Transcript Low emittance gun - Stanford University

Low Emittance Gun as a
Reliable Source for a
XFEL
F. Le Pimpec et al.
PSI-XFEL
SLS symposium - PSI October 2007
PAUL SCHERRER INSTITUT
Emittance () , what is that ?
•  describes the phase space
area/volume occupied by the beam
•  is a measure for the “parallelism”
of the beam.
•  is a measure for the beam quality
( : mm.mrad)
Ellipse equation
  u  2    u  u    u  U
2
2
Area of the ellipse = .   U
u’
0-8
1
U
 U
2
U
7

3
RMS emittance:


x , rms

n , x , rms
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u  u '  uu
2
  
2
2
u  u '  uu
2
2
2
(Erre
ur ! 4
Sourc
e du
5
u
6
U

U
The normalized emittance is
a quantity which is invariant
upon acceleration F. Le Pimpec 2


PAUL SCHERRER INSTITUT
Building an e- source of
low emittance
X-FEL application
PSI-XFEL Electron Gun
Class 1 glove box + CO2 Cleaning
RF waveguide 1.5 GHz
Tesla coil
emittance monitor
LASER focalizing system
Energy spread monitor
Small emittance means :
small size & small divergence
and NOT a beam just perfectly //
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(-)500 kV Pulser Upgradeable 1MV
- 250 ns FWHM pulse length
- Gap 4mm not shorter due to emittance blow up
- Field 125MV/m (250 MV/m)
F. Le Pimpec
3
PAUL SCHERRER INSTITUT
PSI FEL
SCSS
e.g.,
FEL have a peak
- BESSYbrilliance of a few
- FERMI@ELETTRA
order of magnitude
above 3rd
generation light sources (SLS)
Scientifically interesting
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F. Le Pimpec 4
PAUL SCHERRER INSTITUT
Project Goals (comparisons)
Hamburg (De)
Palo-alto(USA)
Beam Energy
E-XFEL
10-20
LCLS
14.3
SCSS
2-8
Peak Current
5
3.4
Bunch Charge
1
Spring8 (Jp)
PSI XFEL
6.0
4.7
GeV
3.5
1.5
1.5
kA
1
1.6
0.2
0.2
nC
1.4
1.5
0.6 (0.8)
0.2
0.2
mm mrad
0.1 (0.085)
0.15
0.1
0.1
0.1
nm
Facility length
3.6
~2
0.75
0.8
<0.7
km
Cost
850
315*
260
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Photocathode
Norm.Emittance
Target h
Thermionic
140 ?
FEA
M€
F. Le Pimpeclinac
5
* existing
PAUL SCHERRER INSTITUT
Electron Source :
1.Field Emitter
1.Array
50μm
● Commercial
● PSI made
2.Single tip FE
2. Photocathode
Cu photocathode
3mm
3. Thermionic
CeB6
4. Hybridizing the sources is also possible
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F. Le Pimpec 6
PAUL SCHERRER INSTITUT
Which source and why ?
Goal :
Emittance < 5.10-8 m.rad
Intensity 5.5 A extracted
Ultimate limit in Accelerators: Thermal emittance of the Electron Source
R: Size of the produced Electron Beam
R 2 Ekin
 n,rms 
2
2 3mc
Ekin: Thermal Agitation of produced electrons
Thermionic Emission
Ekin,r ~
3
kTSolid
2
J  10 A.m
6
T=1500K
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E
2
Photoemission
kin , r
eE
~ h    e
4
J  10 A.m
9
2
Field Emission
Ekin,r ~
0
3
kTSolid
2
J  10 A.m
12
2
T=300K
T=300K
If h  , E  0 – very cold beam, but QE is bad !
F. Le Pimpec 7
PAUL SCHERRER INSTITUT
We choose an electron source based on FE
We want an array of field emitting tips :
1 gate to accelerate the electrons and a 2nd
gate to focus the beamlets and reduce the
emittance …
Initial beamlet 
(after 1st gate)
1 μm
(r~ 20nm; 1 eV)
After 2nd gate
(Focusing)
(r~ 500nm; 0.03 eV)
r’
Overall
initial  if
only 1 gate
r
Overall  after 2nd gate
Any further accelerator optics will access only the overall envelop
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PAUL SCHERRER INSTITUT
Low emittance electron source
Array of 81 tips
Challenge: sufficient current, low emittance
(5.5 A, < 0.05 mm mrad)
1. Field Emitter Arrays (FEA)
5 m
PSI : extracted current: I/tip ~ 20 μA (DC)
Spindt : 120 mA (10ns) over 50.103 tips
2. Single tip field emitter (needle cathode)
→ Pure field emission : 470 mA (2ns)
→ FE triggered by laser : I ~ 2.9 A (16ps)
→ FE in Laser ablation regime : I ~ 5.5 A (30ps)
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F. Le Pimpec 9
PAUL SCHERRER INSTITUT
Field emitter array survival
Local Heat Up
Anode
e-
Heat induced
desorption
+
+ +
Ionization of neutrals
Ion Back bombardment
: Adsorbates
Breakdowns
Most likely to kill
the entire array
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F. Le Pimpec 10
PAUL SCHERRER INSTITUT
Single Gated Field Emitter Array Process
1. Etching mask patterning
4. De-molding
8. Etch-back
SiO2
Si (100)
2. Mold etching
5. Dicing (sawing)
9. Through-hole opening
and polymer removal
6. Deposition of insulator
and gate electrode
Mo
SiO2
3. Multi-layer depositions
and electro-plating
7. Planarization by polymer
Ni
PR
Ti/Pd
Mo
Si (100)
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10. Gate-electrode patterning
(photolithography and etching)
[11. Dicing (sawing)]
F. Le Pimpec 11
Need Metal Based Double Gated Array
Process and a Few Thousand Tips
PAUL SCHERRER INSTITUT
Chips - HD Industry
FEA community
AMD - Intel - Applied material –
KLA Tencor - Maxtorr etc…
B$ spend on FE TV screen in 80’s. Now
small TV screen based on CNT
Usually need a better (speed – reliability)
process every 6 -12 month
Few universities, small companies and
us !
•
•
•
•
•
02.10.2007
With the time
frame
for
• Team of :
Team of :
2 physicist/material scientist –1
the
PSI-XFEL
project
physicist/material
scientist
–
technician
– 0 PhD students
engineer - technician – PhD
Shared R&D lab (Availability &
students
PLAN B • has
to be
Contamination with other material are
Dedicated R&D lab
serious issues)
…
Top of the line equipment initiated
• “Old” equipment (Lab was down for 3
month)
Extensive literature on Si is
existing and know-how readily
available
• Literature on Metal process existing…
lets go dig it out !
$$$$
• $
F. Le Pimpec 12
PAUL SCHERRER INSTITUT
PLAN B : Photocathode electron gun
• Can use a reliable photocathode : Cu , low QE (10-5) but resistant – or
CeTe (others…) higher QE, more delicate
• Photogun needs high acceleration, High E – RF or DC acceleration
• LASER is needed – Laser beam has to be very homogeneous
• LASER has to be synchronized with RF acceleration
• Can produce some low
emittance beam, and seem
easier than the thermionic gun
(mechanic)
LASER beam
homogeneity
Note:
Parameters for photo emission and FE
are chosen such that the accelerator
design is
02.10.2007
the same for both !
©PITZ
F. Le Pimpec 13
PAUL SCHERRER INSTITUT
LEG : 500kV Pulser (a very complex machinery)
SF6
(2bars)
UHV
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F. Le Pimpec 14
PAUL SCHERRER INSTITUT
Diode (500kV) – focusing – RF acceleration (2)
4 mm gap
5.5 A
Focusing - matching solenoid
(coils –counter coils)
Cathode
Anode
e3.5 MeV
6-7 A
2  RF cavity
(1.5GHz – 4.5GHz)
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F. Le Pimpec 15
PAUL SCHERRER INSTITUT
LEG : 500kV Pulser
• Provide
high acceleration field to “freeze” the beam
• Allow fast exchange of FEA cathode via a Load Lock
• Somehow versatile (Cu-Photocathode instead of FEA at 1st) (Plan B)
• Synchronization of E pulse with RF, and now with LASER
• Holding HG without breakdown in operation mode, CHALLENGE
• Minimizing dark current (parasitic electron beam)
Cathode Voltage
( -190kV peak)
XR Scintillator
(arbitrary scale)
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F. Le Pimpec 16
PAUL SCHERRER INSTITUT
The End
• Every XFEL need a low  beam (& high current). Compensate
by beam E increase (Trade between E and   $)
• Thermionic and photoguns are more mature technologies
(FLASH & SCSS have already lased at 13nm & 49nm)
• FEA & HG – PSI challenge and gamble
•
•
•
•
FEA making – Current & pulse production
HG processing inside Pulser without arcing
If laser assisted, insuring no breakdown triggered by laser at HG
Minimizing ion and e- back bombardment, lifetime of e- source (same for
other sources)
• Low Emittance acceleration and transport (pushing the limit of
the simulation codes)
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F. Le Pimpec 17
PAUL SCHERRER INSTITUT
Acknowledgement
• Thanks to my FEL / LMN colleagues for the material
I stole from them …
• Thanks to slides coming from N. Pichoff, L. Staykov,
T. Shintake
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