Steve LidiaICFA Workshop, Chia LagunaJuly, 2002 Flat Beam Photoinjectors for Ultrafast Synchrotron...

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


• 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


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


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


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


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


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).


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.


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]


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


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'


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/



150 A175 A00 A0 A

0

5

10

15

0

5

0 50 100 150 00 50 300

Normalized Emittance0 A Measured


150 A175 A00 A0 A



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.


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)


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


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.


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


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


RF gun developmentANSYS model

Surface electric and magnetic fields

Temperature above cooling water


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.

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Steve LidiaICFA Workshop, Chia LagunaJuly, 2002 Flat Beam Photoinjectors for Ultrafast Synchrotron...

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