STATUS OF E-157: METER-LONG PLASMA WAKEFIELD EXPERIMENT Presented by Patrick Muggli for the E-157 SLAC/USC/LBNL/UCLA Collaboration
OUTLINE Basic E-157 Acelleration, Focusing Plasma Source Diagnostics: Beam, OTR, Cherenkov First run results Focusing Summary
E-157 Goal: Accelerate e - by 1 GeV over 1m, and LWFA: φ 1 GeV/m n e 1 18 cm -3 L 1 mm PBWA: φ 3 GeV/m n e 1 16 cm -3 L 1 cm PWFA: φ 1 GeV/m n e 1 14 cm -3 L 1 m 3 GeV SLC-FFTB * : - The head of the beam excites the 1 GeV/m plasma wake. - The tail of the beam experiences the 1 GeV/m acceleration. - The plasma is 1-m long. - λ pe /4 σ z * Stanford Linear Collider-Final Focus Test Beam
Particle In Cell (PIC) Simulations 1) Longitudinal Field Acceleration SLAC-FFTB * parameters: n e =2 1 14 cm -3 Number of Electrons N e 3.5-4. 1 1 Initial Energy E 3 GeV Bunch Length σ z.6 mm Bunch Size σ x 75 µm σ y 75 µm Emittance γε x 6 mm-mrad γε y 15 mm-mrad Plasma parameters: 12 1 8 6 4 2 Energy Gain (MeV/m) Energy Spread (MeV/m) Charge (pc/ps) 12 c 1 8 6 4 2 Plasma Density n e 2-4 1 14 cm -3 Plasma Length L 1 m Density Uniformity n e /n e <25% Ionization Fraction n e /n >15% Radius r >4 µm PICsim2e14.graph -2-2 -3-2 -1 1 2 3 Axial Distance (mm)
2) Transverse Fields Focusing, Betatron Motion Focusing Strength (T/m) 4-2 -2 z (mm) 2.7.1 -.5 Current Density (1 1 A/m 2 ) Accelerating gradient (GeV/m) Large focusing force in the blowout regime Need to match L=mλ b /2, m=3, 5, or n 1/2 e L fixed
Li Vapor Source Optical Window Heater Wick Optical Window He Cooling Jacket Insulation Cooling Jacket Boundary Layers Pump He Li He n (Li) P He z L L Heating Power
Li Vapor 7 P He =2 mt (prototype) Internal Temperature ( C) 6 5 4 3 2 1 L 2 12.5 cm with 37 W No Li, P=25 W, T ext =751 C With Li, P=265 W, T ext =739 C With Li, P=37 W, T ext =759 C 5 1 15 2 25 3 Axial Distance from Oven Center (cm) Obtained neutral density n =2 1 15 cm -3, L=8 cm, 13 cm
Li Vapor Source Optical Window Heater Wick Optical Window 7 P He =2 mt 6 He Cooling Cooling Insulation Jacket Jacket Boundary Layers He Li He z L Internal Temperature ( C) Pump 5 4 3 2 1 L 2 12.5 cm with 37 W No Li, P=25 W, T ext =751 C With Li, P=265 W, T ext =739 C With Li, P=37 W, T ext =759 C 5 1 15 2 25 3 Axial Distance from Oven Center (cm) n (Li) P He L Heating Power Obtained neutral density n =2 1 15 cm -3, L=8 cm, 13 cm
Li Plasma Ionize Li with 6.45 ev uv photons (ArF laser, 193 nm) 1.5.8.4 UV Transmission.6.4 2 nd -Pass: n e /n e 8%.3.2 n e /n.2 1 st -Pass: n e /n e 36%.1 Profiles2e151mJ.data -2 2 4 6 8 1 12 z (cm) n =2 1 15 cm -3, L= 1m,1mJ/cm 2 2 passes: n e 4.8 1 14 cm -3 (alternative: cylindrical focusing of the beam) n e /n 24% n e /n e 8% Obtained plasma density n e =2.9 1 14 cm -3, L=25 cm (prototype, 1-pass)
Diagnostics 1) e - beam: - Beam Position Monitors (BPM), SLC diagnostics (energy, emitance, beam size, ) 2) Optical Transition Radiation (OTR) DS FWD + US BWD Interference (far field) Down-Stream Near-Field BWD OTR e - Beam BS Pellicle Li Plasma Up-Stream Near-Field BWD OTR Ionizing UV laser Pulse Pellicle Near field: beam position (alignment), and size before/after plasma Far field: beam divergence/emitance (interference between fwd and bwd OTR)
3) Cherenkov radiation: Flat mirror Spherical Mirror Aerogel e - Beam dump e - Air gap start 2% Beam splitter Time Integrated To time resolved Why choose aerogel? cosθ c = 1/βn, n=aerogel index of refraction Energy Resolution: E res Lθ c 15 µm # photon/e - : N γ Lθ c 2 ε res θ c --> want as large a Cherenkov angle as can collect Aerogel parameters: n aerogel = 1.8-1.12 --> θ c =7.2-8.8 deg. N g Cherenkov (photons/e - -mm) = 1.4 >> N g OTR = 1/1 Time integrated: energy spectrum, x-size Streaked : energy, x-size as a function of t or z-slice
First Run Data (preliminary) He (buffer gas) Impact Ionization, downstream OTR N=2 1 1 e -, E 3% n e /n 1.5% @.25T (estimate), L 2.2 m P He =.-.25 T P He =6 T P He =15 T P He =25 T No Impact Ionization Effect @ Operating Pressure At higher pressure: plasma lensing
Plasma Induced Beam Motion (BPM 616, 12. m) N=2 1 1 e -, E 3% n e e - pulse-laser pulse delay Vertical motion of the beam centroid as a function of ionizing laser-e - beam delay i.e., plasma density n e n e,max e -t/σ, σ 27 µs Neutral Density Change ( 1 14 cm -3 ) 1 PlasmaRecombination.graph 1 2 4 6 8 1 12 Time (µs) Time evolution of the plasma density as measured by laser visible interferometry n e decreases by a factor of 2 in 12 µs The plasma kicks around the beam tail observe centroid motion Beam tail due to wake fields in the accelerator
Plasma Induced Beam Motion Beam Dump Image Line-Out The plasma does kick around the beam tail
Plasma Lensing Downstream OTR images N=2 1 1 e -, E 3%, σ z 5 µm 6 1 5 8 Y FWHM (pixel) 4 3 2 Missing Pinch? Y FWHM (µm) 6 4 1 2.1.1 1 n e ( 1 14 cm -3 ) OTRlensing.data The beam pinches by a factor of 2! Consistent with n e,max 1.1 1 14 cm -3, L.8 m, if 2 nd pinch
Time Integrated Cherenkov Images, (focus mode) N=2 1 1 e -, E 3%, σ z 5 µm E Vertical FWHM [pixel] 35 3 25 2 15 No Plasma n e 1.1 1 1 cm -3 Without Laser/Plasma With Laser/Plasma Horizontal FWHM [pix] x 4 35 3 25 2 15 1 Without Laser/Plasma With Laser/Plasma CherenkovFocusM.data 1.1.1 1 n e ( 1 14 cm -3 ) CherenkovFocusM.data 5.1.1 1 n e ( 1 14 cm -3 ) The beam only expands at the Cherenkov location
e - Bunch Length (streak mode) Streak camera measurement of dispersed electron beam Image 8mm (2.7 GeV) field of view @ streak camera 15m away Reflective optics for: Large bandwidth = more sensitivity Low dispersion = 1ps resolution SLAC streak camera (SLC and GTF experience): Intrinsic resolution =.5 ps 125 µm slit contribution =.86 ps Total resolution = 1 ps Charge resolution ~ 1 6 e -
Summary of the First Run Data The experiment is young - Most diagnostics/data acquisition debugged. Clear evidence of beam-plasma interaction: - Focusing due to impact ionization in He. - Focusing (Li plasma) observed at the plasma exit with OTR. - De-focusing (Li plasma) observed with Cherenkov radiation in the dispersion plane. Next runs: - July 12 to August 2 - August 17 to August 31 -???