Search

We describe a new technique for the efficient generation of high-energy ions with electromagnetic ion cyclotron waves in multi-ion plasmas. The discussed ‘three-ion’ scenarios are especially suited for strong wave absorption by a very low number of resonant ions. To observe this effect, the plasma composition has to be properly adjusted, as prescribed by theory. We demonstrate the potential of the method on the world-largest plasma magnetic confinement device, JET (Joint European Torus, Culham, UK), and the high-magnetic-field tokamak Alcator C-Mod (Cambridge, USA). The obtained results demonstrate efficient acceleration of 3He ions to high energies in dedicated hydrogen–deuterium mixtures. Simultaneously, effective plasma heating is observed, as a result of the slowing-down of the fast 3He ions. The developed technique is not only limited to laboratory plasmas, but can also be applied to explain observations of energetic ions in space-plasma environments, in particular, 3He-rich solar flares.

Up till now, the approach to solve the coupled equations arising in modeling wave heating in tokamaks modeling has been to sequentially solve one equation after the other until convergence is reached, a method known as Iterative Substructuring. In this paper we use an elementary model of the physics inside a tokamak, consisting of a simplified wave equation, a simplified Fokker–Planck equation and a diffusion equation. Our aim is to accelerate the solution of the coupled equations using Aitken’s δ 2 method. Results show that a substantial reduction in CPU time can be obtained with this approach. It is hoped that results obtained with the simplified model serve as a proof-of-principle and carry over to more complicated systems of coupled equations used to model tokamaks. [ABSTRACT FROM AUTHOR]

AbstractA brief overview is given of the techniques adopted for modeling plasma heating and current drive as well as the associated particle diffusion. Weaknesses and strengths of each method are highlighted; references are provided for those seeking further information.

AbstractA brief overview is given of the techniques adopted for modeling plasma heating and current drive as well as the associated particle diffusion. Weaknesses and strengths of each method are highlighted; references are provided for those seeking further information.

AbstractA brief overview is given of the techniques adopted for modeling plasma heating and current drive as well as the associated particle diffusion. Weaknesses and strengths of each method are highlighted; references are provided for those seeking further information.

Understanding the physics of internal transport barriers (ITBs) is a crucial issue in developing ITER relevant advanced tokamak scenarios. To gain new information on ITBs, RF power modulation experiments, mainly devoted to the study of electron heat transport through ITBs, have been performed on the JET tokamak. The main physics results have been reported in [1]. The present paper describes in detail the data analysis and numerical modelling work carried out for the interpretation of the experiments. ITBs located in the negative shear region behave as localized insulating layers able to stop the heat wave propagation, thus implying that the ITB is a region of low diffusivity characterized by a loss of stiffness. Various sources of spurious effects affecting the interpretation of the results are analysed and discussed. First principle based models have so far failed to predict the temperature profile in the first place, which prevented their application to modulation results, while empirical transport models have been set up and reproduce the major part of the data.

During the initial operation of the International Thermonuclear Experimental Reactor (ITER), it is envisaged that activation will be minimized by using hydrogen (H) plasmas where the reference ion cyclotron resonance frequency (ICRF) heating scenarios rely on minority species such as helium (3He) or deuterium (D). This paper firstly describes experiments dedicated to the study of 3He heating in H plasmas with a sequence of discharges in which 5?MW of ICRF power was reliably coupled and the 3He concentration, controlled in real-time, was varied from below 1% up to 10%. The minority heating (MH) regime was observed at low concentrations (up to 2%). Energetic tails in the 3He ion distributions were observed with effective temperatures up to 300?keV and bulk electron temperatures up to 6?keV. At around 2%, a sudden transition was reproducibly observed to the mode conversion regime, in which the ICRF fast wave couples to short wavelength modes, leading to efficient direct electron heating and bulk electron temperatures up to 8?keV. Secondly, experiments performed to study D minority ion heating in H plasmas are presented. This MH scheme proved much more difficult since modest quantities of carbon (C) impurity ions, which have the same charge to mass ratio as the D ions, led directly to the mode conversion regime. Finally, numerical simulations to interpret these two sets of experiments are under way and preliminary results are shown.

AbstractA very short overview is given of the methods commonly used to solve the (partial) differential equations describing plasma heating and current drive. Weakness and strength of each method is briefly discussed, and references are provided where the interested reader can find in depth discussions.

Recent research on advanced tokamak in JET has focused on scenarios with both monotonic and reversed shear q-profiles having plasma parameters as relevant as possible for extrapolation to ITER. Wide internal transport barriers (ITBs), r/a~ 0.7, are formed at ITER relevant triangularity ? ~ 0.45 and moderate plasma current, IP= 1.5-2.5?MA, with ne/nG~ 60% when ELMs are moderated by Ne injection. At higher current (IP? 3.5?MA, ? ~ 0.25) wide ITBs sitting at r/a? 0.5, in the positive shear region, have been developed. Generally MHD events terminate these barriers otherwise limited in strength by power availability. ITBs with core density close to Greenwald value, Te~ Tiand low toroidal rotation (4 times lower than standard ITBs) are obtained in plasma target preformed by opportune timing of lower hybrid current drive (LHCD), pellet injection and a small amount of NBI power. Wide ITBs, r/a~ 0.6, of moderate strength, can be sustained without impurities accumulation for a time close to neoclassical resistive time in 3?T/1.8?MA discharges that exhibit reversed magnetic shear profiles and type-III ELMy edge. These discharges have been extended to the maximum duration allowed by JET subsystems (20?s) bringing to the record of injected energy in a JET discharge: E~ 330?MJ. Portability of ITB physics has been addressed through dedicated similarity experiments. The ITB is identified as a layer of reduced diffusivity studying the propagation of the heat wave generated by modulating the ICRF mode conversion (MC) electron heating. Impressive results, QDT~ 0.25, are obtained in these deuterium discharges with 3He minority when the MC layer is located in the core. The ion behaviour has been investigated in pure LHCD electron ITBs optimizing the 3He minority concentration for direct ion heating. Preliminary results of particle transport, studied via injection of a trace of tritium and an Ar-Ne mixture, will be presented.

This paper reports on ITER-relevant ion cyclotron resonance frequency (ICRF) physics investigated on JET in 2003 and early 2004. Minority heating of helium three in hydrogen plasmas-(3He)H-was systematically explored by varying the 3He concentration and the toroidal phasing of the antenna arrays. The best heating performance (a maximum electron temperature of 6.2?keV with 5?MW of ICRF power) was obtained with a preferential wave launch in the direction of the plasma current. A clear experimental demonstration was made of the sharp and reproducible transition to the mode conversion heating regime when the 3He concentration increased above ~2%. In the latter regime the best heating performance (a maximum electron temperature of 8?keV with 5?MW of ICRF power) was achieved with dipole array phasing, i.e. a symmetric antenna power spectrum. Minority heating of deuterium in hydrogen plasmas-(D)H-was also investigated but was found inaccessible because this scenario is too sensitive to impurity ions with Z/A= 1/2 such as C6+, small amounts of which directly lead into the mode conversion regime. Minority heating of up to 3% of tritium in deuterium plasmas was systematically investigated during the JET trace tritium experimental campaign (TTE). This required operating JET at its highest possible magnetic field (3.9 to 4?T) and the ICRF system at its lowest frequency (23?MHz). The interest of this scenario for ICRF heating at these low concentrations and its efficiency at boosting the suprathermal neutron yield were confirmed, and the measured neutron and gammay ray spectra permit interesting comparisons with advanced ICRF code simulations. Investigations of finite Larmor radius effects on the RF-induced high-energy tails during second harmonic (? = 2?c) heating of a hydrogen minority in D plasmas clearly demonstrated a strong decrease in the RF diffusion coefficient at proton energies ~ 1?MeV, in agreement with theoretical expectations. Fast wave heating and current drive experiments in deuterium plasmas showed effective direct electron heating with dipole phasing of the antennas, but only small changes of the central plasma current density were observed with the directive phasings, in particular at low single pass damping. New investigations of the heating efficiency of ICRF antennas confirmed its strong dependence on the parallel wavenumber spectrum. Advances in topics of a more technological nature are also summarized: ELM studies using fast RF measurements, the successful experimental demonstration of a new ELM-tolerant antenna matching scheme and technical enhancements planned on the JET ICRF system for 2006, they being equally strongly driven by the preparation for ITER.

Experiments have been carried out on the JET tokamak to investigate fast 3He and hydrogen minority ion populations accelerated by ion cyclotron range of frequencies (ICRF) waves launched with multiple frequencies (i.e. up to four frequencies separated by up to ?15%). This 'polychromatic' heating is compared with single-frequency, 'monochromatic', ICRF heating of reference discharges with similar power levels. Information on the fast ion populations is provided by two-dimensional gamma-ray emission tomography and the measurements are compared with numerical modelling. Polychromatic heating with resonances in the plasma centre (Rres? R0) and on the low magnetic-field side (LFS) (Rres> R0) is found to produce predominantly high-energy standard trapped ions, while resonances on the high magnetic-field side (Rres< R0) increase the fraction of high-energy passing ions. Monochromatic heating with a central resonance produces stronger gamma-ray emission with the maximum emission in the midplane close to, and on the LFS of, the resonance, in agreement with the calculated radial distribution of fast ion orbits. Both the fast ion tail temperature and energy content are found to be lower with polychromatic waves. Polychromatic ICRF heating has the advantage of producing smaller-amplitude and shorter-period sawteeth, consistent with a lower fast ion pressure inside the q= 1 surface, and higher ion to electron temperature ratios.

AbstractA short overview is given of methods available to solve the (partial) differential equations describing plasma heating and current drive. Weakness and strength of each method is briefly discussed, and references are provided where the interested reader can find in depth discussions.

An overview is given of methods available to solve the (partial) differential equations describing plasma heating and current drive. Weakness and strength of each method is briefly discussed.

A critical review is made of the different possible ion cyclotron resonance heating (ICRH) scenarios to be envisaged for NET. Their relevance is examined with respect to both physical requirements, reactor applicability and specific NET constraints. The impact of the presence of fusion products on these methods is discussed. It is found that larger aspect ratio and operation at higher magnetic field can strongly reduce alpha particle effects. New dielectric tensor expressions relevant to such particle populations are derived; first estimates show that wave damping for realistic distribution functions can be much reduced with respect to Maxwellian results. It is concluded that the most attractive scheme is 2nd harmonic tritium heating rather than 2nd harmonic deuterium heating which is strongly modified by the presence of residual hydrogen and fusion produced alpha particles in the machine.