7.8GHz Dielectric Loaded High Power Generation And Extraction

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7.8GHz Dielectric Loaded High Power Generation
And Extraction

• F. Gao, M. E. Conde, W. Gai, C. Jing, R.
  Konecny, W. Liu, J. G. Power, T. Wong and Z.
  Yusof

Advanced Acceleration Concept Workshop, Santa
Cruz, 2008
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Outline

• Introduction
• Design of the 7.8GHz power extractor
• Cold test
• Beam tests
• i. Single bunch tests
• ii. Bunch train tests
• Summary

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Dielectric loaded two beam acceleration
lower group velocity for RF pulse compression

• Power gt100MW
• Frequency up to 30GHz
• RF pulse length a few nanoseconds to a few tens
  of nanoseconds

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Dielectric loaded waveguide and the power
extractor
transverse view
e-field of the TM01 mode (vp c)
dielectric
vacuum
dielectric
Ez of the TM01 mode (vp c)
wakefield
The power extractor using a circular DL waveguide
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Design of the power extractor the deceleration
waveguide
Frequency
7.8GHz Inner diameter
12.04mm Outer diameter
22.34mm Deceleration section length
266mm Group velocity
0.23c Generated power (Gaussian bunch length
2mm) Single bunch 79MW _at_100nC per bunch Bunch
train (Tb 769ps) 100MW _at_30nC per bunch
280MW _at_ 50nC
per bunch
1.1GW _at_ 100nC per bunch
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Bunch Length MeasurementsExperimental Results
old AWA gun
new AWA gun
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Design of the power extractor the RF output
coupler
mode launcher inside
Power coupling efficiency 91.
S21 -0.41dB _at_7.8GHz
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Experimental setup
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Single bunch test
measured
voltage - Volt
t - ns
simulated
voltage arbitrary unit
t - ns
measured
Power Ps - MW
voltage spectrum
q - nC
f - GHz
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Bunch train test UV laser micropulse train
generation
B
A
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Bunch train test electron bunch train generation
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Charge Phase Scan
50 degree
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Check UV laser bunch train with photodiode
energy meter

• voltage signals from photodiode

v - Volt
v - Volt
train A
train B
t - ns
t - ns

• bunch charge comparison with energy meter

voltage - mV
peak voltage - mV
t - ms
index
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10ns long RF pulse generation

• Beam for 10ns RF pulse generation

769ps
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2.4MW, 10ns long RF pulse generation
voltage spectrum
2.4MW generated, 2.2MW extracted.
v - Volt
t - ns
f - GHz
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22ns long RF pulse generation
1.538ns

• Beam for 22ns RF pulse generation

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4-bunch test for high power generation
769ps

• Simulation shows the power reach flat-top
  saturation level when the drive bunch contains 4
  or more consecutive bunches spaced by 769ps.
• To maximize this power level the UV laser bunch
  was only split into 4 bunches.

the 4th bunch
44MW generated 40MW extracted 26.5nC per bunch
t - ns
f - GHz
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Summary

• Dielectric loaded power extraction has been
  demonstrated. 30MW of power has been generated in
  single bunch tests and 44MW in bunch train tests.
  10ns and 22ns RF pulses have been observed.
• Currently the limitation for higher power
  generation is the beam current, which is further
  limited by the QE of the magnesium photocathode
  (10-4).
• A new cesium telluride photocathode with much
  higher QE (10-2) has been developed, yet to be
  installed and tested in a new gun (AWA G3). 280MW
  of output power are expected to be generated by
  electron charge of 50nC per bunch.
• For much higher charge with the new photocathode,
  space charge effect will be much stronger. Thus
  beam confinement with quadruple magnets may be
  needed.