The increasing need to demonstrate the correctness of computer simulations has highlighted the importance of benchmarks. We define in this paper a representative simulation case to study low-temperature partially-magnetized plasmas. Seven independently developed Particle-In-Cell codes have simulated this benchmark case, with the same specified conditions. The characteristics of the codes used, such as implementation details or computing times and resources, are given. First, we compare at steady-state the time-averaged axial profiles of three main discharge parameters (axial electric field, ion density and electron temperature). We show that the results obtained exhibit a very good agreement within 5% between all the codes. As ExB discharges are known to cause instabilities propagating in the direction of electron drift, an analysis of these instabilities is then performed and a similar behaviour is retrieved between all the codes. A particular attention has been paid to the numerical convergence by varying the number of macroparticles per cell and we show that the chosen benchmark case displays a good convergence. Detailed outputs are given in the supplementary data, to be used by other similar codes in the perspective of code verification. 2D axial-azimuthal Particle-In-Cell benchmark for low-temperature partially ...

This paper provides perspectives on recent progress in the understanding of the physics of devices where the external magnetic field is applied perpendicularly to the discharge current. This configuration generates a strong electric field, which acts to accelerates ions. The many applications of this set up include generation of thrust for spacecraft propulsion and the separation of species in plasma mass separation devices. These "E×B" plasmas are subject to plasma-wall interaction effects as well as various micro and macro instabilities, and in many devices, we observe the emergence of anomalous transport. This perspective presents the current understanding of the physics of these phenomena, state-of-the-art computational results, identifies critical questions, and suggests directions for future research.

The first experimental observation of a radio frequency (rf ) wave-induced particle pinch of trapped ions has been made in experiments at the Joint European Torus with nearly on-axis high power toroidally asymmetric ICRF (ion cyclotron range of frequencies) heating. Significant differences have been detected in discharges when waves have been directed in opposite toroidal directions. In particular, fast-ion-driven Alfvén eigenmode activity, sawtooth behavior, and proton distribution functions have been found to be strongly affected. The analysis of the discharges shows that the observed differences are consistent with an ICRF-induced particle pinch predicted by theory. [S0031-9007(98)06753-2] PACS numbers: 52.50. Gj, 52.55.Fa, 52.65.Ff Heating with waves in the ion cyclotron range of frequencies (ICRF) is one of the main methods for auxiliary heating of tokamak plasmas. On JET [1] ICRF is a well established method and its potential for heating of reactor plasmas was demonstrated during the recent deuterium-tritium campaign [2]. Each of the four ICRF antennas at JET consists of four straps. By applying different phasings to the currents in the straps, it is possible to launch waves not only with a symmetric but also with an asymmetric toroidal mode number spectrum. In this Letter we present experimental evidence for a theoretically predicted wave-induced particle pinch associated with asymmetric spectra.The most commonly used asymmetric phasing at JET is obtained by having 190 ± or 290 ± between the currents in two adjacent straps. For these phasings the toroidal mode number spectrum is asymmetric with a peak around the toroidal mode number jNj 16 (see Fig. 14). Furthermore, the wave propagation is mainly collinear to the toroidal magnetic field and the plasma current for the 190 ± phasing (Fig. 1).Several effects are predicted by theory in the presence of toroidally asymmetric wave particle interaction. First, it is possible to produce minority current drive (MCD) [4]. Experiments with asymmetric spectra and the offaxis cyclotron resonance near the q 1 surface have confirmed that MCD exists and can be used to stabilize sawteeth [5,6].Second, a mechanism for inducing convective radial transport of resonating passing ions, i.e., ions with y k fi 0 along their orbit, has been suggested [7]. Here y k is the velocity component parallel to the magnetic field B. Experimental evidence for this effect has been presented [8]. However, in the case of strong ICRF heating, as in the experiments discussed in this Letter, the resonating ions are mainly trapped and the number of energetic passing ions involved in the radial transport described by the mechanism in Ref.[7] is small.For trapped resonating ions another radial convective transport mechanism has been proposed [9], which depends on the direction of the antenna spectrum (i.e., on the sign of N). This ICRF-induced particle pinch arises because of a fundamental relationship between the change in energy E and toroidal angular momentum P w mRy k B w ͞B 1 Zec an ion recei...

No abstract

The effects of RF-induced transport and orbit topology of resonant ions are analysed for high power ion cyclotron resonance heating (ICRH). These effects are found to play important roles in the details of the high-energy part of the distribution function, and affect the driven current and momentum transfer to the background plasma. The finite drift orbit width broadens the power deposition and leads to losses of high-energy ions intercepted by the wall. RF-induced transport of resonant ions across magnetic flux surfaces appears due to the toroidal acceleration of resonant ions interacting with waves having a finite toroidal mode number. Heating with waves propagating parallel to the current leads to a drift of the turning points of trapped resonant ions towards the midplane. As the turning points meet, the orbits will de-trap, preferentially into co-current passing orbits, which may ultimately be displaced to the low field side of the magnetic axis. Ions with such orbits are a typical feature in plasmas heated with directed toroidal mode spectra of waves propagating parallel to the plasma current. These ions will be subjected to a strong RF diffusion partly caused by the focusing of the wave field and partly by the Doppler shifted cyclotron resonance, as it approaches tangency with the drift orbit. The resonance condition puts a limitation on the achievable energy for these ions, which is more severe than for corresponding trapped ions. This results in a rather flat tail up to a critical energy, above which the tail rapidly decays. Heating with waves propagating anti-parallel with the plasma current curtails the energy of the trapped ions due to a vertical outward drift of the turning points of the trapped ions. Heating with symmetric spectra, in particular with waves with low magnitude of the toroidal mode numbers, gives rise to high-energy trapped ions with wide orbits, of which the maximum energy is either restricted by the fact that the RF diffusion vanishes due to cancellation of the perpendicular acceleration over a gyro orbit or by the drift orbits being intercepted by the wall. In the steady state the main source for momentum transfer to the bulk plasma comes from the finite momentum of the wave for heating with asymmetric spectra. For heating with symmetric spectra the enhanced losses of high-energy trapped ions can produce a net counter-current torque on the plasma.