1. 1 1 Office of Science C. Schroeder, E. Esarey, C. Benedetti, C. Geddes, W. Leemans Lawrence Berkeley National Laboratory FACET-II Science Opportunities Workshop SLAC, Oct 14, 2015 Laser-plasma accelerator-based collider challenges Supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231

2. 2 2 Office of Science Laser-plasma accelerator (LPA) linear collider Plasma density scalings [minimize construction (max. average gradient)] and operational (min. wall power)] indicates operation at n~ 10 17 cm -3 Quasi-linear regime ( a ~1): e + and e -, focusing control Staging & laser coupling into (hollow) plasma channels  tens of J laser/energy per stage  ~10 GeV energy gain/stage Leemans & Esarey, Phys Today (2009) BELLA Multi-GeV expts. BELLA Center Staging expts.

3. 3 3 Office of Science Plasma-based collider requirements: High gradient and Luminosity Acceleration mechanism must produce high average gradient for compact linacs:  >1 GV/m (average or geometric) implies <1 km/TeV Beams must achieve sufficient luminosity: Luminosity requires beam power: Accelerator must be compatible with reasonable (wall-plug) power: Focusability = low emittance (and energy spread); beam quality preservation High charge / bunch High efficiency: Wall-to-driver, driver-to-structure, structure-to-beam, driver energy recovery

4. 4 4 Office of Science Plasma-based collider requirements: Beamstrahlung minimization TeV-scale colliders will operation in high-beamstrahlung regime Beamstrahlung suppressed by using short beams Shorter beams save power: Plasma-based accelerations intrinsically produce short (~few micron) beams Collider power Short beams Beamstrahlung background

5. 5 5 Office of Science Laser diffraction  Self-guiding in nonlinear regime  Guiding in pre-formed plasma channel Laser - particle beam dephasing  Plasma tapering Laser energy depletion (with high accelerating gradient)  LPA staging  Compact driver in-coupling Positron focusing and acceleration (maintaining high beam quality)  Operate in linear regime  Ring-shaped driver High laser to beam efficiency (without energy spread growth)  Shaped particle beams  Laser energy recovery High average laser power  Laser beam combining LPA-based collider challenges and solutions

6. 6 6 Office of Science Scattering in plasma - Strong plasma focusing - Use (near-) hollow channels Emittance growth via ion motion  Quasi-linear regime: Control particle beam density via focusing Beamstrahlung mitigation  Short bunches (bunch trains)  Flat beams Beam break-up  De-tune dipole mode Alignment tolerances / jitter / synchronization - Weaker focusing …  … LPA-based collider challenges and solutions

7. 7 7 Office of Science Positron beam quality preservation in highly-nonlinear regime difficult High intensity ( a 2 >>1) Forms ion cavity Self-trapping present (staging difficult) Strong laser evolution Electron focusing determined by background ion density Positron acceleration and focusing (with high beam quality preservation) difficult (nonlinear accelerating and focusing fields)  In nonlinear regime, laser can be self-guided in plasma and generates large accelerating fields  Condition for guiding:  Peak field: ion cavity a=4.5 e - focus e + focus electron plasma density

8. 8 8 Office of Science Quasi-linear regime: linear e+ focusing and acceleration, Independent control of acceleration and focusing Focusing field Plasma density Accelerating field Quiver momentum weakly-relativistic ( a ~1) Region of accelerating/focusing for both electrons and positrons Stable laser propagation Independent control of accelerating and focusing forces:  Driver transverse profile  Plasma channel profile e - focus e - accel e + focus e + accel e - accel+focus e + accel+focus Schroeder et al., Phys. Plasmas (2013) Cormier-Michel et al., PRST-AB (2011)

9. 9 9 Office of Science Quasi-linear regime in a plasma channel yields higher energy gains (for constant laser driver energy)  Guiding at lower density achieved in quasi- linear regime with channel for fixed laser energy Quasi-linear regime Non-linear regime electron plasma density points= PIC simulation

10. 10 Office of Science Independent control of beam focusing required: Strong focusing from background plasma yields ion motion 0.1 um emittance 100 pC charge 10 17 /cc density k p L b =1 (beam energy/mc 2 ) beam density, n b /n 0  Focusing due to background plasma ions: Ion motion (non-linear focusing head-to-tail; emittance growth) Rosenzweig et al., PRL (2005)

11. 11 Office of Science (near-) Hollow plasma channels: Excellent wakefield properties for ultra-low emittance preservation  Provides structure for laser guiding (determined by channel depth not on-axis density)  Excellent wakefield properties in plasma channel and independent control over accelerating and focusing forces Accelerating wakefield transversely uniform Focusing wakefield linear in radial position and uniform longitudinally  (Near-) hollow plasma channel geometry provides emittance preservation Mitigates Coulomb scattering Control of focusing force and beam density – prevents ion motion Ion motion negligible if ratio of beam density to wall density is less than ion-electron mass ratio For releveant 1 TeV collider parameters: Schroeder et al., Phys. Plasmas (2013)

12. 12 Office of Science Accelerating wakefield set by wall density Focusing (for electrons) wakefield set by channel density Shaping the transverse profile of plasma channel: Independent control of acceleration and focusing k w (laser spot) = 2.3 k w (rms length) = 1 a 0 = 1 channel size: k w r c = 1.5 Accelerating wakefield Focusing wakefield

13. 13 Office of Science  Energy spread minimized using shaped beams Ramped/triangular current distribution: Shaped beams required for high-efficiency acceleration  Beam charge: Lower plasma density, higher bunch charge fraction of peak accel. field wake to beam efficiency Schroeder et al., Phys. Plasmas (2013)

14. 14 Office of Science Positron beams accelerated in hollow plasma channel with external focusing  Acceleration of positron beam in quasi-linear regime:  Focusing for positrons provided by external magnets

15. 15 Office of Science Bunch trains allow ultra-short bunch acceleration with high efficiency, without energy spread growth  Ultra-short beams suppress beamstrahlung Beamstrahlung photons/electron  Improved efficiency using bunch trains Normalized field amplitude Normalized distance behind driver  Using bunch trains, trade-off between efficiency and gradient, with no energy spread growth 1 bunch:6 bunches:

16. 16 Office of Science Improved efficiency using laser energy recovery laser energy recovery plasma laser beam Drive laser deposits energy into plasma wave (frequency red-shifts) Re-use laser in another LPA stage Send to photovoltaic (targeted to laser wavelength) – energy recovery Normalized field amplitude Normalized distance behind driver Additional energy-recovery laser pulse allows for no energy to remain in coherent plasma oscillations after energy transfer to beam – heat management Energy-recovery laser absorbs energy from plasma wave (frequency blue-shifts) Schroeder et al., AAC (2014)

17. 17 Office of Science Staged LPAs: average gradient determined by driver in-coupling distance Length of 1 TeV staged-LPA linac coupling distance: 0.5 m 1 m 5 m 10 m Plasma density [cm -3 ] Length of staged-LPA linac [m]  Number of stages:  Compact laser in-coupling distance (enables high average gradient) Conventional optics: requires many Rayleigh ranges to reduce fluence on optic (avoid damage) Plasma mirror: relies on critical density plasma production (high laser intensity): coupling <1 m

18. 18 Office of Science Power requirements reduced at lower density (Beamstrahlung limits charge/bunch) Charge/bunch:Laser rep. rate (for fixed luminosity):Wall-plug power: Schroeder et al., PRST-AB (2010) Charge/bunch limited by beamstrahlung: Plasma density [cm -3 ] wall-plug power [MW] E cm = 1 TeV L = 2x10 34 s -2 cm -2 η = 6% σ 2 = 100 nm 2 Beam loading limited Beamstrahlung limited

19. 19 Office of Science Collider requires high efficiency laser technology compatible with LPA density range Plasma density [cm -3 ]  Different operational plasma densities require different laser parameters (laser technologies) with varying efficiency Laser duration (bandwidth) requirements: Laser average power requirements:

20. 20 Office of Science Positron creation and injection into LPA requires R&D Leemans & Esarey, Phys Today (2009) Require short (~micron) e+ beam for injection into LPA

21. 21 Office of Science 10 GeV electron beam on high-Z target  positron generation via bremsstrahlung & photo-pair production Following target, e - and e + beams enter high- density plasma Intense, energetic e - beam drives a plasma wave in the linear regime (linear PWFA)  accelerates + focuses positron beam (electron pairs lost)  production of ultra-short relativistic e+ beam Experiment at FACET-II: plasma-based positron beam capture 10 GeV e - -beam FACET-II e-beam (or BELLA e-beam) n beam /n p =0.1 n p =10 19 cm -3 E peak ~1 GV/m Linear PWFA regime requires: high plasma density ( n beam / n p < 1) short electron beams ( k p L beam ~ 1) converter target e-e- e+e+ linear PWFA e+e+

22. 22 Office of Science Experiment at FACET-II: plasma-based positron beam capture  For drive beam to survive: Z target < X 0 = radiation length e.g., X 0 (W) = 3.5 mm  For Z target = 1 mm, (Geant4 modeling with 10 GeV beam and W- target) θ positron = 1.6 mrad θ drive = 0.9 mrad W peak = 5 MeV (27 MeV FWHM)  For Z drift = 2 γ b 2 λ p ~ 2 mm, Position peak of e + distribution at correct phase in wakefield  Conversion efficiency (e - to e + ): 1% z drift z target 10 GeV e - -beam e-e- e+e+ linear PWFA e+e+ converter target

23. 23 Office of Science Future R&D to address challenges for laser-plasma-based linear collider  LPAs have made tremendous progress over the last decade (demonstration of high gradient, multi-GeV acceleration, improved stability, etc.), but require significant R&D to realize LPLC:  Beam quality preservation Plasma target design Plasma channels to mitigate scattering and control focusing (ion motion) Particle beam injection Shaped beam currents enables high efficiency without induced energy spread  Coupling of laser and witness beams between stages Compact transport of witness beam with emittance preservation Compact delivery of drive laser beam (for high average gradient)  Laser technology development High average power lasers (beam combining) High efficiency (fiber lasers)  Development of other collider systems compatible with LPAs Novel methods for generation and cooling electron and positron beams (replace damping rings?) Novel final focus concepts (conventional final focus few km at TeV) Development of novel alignment instrumentation and techniques required …