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

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

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

Concept of Photo-Injector gun: Photo-cathode

TTF INJECTOR BEAM PARAMETERS Quantity Charge per bunch Bunch spacing Bunches per RF pulse Repetition rate TTF spec. 1-8 nC 1 µs Hz Quantity Energy Transverse emittance at 1 nC Transverse emittance at 8 nC TTF spec. 20 MeV 2-3 mm-mrad 15 mm-mrad

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

Oscillator Nd:YLF MHz 2 km optic fiberPockels Cell 1 MHz Multi-pass amplifier Nd-glass Double-pass amplifier Nd-glass 12 nJ/pulse 60 ps 1054 nm 2.5 nJ/pulse 400 ps 800 pulses 2 nJ/pulse 400 ps 100 µJ/pulse 400 ps 0.8 mJ/pulse 400 ps 600 µJ/pulse 400 ps 400 µJ/pulse 4.2 ps 100 µJ/pulse 4.2 ps 532 nm 20 µJ/pulse 4.2 ps 263 nm 10 µJ/pulse 10.8 ps 263 nm LASER (University of Rochester) STACKEDUNSTACKED Spatial filterCompressorBBO CrystalsPulse stacker

UNSTACKED LASER PULSE 4.2 ps FWHM / 20 µJ STACKED LASER PULSE 10.8 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. Mode Resonant frequency Peak field Total energy Peak power dissipation Pulse length Repetition rate Average power dissipation Cooling water flow rate TM010,π 4.5 MeV 2.2 MW 800 µs 10 Hz 28 kW 35 MV/m 4 L/s Q 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. 2A, ~700 Gauss. Bend in the vertical plane Compression ratio ~5 - 6 (theory and measurements)

THE LOW BETA SECTIONTHE WHOLE BEAMLINE SPECTROMETER EXPERIMENT PLASMA WAKEFIELD ACCELERATION

DARK CURRENT STUDIES 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

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)

Round beam Flat beam Effect of the solenoids settings on the dark current

Effect of the vacuum on the dark current

QUANTUM EFFICIENCY STUDIES

Round beam Flat beam Effect of the solenoids settings on the Quantum Efficiency

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

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

Charge Vs. Laser Energy for  = 0.8 mm on the photo-cathode. (Hartman, NIM A340, p , 1994)

TRANSVERSE EMITTANCE MEASUREMENTS Laser RF Gun Capture Cavity 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 Slits width: 50 µm Slits spacing: 1mm

Location of the emittance slits ~ 3.8 m~ 9.5 m~ 6.5 m

Position [mm] Intensity [a. u.] Example: emittance measurement of 8 nC in X3 (z~3.8 m), beamlets in X4 (∆z = 384 mm) BEAM X3BEAMLETS X4

How did we proceed with the emittance measurements ? FIXED PARAMETERS

Q=0.4 nC Q=0.5 nC Q=0.8 nC

Min 205 A

Min 0.5 mm, 260 A

Min 1.2 mm, 260 A

Min 1.4 mm, 255 A

Min 1.6 mm, 245 A

Min 2.1 mm, 225 A

 [mm] Ib=Ip=Is [A] Predicts a decrease of 50% using a 20 ps FWHM laser pulse.

6.5 m 9.4 m

6.5 m 9.4 m

Norm. Emit. Y Z [m]HOMDYNPARMELA ± ± ± ± ± 0.9 Z [m]MeasurementHOMDYNPARMELA ± ± ± ± ± 0.2 CASE Q = 1 nC CASE Q = 8 nC Norm. Emit. Y Norm. Emit. X Norm. Emit. Y Norm. Emit. X Norm. Emit. Y Norm. Emit. X Norm. Emit. Y Norm. Emit. X Norm. 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 cameraOTR screen X6

Time [ps] Intensity [a. u. ] FOCUS MODESTREAK MODE Example: Bunch length measurement of 8 nC in X6 (z~6.5 m)

P>Po P=Po P<Po COMPRESSION PRINCIPLE Po TAIL P>Po HEAD P<Po MOMENTUM PHASE

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 nCQ = 8 nC Prediction MeasurementPredictionMeasurement 13.5 ps10.8 ps 28 ps 10.8 ps 0.7 mm0.8 mm 1.5 mm 1.6 mm 35 deg 40 deg45 deg40 deg 50 MV/m40 MV/m50 MV/m40 MV/m 15 MV/m 12 MV/m15 MV/m12 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 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.