (mm) 3rd stage Zero x 0.15 (mm) 1st stage Zero + 5 x 0.15 (mm) nd stage Zero +.5 x 0.15 (mm) 3rd stage none or 8 pulses 4 pulses

6 Laser pulse Energy [μj] Micro pulses for 0 ps (8 pulses) 1st Pulse Number Electron Charge [nc] Total laser energy = 56 μj Sum of charge of micro pulses = 1.9 nc Charge for all pulses = 0.78 nc Space charge effect

7 Micro 10 ps ( 4 pulses) Total laser energy = 97 μj Sum of charge of micro pulses = 0.78 nc Charge for all pulses = 0.53 nc

8 Simulation of Space Charge Limit 0 ps 8 pulse stack 10 ps 4 pulses stack

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11 Measured beam profile after bending magnet Modulated 1.5 ps Modulated ps

12 Q-scan fitting 1.44 πmmmrad 1.81 πmmmrad 10 ps, 0.38 nc 15 ps, 0.5 nc

13 Q-scan fitting Note that, these data are just examples to show between x- and y- emittance!

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15 Discussions from SP8 to all of you! X emittance 1.4 π mmmrad (@0.4 nc, 10 ps) 1.8 π mmmrad (@0.5 nc, 15 ps).3 π mmmrad (@1.0 nc, 0 ps) Y emittance > X emittance Difficulty to fit some data to quadratic function. Is 0 ps better than 10 ps?

16 Up-to-date topics of high-brightness beam simulations new regime of physical parameters we need computer simulations with better accuracy we must take into account detail physics, which were neglected in previous studies. smaller emittance (ε n < 1mm-mrad) cathode modeling precise calculation of beam dynamics ultrashort pulses (σ t < 100fs) CSR during bunch compression high-average current (I ~ 100mA, CW) ion trapping, formation of beam halo we often see prediction rather than simulation Courtesy of R. Hajima (High-Brightness Gun Meeting in Japan 006)

17 Ready-made or self-made? ready-made simulation codes most of simulation codes used in acc. research are black boxes. these codes have been bench-marked by many people. But, we must be cautious to go beyond the bench-marked regime. we cannot see any physics not implemented there. self-made simulation codes we can extend the code as we learn more about beam dynamics or numerical simulation techniques easy to investigate new physics which become relevant to new machines But, we must check the consistency of the code any time. Courtesy of R. Hajima (High-Brightness Gun Meeting in Japan 006)

18 Artificial noise due to limited number of particles emittance of a 77-pC bunch at 0cm after the anode (space charge included) PASTEL PARMELA ε nx (mm-mrad) ε nx (mm-mrad) error bars : min, max for 10 runs number of particles number of particles the larger numerical noise for the smaller number of particles emittance depends much on the seed of random number, when the number of particles is small. no error bars = deterministic generation of particles almost same results even with Np~100 Courtesy of R. Hajima (High-Brightness Gun Meeting in Japan 006)

19 Particle Generation in PARMELA PARMELA manual (type 9 INPUT = Gaussian distribution) Then the routine generates a series of numbers called the Hammersley s sequence. [ Modified Parmela particle input routine, RLS:05-96, October, 1996.] describes the effect of the Quiet start routine Type 9 INPUT lines. PAC-1993 Random Quiet start Quiet start = Hammersley s sequence Courtesy of R. Hajima (High-Brightness Gun Meeting in Japan 006)

20 Quiet start in FEL simulations FEL is a kind of instability starting from shot-noise. micro-bunching at saturation shot-noise = synchrotron radiation γ d I dωdω e ωρ = 3π c c 1 γ + θ K /3 ( ξ ) + ( 1/ γ ) θ K + θ 1/3 ( ξ ) λ λ λ λ we may overestimate the shot-noise, when we rely on macro-particles. two approaches for quiet start regular interval + tiny fluctuations δ 3n / N n: # of macro particles N: # of electrons δ Hammersley s sequence instead of random numbers γ Ψ C. Penman, B.W.J. McNeil, Opt. Comm. 90 (199) 8 Lecture on Physics of FEL, D. Whittum, Stanford (1996) Courtesy of R. Hajima (High-Brightness Gun Meeting in Japan 006)

21 Low discrepancy sequence (LDS) y 1 x 0 1 N points Monte Calro integration 1 fdv = f i V N ( x ) discrepancy for a -D system D N = sup 0 x, y 1 #( x, y) N xy low-discrepancy sequences Hammersley s sequence Halton s sequence Sobol s sequence Faure s sequence. random LDS Numerical Recipes in C, nd ed. Courtesy of R. Hajima (High-Brightness Gun Meeting in Japan 006)

22 LDS in various applications LDS is used in various applications nowadays. financial analysis; Monte Carlo simulations of trends in bond markets (derivatives) computer graphics; ray tracing Moiré-free LCD panel IBM ThinkPad A30/A30p T. Idé, IBM Tokyo Research Lab Courtesy of R. Hajima (High-Brightness Gun Meeting in Japan 006)

23 PASTEL Simulations with LDS 4 4 ε nx (mm-mrad) random ε nx (mm-mrad) Hammersley in (x,y) regular in z 0.5 error bars : min, max for 10 runs 0.5 error bars : min, max for 10 runs number of particles number of particles (1) generation particles with random Gaussian in (x, y) () random Gaussian in z (1) generation particles in (x, y) with Hammersley s sequence () random shuffling of the particles (3) regular Gaussian distribution in z Courtesy of R. Hajima (High-Brightness Gun Meeting in Japan 006)

24 Summary of physics related to cathode surface Real physics of generation of electrons in the cathode surface Response time of cathode materials should be investigate. Cathode surface structure should be taken into account as a part of optics for laser transport. Some periodical nano-structures on the surface work as a polarizer (sub-wavelengh optics). It could be the cause of polarizationdependence of QE (It is just my personal opinion, do not yet prove these phenomena!). We must pay more attention to improve computer simulations in accuracy and consistency (Especially, for ultra-low emittance against artificial noise). How can we simulate electron bunch just on the cathode surface? Note that, distance between real and image charges is zero! I believe that all specialists should discuss together the topic: Beam behavior near by cathode and physics related to electron bunch generation!