Steve Lidia ICFA Workshop, Chia Laguna July, 2002 Flat Beam Photoinjectors for Ultrafast Synchrotron Radiation Sources Steve Lidia Lawrence Berkeley National Laboratory (and a host of others) WG1, ICFA Workshop, Chia Laguna
Steve Lidia ICFA Workshop, Chia Laguna July, 2002
Flat Beam Photoinjectors for Ultrafast Synchrotron
Radiation Sources
Steve Lidia
Lawrence Berkeley National Laboratory
(and a host of others)
WG1, ICFA Workshop, Chia Laguna
Steve Lidia ICFA Workshop, Chia Laguna July, 2002
• Generate ~ nC bunch in RF photocathode• Produce small vertical emittance from round beam• Accelerate to ~ 100 MeV• Inject into, followed by four passes through, 600 MeV linac• Produce time / angle correlation within bunch• Radiate in insertion devices and bend magnets• Compress x-ray pulse from ps scale to 50 fs scale
Femtosource Layout and Operation
Beam dumpFuture energy recovery path
10 MeV RF gun
110 MeV linac
600 MeV linac
Future energy recovery path
Baseline beam power 25 kw
Use energy recovery for beam
power above ~ 100 kW
Deflecting cavities
Steve Lidia ICFA Workshop, Chia Laguna July, 2002
Femtosecond x-ray pulses from picosecond bunches Reduces problems associated with
ultra-short electron bunches
• Deflecting cavity introduces angle-time correlation into the ~ ps electron bunch
• Electrons oscillate along the orbit
• Crystal x-ray optics take advantage of the position-time correlation, or angle-time correlation to compress the pulse
tail trajectory
Undulator
>> r ’
head trajectory
RF deflecting cavityVoltage U
δy' z = eU
Ebeam
β cavity
β IDsinkrfz
δy z = eUEbeam
β cavityβ bendsinkrfz
Bunch tilt ~ 140 µ-rad (rms)
Radiation opening angle ~ 7 µ-rad @ 1Å
Steve Lidia ICFA Workshop, Chia Laguna July, 2002
Flat electron beam productionCritical technique for producing fs-scale x-ray pulses
• Flat beam transformation– Generate circular cross-section beam from cathode in solenoidal magnetic field– Follow solenoid with quadrupole channel
• Unity transform in x• /2 phase advance in y
– Quadrupole channel transforms beam shear developed on leaving solenoid into linear x,y distribution
• Fermilab/NICADD Photoinjector Laboratory (FNPL) solenoid, k = 12
Bz
p0 / e
β =1 / k
xx 'y
y '
=1 00 10 00 0
0 00 00 β
1/β 0
x0k y0y0
k x0
=
x0
k y0
kβ x01/β y0
⇒
x0k y0x0
k y0
Steve Lidia ICFA Workshop, Chia Laguna July, 2002
Flat beam measurements
Flat beam image on fluorescent screen
Beam image through slits for emittance measurement
Round beam image on fluorescent screen
Flat electron beam productionCritical technique for producing fs-scale x-ray pulses
• Fermilab/NICADD Photoinjector Laboratory (FNPL)– Demonstrated large emittance ratio (50:1) with small emittance 0.9 mm-mrad @ 1 nC
• Limit in vertical emittance will arise from thermal and space charge effects
• LBNL collaborating with Fermilab in flat-beam experiments and modeling – Remote operations from Berkeley– Computer modeling to develop understanding of sensitivity, optimize performance– Develop hardware for operations improvements
Steve Lidia ICFA Workshop, Chia Laguna July, 2002
Emittance Compensation in Angular Momentum Dominated Beams
• Envelope Equation (generic):
R’’ + (’/β2)R’ + (’’β)R + (eBz/2βmc)2R =
{ (p/βmc)2 + (n/β)2 }/R3 + K/R
p = mR2 d/dt + eBzR2/2
• Cyclotron Phase parameterizes variations in RF gun gradient and solenoid field distribution.
d/dt = eBz/m
Steve Lidia ICFA Workshop, Chia Laguna July, 2002
Emittance compensation studies at A0
• Studies were performed to investigate the utility of standard emittance compensation in the angular momentum dominated regime, (p/mc)/thermal > ~20.
• Vertical emittance of the round beam was measured at x3, the insertion point for the skew quad channel.
• The Main solenoid current was set to provide different amounts of initial p, while the Secondary solenoid was scanned over the range of its power supply (0-300A).The bucking coil was turned off.
• Gun RF peak gradient ~40MV/m, 9-cell gradient ~10MV/m -> beam energy at exit ~15MeV. Launch phase at 40° from zero-crossing (optimized value from spectrometer measurements).
Steve Lidia ICFA Workshop, Chia Laguna July, 2002
Cyclotron Phase Advance, HOMDYN Model of A0
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
0 50 100 150 200 250 300
Cyclotron Phase Advance vs. Secondary SolenoidHOMDYN Results
Secondary Solenoid Current [A]
150A
175A
200A
220A
Overlap of phase with varying cathodesolenoid field.
Steve Lidia ICFA Workshop, Chia Laguna July, 2002
Emittance Variations vs. Cyclotron Phase
0
2
4
6
8
10
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5 4
0.88pi
Z_[m]
1
2
3
4
5
6
1
2
3
4
5
6
0 0.5 1 1.5 2 2.5 3 3.5 4
1.1pi
Z_[m]
150 A
150 A
175 A
200 A
175 A
200 A
1
1.5
2
2.5
3
3.5
4
4.5
5
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5 4
1.35pi
Z_[m]
1
2
3
4
5
6
0.5
1
1.5
2
2.5
3
0 0.5 1 1.5 2 2.5 3 3.5 4
1.46pi
Z_[m]
Steve Lidia ICFA Workshop, Chia Laguna July, 2002
Minimum Emittance Vs. Cyclotron Phase
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0.8 0.9 1 1.1 1.2 1.3 1.4 1.5Cyclotron Phase Advance / pi
Overlapping Phases
150A175A200A220A
Approximate Minimum
Steve Lidia ICFA Workshop, Chia Laguna July, 2002
Flat Beam Emittance Scan - A0 Measurements
1
1.5
2
2.5
3
3.5
4
0 50 100 150 200 250 300
RMS Vertical Spot Size, y
[ ]Secondary solenoid A
150 A175 A00 A0 A
0.05
0.1
0.15
0.
0.5
0.3
0.35
0.4
0 50 100 150 00 50 300
y'
[ ]Secondary solenoid A
150 A175 A00 A0 A
: 4Solid X : 5Dashed X
-0.4
-0.
0
0.
0.4
0.6
0.8
1
1.
0 50 100 150 00 50 300
< ( )y y'>1/
[ ]Secondary solenoid A
: 4Solid X : 5Dashed X
150 A175 A00 A0 A
0
5
10
15
0
5
0 50 100 150 00 50 300
Normalized Emittance0 A Measured
[ ]Secondary solenoid A
150 A175 A00 A0 A
: 4Solid X : 5Dashed X
Steve Lidia ICFA Workshop, Chia Laguna July, 2002
A0 Measurements, cont’d.
2
4
6
8
10
12
14
16
18
0 50 100 150 200 250 300
Secondary solenoid [A]
150 A175 A200 A220 A
Solid : X4Dashed : X5
1.1
HOMDYN shows a Minimum at ~1.4p•Simple solenoid model.•Single bunch simulation.•Uniform distribution in r and z.
Steve Lidia ICFA Workshop, Chia Laguna July, 2002
Flat beam emittance ratio, theory
• Theory based on uniform solenoid channel, hard-edged fields, periodic quadrupole channel.
• 4D Emittance is conserved:
Rn2 = 1/4 { <r2>(<r’2> + <r’>2) - <rr’>2 - <r2’>2 }1/2
= {xy}1/2
• Inherited correlations are converted into emittance ratio:
yn/ = βR02/2 x/y = 1 + 4k2R0
2/R0’2 ~ B0
2R04/pz
2Rn,thermal2
• Realistic solenoids and acceleration alter matching condition:
βquad = (Rw2/R0
2) (2pw/eB0)
Steve Lidia ICFA Workshop, Chia Laguna July, 2002
Flat beam modeling
• Develop understanding of limitations and sensitivity of the flat-beam transformation
• Explore designs– Matching lattice parameters– Effects of RF focusing– Space charge
• Analytical model– Characterize circular beam in cylindrical
modes– Transform to x – y modes
• PARMELA modeling
PARMELA model for A0, 1 nC
Steve Lidia ICFA Workshop, Chia Laguna July, 2002
Emittance Ratio, Recent Measurements
Solenoids [A]
Iris (o’clock)
Bunch charge (nC)
Laser Pulse Length [ps]
Emittance Ratio (xL7)
Emittance Ratio (xL8)
0-170-70 10:30 0.2 34 58.72/1.36 = 43 41.93/1.10 = 38
0-170-70 10:30 0.2 10 40.51/2.79 = 15 26.93/0.74 = 36
0-170-70 12:00 0.27 34 49.18/1.76 = 28 49.31/1.88 = 26
0-170-70 12:00 0.27 10 39.98/2.25 = 18 31.66/1.39 = 23
Measurements courtesy Y.Sun, U. Chicago
•10ps laser pulse measurements differ between xL7 and xL8.•34ps laser pulse suffers from gross temporal modulation - spikes + shoulder.
Steve Lidia ICFA Workshop, Chia Laguna July, 2002
RF gun development - key technology that drives pulse repetition rate up to 10-100 kHz
• 64 MV/m on cathode• Three independently phased cells• ~ 8 MeV output beam energy for three cells
– Limit power dissipation <~ 100 W/cm2
Electric field -mode
Solenoidal magnets
Cathode cell
Accelerating cells
Input waveguides
Steve Lidia ICFA Workshop, Chia Laguna July, 2002
RF gun beam dynamics studiesHOMDYN, PARMELA, MAFIA
• 64 MV/m on cathode• 43 MV/m cells 2&3, mode• 10 ps bunch length
60 deg launch phase
1 nC
Steve Lidia ICFA Workshop, Chia Laguna July, 2002
RF gun developmentANSYS model
Surface electric and magnetic fields
Temperature above cooling water
Steve Lidia ICFA Workshop, Chia Laguna July, 2002
Future Studies
• A0 Work– Develop simulation tools to better model pulse structure
and multi-bunch averaging.– Identify matching conditions for different Main solenoid
fields.– Measure emittance ratio, compare to scaling law.
• Femtosource Injector– Complete ANSYS study, finalize gun RF cavity design.– Study solutions from MAFIA, PARMELA, HOMDYN to
optimize solenoid fields.– Design skew quadrupole channel. Look for more robust
solutions than simple triplet.