1. Multi-GeV laser driven proton acceleration in the high current regime Laser-driven acceleration is living a major revolution (PW era)… …but will ever be a serious alternative to RF-based drivers? I.e. is it possible to operate laser driven accelerators in the high current, multi-GeV regime? We discuss a specific proposal based on radiation- pressure acceleration and broadband amplification of the pump laser beam F.Terranova in collaboration with S.V.Bulanov, T.Esirkepov, P.Migliozzi, F.Pegoraro, T.Tajima [LNF, JAERI-Kansai, Prokhorov Inst., INFN Naples, INFM & Univ.Pisa]

2. Why laser acceleration? Well known facts: The absence of breakdown threshold allows to reach field gradients much higher than ordinary RF (>> TV/m!) Acceleration is nearly istantaneous: no need of periodic lattices and long term stability condition Forget about multipole instabilities but do not forget about hydrodynamic ones (e.g. Rayleigh-Taylor) Less known facts: Potential synergy with Inertial Confined Nuclear Fusion (ICF) for: Proof of principle of new acceleration regimes High intensity on target (1-5MJ @ 10 Hz)

3. Three major difficulties An efficient, stable and tunable acceleration mechanism is not available, yet how? Ultra-intense lasers have very low repetition rates (<< 1Hz) Targetry (ion source) is complicated Our basic claim: Inefficiency and lack of tunability comes from the fact that acceleration is due only to space charge effects but what happens at laser intensities (10 23 W/cm 2 ) where radiation pressure dominates? System is self-stabilizing and highly efficient

4. The radiation-pressure dominated regime * (*) S.V. Bulanov et al., Plas. Phys. Rep. 30 (2004) 196 |v e  B|  cE  E  =2  en e l l x ions electrons EE E  > E || > m p  /2  e Independent of x if pulse weist sufficiently large Avoid ion recombination Ions become relativistic in less than one laser cycle

5. Dynamics of the laser-plasma system As the ion-electron bounded system moves more and more rapidly, in its proper frame the incident laser wavelength increases (Doppler shifted). Hence the plasma becomes less and less trasparent (more and more overcritical). As a consequence, the amount of reflected light increases with time (maximum momentum transfer). v=  c l      ions electrons incident reflected mirror ! laser pulse Space charge acts as a field rectifier

6. Dynamics of the laser-plasma system The driving light pressure is P = (E’ 2 L /4  )(1 +|  | 2 -|  | 2 ) = (E’ 2 L /4  ) (  ’/  ) 2 |  (  ’)| 2 and  ’ is the light frequency in the mirror frame  |  (  ’)| 2 (mirror reflectance) grows with time. If we compute the maximum ion energy, accounting for the finite pulse duration we get that the energy increses as t 1/3 and we have: and if the laser energy is >> with respect to the overall rest mass of the protons: The number of protons depends only on the size of the illuminated area of the target The energy depends linearly on the laser intensity and its duration This is the tunability we were looking for !!

7. OK. But does it really work? It seems so. Numerically (*). test of the mirror dynamics (e.g. t 1/3 asymptotic behaviour) Laser ion energy transfer efficiency of about 50% (*) T. Esirkepov et al., Phys. Rev. Lett. 92 (2004) 175003 Tested at a=316 (peak intensity of 10 23 W/cm 2 ), l= =1  m, n e =5.5 10 22 cm -3 This is the light-to ion transfer efficiency we were looking for !!

8. A very unexpected synergy with Inertial Confined Fusion (ICF)! The pressure dominated regime bridges for the first time the R&D done for huge laser-matter energy transfer connected with Inertial Confined Fusion with the one for high power proton drivers S.V.Bulanov et al., Nucl.Instrum.Meth. 540 (2005) 25 Can we really exploit high rep-rate ICF ? All viable candidates (Excimer lasers, Diode-pumped solid state lasers) provide a small bandwidth, high intensity pump pulse. We cannot achieve ~10 fs pulse compression directly! Huge pump with small bandwidth Faint (chirped) signal with large bandwidth Amplifier (OPCPA?) Broadband amplified signal Idler

9. Can we really exploit high rep-rate ICF ? There are region of compatibility for efficient OPCPA amplifications and high rep rate drivers (DPSSL and XeF, not KrF) Anyway, a proof of principle could be done at “early light” (1 beamlet of NIF) with traditional pulse compression techniques (CPA) (based on an idea of T. Tajima and G. Mourou, Phys. Rev. ST Acc. Phys. 2003) Preliminary

10. Conclusions The radiation pressure dominated regime is (to my mind!) the first serious proposal for high intensity laser-driven proton acceleration in the multi-GeV region. This technique is: A real synergy with ICF is possible, provided that pulse compression can be implemented in high repetition rate drivers (we have good candidates…) Efficient (~50% light to ion conversion) Tunable (proton energy decoupled from pulse width and hence from proton intensity) Produces beams of good quality even for injection in lattice Needless to say…