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THz generation and transport

THz generation and transport. Sara Casalbuoni DESY Hamburg. Overview. Motivation Design goals Simulation tools - POP ZEMAX - Mathematica code (B. Schmidt, DESY) THz beam extraction Optical design Simulation of the THz radiation transfer line Summary and outlook.

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THz generation and transport

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  1. THz generation and transport Sara Casalbuoni DESY Hamburg

  2. Overview • Motivation • Design goals • Simulation tools - POP ZEMAX - Mathematica code (B. Schmidt, DESY) • THz beam extraction • Optical design • Simulation of the THz radiation transfer line • Summary and outlook S. Casalbuoni, DESY

  3. Motivation Bunch length measurements with interferometer CDR/CTR 140 m,g = 1000, Eel = 500 MeV S. Casalbuoni, DESY

  4. Design goals • Flat frequency response • Low frequency response limit as low as possible BUT • Finite dimensions of transfer line tube and mirrors (ɸ ≈ 200mm) • Long transfer line ~20m • High frequency structures expected from μbunching up to 30THz(10μm) S. Casalbuoni, DESY

  5. Diamond window 30THz S. Casalbuoni, DESY

  6. Vacuum needed from B. Schmidt S. Casalbuoni, DESY

  7. ZEMAX Commercially available POP (Physical Optic Propagation) B. Schmidt (DESY) Mathematica Simulation Tools 2 CODES Huygens-Fresnel principle: Every point of a wave front may be considered as a centre of a secondary disturbance which gives rise to spherical wavelets, and the wave-front at any later instant may be regarded as the envelope of these wavelets. The secondary wavelets mutually interfere. Kirchhoff integral ≃L ƞ finite size screen y ξ d x L first order second order Fraunhofernear field, Fresnel S. Casalbuoni, DESY

  8. Input for CTR Fourier Transform with respect to the longitudinal coordinate ζ=z-vt of the radial electric field Er of a uniform bunch charge distribution of radius ρmoving with velocity v in straight line uniform motion (M. Geitz, PhD Thesis). S. Casalbuoni, DESY

  9. Valid: - if CTR screen radius ≳ effective CTR radius a ≳ λ -if L ≫λ2⇒ far field (Castellano & Verzilov, Phy.Rev.ST-Accel. Beams ,1998) Ginzburg-Frank ⇒ frequency independent S. Casalbuoni, DESY

  10. a=30mm L=10m =300μm Both conditions satisfied from P. Schmüser S. Casalbuoni, DESY

  11. a=30mm L=20m =1.5mm Finite size CTR screen from P. Schmüser S. Casalbuoni, DESY

  12. a=30mm L=0.5m =300μm Near field from P. Schmüser S. Casalbuoni, DESY

  13. =1.5mm L=0.5m a=30mm Both conditions violated and exact SQRT from P. Schmüser S. Casalbuoni, DESY

  14. ZEMAX Commercially available POP (Physical Optic Propagation) B. Schmidt (DESY) Mathematica Simulation Tools 2 CODES Both make use of scalar Fresnel diffraction theory |x-ξ|,|y-η|<<L Valid if objects and apertures >> λ y ƞ ξ Fourier transform of x L S. Casalbuoni, DESY

  15. Free propagation if a ≲ λand/or L ≲λ2 ⇒ I (x, L,ω) frequency dependent L=1 m; a=10 mm;  =1000 S. Casalbuoni, DESY

  16. Dimensions of the CTR screen window =20mm L=40mm;  =1000 λ=1.5mm ⇒ f=200GHz S. Casalbuoni, DESY

  17. window =20mm CDR screen L=40mm;  =1000 λ=1.5mm ⇒ f=200GHz ɸ=20 mm, slit=1mm ɸ=20 mm 20 mm ʘ ʘ ʘ intensity @window/@screen 0.75 0.55 0.78 S. Casalbuoni, DESY

  18. Optical design: technical drawing from O. Peters S. Casalbuoni, DESY

  19. Optical design diamond window ɸ=20mm M2 f=4500mm ɸ=188mm M3 f=3500mm ɸ=188mm M1 f=630mm ɸ=188mm M4 f=3000mm ɸ=188mm M5 f=100mm ɸ=100mm final screen 40mm 560mm 3410mm 8990mm 3100mm 2400mm 100mm CTR screen ɸ=25mm M1 M4 600mm 100mm M5 2400mm 3410mm 3100mm M2 M3 8990mm S. Casalbuoni, DESY

  20. Simulation of the THz radiation transfer line with ideal thin lenses λ=1.5mm ⇒ f=200GHz;  =1000 CTR screen ɸ=25mm Diamond window ɸ=20mm M1 M2 M3 M4 M5 final screen ɸ=8mm Intensity @final screen/@window=0.49 S. Casalbuoni, DESY

  21. Simulation of the THz radiation transfer line with ideal thin lenses λ=1.5mm ⇒ f=200GHz  =1000 CTRscreenɸ=25mm Diamond windowɸ=20mm Intensity @final screen/@window=0.49 S. Casalbuoni, DESY

  22. Good also for a Gaussian beam λ=1.5mm ⇒ f=200GHz  =1000 CTRscreenɸ=25mm Diamond windowɸ=20mm S. Casalbuoni, DESY

  23. Simulation of the THz radiation transfer line at different frequencies Intensity @final screen/@window 0.49 0.90 0.99 S. Casalbuoni, DESY

  24. Transfer function λ(mm) 3 0.3 0.03 S. Casalbuoni, DESY

  25. Half screen and horizontal polarization: frequency response ʘ Intensity @final screen/@window 0.52 0.91 0.99 S. Casalbuoni, DESY

  26. Gaussian beam λ=500nm w0=1mm Diamond window ɸ=20mm M1 M2 M3 M4 M5 final screen S. Casalbuoni, DESY

  27. Summary and outlook • 2 simulation tools: very good agreement • Optical design for the THz beam line transfer @140m in TTF2: tested for CTR, CDR and Gaussian beam • Outlook -”ideal” mirror surface -tests for stability against beam displacement and mirrors misalignment -effect of tilting CTR screen S. Casalbuoni, DESY

  28. Electric field of a charge in the laboratory system observation point θ ⃔ q S. Casalbuoni, DESY

  29. L ≫ λ2⇒ far field characteristic size of the CTR screen S. Casalbuoni, DESY

  30. Paraboloid mirrors CTR screen ɸ=25mm Diamond window ɸ=20mm M1 M2 M3 M4 M5 final screen S. Casalbuoni, DESY

  31. Paraboloid mirrors S. Casalbuoni, DESY

  32. Transfer function S. Casalbuoni, DESY

  33. Paraboloid mirrors: half screen and horizontal polarization Half CTR screen ɸ=25mm Diamond window ɸ=20mm M1 M2 M3 M4 M5 final screen Intensity @final screen/@window 0.64 S. Casalbuoni, DESY

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