Schottky-Enabled Photoemission in RF Guns

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Schottky-Enabled Photoemission in RF Guns

• Manoel Conde, Zikri Yusof, and Wei Gai
• High Energy Physics Division

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Motivations

• To find a clearer signature of the Schottky
  effect on photocathodes in an RF photoinjector
• To find a more direct determination of the
  field-enhancement factor on the cathode surface
• Scheme Employ Schottky effect in the RF
  photocathode gun and with low energy photons
• This technique may produce an electron beam with
  ultra-low intrinsic emittance (Yusof et al. PRL
  93, 114801 (2004))

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Photoemission
For a typical cathode
However, for a cathode in an electric field E
-Feff
where hn photon energy F
materials bulk work function a a
constant b field enhancement
factor f RF phase E
Electric field magnitude Our scheme is to use
hn lt F, and then employ the Schottky effect to
lower the effective work function Feff, where
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Schottky Effect
0
z
Image potential e2/16pe0z
DF
Electrostatic potential -eEz
F
Feff
Effective potential
EF
Metal
Feff F - DF
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Schematics of Beamline
Light source Frequency-doubled TiSapphire laser
372 nm (3.3 eV), 1 4 mJ, 8ps. Photocathode Mg,
? 3.6 eV. Example of Schottky effect on the
cathode at E(q) 60 MV/m, DF 0.3 eV
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RF Scans E-Field on Cathode Surface
RF Phase
e
e
hn
Laser injection
e
E(q) - Emax sin(q)
Photocathode
RF frequency 1.3 GHz (Period 770 ps) Laser
pulse length 6 8 ps Metallic photocathode
response time fs
We can safely assume that all the photoelectrons
emitted in each pulse see the same E-field
strength
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Charge Obtained from RF Scans
Theoretical RF Scan
Experimental RF Scan
Our scans
X.J. Wang et al. Proc. 1998 LINAC
Theoretical result from PARMELLA simulation of
our RF photoinjector (H. Wang) We see the
expected flat-top profile Full range of charge
detected 130 degrees
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Experimental Results 1 RF Phase Scan
An RF phase scan allows us to impose different
electric field magnitude on the cathode at the
instant that a laser pulse impinges on the
surface, i.e. E(q) Emax sin(q).
New observation
Typical photoinjector conditions
hn 3.3 eV F 3.6 eV Laser beam diameter 2
cm (0.35 mJ/cm2) A noticeable shift of the onset
of photoelectron production with decreasing RF
power.
hn 5 eV, F 3.6 eV No change in the phase
range over all RF power.
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Determination of Field Enhancement Factor
At threshold, Q 0. This allows us to make a
reasonable estimate of the maximum b.
q0 (deg) Emax (MV/m) E0 Emax sin(q0) (MV/m) b
20 28 9.2 6.8
30 17 8.5 7.3
50 14 11 5.8
hn 3.3 eV F 3.6 eV
This is a new and viable technique to
realistically determine the field enhancement
factor of the cathode in a photoinjector
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Experimental Results 2 Intensity

• Parameters
• hn 3.3 eV
• 3.6 eV
• E field on cathode 80 MV/m
• Laser spot diameter 2 cm

As we increase the laser intensity, we detect
more charge. We definitely are detecting
photoelectrons and not dark current!
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Experimental Results 3 Detection Threshold?
Simulated Detection Threshold Using A Sine
Function
Experimental Observation
Emax 28 MV/m
No cutoff
With cutoff
Two different scans with different amount of
charge produced, but with the same RF amplitude,
show the same phase angle for the photoemission
threshold.
The shift in the photoemisson threshold is not
due to the detection threshold.
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Summary

• We have shown the ability to use the Schottky
  effect to tune the RF photoinjector to be right
  at the threshold of photoemission of our cathode.
  This is a new technique for RF photoinjectors.
• The threshold condition used here is also a new
  and viable method to accurately determine the
  maximum field enhancement factor of the
  photoinjector cathode.
• This technique can be used to determine the field
  emission factor for other materials such as Nb
  and Cu, which are common materials for RF
  cavities and accelerating structures.

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Experimental Results
Fitting parameters for 1cm beam a1 0.14 a2
0.96 Fitting parematers for 2cm beam a1 0.11
a2 0.02
Laser beam diameter 1 cm (1.3 mJ/cm2). hn 3.3
eV, F 3.6 eV
Indication that photoemission from 1 cm laser
beam is dominated by 2-photon process while that
from the 2 cm beam is dominated by the
single-photon process.
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Summary of RF Scans
hn 5 eV, F 3.6 eV QE 1.1 x 10-4
hn 3.3 eV, F 3.6 eV, 1 cm laser diameter QE
2 x 10-6
hn 3.3 eV, F 3.6 eV, 2 cm laser diameter QE
0.7 x 10-6