1. RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” NUCLEAR FUSION INSTITUTE Development of the HFS ITER reflectometry (REFLECTOMETRY FOR THE MAIN PLASMA (HFS)) Task- F.09, Packet № 6 Includs: Upper port plug, in-vessel components, equipment after bioshield, acquisition and processing system Location: Upper port № 8 Presented by V.A. Vershkov NFI RRC “Kurchatov Institute”,Moscow, Russian Federation 9-th IRW Meeting, Lisbon, Portugal, 4 – 6 May 2009

2. RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” NUCLEAR FUSION INSTITUTE OUTLINE 1.Principles of HFS reflectometry 2. Advantages and problems 3. Analysis of the physical issues 4. Components of the HFS reflectometry and their characteristics 5.Conclusions

3. 1 – Low frequency extraodinary mode 2 – ordinary mode 3 – Electron cyclotron frequency 4 – Upper frequency extraordinary mode 5 – Second harmonic of the cyclotron frequency Principles of the HFS reflectometry. Advantages and problems Advantages: 1.Using low frequency extraodinary mode it is possible to observe the plasma core even with flat density profile 2.Very week relativistic corrections to the permittivity. 3.Low frequency range (10-80 GHz) with widely available high power generators. Problems: 1.Technical: integration of highly oversized waveguide (20×12 mm) in prescribed geometry 2.Physical: Estimated high level of the phase fluctuations, exceeded in a order of magnitude the typical reflectometry limit of 1.5 radians. RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 3 of 33

4. Physical problems There are several physical problems, arising due to the expected flat density profile with high pedestal value and expected high phase fluctuation level. I.The first problem arises even in the calm plasma and it is connected with the parasitic reflection in O-mode (due to the field line inclination of about 13 0 ) and reflection from the jump of the permittivity at the pedestal. Both parasitic reflection occur in the pedestal zone and make difficult to extract the real reflection in that area. II.The second problem arise from the estimated high level of the phase fluctuations of the reflected signal. This high level of the phase fluctuations influenced all functions of the HFS reflectometry, namely: 1.The accuracy of the density profile measurements 2.Possibility of characterizing the amplitude and Fourier spectrum of the local density fluctuations from the measured one 3.Abilities of the reflectometry to measure MHD and Alfvenic modes. The last two items connected to the fact, that high level of the phase fluctuations result in a 2π jumps and spreading of the spectrum of the reflected wave, giving in the limit to flat spectrum of  -function, which smashes pecularities of the density spectrum. As the most serious problems arise due to the assumptions about the level and wavelength of the density fluctuation at the HFS, it is of primary importance to predict reasonable values and structures of turbulence and properties of the TAE at the HFS!!! RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 4 of 33

5. Enhancement of the linear reflectometry limit 1.5 rad RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 5 of 33 All reflectometer systems for core plasma measurements will operate with strong perturbation of reflected phase due to the plasma turbulence. XL-mode has an advantage in both non-relativistic and relativistic case, but as the enhancement factor over limit is expected 5-8, even XL-mode should have problems.

6. Parameters for XL mode simulation with turbulence Simulation input values: σ n /n was taken according to the mixing criteria from the pressure profile Density profile is flat according to the scenario 2 Results: Edge fluctuations may prevent the XL mode penetration to the reflection layer Strong scattering occurs RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 6 of 33

7. Geometry and turbulence simulations in 2D full wave calculations Z 2D electric field Permittivity (XL-mode) Blue corresponds to ε=1, white – to ε<0. σ n /n=0.47% RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 7 of 33

8. Pulse reflection from unperturbed plasma – parasitic effects RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 8 of 33 Beside the main reflection from cut-off layer, several parasitic reflections could be observed – O-mode reflection due to magnetic field line inclination, reflection from steep gradient (spreading of the pulse due to high dispersion) and secondary reflections.

9. Pulse propagation in turbulent plasmas. 2D full wave simulation. σ(n e )/n e =0.47%, k×ρ i =0.3. Density turbulence level was taken 1/4 of the mixing length criteria of the pressure profile. The characteristics of the LFS were taken, while HFS turbulence has the different nature!!! (Question to theory!!) Several turbulence realization were simulated and avereged. The results showed that the averaged pulse in bistatic and monostatic approached to the limit delay, which about 2 ns less then 1D estimation without turbulence. This time delay should be taken into account in data processing RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 9 of 33

10. Delay shift due to the turbulence 1D geometric optics approach could reveal the nature of delay shift towards the launched/receiving antenna. The delay proportional to ε -1/2, so positive and negative fluctuations near the reflection point give non-symmetric response of plasma permittivity profile near the reflection point. Simulation was made for Gaussian perturbation with 2 cm width, located at cut- off radius. Full wave estimations at high turbulence level are required for qualitative ……..comparison with simulation results. RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 10 of 33

11. Broadening of the measured reflectometry spectrum due to the high level of the phase fluctuations RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 11 of 33 It could be difficult to measure the turbulence spectra in ITER plasma core due to strong phase perturbations in reflected signal even at rather low level of density perturbation. These perturbations appear due to both strong variations of dielectric permittivity near the cut-off layer at flat density profile and small-angle scattering.

12. 2D Simulation of reflectometry sensitivity to AEM modes in unperturbed plasmas. Scen 2 RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 12 of 33 Simulation results shows that in unperturbed plasma reflectometry is sensitive to AEM even at significant distances from mode position to cut-off layer. Reflectometry provides measurements for fluctuations with poloidal m number up to 150 Reflectometry response at XL-mode is sensitive to ratio of density and magnetic filed perturbations.

13. Conclusions from the 2D full-wave simulations for the capabilities of the HFS reflectometry 1.HFS reflectometry is capable of the density profile measurements, even at the highest levels of turbulence with the corrections for the decrease of time delay due to turbulence. 2.Capabilities of the HFS reflectometry for measuring the turbulence characteristics strongly depend on the assumptions about the turbulence properties at the HFS. Thus it is needed theoretical models for estimation HFS turbulence. It should be noted the importance of the decrese of the phase fluctuations with density peaking, which may be the case!! 3.Observation of the MHD and Alfven modes also needed theoretical prediction. Especially: -for the amplitude of such modes (at the HFS, as strong modes asymmetry may exist!) -As the lower extraodinary mode is sensitive to the magnetic field perturbation, the ratio and relative phase of TAE should be known RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 13 of 33

14. Block-schema of the HFS reflectometry RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 14 of 33

15. The components of HFS reflectometry 1.Vacuum chamber elements: - antenna systems - waveguide bends 90 and 40 degrees - waveguide tract at vacuum chamber wall that consists of stainless steel waveguide parts connected with flanges - primary vacuum windows 2. Atmosphere elements before the bioshield - secondary vacuum windows - waveguide tract with N-shape curving to compensate the thermal shifts 3. Frequency combine/divide system in ceiling region between bioshield and gallery. 4. Launching and receiving RF equipment for 7 frequency bends 5. System for diagnostic control, data acquisition, primary data processing and connection with CODAC system RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 15 of 33

16. The schematics of one waveguide routing in the upper port #8 from antenna to the area after bioshield Critical components 1.Antenna 2.90 0 bend after antenna 3.Two “40 0 bends” at the port entrance and output 4.Primary and secondary vacuum windows Bioshield Ceiling Diagnostic equipmentPort Vacuum chamber Primary vacuum window Secondary vacuum window RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 16 of 33

17. Antenna schematics and prototype Design of ITER HFS reflectometry antenna system required developing unique combine horn-mirror system due to strong restrictions of system size and small level of receiving signal. Antenna system prototype was made and successfully tested at HFS reflectometry mock-up in RRC “Kurchatov Institute” (RF). RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 17 of 33

18. Antenna heating simulation with bleckness=1 Thermal simulations were made to estimate the heating of a critical points in antenna system due to neutron flux. Final thermal calculation will be made after finishing antenna design as well as mechanical stress estimations. stainless steel vanadium molybdenum T max =618.5°C T max =569°C T max =495.5°C RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 18 of 33

19. Antenna response in mono and bistatic mode RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 19 of 33

20. Temporal Laboratory Test Facility of HFS reflectometry RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 20 of 33

21. Antenna response in mono and bistatic mode RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 21 of 33

22. Antenna response versus the reflection mirror distance RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 22 of 33 3D simulation Mock-up prototype measurements Both 3D simulation and prototype mock-up test demonstrate the same properties: Strong decrease of monostatic signal at distances above 0.5 m Rise of bistatic signal with distance increase in antenna close region and slow decrease at large distances up to 1.8 m Pulse propagation times were found to be close to geometric optics predictions

23. Optimization of 90° bend RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 23 of 33 Initial non-optimized hyperbolic cosine bend was made and demonstrated good performance. Special optimization of 90° bend was developed to decrease the size of the bend and keep the performance.

24. Experimental transmission of optimized 90° bend in Xl mode Exceptable transmission in XL mode up to 110 GHz, except some spikes at 27 and 52 GHz RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 24 of 33

25. Common problem of EU Plasma shape system and HFS reflectometry Inner size 20×12 mm, wall thickness 1 mm. The same as in EU project Cooperation is urgent. It is preferable to start prototype in 2009 because technology of welding and bending should be developed. Cu Ni stainless steel RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 25 of 33

26. Pulse spreading in the waveguide RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 26 of 33 Slowing of electromagnetic wave in waveguide will be the key issue for working at low frequencies. This effect will be important for both frequency scan and radar technique measurements. Numerical calculations show that for TE 10 wave (XL-mode in plasma) 1 ns pulse broadening due to waveguide dispersion is important for frequencies below 13 GHz. The choice of pulse width for measurements at low frequencies should be the compromise between pulse broadening and initial pulse width.

27. Optimization of “40° bend” and the way to primary vacuum window RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 27 of 33 Optimization of “40° bend” (entrance of waveguide into port) should be made. This work is required blanket module cutting. Waveguide exit from port is required additional simulation to optimize the RF properties of the bends.

28. Primary window Primary vacuum window is made by welding 2 mm quartz plane inside the waveguide. Quartz wedges at both size of plane are using for smooth changes of dielectric permittivity in window. Several prototypes of window make up and test now in N. Novgorod. Choice of design should be made at late 2009-early 2010. RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 28 of 33

29. Primary window characteristics Calculated window attenuation RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 29 of 33 Measured attenuation of The 4-th prototype window Shown the 4-th window example characteristics Problems, which were steadily worked out: 1.Influence of the resonance properties of the measurments waveguides 2.Inaccuracy of the wages fabrication Nevertheless still not match the simulations!!!

30. Secondary window stainless steel waveguide ROHACELL ® RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 30 of 33 Secondary vacuum window is to be made of 2 cm ROHACELL ® foam d=90mg/cm 3 with ε < 1.1 and low RF absorption. ROHACELL ® will be glued inside waveguide with RF dielectric epoxide or conducting compound. Test of secondary vacuum window is to be made till the end of 2009.

31. Quasi-optical system of frequency bands separation and summation in the region between bioshield and gallery RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 31 of 33 Exact principles of the frequency separation system will be chosen later

32. Conclusions 1. The presented preliminary design showed that all components of the HFS reflectomentry are capable to work effectively in the frequency band of Xl mode (10 - 80 GHz) and in O-mode (15 -120 GHz). 2. The last two techical problems should be solved: -The final primary vacuum window design - the production of the SS waveguide 20 x 12 mm, covered inside with 10  of Cupper (which is the common RF and EU problem) 3. The quality of HFS reflectometry measurements : 1.The accuracy of the density profile measurements 2.Possibility of characterizing the amplitude and Fourier spectrum of the local density fluctuations from the measured one 3.Abilities of the reflectometry to measure MHD and Alfvenic modes. Depends fully on the assumptions about the peakedness of the density profile and the level and wavelength of the density fluctuation at the HFS. Thus it is of primary importance to predict reasonable density profile peakedness, values a nd structures of turbulence and properties of the TAE at the HFS!!! RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE” 32 of 33