Joint Ph.D Programme on Fusion Science and Engineering
Experimental Work Microwaves
António Silva
Instituto de Plasmas e Fusão Nuclear
Instituto Superior Técnico
Lisbon, Portugal
http://www.ipfn.ist.utl.pt
2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010
Introduction
• Radio frequency (RF) methods are important basis of
modern diagnostics having a key role in the next
generation of fusion machines (ITER, DEMO).
• They can be passive radiometry or active probing
usually using very low power so that perturbation to
the plasma is negligible.
• The combination of confining magnet field and
electron densities determines the plasma dielectric
properties and the resulting cut-offs and resonances,
so that the optimum wavelengths for plasma probing
are in the range of the millimetre to sub-millimetre
waves (10 GHz to 300 GHz).
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2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010
Introduction (cont.)
• Millimetre wave diagnostics allow the accurate
determination of the electron density and
temperature and their fluctuations both in plasma
core and in the gradient region.
• The development of one- and two-dimensional
detector arrays together with sophisticated
tomographic reconstruction techniques made
imaging feasible and are giving new insights into
turbulence structures present in the plasma.
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2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010
Diagnostics
ne  r 
ne  r 
Interferometry
Polarimetry
Reflectometry
S  k, 
Scattering
q r 
Electron
Cyclotron
Emission
(ECE)
Te  r 
Ti
Active probing
Passive
radiometry
Electron
Cyclotron
Absorption
(ECA)
neTe  pe
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2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010
Passive Components
Rectangular Waveguides
• Rectangular waveguides, as opposed to
circular and elliptical waveguides, are by
far the dominant configuration for the
installed base of waveguides for compact
systems like radar and inside equipment
shelters. It is easier to route and mount in
close quarters.
 fc mn
 m   n 


 

2   a   b 
1
2
2
TE10
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2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010
Millimeter Wave Sources
Tunable oscillators
• Yttrium-Iron Garnet (YIG)
• Hyperabruct Varactor
Oscillator (HTO)
• GUNN
Device
Bandwidth
Tuning
Output
spectrum
Output
Power
Sweeping
YIG
Full (40 GHz)
Current
Narrow
< 20 dBm
> 500 µs
HTO
Full ( 20 GHz)
Voltage
Noisy
< 25 dBm
< 2 µs
20 – 25%
(THz)
Voltage/
Mechanic
Narrow
< 30 dBm
> 100 µs
GUNN
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2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010
Millimeter Wave Sources
Frequency Multipliers
• Passive or active frequency multipliers can be
used to extent the frequency operation of tunable
oscillators like HTO.
• They can multiply by 2, 3, 4 or 6.
• Passive multipliers have conversion loss > 13 dB.
• Active multiplier chains can have conversion gain.
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2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010
Millimeter Wave Sources
Dynamic frequency calibration
• Accurate dynamic frequency calibration is vital for profile
accuracy.
• First: frequency markers that gives a dynamic calibration
curve (up to 25 points).
– Not enough to reproduce details of the HTO tuning characteristic
(bumps in group delay curve from metallic mirror).
• Second: interference fringes of calibrated delay line used to
generate a dynamic calibration.
fb 
F  fb
tsample
tdelay
 1
2 tsample
 1
 F 
2 tdelay
tdelay 
8
2d
c
2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010
Millimeter Wave Sources
Dynamic frequency calibration (cont.)
• Frequency calibration circuit
Frequency multiplier
HTO
20dB
Interference fringes
22dB
Interference [V]
0.1
6dB
(a)
0.0
-0.1
-0.2
-0.3
-0.4
(b)
F [MHz]
20dB
50
14.1643 ns
40
30
20
10
0
1 1.5 17 17.5 18
SDLVA
Comb generator
0.5 – 18 GHz
Frequency Markers
Markers [V]
0.5
(c)
0.4
...
0.3
0.2
0.1
0.0
-0.1
0
2
4
6
8 10 12 14 16 18 20 22 24 26
t [s]
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2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010
Millimeter Wave Sources
Dynamic Frequency Calibration (cont.)
• Instantaneous phase evolution obtained from interference signal with Hilbert Transform.
Frequency step F is obtained from phase evolution.
• Error of the recovered metallic mirror position is reduced from 24.9mm (a) to 7.4mm (b)
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2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010
Antennas
Pyramidal antennas
• Pyramidal horn antennas are
aperture antennas obtained by
enlarging the original waveguide
along the electric and magnetic
field planes.
• To avoid phase interference the
probing zone must be in
Fraunhofer zone.
R  2D2 
• This implies that beam width is about four times the antenna
aperture, reducing space resolution.
• We can reduce D however this cause a degradation of the
radiation diagram leading to gain reduction.
• Two main approaches: lenses or focusing reflecting mirrors.
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2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010
Antennas
Lens Antenna
• A lens antenna with its beam focused at
a finite distance (typically a few lens
diameter’s length in front of the lens)
can be configured for applications
requiring a spot beam focus.
• This option is particularly useful for
near-field applications such as plasma
diagnostics.
• This solution is not appropriate if the
antennas are to be placed inside the
vessel because the dielectric material
used on the lenses is likely to be coated
by plasma impurities.
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2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010
Antennas
Hog-horn
• Focusing brings far field region close to antenna.
• Hog-horn antenna with elliptical mirror focus the beam
and reproduce at focal point same characteristics of nonfocalized horn antenna with same aperture.
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2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010
Antennas
Hog-horn (cont.)
• Hog-horn antennas
installed inside the
ASDEX Upgrade
Tokamak at the HFS.
DC break
Directional
coupler
Waveguide
Reference
pin
Ka antenna
K antenna
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2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010
Passive components
Detectors
• Convert power to voltage.
• The open circuit voltage sensitivity gives an
idea of the detector efficiency (mV/mW).
• Tangential sensitivity gives an idea of the
minimum detectable power and is defined as
the lowest input power for which the detector
will have a 8 dB S/N at the output of the video
amplifier.
• Square-law response means that output
voltage is proportional to the power.
A12  A22
 A1 A2 cos  
VR  t   VP  t   
2
2
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2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010
Passive components
Mixers
• The multiplier type mixers used in radio frequency applications are
formed using non-linear devices. As a result the two signals entering
the circuit are multiplied together - the output at any given time is
proportional to the product of the levels of the two signals entering
the circuit at that instant. This gives rise to signals at frequencies
equal to the sum and the difference of the frequencies of the two
signals entering the circuit.
•
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Single balanced: signal from LO
can may leak to the RF port.
•
Double balanced: RF and LO
are better isolated
2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010
Passive components
Mixers (cont.)
•
Fundamental mixer: LO is in the same range of RF.
– Low conversion loss (< 10 dB)
– Large IF bandwidth
– More expensive
•
Sub-harmonic mixer: operates at a 2nd or 3th harmonic of the LO frequency
– Moderated conversion loss (< 14 dB)
– Large IF bandwidth
– Less expensive
•
Harmonic mixer: operates at a high (5, 6,…) harmonic of the LO frequency
– Higher conversion loss (> 18 dB)
– Small IF bandwidth
– Less expensive
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2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010
Passive Components
Directional Coupler
• A directional coupler is a four port device.
 P1 
Coupling (dB )  10 log 

 P3 
 P3 
Directivity (dB )  10 log 

 P4 
 P1 
ThroughLoss (dB )  10 log 

 P2 
• P3 or P4 can be terminated by a load.
• Typical coupling values are:3, 6, 10, 20 dB.
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2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010
Passive Components
Waveguide Isolator
• The full waveguide band isolator is a
Faraday rotation ferrite device.
• This isolator is offered up to 220
GHz in different waveguide bands.
• The isolator consists of a section of
waveguide containing low loss ferrite
material and impedance matching
elements.
• Should be used in front of any
source to protect the device from
reflected power.
• They must be protected from stray
magnetic fields.
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2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010
Detection techniques
• Single homodyne detection is the most simple scheme
Videoout 
A12  t   A22  t 
2
 A1 A2  t  cos   t  
Directional coupler
Reference pin
Generator
Acos
Detector
Mixer
Acos
• Phase and amplitude are mixed
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2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010
Detection techniques
• Heterodyne quadrature-phase detection.
Directional coupler
Generator
Mixer
I/Q detector
Mixer
IFRef
PLL
90º
IFPlasma
Generator
IFRef
Asin()
I/Q detector
IFPlasma
Acos()
Asin()
Acos()
• Phase and amplitude
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2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010
Conclusion
• RF techniques play an important role in
fusion plasmas diagnostics measuring
mainly the parameters of the electron
distribution with good accuracy (high
spatial and temporal resolution).
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2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010