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**Plasma Wakefield Accelerator**

The Plasma Wakefield Accelerator as a Light Source Driver Patric Muggli University of Southern California, Los Angeles Work supported by US DoE

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**OUTLINE Introduction to the PWFA PWFA Milestones PWFA challenges**

Drive/witness bunch generation PWFA experiments at SLAC Conclusions

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**PWFA Focusing (Er) Defocusing Decelerating (Ez) Accelerating**

+ - electron bunch Accelerating Decelerating (Ez) Focusing (Er) Defocusing Plasma wave/wake excited by a relativistic particle bunch Plasma e- expelled by space charge forces => energy loss + focusing Plasma e- rush back on axis => energy gain Can be optimized for acceleration, focusing, radiation, … Plasma Wakefield Accelerator (PWFA): high-frequency, high-gradient, strong focusing beam-driven, colinear accelerator

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**PWFA CHARACTERISTICS Relativistic, short, dense bunch(es):**

Accelerating gradient: with and (max., single bunch, lin.) Typically for 1GV/m: in Blowout, nonlinear regime: Pure ion column focusing: free of geometric aberrations Wavebreaking field: Combination of large transverse focusing gradient and large accelerating field leads to large energy gain All the beam particles and the wake are ultra-relativistic no dephasing! High energy (per particle) drive bunch

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**PWFA MILESTONES PWFA proposed: Chen, PRL 54 (1985) The demonstration!**

Rosenzweig, PRL 61, 98–101 (1988) The demonstration! Q=2.1nC E0=21-15MeV sz=2.4mm ne= cm-3 Lp=20-35cm

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**e- e+ PWFA MILESTONES Muggli PRL 93, 014802 (2004)**

Blue PRL90, (2003). Rosenzweig, PRL 61, 98–101 (1988) Q=3nC E0=28.5GeV sz=700µm ne=1014cm-3 Lp=1.4m e- e+ 1.5x1014cm-3 1.8x1014cm-3

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**e- PWFA MILESTONES Q=3nC E0=28.5GeV sz=20µm ne=2.7x1017cm-3 Lp=10cm**

Hogan PRL 95, (2005) Muggli PRL 93, (2004) Rosenzweig, PRL 61, 98–101 (1988) e- Q=3nC E0=28.5GeV sz=20µm ne=2.7x1017cm-3 Lp=10cm

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**e- PWFA MILESTONES Lp=0, 13, 22, 31 cm Scaling with length! +14 +8 +4**

Muggli et al., NJP 12, (2010) Lp=0, 13, 22, 31 cm Hogan PRL 95, (2005) Scaling with length! +14 Muggli PRL 93, (2004) +8 Rosenzweig, PRL 61, 98–101 (1988) e- +4 Significant progress Large energy gain with a single bunch, particle acceleration

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**e- PWFA MILESTONES Lp=0, 13, 22, 31 cm Scaling with length! Q=3nC**

Muggli et al., NJP 12, (2010) Lp=0, 13, 22, 31 cm Q=3nC E0=42GeV sz=20µm ne=2.7x1017cm-3 Lp=85cm Hogan PRL 95, (2005) Scaling with length! +14 Muggli PRL 93, (2004) +8 Rosenzweig, PRL 61, 98–101 (1988) e- +4 42 to 84GeV in 85cm of plasma! Energy doubling of an FEL drive bunch?

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**PWFA NEXT STEP Focusing (Er) Focusing (Er) Defocusing**

Decelerating (Ez) Accelerating - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + - - - - - - - - - - - - - - - - - - - + + - - - - - - - - - - - - + - + + - - - - - - - - + + - - + + + + + - - - + + - + + + - - + + + + + + + + + - - - + + + + + + + + + + + + - - + + + + + + + + + + + - + + - - - - - - - - - - - - - + - - - - - - - - - electron beam - - - - - - + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Plasma wave/wake excited by a relativistic particle bunch Plasma e- expelled by space charge forces => energy loss + focusing Single bunch for particle acceleration (∆E/E~1) Plasma e- rush back on axis => energy gain Bunch train (D+W) for bunch acceleration (∆E/E<<1) Optimize for acceleration and/or focusing (plasma lens) Plasma Wakefield Accelerator (PWFA): high-frequency, high-gradient, strong focusing beam-driven accelerator

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**PWFA CHALLENGES Generation of drive/witness bunch train (ATF, FACET)**

Demonstration of bunch acceleration (FACET) Long plasma source for energy doubling (m-scale, ne≈ cm-3 range?) Low energy spread, careful beam loading, longitudinal bunch shaping? Low emittance, preserved over m-scale plasma (linear focusing, low scattering) Beam-plasma matching for low emittance beams

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**DRIVE/WITNEES BUNCH TRAIN GENERATION**

Correlated energy chirp from linac To Plasma e- Detector PLD Muggli et al., PRL 2008 ∆z=434 µm ∆z=226 µm Emittance selection Choose microbunches spacing and widths with mask and beam parameters: N, ∆z, z, Q

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**DRIVE/WITNEES BUNCH TRAIN GENERATION @ SLAC FACET**

Use the same masking method Drive N=6.7x109 e- sz=44µm Witness N=3.3x109 e- sz=13µm Train for proof-of-principle experiments only, ne≈1016cm-3 plasma Need independent control of D and W bunch parameters

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**FACET @ SLAC: BUNCH ACCELERATION**

QuickPIC simulation, D: z=30µm, N=3x1010e- W: z=10µm, N=1x1010e-, r0=3µm ∆z=115µm, ne=1017cm-3 Witness Bunch z=0 28 55 83 cm Drive Bunch Witness Ion Bubble z=0 Drive Bunch Beam 37GV/m (z=0) Wake evolution due to bunch head erosion, but no dephasing Hogan, New J. Phys. 12, (2010)

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**FACET @ SLAC: BUNCH ACCELERATION**

QuickPIC simulation, D: z=30µm, N=3x1010e- W: z=10µm, N=1x1010e-, r0=3µm ∆z=115µm, ne=1017cm-3, E0=25GeV Witness Bunch Drive e-/e- W E0 Wake evolution “bends” energy gain Lp=80cm, gain 25GeV, ∆E/E0≈3%, BUNCH ACCELERATION! D to W energy transfer efficiency ≈30% No bunch shaping, bunches carved out of a single SLAC bunch Hogan, New J. Phys. 12, (2010)

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**NARROW ENERGY SPREAD Effective beam loading with bunch shaping**

Witness bunch with linear density ramp perfect beam loading! No W bunch Very narrow energy spread with linear ramp in witness bunch charge. Tzoufras, Phys. Rev. Lett 2008

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CONCLUSION High gradient PWFA is a good candidate as a compact light source driver Physical parameters have been reached in proof of principle experiments Energy doubler (or more) concept attractive Challenges to produce required bunch quality (energy spread, …)

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