First observation of Current Filamentation Instability (CFI)

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1 First observation of Current Filamentation Instability (CFI) PI - Patric Muggli: University of Southern California Vitaly Yakimenko, Mikhail Fedurin, Karl Kusche, Marcus Babzien: Brookhaven National Laboratory Joana Martins, Luis O. Silva: Instituto Superior Técnico Warren Mori: University of California Los Angeles AAC 2012, Austin TX Chengkun Huang: Los Alamos National Laboratory June 15, 2012 Brian Allen Work supported by DoE and NSF

2 Agenda Overview PIC Simulations Experiment Setup Results Conclusion

3 Current Filamentation Instability (CFI) What is CFI? Particle beam transport in plasmas is subject to Current Filamentation Instability (CFI) CFI results in breakup of the beam into narrow high current filament Enhances/generates magnetic fields and generates radiation Why is it interesting? Basic plasma instability Potential relevance to Astrophysics and Inertial Confinement Fusion (ICF) Afterglow of gamma ray bursts Hot electron transport for Fast Igniter - ICF

4 Characteristics of CFI Characteristics Particular case of the Weibel instability (1) (Temperature anisotropy) Plasma return current s r > c/w pe flows through beam, CFI regime s r << c/w pe return current outside beam, PWFA wakefields Purely transverse electromagnetic instability of relativistic beams - purely imaginary frequency How it occurs Non-uniformities in the transverse beam/plasma profile lead to unequal opposite currents (beam and plasma) and magnetic fields Opposite currents repel each other, leading to instability and filamentation Effects Beam filaments Plasma density perturbations Magnetic field enhancement (or generation) Radiation generation (1) E. Weibel - Phys. Rev. Lett. 2, 83 (1959)

5 Criteria for CFI Criteria for CFI s r >> c/w pe (k p s r >> 1) g 0 >>1 Filament size and spacing ~ c/w pe Growth rate (1) (infinite transverse size): 0 g 0 w pe or 0 w pb g 0 ~ n b ~ Q/(s r2 s z ) s r - Transverse beam size, s z - bunch length, n e - plasma density, n b beam density Q - beam charge, c - speed of light in vacuum, g o - relativistic beam factor Plasma-electron angular frequency: w pe =(n e e 2 /e o m e ) 1/2 Collisionless skin depth: k -1 p =c/w pe Ratio of beam to plasma density: =n b /n e (1) Bret et al., Phys. Rev. Lett. 94, (2005)

6 CFI with ATF Beam ATF Beam Parameters Parameters Value Charge (nc) 1.00 Typical Bunch Transverse Waist Size - s 0x,y (mm) 50 to 100 Bunch Length (ps) 5 Bunch Density (cm -3 ) 6x10 13 Energy (MeV) 58 Normalized Emittance (mm-mrad) 4 to 8(?) g 0 =117 Growth length estimate: =4.2x10 10 s -1 or 7.1 mm at c g 0 >>1, s 0y >> c/w pe for n e > 1.6x10 16 cm -3 CFI should be observable on a cm-length plasma scale (plasma length, L p = 2 cm)

7 Simulations - Beam Filamentation Simulations with QuickPIC Warren Mori ATF Beam and Plasma Simulation Parameters Parameters Value Value Simulation Box - X ( mm, # grids) Simulation Box - Y ( mm, # grids) Simulation Box - Z ( mm, # grids) Plasma Particles/Cell 1 3-D Time Step (mm) 34 Beam Particles - X (#) 512 Beam Particles - Y (#) 512 Beam Particles - Z (#) 128 Relativistic Factor 117 Beam Transverse Waist Size ( mm) 100 Bunch Length (mm) 30 Charge (pc) 200 Plasma Density (cm -3 ) 2.5x10 17 Capillary Length (cm) 2 Skin Depth (c/w pe ) (mm) 10.6 or plasma OFF or plasma ON Filament size 10 µm Filament spacing 20 µm c/w pe ~ 10.6mm

8 Magnetic Energy (A.U.) CFI Growth Rate ( ) Characterize instability by the resulting magnetic energy B perp2 dv Q=400pC Growth rate is beam density (n b ) charge For Q=200 pc, = 8.0x10 10 s -1 or 3.8 mm, agrees with estimated ( =8.6x10 10 s -1 or Q=250pC Q=200pC Q=100pC ATF Case mm) Propagation Distance (cm) Instability appears over available plasma lengths (1 and 2 cm) Parameters: e x,y = 1mm/mrad, all other parameters as shown in simulation parameters table

9 Direct Filament Imaging Micron resolution of transverse bunch Si-window - low scattering, terminates plasma Au coating for Optical Transition Radiation Generation 60 MeV Linac Resolution 3.9mm/line (USAF 1951 Target 50% MTF) Capillary H 2 Gas Si/Au Window Microscope Objective Turning Mirrors 1024 ProEM Princeton Insturments EMCCD Camera e - OTR 15kV Tungesten Beam Stop M. Fedurin Poster WG 5-50

10 Typical Beam and Filament Images Plasma Off Incoming bunch stable: c/w p = 12.3mm Size 10% RMS Charge 1% (cavity) Plasma On Instability/Filamentation Filaments: high count features (higher current density) c/w p = 12.3mm c/w p = 15.4mm c/w p = 15.4mm c/w p = 38.2mm Number, location and position changes On average total count conserved Focusing? Color Maps with background subtraction : a) [0,225], b to f) [0,325]

11 Single to Multiple Filament Transition We Define CFI: Typically multiple filaments CFI Scaling with c/w p Each point may be multiple observations Single Filaments: k p s r <2.2 CFI/Multiple Filaments: k p s r >2.2 1 to 5 observed Merging of Filaments: k p s r >4.5 Observed in simulations Focusing increasing n e Merging

12 Filament Size Scaling High Charge (1.0nC) s 0x =81mm, s 0y =53mm All events c/w p <20mm multiple filaments Low Charge (0.54nC) Scaling for 12<c/w p <42mm Filament merging for c/w p <10mm Low Charge (0.54nC) s 0x =89mm, s 0y =45mm ~n 1/2 b All events single filaments No scaling with c/w p CFI suppressed High Charge (1.0nC) Multiple Single increasing n e Focusing?

13 Summary CFI observed and studied at the ATF Transverse imaging of multiple (1-5) filaments Multiple filaments observed for k p s 0y >2.2 (theory k p s r >1) Filament size and position vary event to event Filaments transverse size scale with plasma skin depth Suppression of CFI with reduced charge ( ~n 1/2 b ) Focusing for k p s r <<1

14 Experiment Experiment on BL #1 Compton Chamber

15 Magnetic Energy (A.U.) Emittance Effects 1 e =1mm-mrad ATF Case Emittance competes with CFI (1) Increased emittance reduces CFI growth rate Similar to temperature effect in Weibel instability (2) 0.1 e =20mm-mrad Plasma Distance (cm) Transverse emittance not an issue for ATF parameters (e N =1-2 mm-mrad) Parameters: e x,y = 1mm-mrad and 20 mm-mrad, all other parameters as shown in simulation parameters table (1) J.R. Cary et al., Phys. Fluids 24, 1818 (1981); (2) L. Silva et. al - Phys. of Plasmas 9, 2458 (2002)