First Observation of SelfModulation Instability Seeding at ATF
First Observation of Self-Modulation Instability Seeding at ATF Yun Fang University of Southern California, Los Angeles, 90089 Patric Muggli Max Planck Institute for Physics, Munich, Germany Warren Mori University of California, Los Angeles, 90095 V. Yakimenko, M. Fedurin, K. Kusche, M. Babzien, C. Swinson, R. Malone Brookhaven National Laboratory, Upton, NY 11973 Work supported by US Dept. of Energy
OUTLINE q. Introduction of Self-Modulation Instability (SMI) q. Investigation of the beam charge through simulation to study energy modulation with the available electron bunch at ATF q. Experimental observation of periodic energy modulation in various plasma densities q. Seeding of SMI through energy modulation q. Study of instability seeding effect involved in the experiment
Motivation of Studying SMI q Proton Bunches produced at LHC will have up to 7 Te. V/particle, 100 k. J/bunch, much higher than the current lepton bunches (60 J/bunch, 100 Ge. V/particle) can be used as the drive bunch in PWFA ? q A. Caldwell proposed the idea of of proton-driven PWFA and demonstrated the possibility of producing a Te. V electron bunch in a single acceleration stage using a short (~100 um) proton bunch driver. (A. Caldwell et al. , Nature Physics 5, 363 (2009); B. E. Blue’s thesis (2003)) q However, such short proton bunches are not available (~12 cm). q Kumar et al. suggested that self-modulation could radially modulate a long bunch into small beamlets on the scale of λpe, resulting in the resonant excitation of large amplitude accelerating wakefields. q SMI is interesting beam-plasma interaction physics q Take advantage of electron bunches and experimental infrastructure available at SLAC and BNL-ATF to study the physics of SMI.
OSIRIS 2. 0 osiris framework · · Massivelly Parallel, Fully Relativistic Particle-in-Cell (PIC) Code New Hybrid algorithm Visualization and Data Analysis Infrastructure Developed by the osiris. consortium ⇒ UCLA + IST New Features in v 2. 0 · · · · Ricardo Fonseca: ricardo. fonseca@ist. utl. pt Frank Tsung: tsung@physics. ucla. edu · http: //cfp. ist. utl. pt/golp/epp/ http: //plasmasim. physics. ucla. edu/ · · High-order splines Binary Collision Module Hybrid code Boosted frame PML absorbing BC Vector processor optimization (SSE) Energy and momentum conserving field interpolation Higher order and dispersion free solvers Open. MP/MPI hybrid 3 D Dynamic Load Balancing
Self Modulation Instability (SMI) z=0 cm Lbeam/ λpe=4 Ez (MV/m) Focusing Field (MV/m) -1 4 × 10 -3 0 1. 5 1 z=13. 5 cm × 10 -4 3 0 SMI 1. 5 4 Growth (z) -4 20 3 Growth (ξ) Resonant Excitation! 0 -4 -20 p Demonstrated by simulation, by never by experiments yet! p. No diagnostics to measure directly the radial modulation p. Energy modulation is measurable, and is the seed for radial modulation
ATF Beam Parameters & Simulation Parameters Electron Bunch ²Lbeam : : 960 um (Square) ²σ r ²E ²ΔE ²ε N ²Q < Front Back : : 120 um : : 58. 3 Me. V : : 0. 481 Me. V (correlated) : : 13 mm-mrad : : 50 p. C / 1 n. C Plasma ²Lplasma : : 20 cm (2 cm in exp. ) ²n 0 : : 1. 941× 1016 cm-3 (variable) (for Lbeam/ λpe=2, nb/np<<1) 2 D Simulation Box ²Ncells : : 320× 300 ²Dims : : 1222 um × 458 um ² 2× 2 beam e-+2× 2 plasma e- 57. 8 58. 3 Energy (Me. V) 58. 8 Peak Ez along z: Q=50 p. C N=1. 36 Q=1 n. C N=27. 2 Saturated @ z=2 cm Low Energy No Growth @ z=2 cm ! q. At z=2 cm: Q=50 p. C has no SMI growth Q=1 n. C reaches the saturation of SMI
“ 50 p. C” vs. “ 1 n. C” at the plasma exit (z=2 cm) Lbeam/ λpe=2 Beam Image ✔ Q=50 p. C r (um) ✗ No radial Modulation! Q=1 n. C 300 Energy Spectrum × 10 -3 0 1 3 2 57. 8 58. 3 58. 8 Lose modulation feature! 200 × 10 -2 Measurable!! 1. 5 0 Not measurable! 0 0 1 1 2 2 100 0 0 500 ct –z (um) 1000 54 58 Energy (Me. V) 62 q Q=50 p. C is chosen in the experiment due to the energy modulation feature!
Energy Modulation-Simple Model (IFEL) Q=50 p. C, Lbeam/ λpe=2 Energy Low Front Deceleration High Ba ck Periodic Ez Energy Gain/Loss: Acceleration Ez decreases from 6 to 1. 2 MV/m q. The energy beamlets are formed when the energy gain/loss is small compared to the initial energy spread of the beam Q=50 p. C q. Lbeam/ λpe Ez
Simulations at Various Plasma Densities In experiment, the plasma density can be varied: 1014 ~1017 cm-3 (capillary discharge) L/λpe Plasma Density (cm-3) nb 0/n 0 1 1. 21× 1015 0. 0030 2 4. 85× 1015 0. 00074 3 1. 09× 1016 0. 00033 4 1. 94× 1016 0. 00019 5 3. 03× 1016 0. 00012 z=2 cm v. At z=2 cm, no SMI growth for various plasma densities v. Initial Ez decreases with n 0, ranging between 4 -2 MV/m, as desired for the energy modulation to be visible.
Experimental Setup Energy Spectrometer Beam Line 2 Plasma Dipole Quadruple Energy Slit Rf Gun Linac e- beam Quadrupole Dipole q. Use the same experimental setup as the PWFA experiments Dipole
Energy Modulation: Seed for SMI q. Seed of self modulation: Energy Modulation Transverse Wakefields Radial Momentum Beam image in vacuum Transverse velocity at z=2 cm a. u 0 vr /c 0 600 High modulation @Lvacuum=7. 5 cm 0. 06 Propagate in Vacuum 300 5 0 1400 Low modulation @Lvacuum=25 cm 0 0. 12 0 500 ct –z (um) 1000 r (um) vθ/c No modulation 400 @Lvacuum=0 cm 200 × 10 -3 -5 Longitudinal Wakefields Self Modulation 700 0 0 500 ct –z (um) 1000 q. Transverse modulation could be observed downstream the plasma! a. u
Beam Transverse Sizes Measured in Experiment Beam image recorded 25 cm downstream the plasma, np=7. 6× 1016 cm-3 σx, off =367 um σx, , on = 489 um σy, off =262 um σy, , on = 438 um q. Defocusing by the plasma also observed in lower plasma densities q. Defocusing might be caused by the overshoot of focused particles q. Need to look into simulations
Instability Seeding: “Sharp” vs. “Round” q. Initial Ez is very important in our previous energy modulation study q. Current rise at the beam front is critical to the initial Ez amplitude q. In the experiments, the bunch current rise is round, instead of being perfectly sharp as assumed in simulations Fix λpe so that L=2 λpe vary ΔL, Ez 0=Ez(ΔL=0) Experiment Image 300 200 ΔL L ΔL/L=0. 06 Ez/Ez 0 Transverse Size (pixel) 400 Back Front L/λpe=1 L/λpe=5 100 0 Experimental region Energy (pixel) ΔL/λpe q. Large ΔL/λpe less rapid current rise lower initial Ez slower saturation q. In the experiment, ΔL/λpe <0. 3 Ez/Ez 0 >0. 9 well seeded instability
Conclusions • Simulations show the 50 p. C ATF beam is subject to periodic energy modulation at 2 cm propagation distance, which is an important evidence of SMI seeding. • Simulations show that SMI does not grow significantly over the 2 cm plasma for 50 p. C ATF beam • Experiment demonstrates the first observation of SMI seeding through energy modulation • Simulations show well-seeded instability in the experiments • Simulations show that 1 n. C ATF beam is radially modulated over 2 cm plasma. • No diagnostics to measure directly the radial modulation
Thank you ! Thank you to ATF!
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