Development of High-Average-Brightness Sources (Injectors, Cathodes

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Development of High-Average-BrightnessSources
(Injectors, Cathodes Lasers)

• D.C. Nguyen
• Los Alamos National Laboratory
• A.M.M. Todd
• Advanced Energy Systems
• LANL work is performed under the auspices of the
  Department of Energy
• and supported by the High-Energy Laser Joint
  Technology Office and NAVSEA PMS-405

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Outline

• Schematic of a Photoinjector
• Requirements for High-power Beams
• State-of-the-Art Photoinjectors
• DC gun Superconducting RF booster
• Normal Conducting RF photoinjector
• Superconducting RF photoinjector
• Photocathode Developments
• Drive Laser Developments
• Conclusion

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Schematic of a Photoinjector
modelocked laser
aperture
harmonic crystal
beam expander
injector
lens
cathode
solenoid
electron beam
RF coupling
RF signal
klystron
circulator
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Requirements of High-Brightness Electron Beams
for High-Power FEL

• Energy 7 MeV
• Charge 0.1 3 nC
• Repetition Rate lt800 MHz
• Average current 0.1 1 A
• rms Trans. Emittance lt10 mm-mr
• Bunch Length lt10 ps
• Energy Spread lt0.25 of final energy
• rms Long. Emittance lt100 keV-ps

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State-of-the-Art Injectors

• DC Gun SRF Booster 100 mA
• Injector for the highest average power FEL
• Baseline design for 100 kW FEL
• Normal Conducting RF Photoinjector 100 mA 1 A
• Highest brightness at duty factor up to 25
• Workhorse for many FELs
• Superconducting RF Photoinjector 1 A
• Holy grail of RF injectors
• Needs most development to reach maturity

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DC Injector with SRF Booster
HELIUM VESSEL (3)
S/C CAVITY (3)
WR1150 WAVEGUIDE INTERFACE (3)
HELIUM RETURN
SOLENOID (Blue)
GUN H/V COLUMN
END CAN
OPTIONAL CELL
GUN SF6 VESSEL
VACUUM VESSEL
COUPLER WAVEGUIDE TO COAX TRANSITION (3)
SPACE FRAME STRUCTURAL SUPPORT
END CAN
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DC Injector with SRF BoosterDemonstrated
Performance Data
Average Current 9.1 mA

• Beam Energy (w/o Booster) 300 keV
• Beam Energy (w/ Booster) 9.2 MeV
• Charge 122-135 pC
• Repetition Rate 75 MHz
• rms Transverse Emittance 15 mm-mr
• Energy Spread 25 keV
• Bunch Length 10 ps
• Beam Power 84 kW

Source Jefferson Laboratory FEL DC Injector,
performance as of Oct 2004
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DC Injector with SRF BoosterProjected Performance
Average Current 100 mA

• Beam Energy (w/o Booster) 500 keV
• Beam Energy (w/ Booster) 7 MeV
• Charge 133 pC
• Repetition Rate 748.5 MHz
• rms Transverse Emittance 1.2 mm-mr
• Energy Spread 35 keV
• Bunch Length 6.3 ps
• Beam Power 0.7 MW

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100 mA Injector Status
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Normal Conducting RF Photoinjector
Solenoid magnets
Cooling
Non-resonant cell for pumping
Photocathode plate
Tapered ridge-loaded waveguides for RF power
input (0.46 MW each)
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Normal-conducting RF PhotoinjectorEmittance RF
Power Consideration
Image charge field
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Normal Conducting RF PhotoinjectorDemonstrated
Performance Data
Average Current 32 mA

• Accelerating Gradient at Cathode 26 MV/m
• Beam Energy 5 MeV
• Charge 1-7 nC
• Repetition Rate/RF Frequency 27 MHz/433 MHz
• rms Transverse Emittance 5-10 mm-mr
• Energy Spread 100-150 keV
• Bunch Length (FWHM) 53 ps
• Beam Power 160 kW

Source Dowell et al., First operation of a
photocathode RF injector at high-duty factor
Appl. Phys. Lett., 63 (15), 2035 (1993).
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Normal Conducting RF PhotoinjectorProjected
Performance Data
Average Current 100 mA

• Accelerating Gradient at Cathode 7 MV/m
• Beam Energy 2.5 MeV
• Charge 1-3 nC
• Repetition Rate/RF Frequency 35 MHz/700
  MHz
• rms Transverse Emittance 2-6 mm-mr
• Energy Spread 27 keV
• Bunch Length (FWHM) 10 ps
• Beam Power 250 kW

Source Nguyen et al., Overview of the 100mA
average current RF photoinjector
NIMA, 528 (1-2), 71-77 (2004).
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High-average-current CW NormalConducting RF
Photoinjector Design
2.5-cell cw RF photoinjector
Magnetic Field vs z
Accelerating Field vs z
Beam Energy vs z
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Normal Conducting RF PhotoinjectorProjected
Performance
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Normal Conducting RF Photoinjector Fabrication
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Superconducting RF Photoinjectors
Advanced Energy Systems 703.75 MHz
ELBE Radiation Source 1.3 GHz (Forschungszentrum
Rossendorf)
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Superconducting RF PhotoinjectorDemonstrated
Performance Data
Average Current 1 mA

• Accelerating Gradient at Cathode 22 MV/m
• Beam Energy 12 MeV
• Charge 77 pC
• Repetition Rate/RF Frequency 13 MHz/1300
  MHz
• rms Transverse Emittance 10 mm-mr
• Energy Spread 55 keV
• Bunch Length (rms) 2.5 ps
• Beam Power 12 kW

Source Teichert et al., Results of beam
parameter measurements of the
ELBE electron accelerator NIMA., 507 (2003),
354-6
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Superconducting RF PhotoinjectorProjected
Performance Data
Average Current 1 A

• Accelerating Gradient at Cathode 20 MV/m
• Beam Energy 2 MeV
• Charge 1.33 nC
• Repetition Rate/RF Frequency 703.75
  MHz/703.75 MHz
• rms Transverse Emittance 7.6 mm-mr
• Energy Spread lt37.6 keV
• Bunch Length (rms) 8 ps
• Beam Power 2 MW

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Photocathode DevelopmentReview of Common
Photocathodes
PHOTOCATHODE GOOD BAD
Metallic (Cu, Mg, Ag, etc.) Easy to obtain/handle Still widely-used QE remains constant for long period of time Fast response time (10-15s) Low dark currents Low QE (10-4) at UV No systematic study of effective cleaning rejuvenating method, especially in-situ at photoinjector
Semiconductor Antimonide (Cs3Sb, CsK2Sb, CsNaK2Sb, etc.) High QE (0.05 0.3) Photoelectrons have lower energy spread (in principle) than metallic Works in the green Requires UHV Surface deteriorates with O2 Longer response time than metallic (10-13s) Initial QE has short lifetime
Negative Electron Affinity (GaAs family, GaP, etc.) Even higher QE than semiconductor (0.1 0.6) Widely used in PMT Possible source of polarized electrons (GaAs) Requires UHV May have problems living in rf environment Even longer response time than semiconductor (10-9s)
Liquid Metal, Superconducting compatible (Nb, NbN, etc.) Possible use in superconducting RF photoinjector Very little is known in accelerator situation QE yet to be determined probably same as metallic
Source J. Lewellen, ANL
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Photocathode DevelopmentDispenser Photocathodes
Dispenser-type cathodes with high-QE surface
dispensers
Subsurface Cs reservoir for CsNaKSb cathode
Source Jensen et al., Measurements and analysis
of thermal emission from a dispenser cathode
PRST-AB., 6 (2003), 083501
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Drive Laser DevelopmentQ-Peak Laser with 8.4 W
at 534nm
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Drive Laser DevelopmentLarge-Mode Area Fiber
Amplifier

• Summary of LMA Laser
• 60 W cw green power
• 110 W cw IR power
• 55 IR-Green conversion
• 10 Wall-plug efficiency

Source Mead et al., 60 W output at 540nm from a
frequency doubled LMA fiber laser SSDLTR 2004.
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Conclusion

• Three state-of-the-art photoinjector designs are
  being fabricated by AES to answer the high-power
  FEL requirements
• In order of near-term to far-term
• DC gun superconducting RF booster
• Normal conducting RF photoinjector
• Superconducting RF photoinjector
• New photocathodes are being developed to fulfill
  the Q.E. and lifetime requirements of a
  high-current injector
• Diode-pumped laser technologies have demonstrated
  the cw green power needed for the
  high-average-current injector.