THz generation and transport

1
THz generation and transport

• Sara Casalbuoni
• DESY Hamburg

2
Overview

• Motivation
• Design goals
• Simulation tools
• - POP ZEMAX
• - Mathematica code (B. Schmidt, DESY)
• THz beam extraction
• Optical design
• Simulation of the THz radiation transfer line
• Summary and outlook

3
Motivation
Bunch length measurements with interferometer
CDR/CTR 140 m, g 1000, Eel
500 MeV
4
Design goals

• Flat frequency response
• Low frequency response limit as low as possible
  BUT
• Finite dimensions of transfer line tube and
  mirrors (? 200mm)
• Long transfer line 20m
• High frequency structures expected from µbunching
  up to 30THz(10µm)

5
Diamond window
30THz
6
Vacuum needed
from B. Schmidt
7
Simulation Tools
2 CODES

• ZEMAX
• Commercially available
• POP (Physical Optic Propagation)

• B. Schmidt (DESY)
• Mathematica

Huygens-Fresnel principle Every point of a wave
front may be considered as a centre of a
secondary disturbance which gives rise to
spherical wavelets, and the wave-front at any
later instant may be regarded as the envelope of
these wavelets. The secondary wavelets mutually
interfere.
Kirchhoff integral
?L
?
finite size screen
y
?
d
x
L
first order second
order Fraunhofer near field,
Fresnel
8
Input for CTR
Fourier Transform with respect to the
longitudinal coordinate ?z-vt of the radial
electric field Er of a uniform bunch charge
distribution of radius ? moving with velocity v
in straight line uniform motion (M. Geitz, PhD
Thesis).
9
Ginzburg-Frank
Valid - if CTR screen radius ? effective CTR
radius
a ? ? ? -if L ? ?2 ? far field
(Castellano Verzilov,
Phy.Rev.ST-Accel. Beams ,1998)
? frequency independent
10
a30mm
L10m
?300µm
Both conditions satisfied
from P. Schmüser
11
a30mm
L20m
?1.5mm
Finite size CTR screen
from P. Schmüser
12
a30mm
L0.5m
?300µm
Near field
from P. Schmüser
13
?1.5mm
L0.5m a30mm
Both conditions violated
and exact SQRT
from P. Schmüser
14
Simulation Tools
2 CODES

• ZEMAX
• Commercially available
• POP (Physical Optic Propagation)

• B. Schmidt (DESY)
• Mathematica

Both make use of scalar Fresnel diffraction theory
x-?,y-?ltltL
Valid if objects and apertures gtgt ?
y
?
?
Fourier transform of
x
L
15
Free propagation
if a ? ?? and/or L ? ? ?2 ? I (x, L,?) frequency
dependent L1 m a10 mm ? 1000
16
Dimensions of the CTR screen
window ?20mm
L40mm ? 1000 ?1.5mm ? f200GHz
17
CDR screen
window ?20mm
L40mm ? 1000 ?1.5mm ? f200GHz
?20 mm, slit1mm
20 mm
?20 mm
?
?
?
intensity _at_window/_at_screen 0.75
0.55
0.78
18
Optical design technical drawing
from O. Peters
19
Optical design
diamond window ?20mm
M2 f4500mm ?188mm
M3 f3500mm ?188mm
M1 f630mm ?188mm
M4 f3000mm ?188mm
M5 f100mm ?100mm
final screen
40mm 560mm 3410mm
8990mm
3100mm 2400mm 100mm
CTR screen ?25mm
M1
M4
600mm
100mm
M5
2400mm
3410mm
3100mm
M2
M3
8990mm
20
Simulation of the THz radiation transfer line
with ideal thin lenses
?1.5mm ? f200GHz ? 1000
CTR screen ?25mm Diamond window
?20mm M1
M2
M3
M4
M5 final
screen ?8mm
Intensity _at_final screen/_at_window0.49
21
Simulation of the THz radiation transfer line
with ideal thin lenses
?1.5mm ? f200GHz ? 1000 CTRscreen?25mm Diamon
d window?20mm
Intensity _at_final screen/_at_window0.49
22
Good also for a Gaussian beam
?1.5mm ? f200GHz ? 1000 CTRscreen?25mm Diamon
d window?20mm
23
Simulation of the THz radiation transfer line at
different frequencies
Intensity _at_final screen/_at_window
0.49 0.90 0.99
24
Transfer function
?(mm)
3 0.3
0.03
25
Half screen and horizontal polarization
frequency response
?
Intensity _at_final screen/_at_window
0.52 0.91 0.99
26
Gaussian beam ?500nm
w01mm Diamond
window ?20mm M1
M2
M3 M4
M5
final screen
27
Summary and outlook

• 2 simulation tools very good agreement
• Optical design for the THz beam line transfer
  _at_140m in TTF2 tested for CTR, CDR and Gaussian
  beam
• Outlook
• -ideal mirror surface
• -tests for stability against beam displacement
  and mirrors misalignment
• -effect of tilting CTR screen

28
Electric field of a charge in the laboratory
system
observation point
?
?
q
29
L ? ?2 ? far field
characteristic size of the CTR screen
30
Paraboloid mirrors
CTR screen ?25mm Diamond window ?20mm
M1
M2
M3
M4 M5
final screen
31
Paraboloid mirrors
32
Transfer function
33
Paraboloid mirrors half screen and horizontal
polarization
Half CTR screen ?25mm Diamond window ?20mm
M1
M2
M3
M4 M5
final screen
Intensity _at_final screen/_at_window 0.64