Search

Laser plasma instabilities (LPIs) cause laser energy loss, asymmetric and insufficient compression, and target preheating, thus are assumed to be among the major concerns of inertial confinement fusion research. Mitigation of LPIs can enhance the laser–target coupling efficiency and optimize the target compression dynamics, which is critical for the realization of robust and high-efficiency fusion ignition. Broadband lasers with polychromatic components or random phases have been investigated for decades as an effective alternative to mitigate LPIs. Here, we present a brief overview on the progress of broadband LPIs, including the models of broadband lasers, the involved physics, the conditions for effective suppression of LPIs, and some schemes to produce broadband lasers.

Laser-driven ion acceleration is an attractive technique for compact high-energy ion sources. Currently, among various physical and technical issues to be solved, the boost of ion energy and the reduction of energy spread represent the key challenges with this technique. Here we present a scheme to tackle these challenges by using a hundred-terawatt-class laser pulse irradiating a cone target. Three-dimensional particle-in-cell simulations show that a large number of electrons are dragged out of the cone walls and accelerated to hundreds of MeV by the laser fields inside the cone. When these energetic dense electron beams pass through the cone target tip into vacuum, a very high bunching acceleration field, up to tens of TV/m, quickly forms. Protons are accelerated and simultaneously bunched by this field, resulting in quasi-monoenergetic proton beams with hundred MeV energy and low energy spread of ~2%. Results exploring the scaling of the proton beam energy with laser and target parameters are presented, indicating that the scheme is robust. This opens a new route for compact high-energy proton sources from fundamental research to biomedical applications.

Astrophysical jets are ubiquitous in the universe and often associated with compact objects, and their interactions with the ambient medium not only dissipate their own energy but also provide ideal circumstances for particle acceleration. By means of theoretical analysis and particle-in-cell simulations, here we study the ion acoustic shock wave (IASW) formation and consequent ion acceleration when electron–positron (e − e +) jets are injected into ambient electron–ion plasmas. It is found that the Buneman instability can be excited first, which induces the formation of an ion acoustic wave (IAW). As the amplitude of the IAW increases, its waveform is steepened and subsequently an IASW is formed. Some ions in the ambient plasmas will be reflected when they encounter the IASW, and thus can be accelerated to form an energetic ion beam. For an initial e − e + jet with the Lorentz factor γ 0 = 100 and the ion–electron mass ratio m i /m e = 1836, the ions can be accelerated up to 580 MeV. This study deepens our understanding of the fireball model of gamma-ray bursts, the shock model of pulsar wind nebulae, the origin of cosmic rays, and other related astrophysical processes.

Bright γ -ray sources are of great significance for fundamental research, medicine, and industry. However, γ -ray production by bremsstrahlung, Compton scattering, and synchrotron radiation is often subjected to large divergence, wide size, and/or low efficiency, making it difficult to achieve high-brightness γ -ray sources. Here, we have found an efficient mechanism to generate γ -rays with, to our knowledge, unprecedented brilliance by use of an ultrarelativistic electron beam with nC charge incident onto a solid surface at grazing incidence. With this interaction configuration, extreme high quasistatic magnetic fields up to the gigagauss level or effective electric fields up to 10 14 V / m are induced by the background electron backflows at the target surface. Subsequently, the electron beam is strongly focused by such fields by over an order of magnitude to submicrometer diameter, and its density is increased beyond the solid density; the induced effective fields are high enough to trigger quantum electrodynamics (QED) effects. These result in the production of extremely brilliant dense γ -ray beams with photon energy reaching multi-GeV, where the electron-to-photon energy conversion efficiency can exceed 60%. This offers a promising route to push the beam–target interaction to the QED regime, leading to ultrabright γ -ray sources for various applications.

Relativistic laser wakefield acceleration is characterized by an unsurpassed accelerating gradient, which is very suitable for electron acceleration over short distances and could be a promising candidate for next-generation compact accelerators. However, using this technique for positron acceleration is still challenging because positively charged particles are naturally defocused in nonlinear wakefields. Here we propose and numerically demonstrate a scheme to accelerate an externally injected positron beam in a nonlinear laser wakefield in a regime where a tail wave is formed behind density cusps of the wakefield. This tail wave can provide a focusing force in addition to longitudinal acceleration for the positrons. Three-dimensional particle-in-cell simulations demonstrate that a trapping efficiency of positrons of nearly 100% in the nonlinear wakefield is possible. This scheme may open a simple way for compact positron acceleration to multi-100 MeV with terawatt-class laser systems at high repetition rates without the need for special laser modes and plasma structures.

Ultra-intense short-pulse light sources are powerful tools for a wide range of applications. However, relativistic short-pulse lasers are normally generated in the near-infrared regime. Here, we present a promising and efficient way to generate tunable relativistic ultrashort pulses with wavelengths above 20 µm in a density-tailored plasma. In this approach, in the first stage, an intense drive laser first excites a nonlinear wake in an underdense plasma, and its photon frequency is then downshifted via phase modulation as it propagates in the plasma wake. Subsequently, in the second stage, the drive pulse enters a lower-density plasma region so that the wake has a larger plasma cavity in which longer-wavelength infrared pulses can be produced. Numerical simulations show that the resulting near-single-cycle pulses cover a broad spectral range of 10–40 µm with a conversion efficiency of ∼2.1% (∼34 mJ pulse energy). This enables the investigation of nonlinear infrared optics in the relativistic regime and offers new possibilities for the investigation of ultrafast phenomena and physics in strong fields.