Physics of Plasmas is dedicated to the publication of original experimental and theoretical contributions in plasma physics. Physics of Plasmas is published by AIP Publishing with the cooperation of the APS Division of Plasma Physics.
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Recent experiments have demonstrated that a freely localized 100 GHz microwave discharge can propagate towards the microwave source with high speed, forming a complex pattern of selforganized filaments. We present threedimensional simulations of the formation and propagation of such patterns that reveal more information on their nature and interaction with the electromagnetic waves. The developed threedimensional Maxwellplasma solver permits the study of different forms of incident field polarization. Results for linear and circular polarization of the wave are presented and comparisons with recent experiments show a good overall agreement. The three dimensional simulations provide a quantitative analysis of the parameters controlling the time and length scales of the strongly nonlinear plasma dynamics and could be useful for potential microwave plasma applications such as aerodynamic flow and combustion control.

Properties of magnetic loop antennas for exciting electron whistler modes have been investigated in a large laboratory plasma. The parameter regime is that of large plasma frequency compared to the cyclotron frequency and signal frequency below half the cyclotron frequency. The antenna diameter is smaller than the wavelength. Different directions of the loop antenna relative to the background magnetic field have been measured for small amplitude waves. The differences in the topology of the wave magnetic field are shown from measurements of the three field components in three spatial directions. The helicity of the wave magnetic field and of the hodogram of the magnetic vector in space and time are clarified. The superposition of wave fields is used to investigate the properties of two antennas for small amplitude waves. Standing whistler waves are produced by propagating two wave packets in opposite directions. Directional radiation is obtained with two phased loops separated by a quarter wavelength. Rotating antenna fields, produced with phased orthogonal loops at the same location, do not produce directionality. The concept of superposition is extended in a Paper II to generate antenna arrays for whistlers. These produce nearly plane waves, whose propagation angle can be varied by the phase shifting the currents in the array elements. Focusing of whistlers is possible. These results are important for designing antennas on spacecraft or diagnosing and heating of laboratory plasmas.

We show through experiments that a transition from laser wakefield acceleration (LWFA) regime to a plasma wakefield acceleration (PWFA) regime can drive electrons up to energies close to the GeV level. Initially, the acceleration mechanism is dominated by the bubble created by the laser in the nonlinear regime of LWFA, leading to an injection of a large number of electrons. After propagation beyond the depletion length, leading to a depletion of the laser pulse, whose transverse ponderomotive force is not able to sustain the bubble anymore, the high energy dense bunch of electrons propagating inside bubble will drive its own wakefield by a PWFA regime. This wakefield will be able to trap and accelerate a population of electrons up to the GeV level during this second stage. Three dimensional particleincell simulations support this analysis and confirm the scenario.

A 3dimensional particleincell/Monte Carlo collision simulation that is fully implemented on a graphics processing unit (GPU) is described and used to determine lowtemperature plasma characteristics at high reduced electric field, E/n, in nitrogen gas. Details of implementation on the GPU using the NVIDIA Compute Unified Device Architecture framework are discussed with respect to efficient code execution. The software is capable of tracking around 10 × 10^{6} particles with dynamic weighting and a total mesh size larger than 10^{8} cells. Verification of the simulation is performed by comparing the electron energy distribution function and plasma transport parameters to known Boltzmann Equation (BE) solvers. Under the assumption of a uniform electric field and neglecting the buildup of positive ion space charge, the simulation agrees well with the BE solvers. The model is utilized to calculate plasma characteristics of a pulsed, parallel plate discharge. A photoionization model provides the simulation with additional electrons after the initial seeded electron density has drifted towards the anode. Comparison of the performance benefits between the GPUimplementation versus a CPUimplementation is considered, and a speedup factor of 13 for a 3D relaxation Poisson solver is obtained. Furthermore, a factor 60 speedup is realized for parallelization of the electron processes.

The object of this review is to summarize the achievements of research on the Alcator CMod tokamak [Hutchinson et al., Phys. Plasmas 1, 1511 (1994) and Marmar, Fusion Sci. Technol. 51, 261 (2007)] and to place that research in the context of the quest for practical fusion energy. CMod is a compact, highfield tokamak, whose unique design and operating parameters have produced a wealth of new and important results since it began operation in 1993, contributing data that extends tests of critical physical models into new parameter ranges and into new regimes. Using only highpower radio frequency (RF) waves for heating and current drive with innovative launching structures, CMod operates routinely at reactor level power densities and achieves plasma pressures higher than any other toroidal confinement device. CMod spearheaded the development of the verticaltarget divertor and has always operated with highZ metal plasma facing components—approaches subsequently adopted for ITER. CMod has made groundbreaking discoveries in divertor physics and plasmamaterial interactions at reactorlike power and particle fluxes and elucidated the critical role of crossfield transport in divertor operation, edge flows and the tokamak density limit. CMod developed the Imode and the Enhanced Dα Hmode regimes, which have high performance without large edge localized modes and with pedestal transport selfregulated by shortwavelength electromagnetic waves. CMod has carried out pioneering studies of intrinsic rotation and demonstrated that selfgenerated flow shear can be strong enough in some cases to significantly modify transport. CMod made the first quantitative link between the pedestal temperature and the Hmode's performance, showing that the observed selfsimilar temperature profiles were consistent with criticalgradientlength theories and followed up with quantitative tests of nonlinear gyrokinetic models. RF research highlights include direct experimental observation of ion cyclotron range of frequency (ICRF) modeconversion, ICRF flow drive, demonstration of lowerhybrid current drive at ITERlike densities and fields and, using a set of novel diagnostics, extensive validation of advanced RF codes. Disruption studies on CMod provided the first observation of nonaxisymmetric halo currents and nonaxisymmetric radiation in mitigated disruptions. A summary of important achievements and discoveries are included.