Search

A simple model is presented to describe how the radio frequency electromagnetic field modifies the plasma density the antenna faces in tokamaks. Aside from “off-the-shelf” equations standardly used to describe wave-plasma interaction relying on the quasilinear approach, it invokes the ponderomotive force in presence of the confining static magnetic field. The focus is on dynamics perpendicular to the (Formula presented.) magnetic field. Stronger fields result in density being pushed further away from the launcher and in stronger density asymmetry along the antenna., SCOPUS: ar.j, info:eu-repo/semantics/published

While far from being technologically mature, fusion is an attractive low-carbon source ofenergy that could play an important role in the future world electricity and heat productionmix. Following this target, the next step after demonstrating a net energy gain in fusion willbe the construction of a demonstration fusion reactor to produce heat or electricity.To reach a burning state, where the energy of the fusion products is sufficient to sustainthe fusion reaction, one first needs to bring the fusion fuel above an energy threshold. Oneof the experimentally proven techniques used in present day tokamaks to heat the plasmafuel launches microwaves in the Ion Cyclotron Range of Frequencies (ICRF) in the low temperatureand density edge of fusion devices using an antenna. This heating method, calledIon Cyclotron Resonance Heating (ICRH), is cost effective, has a mature radio-frequencysource technology and comes with highly flexible and efficient heating scenarios. ICRH alsocovers other important fusion applications like current drive, turbulence stabilization in theplasma, wall conditioning, etc.For these reasons, the installation of an ICRH system is considered for future fusiondemonstration power plants like DEMO or ARC. However, future fusion reactor conditionsare quite challenging for ICRH. The two most stringent aspects of a reactor lies (i) in theexpected low density profile in the edge leading to a low coupling of power to the plasma(ii) the restricted space available for auxiliary components inside the reactor’s chamber andsurrounding wall structures. The other challenges faced are the installation, availability andmaintainability of these components in a reactor environment. On top of these challenges theICRH system should minimize the plasma wall interactions and the impurity release inducedduring its operation.Two very different ICRH systems are proposed for the next reactor generation. One relieson high power high density antennas inserted in a port of the fu, Doctorat en Sciences de l'ingénieur et technologie, info:eu-repo/semantics/nonPublished

Predicting the impact of heating and current drive (H&CD) sources is essential to evaluate the performance of ITER plasmas and to subsequently optimise the scenarios for the four stages of the ITER research plan. This should be done in the context of global transport calculations of complete plasma discharges. For this purpose, a dedicated workflow has been developed in the ITER integrated modelling and analysis suite as a modular component to be used together with transport solvers to quantify the dynamics of H&CD sources for the different phases of a plasma discharge, including possible synergetic effects between the heating sources. This paper presents the results of the combined modelling of H&CD sources for the ITER DT baseline 15 MA/5.3 T scenario including the synergy between neutral beam injection (NBI) of deuterium, fusion-born alpha particles and ion cyclotron resonance heating (ICRH) at the fundamental frequency of deuterium, showing modest synergetic effects. The results of the combined H&CD sources for an ITER 7.5 MA/2.65 T helium plasma of the second pre-fusion power operation phase (PFPO-2) are also shown, exhibiting more significant synergetic effects between the fundamental ICRH minority hydrogen heating and NBI hydrogen beams. Finally, a study of electron cyclotron heating absorption for an ITER helium PFPO scenario at 7.5 MA/2.65 T is also presented with a discussion on the edge parasitic absorption that arises under specific conditions., QC 20211206

Most if not all tokamak heating scenarios involve multiple ion populations being heated simultaneously. To allow the simulation of various aspects of physics dynamics determining the characteristics of operational scenarios in a flexible way, speedy yet sufficiently accurate models are needed, and they should be connected to each other via a ‘backbone’. Under the umbrella of EUROfusion's Integrated Modelling efforts, such a structure is provided. The present paper focuses on one physics aspect: auxiliary heating. After solving the wave equation or beam source equation, this requires solving a set of coupled Fokker–Planck equations for the various populations involved. The adopted modules – enabling accounting for the Coulomb collisional interaction of several non-Maxwellian (minority, majority and beam) populations – are discussed and a practical example of their use is provided: the JET ‘baseline’ scenario heating a minority of 3He ions in a balanced D+T mix heated by D and T neutral beams., QC 20210528

Dedicated experiments to generate energetic D ions and D−3He fusion-born alpha particles were performed at the Joint European Torus (JET) with the ITER-like wall (ILW). Using the 3-ion D-(DNBI)-3He radio frequency (RF) heating scenario, deuterium ions from neutral beam injection (NBI) were accelerated in the core of mixed D−3He plasmas to higher energies with ion cyclotron resonance frequency (ICRF) waves, in turn leading to a core-localized source of alpha particles. The fast-ion distribution of RF-accelerated D-NBI ions was controlled by varying the ICRF and NBI power (P_{ICRF}≈4-6 MW, P_{NBI}≈3-20 MW), resulting in rather high D-D neutron (≈1×10^16/s) and D−3He alpha rates (≈2×10^16/s) at moderate input heating power. Theory and TRANSP analysis shows that large populations of co-passing MeV-range D ions were generated using the D−(DNBI)−3He 3-ion ICRF scenario. This important result is corroborated by several experimental observations, in particular gamma-ray measurements. The developed experimental scenario at JET provides unique conditions for probing several aspects of future burning plasmas, such as the contribution from MeV range ions to global confinement, but without introducing tritium. Dominant fast-ion core electron heating with T_i≈T_e and a rich variety of fast-ion driven Alfven eigenmodes (AEs) were observed in these D−3He plasmas. The observed AE activities do not have a detrimental effect on the thermal confinement and, in some cases, may be driven by the fusion born alpha particles. A strong continuous increase in neutron rate was observed during long-period sawteeth (>1 s), accompanied by the observation of reversed shear AEs, which implies that a non-monotonic q profile was systematically developed in these plasmas, sustained by the large fast-ion populations generated by the 3-ion ICRF scenario.

Nuclear fusion is a promising candidate to supply energy for future generations. At the high temperatures needed for the nuclei to fuse, ions and electrons are no longer bound into atoms. Magnetic fields confine the resulting plasma. One of the heating methods is the ion cyclotron resonant absorption of waves emitted by an external Ion Cyclotron Radio Frequency (ICRF) antenna. The efficiency of ICRF heating is strongly affected by rectified RF electric fields at antenna and other in-vessel components (so-called ‘sheath effects’). The chapter presents an overview of ICRF principles. Attention is given to characterising the detrimental sheath effects through experiments on a dedicated test facility (IShTAR: Ion cyclotron Sheath Test ARrangement). IShTAR has a linear magnetic configuration and is equipped with an independent helicon plasma source. The configuration and capabilities of the test-bed and its diagnostics are described, as well as an analysis of the plasmas.

Inverted3He and D ion cyclotron minority heating scenarios were recently tested in JET-ILW. They confirm the good heating efficiency at low concentrations of ∼3%. The3He minority heating scheme is only modestly affected by the change from a carbon (JET-C) to a Beryllium (JET-ILW) wall but unlike what was the case in JET-C, the intrinsic Be ions D-like particles in terms of charge-over-mass ratio do not prevent the D (or4He) minority regime from being exploited. Direct and indirect evidence of the existence of fast particle subpopulations was found in both cases

Inverted 3He and D ion cyclotron minority heating scenarios were recently tested in JET-ILW. They confirm the good heating efficiency at low concentrations of ~3%. The 3He minority heating scheme is only modestly affected by the change from a carbon (JET-C) to a Beryllium (JET-ILW) wall but unlike what was the case in JET-C, the intrinsic Be ions D-like particles in terms of charge-over-mass ratio do not prevent the D (or 4He) minority regime from being exploited. Direct and indirect evidence of the existence of fast particle subpopulations was found in both cases.

We report on simulations aimed at optimizing external heating using neutral beam injection (NBI) and radiofrequency waves in the ion cyclotron range of frequencies (ICRF) for high fusion yield in the JET tokamak. In this paper, D and DT plasmas are analyzed taking into account the NBI+RF synergy focusing on two different minority ICRF schemes, He and H, respectively. Our results show that by increasing external heating power to the maximum power available, the fusion neutron rate can be enhanced in D plasma by a factor of 2-3 with respect to our reference record D discharge. Regarding the DT plasma we present the external heating performance under the variation of key plasma parameters. We also study the impact of the effects of ICRH to the fusion yield and show that the ICRH power results in an enhanced fusion yield in the whole parameter space studied., This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. Dani Gallart would like to express his gratitude to “La Caixa” for support of his PhD studies., Postprint (author's final draft)

A fusion reactor requires plasma pre-heating before the rate of deuterium-tritium fusion reactions becomes significant. In ITER, radiofrequency (RF) heating of He ions, additionally puffed into the plasma, is one of the main options considered for increasing bulk ion temperature during the ramp-up phase of the pulse. In this paper, we propose an alternative scenario for bulk ion heating with RF waves, which requires no extra. He puff and profits from the presence of intrinsic Beryllium impurities in the plasma. The discussed method to heat Be impurities in D-T plasmas is shown to provide an even larger fraction of fuel ion heating., The authors are grateful to the anonymous referee for his/her valuable suggestion to clarify the effect of extra impurity species. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission., Peer Reviewed, Postprint (author's final draft)

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 reflectedwaves (FuchsV et al 1995 Phys. Plasmas 2 1637-47). 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 ion-ion 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. © 2010 IOP Publishing Ltd., SCOPUS: ar.j, info:eu-repo/semantics/published

ICRF mode conversion heating regime is widely used in present-day tokamaks. The theoretically predicted mode conversion enhancement due to the constructive interference of the fast wave reflected from the evanescence layer with a wave reflected from the high-field side cutoff was first experimentally identified in JET in (3He) D experiments. This effect was recently tested in (3He) H plasmas, which is one of the ICRF heating scenarios considered for the non-activated phase of ITER operation. Due to the presence of the intrinsic D-like impurities in JET (e.g. C6+, 4He) the supplementary conversion layer was produced in the plasma that defined the interference conditions. The different behavior of the conversion efficiency observed at various 3He concentrations is analyzed in the paper on the basis of the theory of the fast wave mode conversion in plasmas with two evanescence layers., SCOPUS: ar.j, info:eu-repo/semantics/published

Recent JET experiments have been devoted to the study of (3He)–D plasmas involving radio frequency (RF) heating. This paper starts by discussing the RF heating efficiency theoretically expected in such plasmas, covering both relevant aspects of wave and of particle dynamics. Then it gives a concise summary of the main conclusions drawn from recent experiments that were either focusing on studying RF heating physics aspects or that were adopting RF heating as a tool to study plasma behavior. Depending on the minority concentration chosen, different physical phenomena are observed. At very low concentration (X[3He] < 1%), energetic tails are formed which trigger MHD activity and result in loss of fast particles. Alfvén cascades were observed and gamma ray tomography indirectly shows the impact of sawtooth crashes on the fast particle orbits. Low concentration (X[3He] < 10%) favors minority heating while for X[3He] 10% electron mode conversion damping becomes dominant. Evidence for the Fuchs et al standing wave effect (Fuchs et al 1995 Phys. Plasmas 2 1637–47) on the absorption is presented. RF induced deuterium tails were observed in mode conversion experiments with large X[3He] (≈18%). As tentative modeling shows, the formation of these tails can be explained as a consequence of wave power absorption by neutral beam particles that efficiently interact with the waves well away from the cold D cyclotron resonance position as a result of their substantial Doppler shift. As both ion and electron RF power deposition profiles in (3He)–D plasmas are fairly narrow—giving rise to localized heat sources—the RF heating method is an ideal tool for performing transport studies. Various of the experiments discussed here were done in plasmas with internal transport barriers (ITBs). ITBs are identified as regions with locally reduced diffusivity, where poloidal spinning up of the plasma is observed. The present know-how on

Experiments on several tokamaks convincingly demonstrated that a deliberate seeding of selected impurities can have a positive effect on the plasma performance. On the one hand, a significant reduction of the head load on divertor plates, one of the main concerns by constructing a thermonuclear reactor, has been achieved due to the increase of edge radiation. On the other hand, in some devices impurity seeding has led to an improvement of the energy confinement and the so called radiation improved (RI) mode has been established with the same or even better confinement than in the H-mode. However, in order to make use of these positive impacts, the behaviour of seeded impurity has to be strictly controlled and such negative developments as the accumulation of impurity ions in the plasma core accompanied by a strong increase of the central radiation losses should be avoided.Plasma heating by radio-frequency (RF) waves has been proven to be a useful tool to control the behaviour of puffed impurities. In order to asses the prospective of impurity control by RF waves in larger devices and under reactor conditions, proper modelling approaches have been developed. One of the important parameters, which should be evaluated, is the averaged energy or temperature of heated impurity ions. The latter determines, in particular, the power transported to the main species, and, thus, the heating efficiency. Besides, the temperature of impurity ions characterizes the intensity of particle losses for heated impurities. An approach to compute the impurity temperature under such conditions is elaborated. It is based on the construction of a hierarchy of approximate solutions to the impurity heat balance equation and takes into account that the density and, thus, the heat conductivity of heated ion species can change by many orders of magnitude with the position in the plasma. The developed method has been incorporated into 1D transport code RITM. Coupled with the full wave code T, Doctorat en sciences, Spécialisation physique, info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/published

Wave-particle decorrelation mechanisms affect the ICRF heating efficiency. Depending on the relative magnitude of the bounce and decorrelation times, three regimes are distinguished. When the decorrelation is due to collisions, these three regimes roughly correspond to thermal, a and RF induced tail particles, respectively. Heating efficiencies may be significantly lower than predicted by models assuming uncorrelated resonance crossings; this has an immediate bearing on superadiabatic α particles which will then absorb less ICRH power., info:eu-repo/semantics/published

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/published