The Fermilab Photo-Injector

Jean-Paul Carneiro (Fermilab & Université Paris XI)For the A0 group (N. Barov, M. Champion, D. Edwards, H. Edwards,

J. Fuerst, W. Hartung, M. Kuchnir, J. Santucci)

Accelerator Physics and Technology Seminars Fermilab, March 23, 2001

OUTLINE

1. Introduction: R&D on linear colliders e+/e- at Fermilab NLC, TESLA2. Layout of the A0 Photo-Injector3. Experiments Dark current Quantum efficiency Transverse emittance Bunch length Compression User experiments5. Conclusion

NLC: 30 km long, copper cavities, 1 TeV COM, luminosity ~1110-33 cm-2 s-1. Collaboration Fermilab/SLAC.

TESLA: 30 km long, superconducting cavities, 0.8 TeV COM, luminosity ~27.510-33 cm-2 s-1. Collaboration between 9 countries and 41 institutions.

R&D on linear colliders e+/e- at Fermilab

THE TESLA ACCELERATOR

• 9-cells superconducting cavities

• Must achieve 40 MV/m to get 0.8 TeV COM.• Today ~ 33 MV/m.

• To develop the technology of TESLA: installation at DESY (Hamburg) of a TESLA TEST FACILITY accelerator.

THE TESLA TEST FACILITY ACCELERATOR

~ 100 meters

• Fermilab contribution to TTF : - design, fabrication and commissioning of the TTF injector (Nov 98).

- design and prototyping of RF couplers for the cavities. - design and prototyping of long-pulse modulators for the klystrons.

THE TESLA TEST FACILITY ACCELERATOR

THE TESLA TEST FACILITY PHOTO-INJECTOR

THE TTF ONDULATO R

• Self-Amplified Spontaneous Emission observed at 209 nm in February 2000.

RF standing wave cavity

Electron bunch

Picosecond UV laser

Concept of Photo-Injector gun:

Photo-cathode

TTF INJECTOR BEAM PARAMETERS

Quantity

Charge per bunchBunch spacingBunches per RF pulseRepetition rate

TTF spec.

1-8 nC1 µs800 10 Hz

QuantityEnergy

Transverse emittance at 1 nCTransverse emittance at 8 nC

TTF spec.20 MeV

2-3 mm-mrad15 mm-mrad

A0 PHOTO-INJECTOR LAYOUT (First beam the 3rd of March 1999)

Oscillator Nd:YLF81.25 MHz

2 km optic fiber Pockels Cell1 MHz

Multi-pass amplifierNd-glass

Double-pass amplifierNd-glass

12 nJ/pulse60 ps

1054 nm

2.5 nJ/pulse400 ps

800 pulses2 nJ/pulse

400 ps

100 µJ/pulse400 ps

0.8 mJ/pulse400 ps

600 µJ/pulse400 ps

400 µJ/pulse4.2 ps

100 µJ/pulse4.2 ps

532 nm

20 µJ/pulse4.2 ps

263 nm

10 µJ/pulse10.8 ps263 nm

LASER (University of Rochester)

STACKED UNSTACKED

Spatial filterCompressorBBO CrystalsPulse stacker

0

5

10

15

20

25

0 10 20 30 40 50 602

4

6

8

10

12

0 5 10 15 20 25 30

UNSTACKED LASER PULSE4.2 ps FWHM / 20 µJ

STACKED LASER PULSE10.8 ps FWHM / 10 µJ

The two regimes of the A0 laser system :

THE PHOTO-CATHODE PREPARATION CHAMBER (INFN-Milano)

• Coat Mo cathodes with a layer of Cs2Te, a material of high quantum efficiency (QE).

• Use manipulator arms to transfer the cathode from the preparation chamber into the RF gun while remaining in UHV.

• Cathodes must remain in ultra-high vacuum (UHV) for its entire useful life, because residual gases degrade the QE. • Contamination can be reversed by rejuvenation: heat cathode to ~230 C for some minutes.

• The same cathode has been used in the RF gun for ~2 years without degradation of its QE (~0.5-3%)

BUCKING SOLENOID

PRIMARY SOLENOID

SECONDARY SOLENOID

THE RF GUN AND SOLENOIDS (Fermilab & UCLA)

• RF gun and solenoids developed by Fermilab and UCLA.

ModeResonant frequency

Peak fieldTotal energyPeak power dissipationPulse lengthRepetition rateAverage power dissipationCooling water flow rate

TM010,π

4.5 MeV2.2 MW800 µs10 Hz28 kW

35 MV/m

4 L/s

Q 240001.3 GHz

Gun parameters

Solenoids parameters

• Bucking & Primary max. Bz --> 2059 G (385A)• Secondary max. Bz --> 806 G (312 A)

• 1.5-cell copper cavity designed for a high duty cycle (0.8%).

RF GUN

THE CAPTURE CAVITY (DESY & SACLAY/ORSAY) & THE CHICANE (Fermilab)

CHICANECAPTURE CAVITY

Capture cavity parameters

Chicane parameters

• 9-cell L-band superconducting cavity of TTF type.• Operated daily at 12 MV/m on axis.

• 4 dipoles of equal strengths, 2 with trapezoid poles and 2 with parallelogram poles.• Operated @ 2A, ~700 Gauss.• Bend in the vertical plane• Compression ratio ~5 - 6 (theory and measurements)

THE LOW BETA SECTION THE WHOLE BEAMLINE

SPECTROMETER

EXPERIMENTPLASMA WAKEFIELD ACCELERATION

DARK CURRENT STUDIES

Idc 150 Vdt

t 150 917 10 9

60 10 6 0.3 mA

X2 Faraday Cup Oscilloscope trace channel 1 Forward power into the gun channel 2 Faraday Cup X2 signal

•Dark current measurement principle : Using a Faraday Cup at X2 (z~0.6 m).

Bucking Ib

Primary Ip

Secondary Is

Comparison of Dark current : March 99 / November 00

0

0.5

1

1.5

2

2.5

3

3.5

0 10 20 30 40 50

03/04/99 Ib=I

p=I

s=0 A

02/11/00 Ib=I

p=I

s=220 A

02/11/00 Ib=I

p=I

s=0 A

RF gun peak field [MV/m]

Edge of the photo-cathode Edge of the photo-cathode

Where does the dark current come from?

•Probably the surface of the photo-cathode.

Photo-cathode & back of the RF gun Dark current spots & photo-current in X6 (z=6.5 m)

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0 1 2 3 4 5 6 7 8

Ib=I

p=I

s=220 A

Ib=0 A, I

p=170 A, I

s=70 A

Time [Hours]

Round beamFlat beam

Effect of the solenoids settings on the dark current

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1 10-9

2 10-9

3 10-9

4 10-9

5 10-9

6 10-9

7 10-9

8 10-9

9 10-9

0 100 200 300 400 500

dark current

pressure

Time [mn]

vanne closed

valveopen

vanneclosed

Effect of the vacuum on the dark current

QUANTUM EFFICIENCY STUDIES

QENumberof electron producedNumberof incident photons 0.47Q[nC]

E[J]

Q [nC] = Charge of the bunch measured byan Integral Current Transformer (X2).

E [µJ] = Energy of the UV laser pulsemeasured by an Energy Meter.

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

0 1 2 3 4 5 6

Ib=0 A, I

p=170 A, I

s=70 A

Ib=I

p=I

s=220 A

Time [Hour]

Round beamFlat beam

Effect of the solenoids settings on the Quantum Efficiency

0

0.5

1

1.5

2

2.5

3

3.5

0 0.5 1 1.5 2 2.5

10,8 ps FWHM4,2 ps FWHM

Laser energy on the cathode [J]

Charge Vs. Laser Energy for 2 longitudinal sizes of the laser beam on the photo-cathode.

Laser transverse size : = 0.9 mm

0

2

4

6

8

10

12

0 0.5 1 1.5 2 2.5 3 3.5 4

x = 2.0 mm

x = 1.7 mm

x = 0.9 mm

Laser energy on the photo-cathode [J]

Charge Vs. Laser Energy for 3 different transverse sizes of the laser beam on the photo-cathode.

Laser longitudinal size : z = 10.8 ps FWHM

0

1

2

3

4

5

6

0 0.5 1 1.5 2 2.5 3 3.5

MeasurementFit with Gauss law (Hartman model, UCLA)

Laser energy on the cathode[J]

Charge Vs. Laser Energy for = 0.8 mm on the photo-cathode.

(Hartman, NIM A340, p.219-230, 1994)

TRANSVERSE EMITTANCE MEASUREMENTS

Q = Charge of the bunch (laser energy) r = Laser pulse transverse size z = Laser pulse length

E0 = Peak field on RF gun 0 = Launch phase Ib, Ip, Is = Current in the solenoids

Ecc = Capture cavity accelerating field cc = Capture cavity RF phase

Laser

RF Gun

CaptureCavity

• The photo-injector is a set of 8 parameters:

• Goal: find for a charge Q, the set of parameters that gives the min. transverse emittance.• Remark: for all the emittance measurements, the chicane was OFF and DEGAUSSED.

• How do we measure the transverse emittance at A0: using slits

u,N beambeam' beam

beamletL

L

• Slits width: 50 µm• Slits spacing: 1mm

beam

beamlet

Location of the emittance slits

~ 3.8 m ~ 9.5 m~ 6.5 m

10

20

30

40

50

60

70

0 2 4 6 8 10 12

u,N beam beam'

180.511

1.8 mm 70.8 m

384 mm

11.7 mm mrad

80

100

120

140

160

0 2 4 6 8 10 12 14 Position [mm]Position [mm]

beam 1.8 mm beamlet 70.8 m

Inte

nsity

[a.

u.]

Inte

nsity

[a.

u.]

Example: emittance measurement of 8 nC in X3 (z~3.8 m), beamlets in X4 (∆z = 384 mm)

BEAM X3 BEAMLETS X4

Ecc = 12 MV/m cc = at the minimum of energy spread z = 10.8 ps FWHM

1/ For a fixed charge Q (0.25, 1, 4, 6, 8 and 12 nC), wetried to find the set of 4 parameters (0, E0, Isol, r) toobtain the minimum transverse emittance at z=3.8 m.

2/ We measured the emittance at z=6.5 m and z=9.4 m.

3/ We compared the results with 2 codes of simulation PARMELA (V5.03 from Orsay, B. Mouton) Known code, slow execution (~15 Hours). HOMDYN ( HTWA21 from Frascati, M. Ferrario) New code, fast execution (~2-3 minutes).

How did we proceed with the emittance measurements ?

FIXED PARAMETERS

1.5

2

2.5

3

3.5

4

4.5

0 100

1 102

2 102

3 102

4 102

5 102

6 102

-100 -50 0 50 100 150 200 250 300

Transmission before experimentTransmission after experiment

Launch phase [Deg]

Emittance Vs. Launch Phase (z=3.8 m)Q=1 nC, E0=35 MV/m,=0.8 mm

øo=40 deg

Q=0.4 nC

Q=0.5 nC

Q=0.8 nC

0

2

4

6

8

10

12

14

16

150 200 250 300

Experiment - 40 MV/m

HOMDYN simulation - 40MV/m

Experiment - 35 MV/m

HOMDYN simulation - 35 MV/m

Experiment - 30 MV/m

HOMDYN simulation - 30 MV/m

Current Ib=I

p=I

s [A]

Emittance Vs. Solenoids Current (z=3.8 m)Q=1 nC, ø0=40 deg, Eo=30, 35, 40 MV/m, =0.8 mm

10

15

20

25

30

35

40

45

140 160 180 200 220 240 260 280

Current Ib=I

p=I

s

10

15

20

25

30

35

40

45

140 160 180 200 220 240 260 280

Experiment - 30 MV/m

HOMDYN simulation - 30 MV/m

Experiment - 40 MV/m

HOMDYN simulation - 40 MV/m

Experiment - 35 MV/m

HOMDYN simulation - 35 MV/m

E0 = 40 MV/m

Emittance Vs. Solenoids Current (z=3.8 m)Q=8 nC, ø0=40 deg, Eo=30, 35, 40 MV/m, =1.6 mm

0

1

2

3

4

5

6

150 200 250 300 350

HOMDYN simulation

PARMELA simulation

Measurement (x = 0.4 mm)

Current Ib=I

p=I

s [A]

Emittance Vs. Solenoids Current (z=3.8 m)Q=0.25 nC, ø0=40 deg, Eo=40 MV/m, =0.4 mm

Min Emit @ 205 A

0

5

10

15

20

180 200 220 240 260 280

x = 1.0 mm

x = 0.5 mm

x = 0.8 mm

Current Ib=I

p=I

s [A]

Emittance Vs. Solenoids Current (z=3.8 m)Q=1 nC, ø0=40 deg, Eo=40 MV/m, =0.5, 0.8 & 1mm

Min Emit @ 0.5 mm, 260 A

0

5

10

15

20

180 200 220 240 260 280 300

Measurement (x = 0.5 mm)

HOMDYN simulation

PARMELA simulation

Current Ib=I

p=I

s [A]

Comparison Measurements / HOMDYN / PARMELACase Q=1nC, =0.5 mm

0

10

20

30

40

50

210 220 230 240 250 260 270 280 290

x = 1.5 mm

x = 1.2 mm

x = 1.8 mm

Current Ib=I

p=I

s [A]

Emittance Vs. Solenoids Current (z=3.8 m)Q=4nC, ø0=40 deg, Eo=40 MV/m, =1.2, 1.5 & 1.8 mm

Min Emit @ 1.2 mm, 260 A

5

10

15

20

25

30

35

40

45

180 200 220 240 260 280 300 320

PARMELA simulation

HOMDYN simulation

Measurement (x = 1.2 mm)

Current Ib=I

p=I

s [A]

Comparison Measurements / HOMDYN / PARMELACase Q=4 nC, =1.2 mm

0

5

10

15

20

25

30

230 240 250 260 270 280

x = 1.8 mm

x = 1.4 mm

x = 1.2 mm

Current Ib=I

p=I

s [A]

Emittance Vs. Solenoids Current (z=3.8 m)Q=6 nC, ø0=40 deg, Eo=40 MV/m, =1.2, 1.4 & 1.8 mm

Min Emit @ 1.4 mm, 255 A

0

10

20

30

40

50

60

180 200 220 240 260 280 300 320

HOMDYN simulation

PARMELA simulation

Measurement (x = 1.4 mm)

Current Ib=I

p=I

s [A]

Comparison Measurements / HOMDYN / PARMELACase Q=6 nC, =1.4 mm

10

15

20

25

30

220 230 240 250 260 270 280

x = 1.8 mm

x = 1.6 mm

x = 1.2 mm

Current Ib=I

p=I

s [A]

Emittance Vs. Solenoids Current (z=3.8 m)Q=8nC, ø0=40 deg, Eo=40 MV/m, =1.2, 1.6 & 1.8 mm

Min Emit @ 1.6 mm, 245 A

10

20

30

40

50

60

70

80

180 200 220 240 260 280 300 320

Measurement (x=1.6 mm)

HOMDYN simulation

PARMELA simulation

Current Ib=I

p=I

s [A]

Comparison Measurements / HOMDYN / PARMELACase Q=8 nC, =1.6 mm

0

20

40

60

80

100

180 200 220 240 260 280 300 320

Measurement (x = 2.1 mm)

HOMDYN simulation

PARMELA simulation

Current Ib=I

p=I

s [A]

Emittance Vs. Solenoids Current (z=3.8 m)Q=12 nC, ø0=40 deg, Eo=40 MV/m, =2.1 mm

Min Emit @ 2.1 mm, 225 A

0

10

20

30

40

50

60

70

80

0 2 4 6 8 10 12 14

Measurement

HOMDYN simulation

PARMELA simulation

Charge [nC] [mm]Ib=Ip=Is [A]

0.4 0.5 1.2 1.4 1.6 2.1205 260 260 255 245 225

Emittance Vs. Charge (z=3.8 m)ø0=40 deg, Eo=40 MV/m

Predicts a decrease of 50% using a 20 ps FWHM laser pulse.

0

0.5

1

1.5

2

2.5

3

3.5

4

0 2 4 6 8 10 12

Measurement (x)

Measurement (y)

HOMDYN simulation (x)

HOMDYN simulation (y)

PARMELA simulation (x)

PARMELA simulation (y)

Longitudinal position [m]

Beam envelope for Q=1 nC.ø0=40 deg, Eo=40 MV/m, =0.8 mm, Ib=Ip=Is=255 A.

Q3=1.32 A, Q4=-2.42 A, Q5=1.32 A.

6.5 m 9.4 m

0

2

4

6

8

10

0 2 4 6 8 10 12

HOMDYN simulation (x)

HOMDYN simulation (y)

PARMELA simulation (x)

PARMELA simulation (y)

Longitudinal position [m]

Beam envelope for Q=8 nC.ø0=40 deg, Eo=40 MV/m, =1.6 mm, Ib=Ip=Is=245 A.

Q3=1.3 A, Q4=-2.6 A, Q5=1.3 A & Q6=2.2 A, Q7=-4.2 A, Q8=2.2 A.

6.5 m 9.4 m

Norm. Emit. Y Z [m] HOMDYN PARMELA

3.8 11 40.76.5 12.5 39.16.5 9.7 40.59.4 8.5 39.39.4 16.4 41.2

10.0 ± 0.111.6 ± 0.5

8.9 ± 0.714.4 ± 0.518.3 ± 0.9

Z [m] Measurement HOMDYN PARMELA3.8 1.7 9.26.5 1.7 9.16.5 1.4 9.29.4 1.6 9.69.4 0.9 9.6

4.1 ± 0.35.0 ± 0.2

5.1 ± 0.26.8 ± 0.25.8 ± 0.2

CASE Q = 1 nC

CASE Q = 8 nC

Norm. Emit. YNorm. Emit. XNorm. Emit. YNorm. Emit. XNorm. Emit. Y

Norm. Emit. XNorm. Emit. YNorm. Emit. XNorm. Emit. Y

Measurement

Transverse Emittance at different locations in the beamline.

BUNCH LENGTH MEASUREMENTS• Principle: - Using a Hamamatsu Streak Camera of 1.8 ps resolution - OTR light at X6 (z=6.5 m)

Streak camera OTR screen X6

18

20

22

24

26

28

30

0 0.5 1 1.5 2 2.5 3 3.5 4127.5

128

128.5

129

129.5

130

130.5

131

131.5

0 20 40 60 80 100

stat 0.17 ps defl 2.55 ps

Time [ps] Time [ps]

Inte

nsity

[a.

u. ]

Inte

nsity

[a.

u. ]

FOCUS MODE STREAK MODE

t defl2 stat

2 2.552 0.172 2.54 ps 0.76 mm

Example: Bunch length measurement of 8 nC in X6 (z~6.5 m)

0

0.5

1

1.5

2

2.5

3

3.5

4

0 2 4 6 8 10 12

Parmela simulationHomdyn simulationMeasurement

Charge [nC]

Bunch Length Vs. Chargeø0=40 deg, Eo=40 MV/m, =2.1 mm, z=10.8 ps FWHM

Ib=Ip=Is= 240 A

P>Po

P=Po

P<Po

COMPRESSION PRINCIPLE

Po

TAILP>Po

HEADP<Po

MOMENTUM

MOMENTUM

PHASE

PHASE

0

1

2

3

4

5

6

-100 -90 -80 -70 -60 -50 -40 -30

Homdyn simulationParmela simulation

Measurement

Relative phase of the superconducting cavity [Deg]

Minimum energy spread

CompressionRatio 3 mm 0.5 mm

6

Compression / Bunch length Vs. 9-cell phaseQ=8 nC, ø0=40 deg, Eo=40 MV/m, =2.1 mm, z=10.8 ps FWHM

Ib=Ip=Is= 240 A

0

5

10

15

20

25

30

35

0 2 4 6 8 10 12

x,n

y,n

Longitudinal position [m]

chicaneentrée sortie

Emittance variation along the beamline of a 8 nC compressed beam.

• Coherent Synchrotron Radiation (CLIC studies with TraFic)

USER EXPERIMENTS

• Electro-Optic Sampling of Transient Electric Fields, M. Fitch (thesis work). - Bunch length measurement using electro-optic detection of the electric field from the passage of a 10 nC bunch (few MV/m). • Crystal Channeling Radiation, R. Carrigan & Co. - Particle acceleration in a thin Si crystal.

• Plasma Wake Field Acceleration in Gaseous Plasma, N. Barov & Co. - Particle acceleration in a plasma: drive bunch makes a plasma wave, witness bunch is accelerated.

• Flat Beams, H. Edwards and Co. - Make emittance much smaller in one direction than in the other. Ratio 1/50 achieved to date. First accelerator to ever produce a flat beam.

• Northern Illinois University, G. Blazey and Co. - Fermilab/NICADD Photo-Injector

Q= 4-8 nC compressed (sigma less 1 mm) / 50 MV/m achieved to date / Plan 150 MV/m.

FLAT BEAMS IMAGES (Q=1 nC)

Before compression

Laser pulse length

Laser transverse size on cathode

Launch phase

Peak field on RF gun

Accelerating field on capture cavity

Transverse normalized RMS emittance

Energy spread

Bunch length

Peak current

After compression

Transverse normalized RMS emittance

Bunch length

Peak current

Q = 1 nC Q = 8 nC

Prediction Measurement Prediction Measurement

13.5 ps 10.8 ps 28 ps 10.8 ps

0.7 mm 0.8 mm 1.5 mm 1.6 mm

35 deg 40 deg 45 deg 40 deg

50 MV/m 40 MV/m 50 MV/m 40 MV/m

15 MV/m 12 MV/m 15 MV/m 12 MV/m

2.5 mm-mrad

0.16%

1.27 mm

80 A

3.02 mm-mrad

1 mm

120 A

3.7 ± 0.1 mm-mrad

0.25 ± 0.02 %

1.6 ± 0.1 mm

75 A

non-measured non-measured

0.55 ± 0.07 mm

218 A

1.2 %

3.1 mm

386 A

15 mm-mrad

19.4 mm-mrad

1 mm

958 A

0.55 ± 0.05 mm

1741 A

330 A

2.9 ± 0.2 mm

12.6 ± 0.4 mm-mrad

0.38 ± 0.02%

Comparison Prediction (Parmela, 1994) and Measurements (1999-->2001)

CONCLUSIONS

CONCLUSIONS (continued)

• The Photo-Injector designed by Fermilab meets its specifications. • Possible future studies of the photo-injector:

- Understand the dark current source. - Understand the dark current and QE “zig-zag” as a function of time for round beam and flat beam settings. - Measure emittance of a non-compressed beam using 20 ps FWHM laser pulse to see if we can decrease the emittance further. - Measure the transverse emittance of a compressed beam to study the predicted emittance increase in the deflection plan (as CERN studies).

- Pursue the user experiments.