Broadband source of coherent THz
radiation based on compact LINAC
Ivan Konoplev, A. Seryi
JAI, Department of Physics, University of Oxford
J. Urakawa, A. Aryshev, K. Lekomtsev, M. Shevelev
High Energy Accelerator Research Organisation, 1-1 Oho
Tsukuba, Ibaraki 305-0801, Japan
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Introduction
• Requirements to the radiation
– Broadband (from 0.1THz to 10s of THz), short-pulse (fs), high-field
intensity
– Tuneable, single mode, single frequency
– Amplification at THz frequencies
– Average Power???
• Requirements to source
– Compact
– Energy efficient and easy to run
– Capable of operating at high-repetition rate
• Environment requirement
– Not sensitive to surrounding environment
– No or low level of ionising radiation
– Low DC Voltage
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Available THz sources
(solid state, not LINAC based)
• Solid State Oscillators (High frequency Gunn, IMPATT and TUNNET diodes
produce 100 mW@100GHz but falls as 1/f2 and at 400 GHz is typically in the range 0.1
to 1 mW)
• Gas and Quantum Cascade Lasers (typically up 100 mW operating in
range from 0.3THz to 5THz ,
– low temperature, large magnetic fields are required for QCLs
– CO2 laser is required to pump the THz Gas laser
• Laser Driven THz Emitters
(operating in 0.2THz to 2THz with average power from nano- to microwatts)
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Available THz sources
(vacuum tubes, not RF LINAC based)
Vacuum electronics:
• Gyrotrons (not tuneable, high >10T magnetic fields is required)
• BWO (in frequency range from 30GHz till 1.2THz,
power up to 100 mW@1.2THz)
• Orotrons (Planar Smith-Purcell oscillators up to
1THz, power from 100mW@1THz to 1W@0.3THz)
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Available LINAC based sources
• Coherent Synchrotron radiation
sources
• Free Electron Lasers
• Coherent Smith-Purcell radiation
(cSPr)
• Coherent Diffraction and
Transition radiation
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10MeV LINAC THz sources - proposal
Schematic cartoon of the LINAC based radiation source with cavity and undulator/grating
10MeV
Bunching of continuous beam
inside cavity
Coherence is insured by either installing a cavity or via pre-bunching electron beam
making dimensions of the bunches smaller as compared with operating wavelength.
 dI 
 dI 
2



  [ N e  N e ( N e  1) | F (  ) | ]
 d  d   N e  d  d   sp
Incoherent radiation
Coherent radiation
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Price of the 40MeV LINAC facilities
X-band RF gun
To produce THz coherent synchrotron radiation (CSR)
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Smith-Purcell radiation

 1
  cos  
 
m 

x
observer

Dispersion relation links radiated wavelength and observation angle 
e-bunch
y
z
x0
 2 x0
 dI 

  F exp  
 d   sp
 e
e 

2





1    sin  sin 
2
2
2
2
For small  and  such that ()<<1/
1/ x0 is the distance between beam and the periodic
structure
2/ e is the electron beam - EM wave coupling parameter
3/ further electron beam is away smaller energy transfer to
EM wave
e 

2
10MeV beam should be 0.1mm away
to generate radiation at 10THz
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Smith-Purcell radiation
E203 experiment at FACET SLAC
Exponential growth of intensity with decrease of beam
grating separation
Beam energy 20GeV
Number of electrons per
bunch 1.8E10 (up to 3nC)
Signal intensity
500um grating
600mm
1
0.8
Toward the beam
0.6
Position (AU)
450mm
Radiation and its
spectral distribution
0.4
Green markers - 1.5 mm grating - approx. range (0.1THz -1THz)
Blue markers – 0.25mm grating - approx. range (5THz- 0.7THz)
Red markers - 0.05 mm grating - approx. range (3THz-17THz)
0.2
0
0
5
10
Frequency (THz)
15
20
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Directionality of cSPr
PiC modelling of cSPr radiation from 500um planar grating
Ez
Ez is parallel to
electron beam
trajectory and located
in x-z plane
x
X


Ey
grating
Y
z
y
Schematic position of the bunch moving
toward us
The divergence angle 2 (approx. 6) of the cSPr from planar 1D grating of 500um
period. Future gratings can be designed to minimise the divergence i.e. simplify the
y1.4mm
optics required to collect the signal.
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Recipes to improve lasing efficiency
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Surface fields excitation on 1D and 2D
cylindrical PSL by electron-beam
Surface fields excitation of by an electron beam. The plots were taken at 0.55ns
(first column), and 0.45ns, 0.4ns (second column Bz, Ez plots respectively).
SF
scattered waves
he
Dashed lines
squares indicate
the location of
PSLs, white line
shows the
boundary of the
beam
ve
SF
scattered waves
ve
he
(a)
Source of coherent SP radiation
Electron bunch having the
following parameters:
- 0.8nc
- Bunch length 800fs
- Electron energy 8MeV
Er
Er
Ez
Ez
FFT(Er)
Source of coherent SP radiation
concept of experiment at LUCX (KEK, Japan)
The 2Q/ time (EM field
decay time inside the
cavity) is less as
compared with distance
between two
neighbouring bunches
1/ The (2Q/) time is
controlled by additional
feedback mirrors
2/ Timing between
feedback loop and
distance between bunches
also can be controlled
3/ Tuning via frequency
selection inside feedback
loop
Cylindrical
Cylindrical
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Conclusion
• Brief overview of THz radiation sources
• Steps toward coherent THz radiation source
• Concept of new THz source of coherent
radiation based on cSPr
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Thank you
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