Joint Ph.D Programme on Fusion Science and Engineering Experimental Work Microwaves

1. Joint Ph.DProgramme on Fusion Science and EngineeringExperimental Work Microwaves António Silva

2. 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).

3. 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.

4. Diagnostics Active probing Passive radiometry

5. PassiveComponentsRectangular 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. TE10

6. MillimeterWave SourcesTunable oscillators • Yttrium-Iron Garnet (YIG) • Hyperabruct Varactor Oscillator (HTO) • GUNN

7. MillimeterWave 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.

8. MillimeterWave SourcesDynamic 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.

9. MillimeterWaveSourcesDynamic frequency calibration (cont.) • Frequency calibration circuit

10. MillimeterWaveSourcesDynamic 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)

11. AntennasPyramidal 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. • 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. AntennasLens 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.

13. AntennasHog-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 non-focalized horn antenna with same aperture.

14. DC break Directionalcoupler Waveguide Reference pin Ka antenna K antenna AntennasHog-horn (cont.) • Hog-horn antennas installed inside the ASDEX Upgrade Tokamak at the HFS.

15. Passive componentsDetectors • 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.

16. Passive componentsMixers • 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. • Single balanced: signal from LO can may leak to the RF port. • Double balanced: RF and LO are better isolated

17. Passive componentsMixers (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

18. Passive ComponentsDirectional 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. Passive ComponentsWaveguide 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.

20. Directional coupler Referencepin Generator Acos(f) Mixer Detector Acos(f) Detection techniques • Single homodyne detection is the most simple scheme • Phase and amplitude are mixed

21. Directional coupler Generator I/Q detector Mixer Mixer IFRef PLL IFPlasma 90º Generator IFRef IFPlasma I/Q detector Acos() Asin() Asin() Acos() Detection techniques • Heterodyne quadrature-phase detection. • Phase and amplitude

22. 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).