Observation of Coherent OTR at LCLS. Unexpected Physics in Standard Beam Diagnostics

Transcription

1 Obsevation of Coheent OTR at LCLS Unexpected Physics in Standad Beam Diagnostics 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

2 Outline Intoduction into LCLS Injecto Coheent OTR Obsevations Analysis and Intepetations Plans 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

3 6 MeV RF Gun Gun Solenoid LCLS Injecto Layout OTR sceens (7) YAG sceens (7) Wie scannes (7) Dipole magnets (8) Beam stoppes () S-band RF acc. sections (5) Gun Spectomete RF Deflecto Emittance Sceens/Wies Emittance Sceens/Wies -km point in 3-km 3 SLAC linac 135 MeV Spectomete X-band RF acc. section BC1 TD11 Stoppe 135 MeV σ z 1 mm 5 MeV σ z. mm 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

4 OTR Diagnostics OTR sceen Aluminum foil Thickness 1um Angle at 45 degees to beam Imaging optics package Telecentic lens, F.8 1 line pais/mm Typical magnification up to 1:.5 Two neutal density filtes with OD.5 and OD1 Beam splitte and eticule fo in situ calibation CCD camea Megapixel CCD Digital with 1bit depth Pixel size 4.65µm Linea esponse within digitizing ange Sensitive between 35nm and 1um Vacuum e-beam OTR YAG Lens CCD Filtes Beam splitte Reticule Illumination 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

5 COTR Obsevations I: Fluctuating Shapes OTR1, downsteam of BC Shot to shot samples, 3pC, max compession w/ L1S & L1X 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

6 COTR Obsevations II: Bunch Compession BLEN Monitos σ = 3μm 3 GHz 1 GHz OTR 5x Wie Pofile L1S Phase 4μm 35pC, max compession 16 Novembe 7 Theoy Goup Meeting OTR Pofiles Henik Loos loos@slac.stanfod.edu

7 COTR Obsevations III: Chage Dependency Numbe of CCD counts 7 x OTR Incoheent OTR Numbe of electons x 1 9 Numbe of CCD counts 1 x OTR satuates OTR Incoheent OTR Numbe of electons x 1 9 nom. compession w/ L1S & L1X 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

8 COTR Obsevations IV: Mico-Bunching Numbe of CCD counts 1 x OTR1 VS QB 9-Aug-7 :5:17 y = A exp((x - B) /C /) + D A = 6.51e+6 B = 1.8 C =.93 D =.8e+6 Incoheent Level 4x Beam size ms (μm) OTR1 VS QB 9-Aug-7 :5:17 Intensity peaks QB Field (kg) QB Field (kg) 1 nc, no compession, BC1 off, scan quad in DL1 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

9 COTR Obsevations Summay OTR light inceases by odes of magnitude when bunch is compessed At high compession, tansvese shape appeas to fluctuate significantly At highest compession, ing shaped light distibution occus OTR light has quadatic dependency on chage at lowe chage Quadatic dependency satuates at high chage Uncompessed beam shows coheence and appaent size change afte DL1 depending on R 56 OTR is not a eliable diagnostic unde cetain conditions. Is expected to be wose fo even shote bunches afte BC 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

10 Optical Tansition Radiation Tansition adiation is scatteing of vitual photons off bounday to geneate eal photons Souce of adiation is Coulomb field Tempoal and spatial fequency components k E( k ) k γ + k / Diffaction and imaging with optical system Angula acceptance of system θ E k sinθ k k / γ + k ( ) k dk J1( k ) Effective souce distibution e k E( ) 1 J ( k sinθ ) K1 γ k [ ] Camea Image Calculation Lens images field distibution fom taget to CCD CCD measues field intensity Incoheent light Convolute souce intensity with beam distibution Coheent light Convolute souce field with beam distibution γ 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

11 Diffaction Limited Souce Distibution 1 7 θ = π/ γ = 5 λ = 1μm θ = (μm) 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

12 COTR fom Electon Bunch Souce field spectal distibution ~ E ( κ ) iq = π v κ ( k γ ) + κ Field on sceen is convolution of souce field with lens diffaction patten E S kd / z ( ) L i ~ d κ κ e E( κ ) = Field of N adiating paticles ~ E N j ~ ikz iκ j ( κ ) = e + E( κ ) j Radiation intensity fo fequency k S ( ) E ( ) = N d dzρ(, z) E( ) N + N ikz ( N 1) d dze ρ(, z) E( ) 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

13 Coheent OTR Imaging Intensity of coheent pat fo fully coheent beam kd / z ( ) L i iq κ EN = d κe κ ρ( κ ) π v ( k γ ) + κ Assume Gaussian chage distibution ρ / ( κ ) =e κ σ Beam size «γλ Image is OTR souce distibution Beam size» γλ Image is gadient of beam distibution If σ > γ/k, beam lage than souce size E N E N kd / z ( ) L i d κe κ ρ( κ ) = q γ π v k q γ π v k ( ) ρ( ) = iκ This gives the field as the gadient of the chage distibution 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

14 Hoizontal Polaization Component of Souce 15 1 γ = 5 λ = 1μm θ =.1 ad Hoizontal Polaization of TR Souce 5 y (μm) x (μm) 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

15 Convolution of Souce with Beam Radial Polaization of TR Souce Hoizontal Polaization of TR Souce y (μm) 1-1 γ = 5 λ = 1μm y (μm) x (μm) Tansvese Distibution of Electon Beam 1 5 σ = μm - - x (μm) COTR Distibution of Electon Beam 1 5 y (μm) y (μm) x (μm) x (μm) 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

16 Measued image Light intensity 3x above incoheent level OTRS:LI1:91 Image on OTR1 Simulation Souce acts like gadient opeato Ceates doughnut shape COTR Distibution of Electon Beam σ = μm y (μm) 4 6 4μm y (μm) μm x (μm) x (μm) 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

17 Max. Compessed Bunch Fom Facto Initial 6keV,.8mm Initial 3keV,.8mm Enegy spead (MeV) Fom facto Paticles Enegy spead (MeV) Fom facto Counts Paticles Counts z (μm) Wave length (μm) z (μm) Wave length (μm) 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

18 Obseved Intensity Estimation Obseved facto 3 enhancement Fully coheent 1 9 enhancement Beam size effect 1 3 suppession Long. fom facto 1 4 1? suppession = Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

19 COTR Chage Dependency Numbe of CCD counts COTR satuates COTR ~ N Incoheent OTR ~ N Numbe of electons Quadatic dependency fo small chage 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

20 Mico-Bunching Simulation Stat w/ enegy modulation in injecto R56 of DL1 geneates tempoal modulation Simulation Modulate beam at 1um x-z tilt on OTR1 Calculate eduction in modulation when QB changes Neglect enegy spead Use full beam and tansvese coe of beam z (μm) QB*1. - x (μm) Phase space x-z 3 QB* % Coe -5 5 x (μm) 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

21 Bunching Facto vs. QB 1 Modulation 1um nom. bunching facto % Coe Beam Measued 1% Beam el. QB stength 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

22 Issues In most cases, coheent effect only visible in incease in light output No significant change in beam shape noticable COTR fom fully tansvese coheent beam should give ing stuctue COTR fom small coheent tansvese faction of beam should give spike Needs many coheent small pats of beam adiating at diffeent wavelengths to explain still Gaussian beam shapes 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

23 Futue Plans Measue spectum of coheence light Gating fo spectally esolved OTR pofiles at visible wavelengths UV and IR diode with filtes fo ange beyond visible Colo CCD? Measue polaization of COTR Polaization dependence of tansvese distibution can indicate tansvese coheence Add adial to linea polaization tansfome? 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

24 Tansmission Gating Spectomete Modifications of pesent setup Add gating in one of the filte slots Use tansmitted light though beam splitte Gating diffaction in fist ode x S = x L G λ/d Tansmission gating in font of lens Distance foil gating: L G = 13mm Gating fequency: g = 1/d = 1/mm Max wavelength: λ = 8nm Spectal esolution Δλ = dλ/dx S 3σ = 3σ/gL G Δλ = 1nm (σ = 3um) Δλ = nm (σ = 6um) Vacuum e-beam Beam splitte OTR YAG Lens CCD Gating Beam splitte Diode Filte Reticule Illumination 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu

25 LCLS Commissioning Team Dave Dowell Paul Emma Joe Fisch Cecile Limbog-Depey Jim Tune Jim Welch Juhao Wu 16 Novembe 7 Theoy Goup Meeting Henik Loos loos@slac.stanfod.edu