Instituto de Plasmas e Fusão Nuclear Instituto Superior Técnico Lisbon, Portugal 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

3 2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010 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.)

4 2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010 Diagnostics Active probing Passive radiometry

5 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. TE 10

6 2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010 DeviceBandwidthTuningOutput spectrum Output Power Sweeping YIGFull (40 GHz)CurrentNarrow< 20 dBm > 500 µs HTOFull ( 20 GHz)VoltageNoisy< 25 dBm< 2 µs GUNN20 – 25% (THz) Voltage/ Mechanic Narrow< 30 dBm> 100 µs Millimeter Wave Sources Tunable oscillators Yttrium-Iron Garnet (YIG) Hyperabruct Varactor Oscillator (HTO) GUNN

7 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.

8 2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010 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 Sources Dynamic frequency calibration

9 2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010 Frequency calibration circuit Millimeter Wave Sources Dynamic frequency calibration (cont.)

10 2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010 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) Millimeter Wave Sources Dynamic Frequency Calibration (cont.)

11 2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010 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. Antennas Pyramidal antennas 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.

12 2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010 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. Antennas Lens Antenna

13 2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010 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. Antennas Hog-horn

14 2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010 Hog-horn antennas installed inside the ASDEX Upgrade Tokamak at the HFS. Antennas Hog-horn (cont.) Directional coupler DC break Ka antenna Waveguide K antenna Reference pin

15 2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010 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 components Detectors

16 2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010 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 components Mixers Single balanced: signal from LO can may leak to the RF port. Double balanced: RF and LO are better isolated

17 2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010 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 components Mixers (cont.)

18 2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010 Passive Components Directional Coupler A directional coupler is a four port device. P3 or P4 can be terminated by a load. Typical coupling values are:3, 6, 10, 20 dB.

19 2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010 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 Components Waveguide Isolator

20 2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010 Single homodyne detection is the most simple scheme Detection techniques Generator Mixer Detector Directional couplerReference pin Acos  Phase and amplitude are mixed

21 2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010 Heterodyne quadrature-phase detection. Detection techniques Phase and amplitude Generator Mixer Directional coupler Mixer Generator PLL I/Q detector IF Ref IF Plasma Asin(  ) Acos(  ) 90º IF Ref IF Plasma Acos(  )Asin(  ) I/Q detector

22 2nd Advanced Course on Diagnostics and Data Acquisition Lisbon, February 2010 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