Studies On Plasma Shutter For Ultraintense Laser-driven Proton Acceleration

In comparison with conventional RF accelerators,ultraintense laser-driven ion acceleration can provide an acceleration gradient up to TV/m in ?m scale,which will significantly reduce the accelerator size and make ”table-top accelerator” feasible.The generated ion beams could be applied in proton radiography,hadron therapy,warm dense matter production and ion-driven fast ignition in inertial confinement fusion,etc,due to their advantages of small emittance,short pulse duration and high brightness.Extensive theoretical and experimental efforts have been devoted to the high-quality proton beam generation and optimization based on the target normal sheath acceleration scheme(TNSA).In recent years,with the rapid development of laser technology and micro-nano target fabrication technology,studies of laser-plasma interaction has entered a new regime.It enables the experimental studies on advanced ion acceleration mechanisms(such as BOA and RPA)and novel radiation source generation(such as HHG and X-ray).However,the issues of laser prepulse on laser-plasma interaction becomes a new challenge.High temporal contrast is prerequisite to both TNSA and other novel mechanisms.This thesis will focus on how to improve the contrast of laser pulses by using an ultrathin nanometer-foil as a plasma optical shutter,tune the spatial density distribution of pre-plasma at the same time,and therefore application in enhancing the proton acceleration and the proton beam quality.The thesis is organized into four parts as follows:In the first part(chapter 1 and chapter 2),the development of laser technology,the basic theories of laser-plasma interaction,ion acceleration mechanisms and the relevant applications are briefly introduced.Then I will present the main experimental methods and diagnosis in laser-plasma interaction.The effect of laser prepulse on ion acceleration,target fabrication and transverse optical probing are emphasized.In the second part(chapter 3 and chapter 4),a plasma optical shutter concept to improve the laser pulse contrast and modulate the spatial density distribution of the preplasma by using an ultrathin foil is proposed.Firstly,the characteristics of plasma shutter,including laser energy transmittance,the temporal shape and spectrum with phase of transmitted laser pulse and the plasma shutter density distribution are measured and optimized.It is found that the plasma shutter can shape the front edge and modulate the spectrum of the transmitted laser pulse.This is strongly dependent on the shutter foil thickness.When the shutter foil is relatively thin(?50nm),the front edge of the transmitted laser pulse becomes steeper and the pulse duration becomes narrower.With the increase of shutter foil thickness,the spectrum of transmitted laser pulse is narrowed.The obvious intensity suppression at the long-wave side is gradually shifted to the short-wave side.The laser energy transmittance is nearly kept at 40%.Secondly,the plasma shutter is placed in front of a main target of interest and separated them by a few tens of microns-thick vacuum.The preplasma profiles from the double-foil design observed are similar to that produced from a single-layer reference target irradiated by a high-contrast laser,and can be finely tuned by varying the shutter thickness.Proton beams with significantly reduced divergence and higher flux density were measured experimentally using the double-foil design.The reduction in beam divergence is a characteristic signature of higher contrast laser production as a combined consequence of less target deformation and flatter sheath-acceleration field,which results from the decrease of ASE prepulse intensity and the increase of hot electrons injection angle at the target front surface.This is demonstrated by 2D hydrodynamic and particle-in-cell simulations.With the increase of the shutter target thickness,the maximum energy of the proton beam improves and the beam divergence angle remains stable within the moderate thickness range.In the third part(chapter 5),the enhancement on proton acceleration by employing a plasma shutter is studied,which is carried out on the high-energy PW laser system.The maximum energy of the proton beam and the laser-to-proton energy conversion efficiency are both significantly increased when inserting the plasma shutter.The proton energy spectra are also modulated from plateau-shape profiles to exponential distributions.The measured annular x-ray emissions from both target front and rear sides and the 2D analytical numerical model show that the optimal proton acceleration occurs when the front shutter foil is right swelled onto the front surface of the rear source foil by the prepulses at the arrival of the main laser pulse.The enhancement on proton acceleration efficiency is mainly contributed to the moderate preplasma density distribution,which is supported by the 1D hydrodynamic simulation.In addition,it is also shown that the plasma shutter is universal.This cascade thin-foil target design can be easily popularize to other laser-plasma physics researches which requires high contrast laser pulse.In the fourth part(chapter 6),the proton acceleration from vacuum-gapped double-foil target with initially high contrast laser irradiation and controlled femtosecond prepulse is studies.Two proton beams with uniform spatial distribution and small divergence angle are observed along the target normal direction and laser propagation direction,which is considered to come from the second-layer proton source target and the first-layer shutter foil,respectively.With the increase of femtosecond prepulse intensity,the flux density of proton beam in the direction of laser propagation is gradually increased than that in target normal direction.The possible reason is that the femtosecond prepulse produces a preplasma state with density profile changing from high-density small-scale to near-critical-density large-scale,which leads to the shift from TNSA mechanism to CSA or BOA.The spatial intensity distribution of the reflected laser pulse and the temporal shape and spectrum of the transmitted laser pulse through the plasma shutter support this hypothesis.Underlying physics requires intensive theoretical analysis and further experimental verification.