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Photocathode based Electron
Sources for Particle Accelerators –
Yesterday, Today and Tomorrow
B.L. Militsyn
STFC ASTeC, UK
European Workshop on Photocathodes for
Particle Accelerator Applications,
6-8 June 2016, Daresbury, UK
Outline
• Days before yesterday – discovery of the photoeffect
• Yesterday – first photocathode based electron
sources
– DC photocathode guns
– RF photocathode guns
• Today – evolution of photoelectron sources
– DC
– RF (SRF)
– VHF
• Tomorrow – what do we expect from the future
photocathodes
B.L. Militsyn, EWPPAA, Daresbury, UK
6 June 2016
Days before yesterday.
Discovery of the Photoeffect.
The Nobel Prize in Physics 1921 was awarded to Albert
Einstein "for his services to Theoretical Physics, and
especially for his discovery of the law of the
photoelectric effect".
Heinrich Rudolf Hertz
(1847-1894)
Philipp Eduard Anton
von Lenard (1862-1947)
Albert Einstein (1879-1955)
B.L. Militsyn, EWPPAA, Daresbury, UK
6 June 2016
Yesterday. SLAC DC photocathode gun
Photocathode used,
Maximum polarization
B.L. Militsyn, EWPPAA, Daresbury, UK
6 June 2016
GaAs
40%
Yesterday. Los Alamos Photocathode RF gun
Operation Frequency
Photocathode used
Peak surface field
Bunch charge
Peak beam current
Beam emittance
B.L. Militsyn, EWPPAA, Daresbury, UK
6 June 2016
1.3 GHz
Cs3Sb
60 MV/m
12 nC
100-150 A
32 mm·mrad
Yesterday. BNL RF Gun
Photoctahode used
Beam emittance
Cu
3 mm·mrad
B.L. Militsyn, EWPPAA, Daresbury, UK
6 June 2016
Today: Very general classification of the
photoelectron sources
• Out of the gun photocathode preparation and
exchange with a Load-Lock system
• Electron emission
– Photocathodes
• Metal
• Amorphous semiconductor
• Crystalline semiconductors
• Electron acceleration
– DC high voltage
– Radiofrequency acceleration
• Normal conductive acceleration
• Superconductive acceleration
– VHF field acceleration
• Normal conductive acceleration
• Superconductive acceleration
B.L. Militsyn, EWPPAA, Daresbury, UK
6 June 2016
Today: Modern DC gun designed for ERL
• Maximum voltage
achieved – 500 kV
• Maximum design current –
10 mA
• Photocathode – GaAs/Sb
• Photocathode preparation
system (not shown)
integrated with the gun
• For protection of the
ceramic insulator from
field emission and
scattered electrons it is
made segmented
B.L. Militsyn, EWPPAA, Daresbury, UK
6 June 2016
Today: NC RF CLARA Gun-400
H shape coupler
RF probe*
Cathode plug and
spring retaining plate.
B.L. Militsyn, EWPPAA, Daresbury, UK
6 June 2016
Cooling channels
Gun Cavity
Today. Design parameters of the Gun-400
Units
Beam Repetition Rate
Operating mode
100 Hz
400 Hz
Frequency
GHz
2.9985
RF peak power
MW
<10
RF average power
kW
10
Gun electric field
MV/m
Bunch charge
pC
Operating mode
RF feedback
B.L. Militsyn, EWPPAA, Daresbury, UK
6 June 2016
120
100/80
20-250
250
Pulsed/Train
pulsed
Pulsed
Required
Today: The LBNL VHF photocathode gun
Photocathode used Metal/Te/Sb
B.L. Militsyn, EWPPAA, Daresbury, UK
6 June 2016
Today: ELBE SRF Photocathode gun
RF frequency
RF scheme
Operation mode
Photocathode used
B.L. Militsyn, EWPPAA, Daresbury, UK
6 June 2016
1.3 GHz
3.5 cells
CW
Cs2Te/Metal
Today: Photocathode Materials
Class
Mate
rial
QE
Response
time
Intrinsic
Energy
Wave
Length
Gun
Application
Normally
conducting
metals
Cu,
Mg
10-510-4
10’s fs
100’s mV – 1’s V
UV
NC-RF,
VHF
Low rep rate
FELs
Super
conducting
metals
Nb,Pb
10-510-4
10’s fs
100’s mV – 1’s V
UV
SC-RF
High rep rate
FELs
Positive eaffinity Tebased
Cs2Te
0.10.2
ps’s
100’s mV
UV
NC-RF,
SC-RF
VHF
DC
High rep rate
FELs
Positive eaffinity Sbbased
Cs3Sb
etc
0.10.2
ps’s
100’s mV
Visible
DC
VHF,
SCRF(?)
NCRF(???)
ERLs, High rep
rate FELs
Negative eaffinity
semiconductor
GaAs,
etc
0.10.35
1’s ps – 100’s ps
10’s mV – 100’s mV
IR- Visible
DC (XHV)
Polarized
sources, ERLs
B.L. Militsyn, EWPPAA, Daresbury, UK
6 June 2016
Tomorrow: general requirements for
electron accelerators:
• Beam energy – 100’s keV – GeV’s
• Bunch peak current – Amps – kA’s
– Bunch charge – fC’s – nC’s
– Bunch duration – fs’s – ns’s
• Beam average current – nA’s –A’s
– Bunch charge – fC’s – nC’s
– Bunch repetition rate – Hz – 100’s MHz
• Beam energy spread – 0.1 %’s
• Beam emittance – sub µm’s
• Beam synchronisation – less than10’s of fs
B.L. Militsyn, EWPPAA, Daresbury, UK
6 June 2016
Compact Linear Accelerator for Research
and Applications CLARA
FEL OUTPUT
STUDIED
VELA RF Photoinjector Test
Facility
Notes:
1. CLARA uses European RF frequencies
2. X-band cavity is used for phase space
linearisation
CLARA Phase I
Commissioning starts September 2016
B.L. Militsyn, EWPPAA, Daresbury, UK
6 June 2016
Tomorrow: Requirements to the
photocathodes for the future accelerator
•
•
•
•
High Quantum efficiency
High Robustness
Low intrinsic emittance
Fast response time
We are waiting your new ideas!
B.L. Militsyn, EWPPAA, Daresbury, UK
6 June 2016
Thank you for your
attention!