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Erdem Oz* USC
E-164X,E167 Collaboration
Plasma Dark Current in Self-Ionized Plasma Wake Field Accelerators
What is Dark Current and Why is it Important?
• One of the fundamental limits to high accelerating gradientsin conventional metallic particle accelerators ( < 100 MeV/m)
Damaged CD
Simulation of Dark Current in a conventional accelerator cavity
Particle Rides on the longitudinal field
•Lead to active research on Plasma Accelerators (10-100 GeV/m)
Metallic accelerator walls breakdown just like a CD in a microwave
Is there a corresponding limit in Plasma Accelerators?
Wave Breaking
The wake grows from an instability, therefore the onset of trapping is not controllable
Self Modulated Laser Wake Field Accelerator
Laser
Self Trapped Plasma Electrons
OTRLi oven
e- beam
Plasma
windows
spectrographThin lens
Gated ccd cameraPlane mirror
slit
OTR* Foil Plasma LightOTR LightCerenkov Light
*Optical Transition Radiation
LIGHT COLLECTION
TOROID
CherenkovGas Cell
ccd
Clear thresholdat ~7 GV/m
• Evidence for two (or more) short bunches of trapped particles
• Excess charge of the order of the beam incoming charge (1.6x1010 e-)
Interference of Coherent Radiation from Trapped Bunches
• Trapping above a threshold wake amplitude as measured by average energy loss or decelerating field: ≈7GV/m
.64,
,70
2
pwavelengthplasma
cSpacingBunch
Density=1.6x1017cm-3
Simulation of the Experiment with OSIRIS*
*2-D Object Oriented Fully Parallel PIC (Particle In Cell) Code
e- beam
-Li Profile-He Profile
Parameters of OSIRIS Simulation For The Full PWFA Experiment
Beam Spot Size rGaussian
12
Beam FWHM
(non-Gaussian longitudinal distribution)
70
eam 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 (1p 0.0286
Cell Size z xr 0.09 x 0.04 c/p
OSIRIS Simulation: Real Space (r-z) Of Li & He Electrons
short Bunches~3 z
Lithium electrons support the wake
He electrons trapped inside the wake
Li atz=11.3 cm
He atz=11.3 cm
e- beam
total number of trappedHe at this point0.6x1010
0.3x1010 0.25x10100.05x1010
OSIRIS Simulation: Phase Space (Pz-z) Of Li & He Electronsand the on–axis line out of the Ez
Li atz=11.3 cm
He atz=21 cm
Li electrons do not get trapped
He electrons do and reach energies up to 2.5 GeV
E
28.5GeV Beam
High-energyTrapped e-
Cherenkov Cell Image
TRAPPING OF PLASMA e-
• High-energy, narrow ∆E/E trapped particle bunches
Lp=32cm, ne=2.6x1017 cm-3
500
1000
1500
2000
2500
3000
3500
100 200 300 400 500 600 700 800 900
12180ct231142n
d C
her.
Rin
g E
nerg
y (M
eV)
CTR Energy (a.u.) 1/ z
Courtesy of P. Muggli
e- : Preionized
e- : Ionized inside the wake
-Vp
-Vp
min
zE
max
p
LongitudinalWake Amplitude
Potential z
ctz
Just like marbles rolling over a hill, It’s easier to turn the marble starting at the bottom around
Vp: Plasma Wake Phase Velocity
zE
k :e-field slope
Beam charge is varied from 0.4 to 1 times that of original beam from left to right
Vertical Lines are the analytic estimates each corresponds to a different simulation
'max 2kE
: calculated from linear fits to Ez
from simulations'k
: calculated from simulationsPeak Field
const;
c
qPmc z
Constant of motion for arbitrarywave potentials of the form,A= A(z-ct), (z-ct)
peak at 60690
peak at 61170
Pz(
mc)
z
black and yellowrepresents simulation with buffer
Trapped particles load the wake causing less energy gain
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*
*T. Katsouleas et al. Phys Fluids b 1990
Real space of plasmaelectrons
e- e+
kpr
kpz