Theoretical And Numerical Studies On Cascading Ion Acceleration Driven By Intense Lasers

As one of the most important potential applications of ultra-short ultra-intense laser pulses,laser-driven ion acceleration has been drawing more and more attention,and immense progresses have been achieved both in theoretical and experimental research in two decades.Compared with conventional radio-frequency(RF)accelerators,laser-driven accelerators have the features of high gradient accelerating field,compact size and low cost.Meanwhile,it provides a new technical approach to constructing particle colliders.With the invention of chirped pulse amplification technology,the laser intensity has entered into the relativistic regime,in which the laser oscillation velocity in the laser electric field is close to the speed of light in the vacuum.In the interactions of intense laser pulses with plasmas,a large number of energetic electrons can be generated via various heating mechanisms.When these energetic electrons expand into vacuum(such as in TNSA)or move forward coherently(RPA),a strong charge separation field is induced which can be used for the generation of energetic protons or ions.The ion beams driven by lasers have many prospective applications in fundamental scientific researches,medical science,industry and so on.In order to obtain high-quality and high-energy ion beams,a great diversity of laser-driven ion acceleration mechanisms have proposed.Up till now,the main mechanisms for laser-driver ion acceleration include target normal sheath acceleration(TNSA),coulomb explosion(CE),radiation pressure acceleration(RPA),collisionless shock acceleration(CSA),and so on.However,many applications have strict requirements upon the energy and the quality of ion beams,and yet the ion beams driven by lasers usually have low energy and broad energy spread.Therefore,my thesis is contributed to the further enhancement of the energy and quality of laser-driven ion beams by cascading TNSA,CE or using two-color laser pulses in hole-boring RPA.In chapter one,I will introduce the development of laser techniques,some basic concepts and parameters in laser plasma physics,the potential applications of laser-driven ion beams,and the numerical method of particle-in-cell simulation.In chapter two,I will present a simple review on some main mechanisms for laser-driven ion acceleration and their cascaded and hybrid scheme.In chapter three,the Coulomb expansion in the interaction of ultraintense laser pulses with the micro-tube will be studied.Based on such a Coulomb expansion,I propose a cascaded ion acceleration scheme.This cascaded scheme can enhance both the energy and the quality of an injected ion beam.At first,we have established a model for the electric potential and field distribution of a micro-tube irradiated by laser pulses.Irradiated by intense laser pulses,a micro-tube will be ionized instantaneously and its electrons will be quickly blown away.Due to the expansion of electrons,a radial charge-separation electric field will be generated,which is inward inside the microtube.So it can play a unique role in the focusing of proton beams.The lag-behind ions will form a positively charged hollow cylinder,which brings out a strong axial electric field outward along the axis.We have implemented three-dimensional PIC simulations to verify the accelerating and focusing of an injected proton beam by this Coulomb expansion..By controlling the arrival and departure time of an injected pre-accelerated proton beam to guarantee that this proton beam departs the microtube after the generation of axial and radial electric fields,we show that this proton beam can be further accelerated and focused in this cascaded ion acceleration scheme.We also demonstrate that this cascaded ion acceleration scheme works well with the injected proton beams of energies up to 100 MeV.More importantly,the re-accelerated proton beam will have the characteristics of both low divergence angle and low energy spread.At last,we have also studied the scenario of the laser pulse irradiating the microtube along its axial direction,in which the injected ion beam can be accelerated and focused as well,and the energy conversion efficiency from the laser pulse to the ion beam is obviously enhanced.In chapter four,we study the mechanism of target normal sheath acceleration,.and have proposed a cascaded TNSA scheme.This cascaded TNSA scheme can enhance the quality of the injected ion beams by compressing their longitudinal size.When a short intense laser pulse obliquely irradiates a thin solid foil target,a large number of hot electrons will be generated via collisionless heating mechanisms.A strong charge-separation electrostatic field can be stimulated at the rear sheath when some hot electrons propagate through the target and expand into the vacuum.In particular,it takes a considerable time before this sheath field attains its maximum strength.If an ion beam is injected into this laser-irradiated foil when its sheath field is rising,one can imagine that the back-end ions of the beam will feel a stronger sheath field than the front-end ions.Therefore,the back-end ions may be accelerated to higher velocities even if their initial velocities are lower than those of the front-end ions.Consequently,the back-end ions will overtake the front-end ions.Concerning this whole acceleration process,a half-cycle rotation of the ions in the phase space is accomplished.Using particle-in-cell(PIC)simulations,we verify that the longitudinal size of the injected ion beam can be well controlled via such a phase rotation,i.e.,the ion beam bunching is achieved.Meanwhile,we have compared two ion beams with different incident time in the integrated simulations of two successive acceleration stages,where one ion beam has phase rotation and the other does not have.It is found that the bunched ion beam is more suitable to the cascaded acceleration and its energy spread can be always maintained in low level.In chapter five,the development and evolution of transverse Rayleigh-Taylor instability in the hole-boring RPA has been studied.We have proposed to suppress the Rayleigh-Taylor instability occurred at the front surface of the target by using two-color circularly-polarized laser pulses,including a main pulse and another assistant pulse with relatively lower intensity and lower frequency,.Theoretically,it is found that the hot electrons will be generated in-situ at the target front when the target is irradiated simultaneously by two circularly-polarized laser pulses,and these hot electrons will enhance the transverse diffusion of ions.Consequently,the transverse ion movement can smooth the periodical structure at the front target surface in the characteristic time of the instability development.2D PIC simulations verify that the front target surface becomes smoother and the growth rate of Rayleigh-Taylor instability can be obviously decreased by using two-color laser pulses.Meanwhile,the ion phase space has a stable platformshaped structure,and most of ions can be consistently accelerated to a speed twice the holeboring speed.The spectrum with a low energy spread also verifies the validity of suppression of transverse Rayleigh-Taylor instability by applying two color circularly-polarized laser pulses.In chapter six,a brief summary of the thesis and prospect of future work are included.