Nicolas Fagnoni – Cosmology on Safari – 14 th February 2017

1. The “Hydrogen Epoch of Reionization Array” (HERA)-Simulation of the chromatic effects of the antenna and impact on the detection of the EoR power spectrum Nicolas Fagnoni – Cosmology on Safari – 14th February 2017 nf323@mrao.cam.ac.uk – Department of Physics, University of Cambridge, UK

2. Summary 1. Detection of the EoR signal with HERA 2. The problematic of chromatic effects 3. Electromagnetic and electrical co-simulation of HERA 4. Design improvement

3. Detection of the redshifted hydrogen 21cm signal with HERA

4. Detection of the redshifted hydrogen 21cm signal with HERA • EoR signal contaminated by the foreground signal • Galactic synchrotron emission + extra-galactic radio sources • Foreground ~ 105 more intense than the EoR signal • Detection of the signal: • “Foreground subtraction”method • Complicated, requires an excellent knowledge of the foreground properties and chromatic effects induced by the telescope • “Foreground avoidance”method • “Smooth” foreground spectrum vs varying EoR spectrum • Study of a specific region of the EoR signal not contaminated by the foreground

5. Power delayspectrum • Delay spectrum • Power delayspectrum • Components of the wave vector k

6. EoR window and “wedge” • Delay power spectrum averaged in a cylindrical cosmic volume • k⊥ = parameter associated with the spatial scale of the observed region in the plane of the sky • Small k⊥ ⇒ large region probed • Proportional to the baseline • k∥ = parameter associated with the time scale of the Reionization • Depends on the redshift and baseline delay of the received signal Credit: Liu, A. et al. (2014)

7. The problematic of chromatic effects

8. The problematic of chromatic effects • PRISim • Foreground sky model + EoR model (21cmFAST) • Convolved with antenna beam models • Achromatic beam • Foreground contamination limited by the maximum signal delay associated with the baseline (i.e the “horizon delay limit”) • Chromatic beam • Foreground leakage at high k-modes • Contamination of the EoR window Thyagarajan, N., et al. (2016)

9. Sources of chromatic effects Mutual coupling Feed-vertex reflections Crucial to understand, model and limit these chromatic effects

10. Electromagnetic and electrical co-simulation

11. Antenna model with CST

12. RF-front end model • RF front-end: active balun + transmission cables + analogue receiver • Electrical circuits simulated with Genesys • Generation of the S-parameters

13. Electromagnetic / electrical co-simulation • Antenna model + front-end S-parameters • Simulation excited by a plane wave coming from the zenith • Gaussian pulse centred on 150 MHz and with a bandwidth of 100 MHz • Transient solver (time domain simulation => solution for all freq. in 1 run) • “Finite Integration Technique” • Hexahedral mesh (18 million cells) • Simulation time: 1000 ns with a time step of 0.003 ns • Simulation of the output voltage after the receiver

14. Antenna output signal Main signal Dish-feed reflections Cable reflections

15. Antenna voltage response

16. Effect of the reflections on the EoR signal • Constraints on the attenuation level of the reflected signal as a function of the delay of the reflections • White area the foreground spillover should not impact the EoRdetection • Ideal scenario: signal attenuated by 60 dB after 60 ns • k∥-mode non-detectable up to 0.2 h/Mpcbecause of reflections in cables, otherwise 0.15 h/Mpc Credit:Thyagarajan, N., et al. (2016)

17. Impedancemismatch • Reflectionscaused by a problem of impedancemismatchbetween the balun and the antennatermination • Balunimpedance: close to 65 – 30i ohm • BUT the antennaimpedance varies a lot • Historical reason: RF front-end optimised for PAPER

18. Are simulations reliable? 100-ohm termination

19. Coupling in HERA 19 • Antennas excited by a plane wave • Antenna response • Central antenna excited • S-parameters and beam model

20. Antenna response 100-ohm termination

21. Beam • Withcoupling • Slighltyhighersidelobes and lower gain • Sidelobesnot smoothat all • Ohtereffectsunder investigation At 150 MHz

22. Design improvement

23. Matching network • New electrical matching circuit to be inserted between the antenna and receiver • Smooth the impedance transition • Made up of 10 lumped elements (inductors + capacitors) • Decrease the reflections by ~ 10 dB (k// modes above 0.1h/Mpc may be detectable, if reflections in cable avoided) • BUT additional losses(between 0.2 and 0.9 dB) • Noise figure of the amplifiers modified

24. Central parabolic cone • Central cone • Flatten the antenna impedance • Impedance matching easier (reflections decreased by 20 dB) • BUT increase the sidelobe level by 5 – 10 dB

25. Development of a new Vivaldi feed • Vivaldi feed • Larger bandwidth: 50 – 250 MHz (z = 4.7 – 27.4) • 115 million and 1.3 billion years after the Big Bang • Null experiment at high freq. • Probe the Cosmic Down at low freq. • “Travelling-wave” antenna • Impedance and beam more stable over a large band

26. Conclusion

27. Conclusion • The study of the EoR signal is the key to understand the birth and evolution of the first galaxies and stars • Astrophysical results are limited by the hardware • Essential to properly understand and limit the impacts of the instrument on the data • Now it is possible to reach a good level of precision using end-to-end computer simulation • Same method applied to SKA-LA

28. Thankyou for your attention. Questions?