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 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~ 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 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) 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 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 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 Office of Science Independent control of beam focusing required: Strong focusing from background plasma yields ion motion 0.1 um emittance 100 pC charge /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 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 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 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 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 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 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 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