Analytical model for ion acceleration by high-intensity laser pulses

We present a general expression for the maximum ion energy observed in experiments with thin foils irradiated by high-intensity laser pulses. The analytical model is based on a radially confined surface charge set up by laser accelerated electrons on the target rear side. The only input parameters are the properties of the laser pulse and the target thickness. The predicted maximum ion energy and the optimal laser pulse duration are supported by dedicated experiments for a broad range of different ions.     

Forward ion acceleration in thin films driven by a high-intensity laser

A collimated beam of fast protons, with energies as high as 1.5 MeV and total number of greater, similar10(9), confined in a cone angle of 40 degrees +/-10 degrees is observed when a high-intensity high-contrast subpicosecond laser pulse is focused onto a thin foil target. The protons, which appear to originate from impurities on the front side of the target, are accelerated over a region extending into the target and exit out the back side in a direction normal to the target surface. Acceleration field gradients approximately 10 GeV/cm are inferred. The maximum proton energy can be explained by the charge-separation electrostatic-field acceleration due to "vacuum heating."     

Proton acceleration with high-intensity ultrahigh-contrast laser pulses

We report on simultaneous measurements of backward- and forward-accelerated protons spectra when an ultrahigh intensity (approximately 5 x 10(18) W/cm(20), ultrahigh contrast (>10(10)) laser pulse interacts with foils of thickness ranging from 0.08 to 105 microm. Under such conditions, free of preplasma originating from ionization of the laser-irradiated surface, we show that the maximum proton energies are proportional to the p component of the laser electric field only and not to the ponderomotive force and that the characteristics of the proton beams originating from both target sides are almost identical. All these points have been corroborated by extensive 1D and 2D particle-in-cell simulations showing a very good agreement with the experimental data.     

Motion and acceleration of electrons in high-intensity laser standing waves

The motion and the energy of electrons driven by the ponderomotive force in linearly polarized high-intensity laser standing wave fields are considered. The results show that there exists a threshold laser intensity, above which the motion of electrons incident parallel to the electric field of the laser standing waves undergoes a transition from regulation to chaos. We propose that the huge energy exchange between the electrons and the strong laser standing waves is triggered by inelastic scattering, which is related to the chaos patterns. It is shown that an electron's energy gain of tens of MeV can be realized for a laser intensity of 10²⁰ W/cm².     

Proton acceleration from high-intensity laser interactions with thin foil targets

Measurements of energetic proton production resulting from the interaction of high-intensity laser pulses with foil targets are described. Through the use of layered foil targets and heating of the target material we are able to distinguish three distinct populations of protons. One high energy population is associated with a proton source near the front surface of the target and is observed to be emitted with a characteristic ring structure. A source of typically lower energy, lower divergence protons originates from the rear surface of the target. Finally, a qualitatively separate source of even lower energy protons and ions is observed with a large divergence. Acceleration mechanisms for these separate sources are discussed.     

Electron acceleration and the propagation of ultrashort high-intensity laser pulses in plasmas

Reported are interactions of high-intensity laser pulses ( lambda = 810 nm and I相似文献   

Laser-induced ion acceleration at ultra-high laser intensities

Ion acceleration by ultrashort laser pulses of very high intensities of the order 10²²?W/cm² is studied by two-dimensional Particle-In-Cell simulations. We show that laser normal incidence is preferred for such high intensities. For linearly polarized laser radiation, higher maximum proton/ion energies are achieved than for circular polarization. For linear polarization, the transition from the target normal sheath acceleration to the acceleration on the target front side by the radiation pressure is analyzed in detail. The transition intensity is increasing with the target thickness. The radiation pressure dominated regime leads to considerably higher number of accelerated protons and thus to a higher acceleration efficiency.     

Instability-free ion acceleration by two laser pulses

We demonstrate the instability-free ion acceleration regime by introducing laser control with two parallel circularly polarized laser pulses at an intensity of I = 6.8 × 10²¹?W/cm², normally incident on a hydrogen foil. The special structure of the equivalent wave front of those two pulses, which contains Gaussian peaks in both sides and a concavity in the centre (2D), can suppress the transverse instabilities and hole boring effects to constrain a high density ion clump in the centre of the foil, leading to an acceleration over a long distance and gain above 1GeV/u for the ion bunches.     

High-intensity laser induced ion acceleration from heavy-water droplets

Fusion neutrons from a heavy water droplet target irradiated with laser pulses of 3 x 10(19) W/cm(2) and from a deuterated secondary target are observed by a time-of-flight (TOF) neutron spectrometer. The observed TOF spectrum can be explained by fusion of deuterium ions simultaneously originating from two different sources: ion acceleration in the laser focus by ponderomotively induced charge separation and target-normal sheath acceleration off the target rear surface. The experimental findings agree well with 3D particle-in-cell simulations.     

激光加速实验超薄类金刚石碳靶的制备

采用过滤阴极真空弧法,制备了满足激光稳相加速机制要求的超薄自支撑类金刚石碳靶。室温下沉积,基片偏压为-32 V,薄膜沉积速率约为每脉冲0.002 nm。选取NaCl膜作脱膜剂,采用漂浮法进行脱膜。打捞板孔径为1 mm时,自支撑厚度范围为5~50 nm,自支撑成功率约为70%。利用拉曼光谱仪及原子力显微镜等仪器,测量了薄膜的结构、表面粗糙度等关键参数。     

激光加速实验超薄类金刚石碳靶的制备

采用过滤阴极真空弧法,制备了满足激光稳相加速机制要求的超薄自支撑类金刚石碳靶。室温下沉积,基片偏压为-32 V,薄膜沉积速率约为每脉冲0.002 nm。选取NaCl膜作脱膜剂,采用漂浮法进行脱膜。打捞板孔径为1 mm时,自支撑厚度范围为5~50 nm,自支撑成功率约为70%。利用拉曼光谱仪及原子力显微镜等仪器,测量了薄膜的结构、表面粗糙度等关键参数。     

Plasma ion evolution in the wake of a high-intensity ultrashort laser pulse

Experimental investigations of the late-time ion structures formed in the wake of an ultrashort, intense laser pulse propagating in a tenuous plasma have been performed using the proton imaging technique. The pattern found in the wake of the laser pulse shows unexpectedly regular modulations inside a long, finite width channel. On the basis of extensive particle in cell simulations of the plasma evolution in the wake of the pulse, we interpret this pattern as due to ion modulations developed during a two-stream instability excited by the return electric current generated by the wakefield.     

Ponderomotive ion acceleration in dense plasmas at super-high laser intensities

Short laser pulses at super-high intensities such as those proposed in the Extreme Light Infrastructure (ELI) project open new prospects for efficient acceleration of ions in overdense plasmas. A simple analytical model and numerical simulations demonstrate that pulses with intensities exceeding 10²² W/cm² may efficiently accelerate ions to high energies up to a few GeV. Maximum ion energy and the energy spectrum of the accelerated ions can be tuned by an appropriate choice of laser pulse intensity and duration at a given plasma density distribution.     

Optimization of laser propagation in optical-field-ionization plasmas for X-ray laser generation

Optical-field-ionization X-ray lasers are investigated numerically with a three-dimensional wave propagation code considering the effects of photoionization, energy depletion due to ionization, and refraction on pump laser pulses in spatially and temporally varied plasmas. By focusing the pump laser with small f-number optics at the optimal position, simulations show that diffraction and ionization-induced refraction in plasmas are compensated to keep the pump beam propagating at the optimal size for a longer distance. An amplification length as long as 5 mm can be achieved in Pd-like xenon and Ni-like krypton X-ray lasers at a pump energy of 160 mJ in 50-fs and 30-fs pulses, respectively. The significant reduction of the pump energy is a desirable step toward low-threshold and practical high-repetition-rate operations. PACS 42.55.Vc; 52.50.Jm     

Collimated multi-MeV ion beams from high-intensity laser interactions with underdense plasma

A beam of multi-MeV helium ions has been observed from the interaction of a short-pulse high-intensity laser pulse with underdense helium plasma. The ion beam was found to have a maximum energy for He2+ of (40(+3)(-8)) MeV and was directional along the laser propagation path, with the highest energy ions being collimated to a cone of less than 10 degrees. 2D particle-in-cell simulations show that the ions are accelerated by a sheath electric field that is produced at the back of the gas target. This electric field is generated by transfer of laser energy to a hot electron beam, which exits the target generating large space-charge fields normal to its boundary.     

Third-harmonic generation in a vacuum at the focus of a high-intensity laser beam

A new physical effect lying in the harmonic generation by a focused high-intensity (I ～ 10²⁷ W/cm²) laser beam in a vacuum is predicted. The probability for the third-harmonic generation is calculated for a specific model of a monochromatic laser beam (symmetric beam) with the optical frequency.     

Hybrid model of ion acceleration in laser plasma of flat heterogeneous target

We have constructed a two-phase analytical model of acceleration of ions in a two-layer laser target. The first phase of acceleration is isothermic and covers the action time of the laser pulse, while the second phase is adiabatic and occurs after the end of the laser pulse. The maximal ion energy is obtained as a function of parameters of the laser pulse and target. We compare analytical results with PIC calculations and show that the theory is adequate.     

Steady state ion acceleration by a circularly polarized laser pulse

The steady state ion acceleration at the front of a cold solid target by a circularly polarized flat-top laser pulse is studied with one-dimensional particle-in-cell (PIC) simulation. A model that ions are reflected by a steady laser-driven piston is used by comparing with the electrostatic shock acceleration. A stable profile with a double-flat-top structure in phase space forms after ions enter the undisturbed region of the target with a constant velocity.