Review of experimental results on photoemission electron sources

1
Review of experimental results on photo-emission
electron sources

• Ph. Piot, DESY Hamburg

Introduction RF-guns DC-guns Conclusions
2
Introduction

• Application of high-brightness photo-injectors -
  high energy linear colliders (needs flat beam
  ey/ex ltlt1) - radiation sources (FELs,
  linac-based SR) - X-rays production (XTR,
  Thomson) - plasma-based electron
  sources-drivers,
• Many accelerator test facilities in operation
  based on photo-injectors - dedicated to beam
  physics (BNL, UCLA, DESY-Z, NERL...) - drive
  user-facility (Jlab, DESY-HH,)
• Figure-of-merits emittance (FELs requires eltl) ,
  peak current, average current (photon flux),
  local energy spread, bunch length (e.g. for
  probing ultra-fast phenomena)

3
Photo-emission from metals and semi-conductors
semi-conductor
metal
(from Spicer et al. SLAC-PUB 6306)
4
Few words on Lasers

• For metal, typical laser energy required 5-500
  mJ/pulse
• For semi-conductor 0.5 mJ/pulse
• Metallic cathodes are bad candidates for
  high-average power machine one might need an
  FEL-based photo-cathode laser to have 100 W level
  in the UV. E.g. see Zholentss talk at BNL PERL
  workshop 01/2001

5
Emittance and Brightness
Phase-space emittance (Liouvilian invariant)
(what codes give)
Trace-space emittance (experimentally measurable)
Normalized brightness
beam current
6
Thermal Emittance
Electrons are emitted with a kinetic energy Ek
laser spot assumed uniform
with radius r
Example of measurement for Cu-cathode
(Courtesy of W. Graves)
Nonlinear fit gives brf3.1/-0.5,
Fcu4.73/-0.04 eV, and Ek0.40 eV
Linear fit gives Ek0.43 eV
7
Thermal Emittance (CNTD)
To date no thermal emittance measurement for
Cs2Te cathodes has beenperformed plan at INFN
Milano are underway
Several groups have measured thermal emittance of
GaAs
Duhnam et al., on the Illinois/CEBAF polarized
beam (PAC1993) at room temperature
Orlov et al., at Heidelberg (Appl. Phys. Lett.
78 2171 (2001)) at 70 K
The measurements indicate that a reduction of the
cathode temperature results in a lower
transverse kT for the emitted e-. This is
particular to NEA cathodes where electrons from
thermalized population can escape. The price to
pay is the long emission time of 10-20ps
8
Generic photo-injectors
Split injectors

• 1-1/2, 2-1/2 cell cavity with high E-field
• booster section downstream of the gun
• E.g. BNL-gun, FNAL, AWA, DESY,

booster
gun
Integrated injectors

• typically 10-1/2 cell cavity with moderate
  E-field
• long solenoid lens
• E.g. AFEL, PEGASSUS

gun
DC-gun

• DC column with HV 500 kV and higher achieved
• Solenoids rf-buncher
• Booster section
• E.g. IR-Demo

booster
gun
9
Frequency Scaling of photo-injectors
(Rosenzweig and Colby PAC95 Also L C.-L. Lin et
al., PAC95)

• If the operating parameters are scaled following
  the Table, one would expect Brightness w2
• this assumes E-field w1
• Naively scaling the present BNL gun (120 MV/m)
  e.g. to 17 GHz would imply E-field 720
  MV/m!!!

10
MIT 17 GHz gun
Mission Advanced ultra-bright accelerator
developments 1/ has commissioned a 1.5 cell
gun 2/ work on a 2.4 cell gun (gt2 MeV)
(PR. ST. AB vol. 4083501 (2001))
11
MIT 17 GHz gun

• Measured emittance at 50 pC to be 1mm-mrad at the
  gun exit
• Brightness80 A/(mm-mrad)2
• It will be boosted to 800 A/(mm-mrad)2 after
  emittance compensation

• Emittance compensation presently non effective
  (velocity spread) need to increase the beam
  energy at gun exit (will use a 2.4 cell gun)

(PR. ST. AB vol. 4083501 (2001))
12
3 GHz CLIC drive beam photo-injector
Operational since 1996. About 1000h of running
each year since, mainly for CLIC 30 GHz power
production
(Courtesy of H Braun)
13
BNL/UCLA/SLAC gun

• Popular design, used at BNL (ATF SDL), SLAC
  (GTF), ANL (LEUTL), Tokai (NERL),
• Since its first design the gun has undergone
  improvements latest foreseen are a mode-lock
  system and a split symmetric RF input coupler

14
Recent results from ATF, BNL
Example of fitted envelope at 70 MeV

• Beam based alignment of quad to center beam in
  the TWS
• Optimized optics (with a high-b) to overcome
  problems inherent to the screen resolution
• Measured beam emittance using the multi-monitor
  technique
• Obtained e0.8 mm-mrad for Q0.5nC and I200 A

mis-steered beam
centered beam
focused spot
30 um wire
(Courtesy of V. Yakimenko)
15
Recent results from ATF, BNL

• Measurement of impact of transverse
  non-uniformity on emittance
• Used a mask
• Q0.5 nC (kept constant)
• Emittance for uniform beam is about 1.5 mm-mrad
• Long. Length is 3 ps FWHM

(extracted from ATF News Letter 03/2002)
50
60
70
80
90
100
100
90
60
50

• As predicted by simulation, uniform beam gives
  the best emittance
• Emittance doubles for the 50 modulation case

16
Recent results from SDL, BNL
y
Slice emittance measurements

• Parametric study of emittance (projected slice)
  vs various parameters
• Preliminary data indicate brightness improves as
  charge is decreased

t
(Courtesy of W. Graves et al.)
200 pC
10 pC
17
Recent results from SDL, BNL
Observation of sub-picosecond compression by
velocity bunching

• Used the TWS tank downstream of the rf-gun as a
  buncher (operated far off-crest) see M.
  Ferrarios talk

• Measurement were performed using both frequency-
  and time-domain technique

(Piots talk in Working Group I)
18
Recent results GTF, SLAC

• Parametric study of emittance versus bunch charge
• Achieved LCLS project parameters (1.5 mm-mrad for
  I100 A)

• Reconstructed the longitudinal phase space from a
  set of energy profile measurement.
• For Q200 pC, FWHM d8, FWHM t3 ps (initial
  laser FWHM4.3 ps)

(Courtesy of J. Schmerge)
19
2.856 GHz PWT gun (under commissioning at UCLA)

• Integrated injector installed at the PEGASUS
  facility, UCLA
• Exit energy 20 MeV
• E-field 40-60 MV/m
• Charge 1 nC
• Input power 20 MW

Proposed to be used to produced polarized
electron beam using GaAs (which requires 1E-11 T
vacuum) because of the better vacuum conductance
compared to usual cavity-based photo-injector Cle
ndenin et al. SLAC-PUB-8971
(Telfer et al., PAC 2001)
20
FNPL(FNAL) TTF injector II (DESY)
typical parameters for TTF 1-FEL
see also S. Schreibers talk
repetition rate 1 Hz pulse train
length 1-800 µs bunch frequency 1-2.25
MHz bunch charge 1-3 nC bunch length
(rms) 3 mm ( 1 nC, after booster )
norm. emit., x,y 3-4 µm ( _at_ 1nC) dpp
0.13 rms ( _at_ 17 MeV ) injection energy 17
MeV
(Schreiber et al. EPAC2002)
21
Results at TTF Injector 2 (1nC setup)
Emittance measurements
Bunch length measurement (streak cam.)
(Schreiber et al. PAC2001)
(Honkaavara et al. PAC2001)
22
VUV-FEL driven TTF injector

• Primary electron bunches (charge 3nC) are
  produced by laser-driven rf gun
• During single pass of the undulator primary
  bunch produces powerful VUV radiation (l95 nm)
• Radiation is reflected by plane SiC mirror and
  is directed back to the photocathode of rf gun
• Electron bunch produced by SASE radiation
  (charge up to 0.5 nC) is accelerated

(Faatz et al. FEL2002)
23
Results at FNPL, FNAL
Transverse Emittance Studies

• Systematic optimization of the rf-gun parameters
  (solenoids, laser radius) for various charges
• Estimate of brightness indicates it improves with
  decreasing charge

Production of Flat beams

• Used the inverse Derbenev transform to convert a
  magnetized round beam in a flat beam see S.
  Lydias talk
• High ratio of ex/ey50 demonstrated

(Courtesy of J.-P Carneiro)
24
DESY 1.3 GHz gun
 

• Second generation of gun for TTF user facility
• Fully symmetrized cavity using a coaxial
  input-coupler

• Test facility at DESY-Z just commissioned
• Cs2Te thermal emittance measurement are foreseen

25
LANL AFEL Facility
Mission Advanced free-electron laser experiment
at Los Alamos. The gun has driven a IR SASE-FEL

• 1.3 GHz, 101/2 cells
• E-field20 MV/m
• Typical charge 1 to 4 nC
• Exit energy 15-20 MeV
• Macropulse current up to 400 mA

(from Nguyens talk at PERL workshop BNL, Jan
2001)
26
Results, LANL AFEL

• Measure slice emittance using a combined
  quadrupole scan with a streak camera
• Measured slice emittance of 1.6 mm-mrad at 1nC
• PARMELA predicts 0.6 mm-mrad (without thermal
  emittance)

(from S. Giermans Thesis -- UCSD)
27
SRF gun (DROSSEL collaboration)
First phase proof-of-principle observe
photo-emission of a cathode in a superconducting
rf-cavity Later built a real gun that could be
used for CW operation of the ELBE free-electron
laser based at Forschungszentrum Rossendorf

• frequency1.3 GHz
• Number of cell 0.5
• Half-cell is a TESLA cavity shape with a shallow
  cone
• Use a Cs2Te
• No solenoid gt focusing provided by rf
  (conic-shaped back plate)
• First photo-electrons observed last March

(Courtesy of P. Janssen et al.)
28
SRF gun (DROSSEL collaboration)
(Courtesy of P. Janssen et al.)
29
SRF gun (DROSSEL collaboration)
(Courtesy of P. Janssen et al.)
30
The APLE BOEING (decommissioned)

• 0.433 GHz, 2 cells
• E-field25 MV/m
• Typical charge 1 to 5 nC
• Exit energy 2 MeV
• Laser 53 ps (FWHM), 5 mm radius
• K2CsSb cathode

Bucking coil

coil

• duty cycle 25
• Macropulse frequency 30 Hz
• Macropulse length 8.3 ms
• Micropulse frequency 27 MHz

(Courtesy of D. Dowell)
31
Recent results, ELSA-2 Bruyeres-le-chatel

• 0.144 GHz, 2 cells
• E-field25 MV/m
• Typical charge 1 to 10 nC
• Exit energy 2.6 MeV
• Laser 60 ps (FWHM), 4 mm radius

• Macropulse frequency 10 Hz
• Macropulse length 150 ms
• Micropulse frequency 14.4 MHz

(Courtesy of Ph. Guimbal)
32
DC-GUN, JLab IR-Demo
insulating ceramic

• DC gun with GaAs photo-cathode
• Buncher needed despite the 20 ps laser
• In the Ir-Demo gun is coupled to a ¼ cryounit (2
  CEBAF-type 5-cell SRF cavities at 10 and 9 MV/m)
• advantage ran CW at 75 MHz (1/80th of 1497 MHz)
• Recently developed laser (M. Poekler PAC 2001)
  allows CW ope. _at_ 1.5GHz

photo-cathode
anode
solenoid
(D. Engwall et al. PAC1997, Ph. Piot et al.
EPAC1998)
laser
33
DC-gun, JLab IR-Demo

• High voltage operation of DC-gun limiter by
  field-emission
• Collaboration Jlab College of William Mary
  study reduction of field-emission by Nitrogen
  ions implantation on the electrodes
• Experiment performed in a test chamber
  demonstrate the benefits of ion implantation up
  to 25 MV/m DC-field could be achieved with less
  than 40 pA dark current.

(C.K. Sinclair et al. PAC2001)
34
Comparison of Peak brightness
35
Conclusions

• ATF at BNL has set new record in brightness
• Both BNL-type and DESY-type gun have driven short
  wavelength single-pass FELs to saturation (LEUTL,
  TTF-1).
• ELSA-2 at Bruyeres-le-Chatel has demonstrated the
  targeted emittance number of 1 mm-mrad at 1 nC
  (to the expense of bunch length)
• Presently achieved performances with a DC gun are
  comparable to rf-gun running with high duty cycle
  (in term of brightness). - better
  candidate to drive high photon-flux based on
  ERL? - largest average brightness - and E-field
  of 25 MV/m have been achieved in experiment
• Many other developments I have not addressed
  (hybrid DC/RF guns, hybrid plasma/photo-emission
  guns, needle cathodes, etc)

36
Grazie Mile!
Thanks to all the individual aforementioned for
their contributions To M. Ferrario, K.
Floettmann , W. Graves , P. Hartmann, C. Sinclair
for discussions