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C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Optimum electron distribution for space charge dominated beams
C.Limborg-Deprey
Minimum emittance Ellipsoidal electron bunch
Comparison with “Beer Can” nominal case Optimization for S-Band and L-Band guns Sensitivity
Generation of 3D-ellipsoidal laser pulseTolerances on “stacker”Simulation
ConclusionsIs the production of such a pulse realistic?
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Minimum emittance: Intrinsic limits
Intrinsic emittance for metal cathodefor copper measured 0.6 mm.mrad per mm [1,2,3]
theoretical is 0.3 mm.mrad per mm
Space Charge Limit Minimum radius or electrons cannot leave cathode for metal cathodes
rminimum = 0.82/0.26 mm at 54 MV/m for a 1/0.1nCrminimum = 1.34/0.42 mm at 20 MV/m for a 1/0.1nC
RF emittance 10 degrees acceptable for small RF , gives 0.15 mm.mrad for S-BandFor a single slice out of 100, it reduces to nearly 0
Space charge Strong non-linear effects if local charge density varies
Ideal Emitted pulse = Uniform charge density inside 3D-Ellipsoid volumeCharge density remains constant as the space charge force is linear Perfect emittance compensation is achieved with linear optics elements space charge ==0
sin
2 peakoo
Er
QE
2arg
22echspaceRFcathodetot
??22 roughnessthermalcathode
cathodetot
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Ellipsoidal Emission pulse
Beer Can shape is not the optimum
Ideal Emitted pulse = Ellipsoid Ideally differs from Laser pulse by Shottky effect Electrons are uniformly distributed inside a 3D ellipsoid volume
22
2
2
2
2
2
Ac
z
b
y
a
x .const
dzdydx
N
rmax = 1.2 mm
Pulse length
Radius
fwhm = 10 ps
Pulse length
Line
Density
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Gun S1 S2 L0-119.8MV/m
L0-224 MV/m
‘Laser Heater’
‘RF Deflecting cavity’ TCAV1
3 screen emittance measurement
6 MeV = 1.6 m ,un. = 3keV
63 MeV = 1.08 m ,un. = 3keV
135 MeV = 1.07 m ,un. = 3keV
DL1
135 MeV = 1.07 m ,un. = 40keV
Spectrometer
Lina
c tu
nnel
UV Laser 200 J, = 255 nm, 10ps, r = 1.2 mm
Spec
trom
eter
S-Band Gun
Epeak~120MV/m
S1L0-1~20 MV/m
6 MeV
63 MeV
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Comparison between“beer can” &“3D ellipsoid”
rmax = 1.2mm
r= 1.2mm
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Comparison between“beer can” &“3D ellipsoid”
= 1.02 mm.mrad; 80% = 0.95 mm.mrad, r = 1.2mm
= 0.71 mm.mrad ; 80% = 0.71 mm.mrad; r = 1mm
3D ellipsoid is even better optimized with rmax = 1mm
Larger slice emittance for beer can with r = 1mm
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Gaussian
Square pulse
Ellipsoid
with thermal
= 1.16 mm.mrad
= 2.34 mm.mrad
= 0.75 mm.mrad
= 0.15 mm.mrad
no thermal
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Linear longitudinal Phase Space Beer can
3D-ellipsoid
Exit gun Entrance L01
Exit L01
Longitudinal Phase Space
Ek [MeV] vs T [ps]
• The longitudinal phase space gets linear
• Unfortunately, does not prevent the production of large spikes after bunch compressor
those spikes come from wakefield which follow’
☺☼ lower current density on head and tail might help
☼ LCLS will benefit from lower slice emittance anyway, and better match
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Optimization
= ½(o-2o+o)
Smaller r more mismatch
Too short bunch large mismatch
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Optimization vs pulse length and radius
Vs laser pulse length Vs laser spot size radius
• Increasing pulse length reduces emittances
suggests running 12ps , but too long in LCLS later pulse compression
• Optimum radius would be between 0.8 and 1mm
unfortunately at 0.8 mm , too strong image charge distorts too much the bunch
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Much less sensitive !!
Solenoid 1 0.3%
gun 2.5
Solenoid 2%
“Beer can”
> 7
Ellipsoid
how much distorsion on that perfect shape until we start losing this low sensitivity ?
Very low sensitivity to Shottky – Anyway can be compensated for as pulse generated
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Optimization for L-Band Gun1nC, with little effort in optimizing/retuning
=1.42 mm.mrad; 80% = 1.34 mm.mrad
= 0.93 mm.mrad ; 80% = 0.96 mm.mrad
= 1.02 mm.mrad; 80% = 1.03 mm.mrad
L-Band gun 40MV/m, = 33,
at 140 MeV
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Stacking pulses
6+6 beamlets of different radii
Gaussians Wash out discrete steps of rms value
Interferences
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Fighting interferences in Stacker
Alternating polarization + appropriate choice of , interference effect is minimized
No interference Interferences random phases
+/- 15 %
i
i
i
i
tt
ip eeAE 242
2
22
i
i
i
i
tt
is eeAE 2412
2
212
sp III
*. ppp EEI *. sss EEI
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
PARMELA simulations using stacker distributions
Beer Can
Direct beer can
Ellipsoid ideal
50 Beamlets no interferenceStacker
12 Beamlets and random phase
= 1.02 mm.mrad; 80% = 0.95 mm.mrad
= 0.71 mm.mrad ; 80% = 0.71 mm.mrad
= 0.80 mm.mrad; 80% = 0.80 mm.mrad
IDEAL
NOT IDEAL
IDEAL
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Stacker Layout
Profile shaper
pulse energy control
delayline
imaging optics
collimator-magnifier
delayslide
gratinggrating
spectralfilters
photocathode
halfwaveplate
polarizing cube
launch mirror
To the Courtesy of P.Bolton
“what to try to avoid…” from P.Bolton
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Production of the 3D ellipsoidal pulseTwo solutions proposed
Pulse StackerToo complexToo lossy Uses too much space Technically feasible with many $$$$$$$ for
controls, to achieve alignment , timingmeasurement to adjust amplitude coefficient
Spectral Control technique UV shaping using Four-gratings with masking array in dispersive environment
Principle : for highly chirp beam
projects (t,x) into a 2D surface Use masking matrix (2D) Projects back in (t,x)
A second pair of gratings : same for (t,y)masking technology for UV needs to be developed (transmissive or reflective scheme)fluence limits on optics (even worse upstream)efficiency lowprobably better for space and money than previous solution
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Spectral Control
z
yx
X mask y mask
Chirped input,
temporally
t
x
y
t
To cathode
In z,y plane
In z,x plane
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
ConclusionIdeal emission pulse = “ellipsoid” not “beer can”
Perfect emittance compensation in high charge regimeWorks better at high gradient Impressively less sensitive to tuning parameter
tolerances are 1 order of magnitude above those defined for “beer can” pulse even using “stacker pulse”
More exploration required for L-Band gun
Ellipsoidal Laser pulse is aTechnical challenge maybe not worse than “beer can” generation? if direct UV shaping has to considered anyway for “beer can”, the ellipsoid generation shares many of the same difficulties
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
References
[1] , Sumitomo Industries
[2] B.Graves, DUVFEL
[3] J.Schmerge, GTF
[4] O.J.Luiten, et al. “How to realize uniform 3-dimensional ellipsoidal electron bunches”, Phys.Rev. August 2004
[5 ]
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Computation of of stacker
Alternating polarization + appropriate choice of , interference effect is minimized
No interference
Interferences assuming random phases
+/- 15 %
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
S-Band Gun
Epeak~120MV/m
S1L0-1~20 MV/m
6 MeV
63 MeV
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Computation using stackerDistributions with 12 Gaussians run and random phase with PARMELA for the
LCLS set-up
r = 0.8 mm
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Computation using stackerDistributions with 12 Gaussians run and random phase with PARMELA for the
LCLS set-up
= 1.02 mm.mrad; 80% = 0.95 mm.mrad
= 0.71 mm.mrad ; 80% = 0.71 mm.mrad
= 0.80 mm.mrad; 80% = 0.80 mm.mrad
= 1.02 mm.mrad; 80% = 0.95 mm.mrad
= 0.69 mm.mrad ; 80% = 0.69 mm.mrad
= 0.79 mm.mrad; 80% = 0.79 mm.mrad
r = 1mm r = 0.9 mm
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Production of the 3D ellipsoidal pulseTwo solutions proposed
pulse stackerToo complexToo lossy Uses too much space Technically feasible with many $$$$$$$
For controls, to achieve alignment , timingFor measurement to adjust coefficient
UV shaping using Four-gratings with masking array in dispersive environment ELL_Shaper
Principle First pair of gratings :
projects (t,x) into a 2D surface Use LCD matrix (2D) for partially masking Projects back in (t,x)
A second pair of gratings : same for (t,y)
Requires high input power (as low efficiency)
C.Limborg-Deprey
ERL Workshop , Jefferson Laboratory [email protected]
March 20th 2005
Stacking pulses
Alternating polarization + appropriate choice of , interference effect is minimized
No interference Interferences random phases
+/- 15 %
i
i
i
i
tt
ip eeAE 242
2
22
i
i
i
i
tt
is eeAE 2412
2
212
sp III
*. ppp EEI
*. sss EEI
12 beamlets
Gaussians
Wash out discrete
steps