Research Of The Generation,control And Energy Deposition Of Ultra-intense Laser-driven Energetic Ions

Energetic ions have potential applications in inertial confinement fusion,proton imaging,biomedical therapy,production of warm dense matter and nuclear and particle physics.People have been looking for the high-quality ion sources in laboratories to satisfy the applicative requests.The conventional particle accelerators have developed nearly a hundred of years,which are approaching the limit of economy,technology and resource and the acceleration gradient is limited to ~ 100 MV/m.With the development of laser technique,generation and transport of laser-driven ions have attracted significant interest in recent years.The ultra-high acceleration gradient of ~ 10 TV/m excited within plasma provides a great opportunity to develop the small scale particle accelerators.Nowadays,many ion acceleration mechanisms have been proposed in succession.Especially,target normal sheath acceleration(TNSA)and radiation pressure acceleration(RPA)have been verified to be most efficient acceleration ways by a large number of experiments.However,their practical applications are extremely limited due to the intrinsic defects.For example,energy conversion efficiency in TNSA is relatively low and the beam quality is also bad,and the transverse instabilities in RPA easily lead to the stopping of stable ion acceleration process.My thesis focuses on the generation and control of ultra-intense laser driven energetic ions.Besides,the stopping of laser-driven ions in fully ionized doped plasma and the effect of heavy ion dopants on fast ignition are also investigated.The thesis structure is as follows,Firstly,the enhanced TNSA mechanism based on the laser self-focusing effect is investigated by the theoretical analysis and particle-in-cell(PIC)simulations.We find that the hot-electron temperature in the underdense plasma is greatly increased due to the occurrence of resonant absorption while the electron-betatron-oscillation frequency is close to its witnessed laser frequency.While these hot electrons penetrate through the backside solid target,a strong sheath field at the rear surface of the target is induced,which can accelerate the protons to a higher energy.It is also shown that the optimum length of the underdense plasma is approximately equal to the self-focusing distance.Secondly,we propose a new method which provides a ~ 10-fold energy increase in laser-driven ion acceleration.It is shown that a plasma micro-channel target allows simultaneous acceleration of protons,up to ~ 30 MeV,and carbon-ions,up to ~ 60 MeV,for a laser intensity of ~ 1020 W/cm2.When an intense,short laser pulse propagates deep into the channel,two trains of dense electron bunches are thrown out from the channel.After being accelerated forward and penetrating through the attached plastic substrate,a strong sheath field is induced which sharply enhances ion acceleration.The optimal channel parameters for the experimental design and the scalings of the maximum ion energies are obtained.We find that 60-250 MeV proton beams and carbon-ions up to multi-100 MeV,relevant for medical therapy,can be realized by currently available petawatt-class laser systems.Thirdly,it is demonstrated that TNSA protons can be well controlled by using a guiding cone.Compared to a planar target,both the collimation and number density of the proton beams are substantially improved,giving a high beam quality which can be maintained for a longer distance without degradation.This effect is attributed to the presence of the radial electric field resulting from the charge due to hot target electrons propagating along the cone surface.This field can effectively suppress the spatial spread of the protons after the expansion of the hot electrons.Fourthly,we investigated the laser shaping of a relativistic circularly polarized laser pulse in ultra-intense laser thin-foil interaction.It is found that the plasma foil as a nonlinear optical shutter has an obvious cut-out effect on the laser temporal and spatial profiles.Two-dimensional simulations show that,the high intensity part of a Gaussian laser pulse can be well extracted from the whole pulse.The transmitted pulse with longitudinal steep rise front and transverse super-Gaussian profile is obtained which is beneficial for the radiation pressure acceleration scheme.The Rayleigh-Taylor-like instability is observed in the simulations,which destroys the foil and results in the cut-out effect of the pulse in the rise front of a circularly polarized laser.Fifthly,a foil-in-cone target is proposed to realize stable RPA proton acceleration and dynamic control of the accelerated protons.It is found that the oscillating electric field of focused circularly polarized laser pulse with transverse Gaussian profile pulled out electrons from the cone walls,which leads to a strong sheath field within the cone.Co-moving with the accelerated foil,this sheath field tends to persistently focus the accelerated proton bunch and effectively suppresses undesirable transverse explosion.Compared to a planar target,the guiding cone can substantially improve the spectral and spatial properties of the ion beam and lead to better preservation of the beam quality.The high-quality proton beams can be obtained at the exit of the cone.Finally,we investigated the transport of the energetic protons in a compressed deuterium-tritium pellet mixed with heavy ion dopants.We show that with increasing charge number and mixed density ratio the proton stopping power is substantially enhanced,leading to a shorter penetration distance and an earlier appearance of the Bragg peak with higher magnitude.The effect of the hot-spot size on proton-driven fast ignition is also discussed.We find that the required ignition time can be significantly reduced with slightly increased beam energy,while the fuel temperature is difficult to maintain for a long time due to the increasing mechanical work and thermal conduction loss.