Synchronization Performance of S-band Linac with Photocathode RF injector

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Synchronization Performance of S-band Linac with
Photocathode RF injector
H. Iijima, M. Uesaka1, H. Tomizawa2, N. Kamagai2,
T. Ueda1 National Institute of Radiological
Sciences (NIRS) 1Nuclear Engineering Research
Laboratory, University of Tokyo 2Japan
Synchrotron Radiation Research Institute
(Spring-8)
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Outline

• The S-band linac with Mg photocathode RF
  injector has been developed for radiation
  chemistry.
• The radiation chemistry experiment requires a
  time resolution in a range of sub-picosecond.
• The time resolution is defined by
• pulse duration of pump-beam and probe-laser,
  synchronization between the beam and laser,
• and the beam intensity.

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Chemical Reaction of Water
Final target !
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Radiation Chemistry
Chemical reaction of water
Pulse radiolysis method
Systematic time-resolution is 12 psec !
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Systematic Time Resolution
Factors of time resolution I Pulse duration
of electron and laser II Stability of
synchronization between pump-beam and
probe-laser III Thickness of water cell
Water cell (Refractive index 1.33)
Probe-Laser
1 mm 1 psec
Pump-Beam
Thickness of water cell causes a time difference
between the pump-beam and probe laser.
Thin water cell Intense pump-beam
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Requirements
Pulse radiolysis in a time range of sub-picosecond

• I Ultra-short bunch and laser
• Pump-beam Utilization of a chicane-type
  magnetic compressor
• Probe-laser Femtosecond laser
• II Stable synchronization
• Jitter Synchronization lock frequency
• Drift Laser transport line
• III Intense electron bunch
• High QE Mg cathode Laser cleaning (future
  plan)

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Target of Time Resolution
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Linac System
Beam Spliter (11)
Laser Transport Line (50 m)
Compressor THG
Compressor
Chicane-type Magnetic Compressor
Solenoid Coil
Water Cell
Accelerating Tube
RF Injector
Klystron (15 MW max.)
Q-Magnet
TiSapphire Laser System
Master Oscillator

• The Mg photocathode is used as the injector.
• The electron is accelerated up to 22 MeV by a
  S-band accelerator.
• The electron bunch is compressed by a
  chicane-type magnetic compressor.
• The TiSapphire laser is used for the driven
  laser of the injector and the probe-laser.

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Laser System
Laser Transport Line (50 m, 80)
100mJ/pulse, 3 psec
Compressor THG
Compressor
Beam Spliter (11)
Driven laser
12mJ/pulse, 100fs
10Hz, 30mJ/pulse
Probe-laser
Regenerative Amp. (pulse selector) Multipath
Amp.
YAG Laser (pump)
LD Laser (seed)
CW,5W
Stretcher
TiSapp. Laser
79.33 MHz
Master Oscillator
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Performance of RF Injector
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Ultra-Short Bunch Laser

• The pulse duration of probe-laser is 100 fsec
  using TiSapp. Laser.
• The short bunch of pump-beam is obtained by the
  chicane type magnetic compressor.
• The bunch duration was achieved down to 0.7 psec.

Streak image of electron bunch (Charge
1.5nC/bunch)
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Stable Synchronization
Higher Harmonic Synchronization Lock
LD Laser (seed)
CW,5W
Stretcher
TiSapp. Laser
79.33 MHz
Master Oscillator
79.33 MHz
New 714 MHz
1/6
? 9
473 MHz
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Result of Higher Harmonic Lock
Timing interval between RF and laser
Timing drift of long term was left.
Timing jitter was suppressed.
2 hours
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Timing Drift
Mechanism of timing drift
The laser transport line is 50 m long, and 14
bellows are used.
Transport line chamber
Bellows
Mirror
(iii) The mirror chamber with flexibility due to
the bellows is moved by the pressure.
Vacuum
Air
(i) The chamber of transport line is expanded and
contracted by a change of temperature.
(ii) The pressure difference between inside and
outside of the transport line chamber applies the
force to the mirror chamber.

• In the chembers Vacuum Atmospheric pressure N2
  gas.

To suppress the pressure effect!
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Stable Synchronization
Result of transport line improvement
The pressure effect due to evacuated transport
line was suppressed. The expansion and
contraction effect due to temperature was left.
1.4 psec (rms) for 1 hour
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Intense Electron Bunch (Future Plan)
Mg cathode (High QE, 10-3)
Our cathode QE 1.3 ? 10-4 (at present)
Thin water cell High intensity of electron beam
Laser cleaning of the cathode
Craters due to the RF discharge on the cathode
surface
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Summary

• We have developed the linac system for the
  radiation chemistry.
• The pump-and-probe method for the chemical
  reaction of water requires ultra-short bunch,
  stable synchronization and intense electron
  bunch.
• The ultra-short bunch is realized to use the
  femtosecond laser and chicane-type compressor.
• The synchronization is achieved to be 1.4 psec
  for 1 hour.
• The laser cleaning is planning to obtain high
  charge electron bunch.