17 research outputs found
Mode Conversion of Waves in the Ion-Cyclotron Frequency Range in Magnetospheric Plasmas
Waves in the ion-cyclotron range of frequencies with linear polarization detected by satellites can be
useful for estimating the heavy ion concentrations in planetary magnetospheres. These waves are
considered to be driven by mode conversion (MC) of the fast magnetosonic waves at the ion-ion hybrid
resonances. In this Letter, we derive analytical expressions for the MC efficiency and tunneling of waves
through the MC layer. We evaluate the particular parallel wave numbers for which MC is efficient for
arbitrary heavy ion/proton ratios and discuss the interpretation of the experimental observations
Tunneling and mode conversion of fast magnetosonic waves in the magnetospheres of Earth and Mercury
Narrow-band linearly polarized waves, having a resonant structure and a peak
frequency between the local cyclotron frequency of protons and heavy ions, have
been detected in the magnetospheres of Earth and of Mercury. Some of these wave
events have been suggested to be driven by linear mode conversion (MC) of the
fast magnetosonic waves at the ion-ion hybrid (IIH) resonances. Since the
resonant IIH frequency is linked to the plasma composition, solving the inverse
problem allows one to infer the concentration of the heavy ions from the
measured frequency spectra. In this paper, we identify the conditions when the
MC efficiency is maximized in the magnetospheric plasmas and discuss how this
can be applied for estimating the heavy ion concentration in the magnetospheres
of Earth and Mercury.Comment: 13 pages, 5 figure
Impurity transport in Alcator C-Mod in the presence of poloidal density variation induced by ion cyclotron resonance heating
Impurity particle transport in an ion cyclotron resonance heated Alcator
C-Mod discharge is studied with local gyrokinetic simulations and a theoretical
model including the effect of poloidal asymmetries and elongation. In spite of
the strong minority temperature anisotropy in the deep core region, the
poloidal asymmetries are found to have a negligible effect on the turbulent
impurity transport due to low magnetic shear in this region, in agreement with
the experimental observations. According to the theoretical model, in outer
core regions poloidal asymmetries may contribute to the reduction of the
impurity peaking, but uncertainties in atomic physics processes prevent
quantitative comparison with experiments.Comment: 32 pages, 12 figure
Influence of impurities on the transition from minority to mode conversion heating in (3He)âH plasmas
Ion cyclotron resonance heating (ICRH) is one of the main auxiliary heating systems used in present-day tokamaks and is planned to be installed in ITER. In the initial full-field phase of ITER operating with hydrogen majority
plasmas, fundamental resonance heating of helium-3 ions is one of a few ICRH schemes available. Past JET experiments
with the carbon wall revealed a significant impact of impurities on the ICRH performance in (3He)âH plasmas.
A significant reduction of the helium-3 concentration, at which the transition from minority ion to mode conversion
heating occurs, was found to be due to a high plasma contamination with carbon ions. In this paper we discuss the effect of Be and another impurity species present at JET after the installation of a new ITER-like wall on the transition helium-3 concentration in (3He)âH plasmas. We suggest a potential method for controlling helium-3 level needed for a specific ICRH regime by puffing an extra helium-4 gas to the plasma
Poloidal asymmetries due to ion cyclotron resonance heating
The poloidal density asymmetry of impurity ions in ion cyclotron resonance heated (ICRH) discharges is calculated. The link between the asymmetry strength and ICRH and plasma parameters is quantified. The main parameter governing the asymmetry strength is identified to be the minority ion temperature anisotropy. Through numerical simulations with the full-wave TORIC code coupled to the FokkerâPlanck quasilinear solver SSFPQL, the dependence of the anisotropy on various parameters, such as ICRH power, background density and temperature, minority and impurity concentration and toroidal wavenumber has been investigated. An approximate expression for the poloidal asymmetry of impurities as a function of plasma parameters, resonance location and ICRH power is given. A quantification of the link of the impurity asymmetry and ICRH heating is valuable not only for understanding the changes in the cross-field transport but also for the possibilities to use the asymmetry measurements as diagnostics
Modelling of the ion cyclotron resonance heating scenarios for W7-X stellarator
The construction of the world largest superconducting stellarator Wendelstein 7-X (W7-X) has reached the final stage. One of the main scientific objectives of the W7-X project is to prove experimentally the predicted good confinement of high-energy ions. Ion cyclotron resonance heating (ICRH) system is considered to be installed in W7-X
to serve as a localized source of high energy ions. ICRH heating scenarios relevant for hydrogen and deuterium phases of W7-X operation are summarized. The heating efficiency in (3He)-H plasmas is qualitatively analyzed using a modified version of the 1D TOMCAT code able to account for stellarator geometry. The minority ion absorption is shown to be maximized at the helium-3 concentration ~2% for the typical plasma and ICRH parameters to be available during the initial phase of W7-X
Enhanced ICRF (ion cyclotron range of frequencies) mode conversion efficiency in plasmas with two mode conversion layers
The ICRF (ion cyclotron range of frequencies) mode conversion regime efficiently provides local electron heating. The efficiency of mode conversion could be enhanced due to the interference between the reflected waves (Fuchs V et al 1995 Phys. Plasmas 2 1637â47). Plasmas of large-scale tokamaks can include multiple mode conversion layers which results in a complicated picture of mode conversion. The 1D theory of mode conversion in plasmas with two ionâion hybrid resonance layers is presented. Using the phase-integral method the analytical expression for the conversion coefficient is derived within a cold plasma model. The possible enhancement of the mode conversion coefficient in such plasmas is shown. The developed theory is used to analyze the role of carbon ions in the (3He)H scenario of ICRF heating. As hot plasma effects may decrease the amount of power ultimately ending up on mode converted waves, a brief discussion of numerically obtained results but relying on a hot plasma model is included
Energy-selective confinement of fusion-born alpha particles during internal relaxations in a tokamak plasma
Long-pulse operation of a self-sustained fusion reactor using toroidal magnetic containment requires control over the content of alpha particles produced by D-T fusion reactions. On the one hand, MeV-class alpha particles must stay confined to heat the plasma. On the other hand, decelerated helium ash must be expelled before diluting the fusion fuel. Here, we report results of kinetic-magnetohydrodynamic hybrid simulations of a large tokamak plasma that confirm the existence of a parameter window where such energy-selective confinement can be accomplished by exploiting internal relaxation events known as sawtooth crashes. The physical picture â a synergy between magnetic geometry, optimal crash duration and rapid particle motion â is completed by clarifying the role of magnetic drifts. Besides causing asymmetry between co- and counter-going particle populations, magnetic drifts determine the size of the confinement window by dictating where and how much reconnection occurs in particle orbit topology
ICRF Plasma Production with Hydrogen Minority Heating in Uragan-2M and Large Helical Device
ORCIDă0000-0003-4948-0896This report compares results ion-cyclotron range of frequencies (ICRF) plasma production at hydrogen minority regime in Uragan-2M (U-2M) and Large Helical Device (LHD). The condition of the presence of the fundamental harmonic ion cyclotron resonance zone for the hydrogen inside the plasma column should be fulfilled for this method. The scenario is successful at both machines and weakly sensitive to the variation of the hydrogen concentration in the H2+He gas mixture. It should be noted that at LHD the start up is slower than at U-2M. The comparison of plasma production in ICRF with hydrogen minority at U-2M and LHD indicate that this scenario can be scaled to larger stellarator devices. The experiments made are the base for the proposal for usage this scenario for plasma production in ICRF at Wendelstein 7-X at magnetic field reduced to 1.7 T