Workshop: Plasma Based Colliders for Future Experiments

1. SLAC, 2015-10-15 Workshop: Plasma Based Colliders Multi-stages acceleration required future experiments Bernhard Hidding et al. Scottish Centre for the Application of Plasma-Based Accelerators SCAPA, Department of Physics, University of Strathclyde & Department of Experimental Physics, University of Hamburg

2. Non-exhaustive overview on key HEP acc. strategies • e-e+ purelylaserplasma Leemans & Esarey, Phys. Today, 2009 • ILC: e-e+ purelyconventional Rosenzweig et al., NIMA 410, 1998 • γγ PWFA rf-driven • e-e+ PWFA rf-driven w/ recirc. option • e-e+ PWFA rf-driven Seryi et al., PAC 2009 Adli et al., 2014

3. Potential strategy: explore plasma building blocks step by step and work synergistically with the linear collider community to avoid friction

4. Injector: Trojan Horse has potential to generate 1e-9 mrad norm. emittance electron beams replace conventional rf-injector and damping ring with TH injector?

5. Why witness bunches ultracold? Laser kick contrib. to norm. emittance: 72 GV/m -72 GV/m residual momentum source size crude scaling • normalized emittance n down to 10-9 m rad • Because the laser pulse intensity is 2-3 orders of magnitude lower than in LWFA! a0=0.018 instead of a0>1 • Because the initial phase space volume is low • Because the electrons are rapidly accelerated (space charge impact decreases as γ-2) • Because initial ion shielding by released He ions • Polarization big unknown B. Hidding et al., PRL 108, 035001, 2012,Y. Xi et al., PRSTAB 2013, DE patent 2011, US patent 2012

6. Injector: Trojan Horse has potential to generate 1e-9 mrad norm. emittance electron beams – this may allow for enhanced luminosity via excellent focusability • luminosity and event rate can be enhanced if final focus size is reduced due to emittance values otherwise only accessible with damping rings • Need high charge: beam loading “limit” is driver charge (wakefield then cancelled out by witness). But also Nb ~ ne-1/2, i.e. needs larger blowouts & moderate plasma densities. This is (technically) challenging as regards plasma production (wide plasma channel) but has advantages, e.g. of reducing synchronization requirements. Simulations suggest ~nC-scale charge may be possible. Other, connected challenge: beam loading to reduce energy spread (important for extraction from plasma, but also (to a lesser extent because this is only the injector) for final focusing etc.) Katsouleas Part. Acc. 1987 • Need high rep rate: ion motion, recombination, heating, scattering, kicker/deflector speed and recombination time scales come into play • Bunch has to be short enough to fit into the blowout (need short bunches anyway)

7. Avoid hydrodynamic (i.e. gas profile) shaping: the low peak E-fields (<100 GV/m) of PWFA allow for optical shaping of the plasma density profile • Use diffractive optics (e.g. axilens) to produce plasma supporting the PWFA (e.g. hydrogen) • Longitudinal: laser intensity downramp (for TH) and upramp (for staging) translated into plasma profile. • Transversal: intensity/plasma profile region has to be wide enough to accommodate blowout size: this is in some contradiction to aiming at large blowouts to accommodate large (nC) charge longitudinal hydrogen downramp

8. Avoid hydrodynamic (i.e. gas profile) shaping: the low peak E-fields of PWFA allow for optical shaping of the plasma density profile • Hydrodynamic gas profile does not matter, as plasma interaction region and ramps are determined by intensity profile • Rep rate limited by recombination rate (few ns), heating, ion motion, scattering, drive beam rep rate.. but at least not by hydrodynamic movement (µs to ms) time scales ~1 J Ti:Sa H, alkali, Xe or else (or He/H for injector stage) gas reservoir, windowless, confined by diff. pumping driver beam quads quads quads fast kicker diff. optics + holed mirror • TH stage needs LIT and HIT media (H and He?), following stages can be single medium (high-Z component to stabilize ion background?)

9. TH injector setup in collinear geometry driver beam longitudinal hydrogen (or other LIT medium) downramp to be tailored for emittance preservation

10. TH injector setup maximum blowout width, otherwise blowout collapses driver beam full ionization region Limits minimum plasma density

11. Use TH bunches as acid test for emittance preservation during extraction and staging • Produce 1e-9 mrad bunches and test emittance preservation on stage ramps • Shape electron bunches and energy chirp/spread • Use plasma photocathode laser to shape electron bunch(es): • Change laser direction, focusing, intensity, polarization, wavelength, duration, shape, mode, transverse chirp, use multiple laser pulses… E210 currently: 90° geometry German/US patent (UCLA/RadiaBeam) 2011 ff.

12. One option to realize multi-bunch production Spatial footprint per pulse: approx. 30x30 cm, may be less if optimized

13. Electron driver sources? Use LWFA to produce driver beams for PWFA

14. Alternative to rf-generated drive beams: Hybrid LWFA  PWFA One inherent advantage of LWFA is to produce high charge, many kA, short bunches – use those as driver bunches for PWFA? See e.g. PRL 104, 195002, 2010 “Monoenergetic energy doubling in a hybrid plasma acc.”

15. Alternative to rf-generated drive beams: Hybrid LWFA  PWFA • LWFA-generated electron beam, optimized for high current – while substantial energy spread and energy jitter can be tolerated • Refocusing/matching helpful, plasma lensing very helpful to reach high norm. • ~100 µJ Ti:Sapphire TH laser pulse (inherently synchronized) used for underdense photocathode

16. Hybrid accelerator driven by electron beams from LWFA stages state-of-the art LWFA stage or LWFA-PWFA-TH (latter is more complicated, but may allow for higher control/stability) Note: partially conservative parameters (e.g. charge)

17. Intense Electron Sources: Hybrid LWFA  PWFA • System works as quality transformer (e.g. energy spread, brightness by orders of magnitude) • Levels out jitters of LWFA electron bunch, which is often an exp. reality as of today (energy, energy spread, even charge..) • E.g. put in driver with ~500 MeV, up to 50% energy spread, 1e-6 mrad scale norm emittance  extract witness bunch of approximately same energy, sub-% level energy spread, 1e-8 to 1e-9 mrad scale norm emittance Example: witness energy & energy spread w/ substantially different drivers: • System even levels out significant jitter in driver current/charge density due to trapping dynamics & plasma lensing  Highly promising to stabilize LWFA output and at the same time to boost quality! Rep rate a problem – combination of fiber lasers and / or OPA may come to the rescue Also transporting large energy spread drive beam on axis.. O. Karger et al., AAC 2014 & to be submitted

18. Positron source • In the ILC baseline design, positron source is dependent on ILC e- bunches – this means that the integrated luminosity is decreased • Using a plasma –based electron beam to produce the positron beam (either “conventionally” on tungsten target, or via photons from a helical undulator, or from Compton backscattering) allows for flexibility, may not decrease the integrated luminosity, is independent, and the electron beam used to generate the positrons can be shaped (e.g. use low emittance of TH bunches?)

19. Positron source • Worthwile to look into channeling crystals (considered at AAC 2014 for emittance measurement)?

20. Final focusing Final focusing via plasma lens e.g. reduction of beam-beam disruption effect

21. Hybrid plasma-ILC: Electron bunches from plasma accelerator, positron bunches conventional? plasma-based e- Or e-e- collider purely plasma-based?

22. Conclusions • Injector: Plasma photocathode beams may allow to omit damping ring, if high charge and ultralow emittance can be maintained  exp.: do parametric studies to explore scaling • Shape witness bunch by laser(s) to get proper beam loading, demonstrate collinear TH, demonstrate multi-bunch production • Emittance and beam quality preservation: Optical shaping of plasma profile may allow for emittance preservation, also for high rep rates compared to hydrodynamic solutions  use diffractive optics to shape longitudinal ramps • Staging: Explore optical shaping and staging in heavier media than hydrogen • Minimize number of stages to avoid accumulation of (potential) emittance growth • Use TH beams as acid test for emittance preservation – shape electron beams (chirp, emittance etc.) • Diagnostics: Measure e.g. emittance at 1e-9 mrad levels.. • Positrons: Independent, tunable, low-emittance electron source may be of use to generate electrons? • Many synergies with FEL and light source R&D at FACET..