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 S. Casalbuoni, DESY

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

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

Diamond window 30 THz S. Casalbuoni, DESY

Vacuum needed from B. Schmidt S. Casalbuoni, DESY

Simulation Tools 2 CODES ZEMAX Commercially available POP (Physical Optic Propagation) B. Schmidt (DESY) Mathematica 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 ƞ d ξ y x finite size screen ≃L L first order Fraunhofer second order near field, Fresnel S. Casalbuoni, DESY

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 ρ moving with velocity v in straight line uniform motion (M. Geitz, Ph. D Thesis). S. Casalbuoni, DESY

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

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

a=30 mm L=20 m =1. 5 mm Finite size CTR screen from P. Schmüser S. Casalbuoni, DESY

a=30 mm L=0. 5 m =300μm Near field from P. Schmüser S. Casalbuoni, DESY

L=0. 5 m a=30 mm =1. 5 mm Both conditions violated and exact SQRT from P. Schmüser S. Casalbuoni, DESY

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

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

Dimensions of the CTR screen window =20 mm L=40 mm; =1000 λ=1. 5 mm ⇒ f=200 GHz S. Casalbuoni, DESY

window =20 mm L=40 mm; =1000 λ=1. 5 mm ⇒ f=200 GHz ɸ=20 mm ʘ intensity @window/@screen 0. 75 CDR screen 20 mm ʘ 0. 55 ɸ=20 mm, slit=1 mm ʘ 0. 78 S. Casalbuoni, DESY

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

Optical design diamond window M 1 f=630 mm ɸ=20 mm ɸ=188 mm 40 mm 560 mm M 2 f=4500 mm ɸ=188 mm 3410 mm M 3 f=3500 mm M 4 f=3000 mm ɸ=188 mm M 5 f=100 mm ɸ=100 mm final screen 8990 mm 3100 mm 2400 mm 100 mm CTR screen ɸ=25 mm 600 mm M 1 M 4 100 mm 2400 mm M 5 3410 mm 3100 mm M 2 8990 mm M 3 S. Casalbuoni, DESY

Simulation of the THz radiation transfer line with ideal thin lenses λ=1. 5 mm ⇒ f=200 GHz; =1000 CTR screen ɸ=25 mm M 3 Diamond window ɸ=20 mm M 4 M 1 M 5 Intensity @final screen/@window=0. 49 M 2 final screen ɸ=8 mm S. Casalbuoni, DESY

Simulation of the THz radiation transfer line with ideal thin lenses λ=1. 5 mm ⇒ f=200 GHz =1000 CTRscreenɸ=25 mm Diamond windowɸ=20 mm Intensity @final screen/@window=0. 49 S. Casalbuoni, DESY

Good also for a Gaussian beam λ=1. 5 mm ⇒ f=200 GHz =1000 CTRscreenɸ=25 mm Diamond windowɸ=20 mm S. Casalbuoni, DESY

Simulation of the THz radiation transfer line at different frequencies Intensity @final screen/@window 0. 49 0. 90 0. 99 S. Casalbuoni, DESY

Transfer function 3 0. 3 λ(mm) 0. 03 S. Casalbuoni, DESY

Half screen and horizontal polarization: frequency response ʘ Intensity @final screen/@window 0. 52 0. 91 0. 99 S. Casalbuoni, DESY

Gaussian beam λ=500 nm w 0=1 mm M 3 Diamond window ɸ=20 mm M 4 M 1 M 5 M 2 final screen S. Casalbuoni, DESY

Summary and outlook • 2 simulation tools: very good agreement • Optical design for the THz beam line transfer @140 m in TTF 2: 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

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

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

Paraboloid mirrors CTR screen ɸ=25 mm M 3 Diamond window ɸ=20 mm M 4 M 1 M 5 M 2 final screen S. Casalbuoni, DESY

Paraboloid mirrors S. Casalbuoni, DESY

Transfer function S. Casalbuoni, DESY

Paraboloid mirrors: half screen and horizontal polarization Half CTR screen ɸ=25 mm Diamond window ɸ=20 mm M 3 M 4 M 1 M 5 Intensity @final screen/@window 0. 64 M 2 final screen S. Casalbuoni, DESY