the new superconducting rf photoinjector a high-average...

Institute of Radiation Physics Jochen Teichert www.fzd.de Forschungszentrum Dresden-Rossendorf

Radiation Source ELBE

Jochen Teichert for the BESSY-DESY-FZD-MBI collaboration and the ELBE crew

The New Superconducting RF Photoinjector a High-Average Current & High-Brightness Gun

High-Power Workshop, UCLA, Los Angeles 14 – 17 January 2009



Generation of high-brightness electron beams

LASER

PHOTO CATHODE

1. direct production of short pulses:

laser & photo cathode

2. high acceleration field at cathode:

radio frequency field

3.  CW operation for high average current:

superconducting cavity

SRF PHOTO INJECTOR

RF FIELD

SC NIOBIUM CAVITY

e-

INTRODUCTION



DESIGN & PARAMETERS

e-beam

Cathode

Choke Filter Half Cell

3 TESLA Cell

design value: Epeak = 50 MV/m (TESLA cavities at DESY) obtained: Epeak ≈ 20 MV/m

MAIN COMPONENTS

The heart – superconducting niobium cavity In liquid He tank Top = 2.0 K (31 mbar)

SC Nb: Tc = 9.3 K superfluid He: TSF = 2.17 K

Maximum RF acceleration field is limited by particle pollution: quenches & field emission

Field value sufficient for gun operation, but to meet the design values fabrication of new cavities with advanced design is under way



cathode

cathode cooler

LN2 reservoir

Ø10 mm Cs2Te cone for positioning & thermal contact

pressure spring bayonet fixing

Requires special support and cooling system •  no mechanical contact with Nb cavity •  cooling with liquid nitrogen (77 K)

MAIN COMPONENTS

The photo cathode •  normal conducting •  semiconductor (NEA) Cs2Te advantages: high quantum effic. > 5 % @ λ= 262 nm long life time robust (UHV 10-9 mbar)


Radiation Source ELBE ELBE USER FACILITY

•  New Injector for the ELBE SC Linac •  Test Bench for SRF Gun R&D

150 TW Laser


Radiation Source ELBE COMMISSIONING - FIRST ELECTRON BEAM

Cu cathode

First beam of the 3½ cell superconducting rf photo gun on November 12th, 2007

Beam spot on the first YAG screen in the BESSY diagnostics beamline

RF: Eacc = 5 MV/m f = 1300. 38327 MHz, 150 Hz bandwidth Pdiss = 6 W Laser: 263 nm, 100 kHz reprate 0.4 W power (4 µJ) temporal profile: 15 ps FWHM Gaussian lateral profile: 4 mm x 6mm spot, Gaussian Cathode: Cu, Q.E. ≈ 10-6

Electron beam: 2.0 MV energy 50 nA average current,0.5 pC bunch charge


Radiation Source ELBE Cs2Te PHOTO CATHODES

Photo cathode preparation lab at FZD

preparation process storage & recovery

May 08: First set of Cs2Te cathodes in the SRF gun

Quantum efficiency scan in SRF gun

Q.E. = 10-3

insufficient vacuum in transfer chamber during manipulation


Radiation Source ELBE BEAM PARAMETER MEASUREMENT

Schottky scan – energy & energy spread screen DV04 (YAG) 4.4 m from cathode

screen DV05 same optical path as DV04

-160°, σx = 600 µm

energy energy spread

15 pC


Radiation Source ELBE BEAM PARAMETER MEASUREMENT

Transverse Emittance – Solenoid scan

screen DV02 screen DV01

solenoid for emittance compensation, field precisely measured

Measurement: 2 MeV energy

laser: temporal:15 ps FWHM Gaussian lateral: 2.7 mm diam. sharp edge

-160° 20 pC

σx = 320 µm



parameter present cavity new “high gradient cavity” measured ´08 ELBE high charge ELBE high charge

final electron energy 2.1 MeV 3 MeV ≤9.5 MeV peak field 13.5 MV/m 18 MV/m 50 MV/m laser rep. rate 1 – 125 kHz 13 MHz 2 – 250 kHz 13 MHz ≤500 kHz laser pulse length (FWHM)

15 ps 4 ps 15 ps 4 ps 15 ps

laser spot size 2.7 mm 5.2 mm 5.2 mm 2 mm 5 mm bunch charge ≤ 200 pC 77 pC 400 pC 77 pC 1 nC max. aver. Current 1 µA 1 mA 100 µA 1 mA 0.5 mA peak current 13 A 20 A 26 A 20 A 67 A transverse. norm. emittance (rms)

3±1 mm mrad @ 80 pC

2 mm mrad 7.5 mm mrad 1 mm mrad 2.5 mm mrad

SRF Gun Parameter

BEAM PARAMETERS


Radiation Source ELBE SUMMARY

First Run of SRF Gun operation: about 100 h with Cu cathode, 400 h with Cs2Te Iav = 1 µA, total 15 C (diagnostic mode & radiation safety permission)

Problems during commissioning: •  Cavity cleaning and low gradient •  wrong cavity π-mode frequency at 2 K (corrected in winter shut-down) •  insufficient vacuum in cathode transfer system (improved in winter shut-down)

Answers to the „big“ questions: •  basic principle (NC photo cathode) works well, no limits found •  high current operation: answer will be given in the first run in 2009 •  high gradient and higher brightness: needs an improved cavity Future: •  Oct.- Jan. 09: correction of π-mode frequency for operation at ELBE •  2009: connection to ELBE for tests and user operation run with high current •  test bench for SRF gun R&D (FP7, EuCARD) emittance compensation methods, alternative photo cathodes (GaAs) •  fabrication of two improved cavities, funded by BMBF, replacement in 2010

the new superconducting rf photoinjector a high-average...

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