The interaction of a petawatt laser with a small solid-density plasma bunch is studied by particle-in-cell simulation. It is shown that when irradiated by a laser of intensity $>{10}^{21}\mathrm{W}∕{\mathrm{cm}}^2$, a dense plasma bunch of micrometer size can be efficiently accelerated. The kinetic energy of the ions in the high-density region of the plasma bunch can exceed ten MeV at a density in the ${10}^{23}\text{−}{\mathrm{cm}}^{-3}$ level. Having a flux density orders of magnitude higher than that of the traditional charged-particle pulses, the laser-accelerated plasma bunch can have a wide range of applications. In particular, such a dense energetic plasma bunch impinging on the compressed fuel in inertial fusion can significantly enhance the nuclear-reaction cross section and is thus a promising alternative for fast ignition.

Intense laser-plasma interaction can be a source of various electronic instabilities. Recently, stimulated backscattering from a trapped electron-acoustic wave (SEAS) [Montgomery et al., Phys. Rev. Lett. $87,$ 155001 (2001)] was proposed to reinterpret spectra previously attributed to stimulated Raman scattering (SRS) from unrealistically low densities. By particle simulations in a uniform plasma layer, which is overdense for ordinary SRS, strong reflection by SEAS at the electron plasma frequency is found. Transient SEAS reflectivity pulsations are followed by strong relativistic heating of electrons. Physical conditions are explained by three-wave parametric coupling between laser light, standing backscattered wave and slow electron-acoustic wave. Regions in which SEAS reflection can dominate over SRS are singled out.

The formation of an electrical potential difference between hot and cold plasmas is studied by means of a particle simulation. It is found that a double layer structure is formed on the hot‐plasma side of the contact surface between the cold and hot plasmas. The potential difference is given approximately by φ_DL∼T_eh/2e, where T_eh and e are the hot‐electron temperature and the electronic charge, respectively. The double layer is accompanied by a negative potential dip on the low potential side (cold plasma) of the double layer, the depth of which is φ_dip∼2T_ec/e, where T_ec is the cold‐electron temperature. Interestingly, however, the positive potential difference and the negative potential dip are created independently.

It is first demonstrated that a V-shaped dc potential structure is created by a current-driven electrostatic ion-cyclotron instability by means of an open two-dimensional particle simulation model. A positive dc potential difference along magnetic field lines is generated by anomalous resistivity caused by the ion-cyclotron instability. In the direction across the magnetic field lines, the dc potential rises from the high-current region to the low-current region.

We present initial results in the development of a gyrokinetic particle-in-cell code for the whole-volume modeling of stellarators. This is achieved through two modifications to the X-point Gyrokinetic Code (XGC), originally developed for tokamaks. One is an extension to three-dimensional geometries with an interface to Variational Moments Equilibrium Code (VMEC) data. The other is a connection between core and edge regions that have quite different field-line structures. The VMEC equilibrium is smoothly extended to the edge region by using a virtual casing method. Non-axisymmetric triangular meshes in which triangle nodes follow magnetic field lines in the toroidal direction are generated for field calculation using a finite-element method in the entire region of the extended VMEC equilibrium. These schemes are validated by basic benchmark tests relevant to each part of the calculation cycle, that is, particle push, particle-mesh interpolation, and field solver in a magnetic field equilibrium of Large Helical Device including the edge region. The developed code also demonstrates collisionless damping of geodesic acoustic modes and steady states with residual zonal flow in the core region.