Proton-driven plasma wakefield acceleration

Transcription

1 Proton-driven plasma wakefield acceleration Konstantin Lotov Budker Institute of Nuclear Physics SB RAS, Novosibirsk, Russia Novosibirsk State University, Novosibirsk, Russia AWAKE Collaboration

2 Motivation and first idea Plasma wakefield accelerators are subject to transformer ratio limit (ΔW e < W d ). ~ To reach TeV scale electron energies, we need either staging of many electron drivers or proton beams. Background: energy content <40 J (laser), <120 J (electrons) 20 kj (SPS), >150kJ (LHC) A. Caldwell, K. Lotov, A. Pukhov, F. Simon, Nature Physics 5, 363 (2009). K. Lotov, Phys. Rev. ST Accel. Beams 13, (2010). 1 TeV proton driver indeed boosts electrons to sub-tev, but... witness-to driver charge ratio Drive beam: p + E=1 TeV, N p =10 11 σ z =100 μm,σ r =0.42 mm σ θ =0.024 mrad, ΔE/E=10% Witness beam: e E 0 =10 GeV Plasma: n p =10 15 cm 3... but if the driver is compressed longitudinally to sub-millimeter scale (the driver must be short as compared with the local plasma wavelength).

3 Second key idea: multi-bunch wakefield excitation Single bunch: LHC beam: 7.6 cm, energy spread = 0.01% Conservation of phase volume: energy spread = 7.6% 500 GeV for 7 TeV beam compressed to 0.1 mm ILC-scale compression section may be too expensive... Bunch train: For, the length of the 10 bunch train is 1 cm. The required correlated energy spread is 0.076% * 2 (we use 2 ) * 4 (for bunching) = 0.6% = 42 GeV (reasonable). Self-modulating beam (cheap and easy): No compressor, no chopper, just let the plasma to modulate the beam via the transverse two-stream instability: modulation of the beam radius produces modulation of the (de)focusing force, which further changes beam radius etc.

4 Road to the experimental test AWAKE collaboration (>60 physicists from ~25 institutes) aimed at experiments with 400 GeV proton beam of SPS synchrotron (CERN), now at conceptual & technical design stage. most probable site to replace CNGS facility

5 New physics: controlled self-modulation of the beam If developing from a noise, the transverse two-stream instability just breaks the beam, and there is no strong wakefield If the axisymmetric mode is externally seeded, then the beam is bunched, and strong wakefield is excited which is phase locked to the seed. accelerating wakefield beam density (N.Kumar, A.Pukhov, K.Lotov, Phys. Rev. Lett , 2010) plasma with the wakefield laser pulse proton beam neutral gas (Rb)

6 New physics: seeding the instability by co-propagating ionization front wakefield the beam as is seen by the plasma original beam laser pulse

7 New physics: side injection of electrons into the wave At the stage of wakefield growth, the phase velocity of the wakefield is substantially lower than c, so that electrons can be accelerated only after the beam is fully bunched Side injection of electrons at some shallow angle is a natural choice: cm Final energy spectra are sensitive to place and energy of injection: mm 12 MeV, 3 mrad, 7 m, 6 cm delay 16 MeV, 9 mrad, 4 m, 13 cm delay can accelerate electrons (A.Pukhov, et al. Phys. Rev. Lett , 2011) (K.Lotov, J. Plasma Physics , 2012)

8 Technical challenge: the most uniform plasma ever Accelerated electrons are very sensitive to variations of plasma density which cause shifts of the wakefield phase number of wakefield periods ahead electrons Acceptable density non-uniformity < 0.2% (K.Lotov, A.Pukhov, and A.Caldwell, Phys. Plasmas , 2013) Solution: instant ionization of highly uniform neutral gas by a short laser pulse (now developed by MPI for Physics, Munich)

9 Future steps: shorter beam (compression in SPS ring) Maximum accelerating field: 0.3 GeV/m 1.1 GeV/m 2 GeV/m 4 GeV/m

10 Future steps: two-staged plasma A small step in plasma density saves from quick decrease of the wakefield (illustrated with 7 TeV LHC beam) density step uniform plasma (K.Lotov, Phys. Plasmas , 2011) Stagable alternatives to laser ionization are studied (helicon plasma source) electron energy (A. Caldwell, K. Lotov, Phys. Plasmas , 2011) (now developed by IPP MPI, Greifswald)

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12 Thank you

13 Proton-driven plasma wakefield acceleration is approaching the experimental test The energy content of the driver: <40 J (laser), <120 J (electrons) 20 kj (SPS), >150kJ (LHC) Expected electron spectra for the 1 st stage of the experiment: late injection early injection In perspective: most probable site to replace CNGS facility electron energy

14 New physical effects or techniques to be tested in AWAKE: Self-modulation of the proton beam controlled by the co-propagating ionization front: Side injection of electrons at some shallow angle: plasma with the wakefield laser pulse proton beam neutral gas (Rb) wakefield the beam as is seen by the plasma original beam laser pulse The most uniform plasma ever: δn<0.2% at 10 meters