joint ph.d programme on fusion science and engineering experimental work microwaves

Instituto de Plasmas e Fusão NuclearInstituto Superior TécnicoLisbon, Portugalhttp://www.ipfn.ist.utl.pt

2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010

Joint Ph.D Programme on Fusion Science and Engineering

Experimental Work Microwaves

António Silva

2 2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010

• 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).

Introduction


• 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.

Introduction (cont.)


Diagnostics

Active probing

Passive radiometry

Interferometry

Polarimetry

Reflectometry

Scattering

Electron Cyclotron Emission

(ECE)Electron Cyclotron

Absorption(ECA)

en r

en r

,S k

q r

eT r

iT

e e en T p


Passive ComponentsRectangular Waveguides

2 21

2c mn

m nfa b

• 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.

TE10


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

GUNN 20 – 25% (THz)

Voltage/ Mechanic

Narrow < 30 dBm > 100 µs

Millimeter Wave SourcesTunable oscillators

• Yttrium-Iron Garnet (YIG)• Hyperabruct Varactor

Oscillator (HTO)• GUNN


Millimeter Wave SourcesFrequency 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.


• 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.

Millimeter Wave SourcesDynamic frequency calibration

12

12

2

bsample

sampleb

delay delay

delay

ft

tF f F

t t

dtc


• Frequency calibration circuit

Millimeter Wave SourcesDynamic frequency calibration (cont.)

HTO

14.1643 ns

20dB

22dB

Frequency multiplier

Interference fringes

6dB

Frequency Markers

20dB

Comb generator0.5 – 18 GHz

SDLVA

0.5 1 1.5 17 17.5 18...

0 2 4 6 8 10 12 14 16 18 20 22 24 26-0.1

0.0

0.1

0.2

0.3

0.4 (c)

Mar

kers

[V]

t [s]

-0.4

-0.3

-0.2

-0.1

0.0

0.1

Inte

rfere

nce

[V] (a)

0

10

20

30

40

50 (b)

F [M

Hz]


• Instantaneous phase evolution obtained from interference signal with Hilbert Transform. Frequency step F is obtained from phase evolution.

0 2 4 6 8 10 12 14 16 18 20 22 24 2645

50

55

60

65

70

75Vu calibration curve and markers

t [s]

• Error of the recovered metallic mirror position is reduced from 24.9mm (a) to 7.4mm (b)

Millimeter Wave SourcesDynamic Frequency Calibration (cont.)


• 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.

AntennasPyramidal antennas

22R D

• 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.


• 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.

AntennasLens Antenna


• 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 non-focalized horn antenna with same aperture.

AntennasHog-horn


• Hog-horn antennas installed inside the ASDEX Upgrade Tokamak at the HFS.

AntennasHog-horn (cont.)

Directional coupler

DC break

Ka antenna

Waveguide

K antenna

Reference pin


• 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.

Passive componentsDetectors

2 2

2 1 21 2 cos

2R PA AV t V t A A


• 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.

Passive componentsMixers

• Single balanced: signal from LO can may leak to the RF port.

• Double balanced: RF and LO are better isolated


• 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

Passive componentsMixers (cont.)


Passive ComponentsDirectional Coupler

• A directional coupler is a four port device.

1( ) 10log3

3( ) 10log4

1( ) 10log2

PCoupling dBPPDirectivity dBP

PThroughLoss dBP

• P3 or P4 can be terminated by a load.• Typical coupling values are:3, 6, 10, 20 dB.


• 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.

Passive ComponentsWaveguide Isolator


• Single homodyne detection is the most simple scheme

Detection techniques

2 2

1 21 2 cos

2out

A t A tVideo A A t t

Generator

MixerDetector

Directional coupler Reference pin

Acos

Acos

• Phase and amplitude are mixed


• Heterodyne quadrature-phase detection.

Detection techniques

• Phase and amplitude

Generator

Mixer

Directional coupler

Mixer

Generator

PLL

I/Q detectorIFRef IFPlasma

Asin() Acos()

90º

IFRef

IFPlasma

Acos() Asin()

I/Q detector


• 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).

Conclusion

joint ph.d programme on fusion science and engineering experimental work microwaves

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