E-162: Positron and Electron Dynamics in a Plasma Wakefield Accelerator

Transcription

1 E-162: Positron and Electron Dynamics in a Plasma Wakefield Accelerator Presented by Mark Hogan for the E-162 Collaboration K. Baird, F.-J. Decker, M. J. Hogan*, R.H. Iverson, P. Raimondi, R.H. Siemann, D. Walz Stanford Linear Accelerator Center B.E. Blue, C.E. Clayton, E.S. Dodd, C. Joshi *, K.A. Marsh, W.B. Mori, S. Wang University of California at Los Angeles T. Katsouleas *, S. Lee, P. Muggli University of Southern California R. Assmann CERN Extraordinarily high fields developed in beam plasma interactions Many questions related to the applicability of plasmas to high energy accelerators and colliders E-157: First experiment to study Plasma Wakefield Acceleration (PWFA) of electrons over meter scale distances Physics for positron beam drivers qualitatively different (flow-in vs. blow-out) E-162

2 Outline q Motivation for positrons q Differences between positrons and electrons in PWFA Flow-in vs. blow-out regimes q Transverse wakes in homogeneous plasmas Opportunity to do unique physics with minimal investment q Longitudinal wakes in hollow and homogeneous plasmas Hollow channels to optimize the accelerating wake for positrons Required improvements to the experimental apparatus q Study matched beam propagation and longitudinal wakes of electrons

3 Why Positrons? For Example -- The Afterburner Idea apple Double the energy of Collider w/ short plasma sections before IP apple 1 st half of beam excites wake --decelerates to apple apple 2 nd half of beams rides wake--accelerates to 2 x E o Make up for Luminosity decrease N 2 /σ 2 by halving σ in a final plasma lens LENSES 5 GeV e - e - WFA e + WFA 5 GeV e + IP

4 Fundamentally Different Physics for Electron and Positron Wakes: Blow-out vs. Flow-in regime

5 Physical Principles of the Plasma Wakefield Accelerator Space charge of drive beam displaces plasma electrons Ez electron beam Plasma ions exert restoring force => Space charge oscillations Wake Phase Velocity = Beam Velocity (like wake on a boat) Wake amplitude N b σ 2 1 ( for 4σ ) z z λp n o

6 e - electron Radius ELECTRON PLASMA Blow-out -14 positron ELECTRON PLASMA -.5 e + Radius Flow-in -3 Z

7 3D Positron Beam Modeling Transverse Dynamics Electrons Positrons Radius [a.u.] Time [ps] Time [ps]

8 Why Positrons? Flow-in vs. Blow-out Regime Transverse Spot Size [µm] Electron Simulation Transverse Spot Size [µm] Positron Simulation Distance Into Plasma [m] 1.4 Transverse Spot Size [µm] DS OTR Electron Data Missing Positron Data? /2 Phase Advance K*L n e

9 Longitudinal Wake Optimization for Positron Beams: Homogeneous Plasma Hollow Plasma Channel

10 Wakefields of positron and electron 1 N = 2 1, σ = 4. mm(~ 13. ps), σ = 75µ m n p = z cm 14 3 r electron positron E 1 (GV/m) E z = 834 MV/m E 1 (GV/m) E z = 211 MV/m z (cm) z (cm)

11 ELECTRON For a positron drive beam and a homogeneous electron plasma the accelerating wakes have a lower amplitude than electron beam driven plasma waves require a high resolution imaging spectrometer Average Energy per 35µm bin (GeV) average <p z > x (3a) Electrons 34 MeV/m POSITRON z (cm) Relative Position Along Electron Bunch [mm] x number(#)/2.1747e15 sec bin Number of Electrons per 35µm bin (1 8 ) Average Energy per 35µm bin (GeV) x P z = 192 MeV/m (268.8MV/1.4m) z (cm) average <p z > (3b) Positrons x Relative Position Along Electron Bunch [mm] number(#)/2.1747e15 sec bin Number of Electrons per 35µm bin (1 8 )

12 q Why are positron wakes smaller? τ 3 τ 2 τ 1 τ 1 τ 2 τ 3 electron positron q Phase mixing due to different arrival time of sucked-in electrons q Gradient can be made larger by using a hollow plasma channel

13 electron Wakes in Hollow channel electron positron positron E 1 (MV/m) E zmax = 34 MV/m z (cm) E 1 (MV/m) E z = 27 MV/m z (cm)

14 Hollow channel plasmas can be optimized (plasma density vs. channel radius) for positron acceleration Accelerating Wake E z (MV/m) e - e Normalized Hollow Channel Radius r o ω p /c Accelerating wake for electron and positron beams as a function of the hollow plasma channel radius. For the electron case (filled blue circles) the wake amplitude decreases with channel radius, where as for positrons (open red circles) the wake has an maximum for a channel radius equal to c/ω p.

15 The accelerating wake for a positron beam driven plasma wave can be optimized by using a hollow channel plasma UV profiles.3 m (a) and 1.3 m (b) from a damaged UV optic with ~ 5 µm hole in reflective coating (center of images) as well as other forms of damage. The structure of the resultant mask in UV fluence is preserved over the required length to photo-ionize a hollow channel plasma.

16 Optimization of the Experimental Set-up Imaging spectrometer Matched beam propagation for electrons

17 Aerogel Dump magnets Quad

18 Move experiment from IP1 to IP where optics are available to: Image plasma entrance and exit onto aerogel and have a true imaging spectrometer Match beam into plasma E-157 E-162 Cherenkov

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20 IP allow matching into the plasma σ (µm) β beam =σ 2 /ε=1/k=β plasma σ =1.5 µm ε N =5 1-5 m rad n e = cm -3 Plasma SoptSizeZXmatchedX11.25µ.graph z (m) DS OTR σ Plasma Exit (µm) MatchedBeamPLExit.graph Plasma transparency condition broadened (@ plasma exit) Use imaging spectrometer µm n e = cm -3 σ =1.5 µm ε N =5 1 5 m rad β =13 cm σ=1.5µm±2.4% 1.4<n e < cm -3 n e ( 1 14 cm -3 )

21 Conclusions: q PWFA physics for positrons fundamentally different than for electrons q E-157 has produced a number of significant results with electrons q E-162 will be the first experiment to study the issues of PWFA with positron beams q Strong collaboration eager to use the facility developed in the FFTB to study PWFA

22 Experimental Program Run 1: A First Look at Positron Propagation in Long Homogeneous and Hollow Plasmas Use working E-157 apparatus Positrons in homogeneous and hollow plasmas Transverse dynamics (time integrated & time resolved) in the flow-in regime Run 2: High Resolution Energy Gain Measurements of Positrons Move to new location in FFTB to build true imaging spectrometer Positrons in homogeneous and hollow plasmas Detailed structure of longitudinal wakes (acceleration) Run 3: High Resolution Energy Gain Measurements of Electrons Electrons in homogeneous and hollow plasmas Matched beam propagation in a long plasma Higher resolution acceleration measurements