Low Emittance Gun as a Reliable Source for a XFEL - PowerPoint PPT Presentation


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Low Emittance Gun as a Reliable Source for a XFEL


Parameters for photo emission and FE are chosen such that the accelerator design ... Holding HG without breakdown in operation mode, CHALLENGE ... – PowerPoint PPT presentation

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Title: Low Emittance Gun as a Reliable Source for a XFEL

Low Emittance Gun as a Reliable Source for a XFEL
  • F. Le Pimpec et al. PSI-XFELSLS symposium - PSI
    October 2007

Emittance (?) , what is that ?
Ellipse equation
  • ? 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)

Area of the ellipse ?. ? ? U
The normalized emittance is a quantity which is
invariant upon acceleration
PSI-XFEL Electron Gun
Building an e- source of low emittanceX-FEL
Class 1 glove box CO2 Cleaning
RF waveguide 1.5 GHz
Tesla coil
emittance monitor
LASER focalizing system
Energy spread monitor
  • (-)500 kV Pulser Upgradeable 1MV
  • 250 ns FWHM pulse length- Gap 4mm not shorter
    due to emittance blow up- Field 125MV/m (250

Small emittance means small size small
divergence and NOT a beam just perfectly //
FEL have a peak brilliance of a few order of
magnitude above 3rd generation light sources
(SLS) Scientifically interesting
Project Goals (comparisons)
Hamburg (De)
Spring8 (Jp)
Photocathode Thermionic FEA
existing linac
Electron Source
  • Field Emitter
  • Array
  • ? Commercial
  • ? PSI made
  • Single tip FE
  • Photocathode
  • Thermionic
  • Hybridizing the sources is also possible

Which source and why ?
Goal Emittance lt 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
Ekin Thermal Agitation of produced electrons
If h? ? ?, E ? 0 very cold beam, but QE is bad !
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) (r 20nm 1
After 2nd gate (Focusing) (r 500nm 0.03 eV)
Overall initial ? if only 1 gate
Overall ? after 2nd gate
Any further accelerator optics will access only
the overall envelop
Low emittance electron source
Challenge sufficient current, low emittance
(5.5 A, lt 0.05 mm
  • 1. Field Emitter Arrays (FEA)
  • 2. Single tip field emitter (needle cathode)

PSI extracted current I/tip 20 µA
(DC) Spindt 120 mA (10ns) over 50.103 tips
? 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)
Field emitter array survival
Single Gated Field Emitter Array Process
Need Metal Based Double Gated Array Process and a
Few Thousand Tips
Chips - HD IndustryAMD - Intel - Applied
material KLA Tencor - Maxtorr etc Usually
need a better (speed reliability) process every
6 -12 month
FEA community B spend on FE TV screen in 80s.
Now small TV screen based on CNT Few
universities, small companies and us !
With the time frame for the PSI-XFEL projectPLAN
B has to be initiated
  • Team of 2 physicist/material scientist 1
    technician 0 PhD students
  • Shared RD lab (Availability Contamination
    with other material are serious issues)
  • Old equipment (Lab was down for 3 month)
  • Literature on Metal process existing lets go dig
    it out !
  • Team of physicist/material scientist engineer
    - technician PhD students
  • Dedicated RD lab
  • Top of the line equipment
  • Extensive literature on Si is existing and
    know-how readily available

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
  • LASER has to be synchronized with RF acceleration
  • Can produce some low emittance beam, and seem
    easier than the thermionic gun (mechanic)

Note Parameters for photo emission and FE are
chosen such that the accelerator design is the
same for both !
LEG 500kV Pulser (a very complex machinery)
SF6 (2bars)
Diode (500kV) focusing RF acceleration (2?)
4 mm gap5.5 A
Focusing - matching solenoid (coils counter
3.5 MeV6-7 A
LEG 500kV Pulser
  • Provide high acceleration field to freeze the
  • Allow fast exchange of FEA cathode via a Load
  • Somehow versatile (Cu-Photocathode instead of
    FEA at 1st) (Plan B)
  • Synchronization of E pulse with RF, and now with
  • Holding HG without breakdown in operation mode,
  • Minimizing dark current (parasitic electron beam)

Cathode Voltage( -190kV peak)
XR Scintillator (arbitrary scale)
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)

  • 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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