Transcript PPT - SLAC
Plasma Dark Current in Self-Ionized Plasma Wake Field Accelerators Erdem Oz* USC E-164X,E167 Collaboration *[email protected] What is Dark Current and Why is it Important? • One of the fundamental limits to high accelerating gradients in conventional metallic particle accelerators ( < 100 MeV/m) Particle Rides on the longitudinal field Simulation of Dark Current in a conventional accelerator cavity Metallic accelerator walls breakdown just like a CD in a microwave •Lead to active research on Plasma Accelerators (10-100 GeV/m) Damaged CD Is there a corresponding limit in Plasma Accelerators? Wave Breaking Self Modulated Laser Wake Field Accelerator Laser Self Trapped Plasma Electrons The wake grows from an instability, therefore the onset of trapping is not controllable Plasma ebeam Cherenkov Gas Cell Li oven OTR TOROID ccd LIGHT COLLECTION slit Thin lens spectrograph Plane mirror Gated ccd camera l OTR* Foil windows *Optical Transition Radiation Plasma Light OTR Light Cerenkov Light Clear threshold at ~7 GV/m Interference of Coherent Radiation from Trapped Bunches 2 Density=1.6x1017cm-3 Bunch Spacing c 70 , plasma wavelength, l p 64 . • Trapping above a threshold wake amplitude as measured by average energy loss or decelerating field: ≈7GV/m • Excess charge of the order of the beam incoming charge (1.6x1010 e-) • Evidence for two (or more) short bunches of trapped particles Simulation of the Experiment with OSIRIS* -Li Profile -He Profile ebeam *2-D Object Oriented Fully Parallel PIC (Particle In Cell) Code Parameters of OSIRIS Simulation For The Full PWFA Experiment Beam Spot Size (sr) Gaussian 12 Beam FWHM (non-Gaussian longitudinal distribution) 70 Beam Energy 28.5 GeV Number of Beam e- 1.88 x 1010 Li Gas Density (n0) 1.6 x 1017 cm-3 Number of Simulation Cells 500 x 600 moving Beam Particles/cell 25 Gas Particles/cell 1 dt (1/p) 0.0286 Cell Size z x r 0.09 x 0.04 c/p OSIRIS Simulation: Real Space (r-z) Of Li & He Electrons Li at z=11.3 cm He at z=11.3 cm Lithium electrons support the wake ebeam total number of trapped He at this point 0.6x1010 0.05x1010 0.3x1010 short Bunches ~3 sz 0.25x1010 He electrons trapped inside the wake OSIRIS Simulation: Phase Space (Pz-z) Of Li & He Electrons and the on–axis line out of the Ez Li at z=11.3 cm Li electrons do not get trapped He at z=21 cm He electrons do and reach energies up to 2.5 GeV TRAPPING OF PLASMA eLp=32cm, ne=2.6x1017 cm-3 E 28.5GeV Beam 2nd Cher. Ring Energy (MeV) Cherenkov Cell Image High-energy Trapped e- 3500 3000 2500 2000 1500 1000 1 21 80 ct2 31 14 500 100 200 300 400 500 600 700 800 900 CTR Energy (a.u.) -1/ sz • High-energy, narrow ∆E/E trapped particle bunches Courtesy of P. Muggli Longitudinal E z Wake Amplitude Ez x Potential F-Az k :e-field slope xp x z - ct x Just like marbles rolling over a hill, It’s easier to turn the marble starting at the bottom around e- : Preionized e- : Ionized inside the wake -Vp Vp: Plasma Wake Phase Velocity xmax -Vp xmin x Vertical Lines are the analytic estimates each corresponds to Constant of motion for arbitrary a different simulation wave potentials of the form, ' A= A(z-ct), FF(z-ct) max E 2k mc - Pz q const; c k ' : calculated from linear fits to Ez from simulations Peak Field : calculated from simulations Beam charge is varied from 0.4 to 1 times that of original beam from left to right Trapped particles load the wake causing less energy gain Pz(mc) black and yellow represents simulation with buffer peak at 60690 peak at 61170 z Trapping could be more important for Positrons Plasma electrons are dragged out of the plasma by positron beam and can become as dense as the positron beam* kpr Real space of plasma electrons kpz e- *T. Katsouleas et al. Phys Fluids b 1990 e+