Ion Cyclotron Radio Frequency Heating Research Articles

Frontiers in data analysis methods: from causality detection to data driven experimental design

On the route to the commercial reactor, the experiments in magnetical confinement nuclear fusion have become increasingly complex and they tend to produce huge amounts of data. New analysis tools have therefore become indispensable, to fully exploit the information generated by the most relevant devices, which are nowadays very expensive to both build and operate. The paper presents a series of innovative tools to cover the main aspects of any scientific investigation. Causality detection techniques can help identify the right causes of phenomena and can become very useful in the optimisation of synchronisation experiments, such as the pacing of sawteeth instabilities with ion cyclotron radiofrequency heating modulation. Data driven theory is meant to go beyond traditional machine learning tools, to provide interpretable and physically meaningful models. The application to very severe problems for the tokamak configuration, such as disruptions, could help not only in understanding the physics but also in extrapolating the solutions to the next generation of devices. A specific methodology has also been developed to support the design of new experiments, proving that the same progress in the derivation of empirical models could be achieved with a significantly reduced number of discharges.

Simulations of ICRF heating of fusion oriented plasmas in plane-stratified and full toroidal geometry

We compare results from the fully toroidal TORIC–SSFPQL package (Brambilla and Bilato 2006 Nucl. Fusion 46 s387) and from the plane-stratified geometry solver FELICE (Brambilla 1989 Plasma Phys. Control. Fusion 31 723) to illustrate the advantages and the limitations of the two approaches for the simulations of Ion Cyclotron Radio Frequency heating of tokamak plasmas. We point out that some of the predictions of the plane-stratified models do not straightforwardly apply to the true toroidal configuration. This is the case, in particular, for the excitation of guided modes between the plasma surface and the vessel.

New assessment of the fast ion energy in ASDEX upgrade H-mode discharges

Confinement scaling laws such as IPB98(y, 2) are widely used to extrapolate the performance of present tokamaks to next-step devices such as ITER or DEMO. The thermal energy of the plasma (W th), which is used to determine the energy confinement time for most scaling laws, is difficult to measure, due to the sizeable uncertainties in the experimental kinetic profiles. The common approach in the tokamak community is to derive W th as the difference between the measured magnetohydrodynamic (MHD) energy and some simulation-based estimate of the fast ion energy W fi. In H-mode plasmas W fi can be as high as W th, in presence of neutral beam injection (NBI) or ion cyclotron radio frequency heating (ICRF), therefore an accurate assessment of W fi is crucial to have a somewhat reliable H-factor, regardless of the power-scaling of a given scaling law. In this paper we aim at evaluating the current approach to estimate W fi, by comparing its predictions with a wide database of calculations using validated NBI codes. Systematic deviations and trends, as well as statistical scatter are discussed. We use a comprehensive database of AUG H-mode deuterium plasmas, with significant variations of plasma current, NBI power and plasma density. We neglect thereby the fast-ion losses caused by MHD modes and the synergy effect between NBI and ICRF. A new approach is proposed based on the newly developed fast NBI code RABBIT.

Experimental Results of Core Ion Temperature and Neutral Density Measurements on ADITYA Tokamak using Four Channels Neutral Particle Analyzer

Core-ion temperature measurements are based on resolving the energy components and the analysis of fast neutrals coming out of the magnetic confinement of plasma. ADITYA-tokamak (R/a = 75 cm/25 cm) uses Neutral Particle Analyzer (NPA) based charge exchange diagnostic system for measurement of the core ion-temperature in the range of 100–300 eV of Aditya plasma. Several ohmic plasma discharges are investigated for the Aditya tokamak, which provides an estimate of core ion-temperature (Ti0) evaluation with time and its comparison with the core electron-temperature (Te0). The analysis shows the ratio of Ti0/Te0 is typically in the rage of 30%–40%. Attempts have been made to estimate the neutral hydrogen (nH°) density in the core regime and its evolution with time using a simple approach. The core neutral densities estimated in the order of 108–109 cm−3 for ohmic discharges in ADITYA-tokamak. Effect of the ion cyclotron radio frequency heating (ICRH) on the experimental charge-exchange spectrum is also observed, which shows a typical increase of the perpendicular ion temperature (Ti⊥) by ~ 50 eV.

ICRF heating schemes for the ITER non-active phase

ITER plasma operation requires a non-active phase for tokamak initial commissioning, covering First Plasma and Pre-Fusion Power Operation phases, PFPO-1 and PFPO-2. Non-active operation consists of hydrogen and helium plasmas to minimize the neutron production rate. The present document describes some Ion Cyclotron Radio Frequency (ICRF) heating schemes in terms of their predicted performance for the main foreseen scenarios of the ITER non-active phase in hydrogen and helium. Emphasis is given on remaining issues and physics uncertainties to be addressed for successful ICRF heating in ITER.

Modelling third harmonic ion cyclotron acceleration of deuterium beams for JET fusion product studies experiments

Recent JET experiments have been dedicated to the studies of fusion reactions between deuterium (D) and Helium-3 (3He) ions using neutral beam injection (NBI) in synergy with third harmonic ion cyclotron radio-frequency heating (ICRH) of the beam. This scenario generates a fast ion deuterium tail enhancing DD and D3He fusion reactions. Modelling and measuring the fast deuterium tail accurately is essential for quantifying the fusion products. This paper presents the modelling of the D distribution function resulting from the NBI+ICRF heating scheme, reinforced by a comparison with dedicated JET fast ion diagnostics, showing an overall good agreement. Finally, a sawtooth activity for these experiments has been observed and interpreted using SPOT/RFOF simulations in the framework of Porcelli’s theoretical model, where NBI+ICRH accelerated ions are found to have a strong stabilizing effect, leading to monster sawteeth.

Conceptual study of an ICRH traveling-wave antenna system for low-coupling conditions as expected in DEMO

For the central heating of a fusion reactor ion cyclotron radio frequency heating (ICRH) is the first choice method as it is able to couple RF power to the ions without density limit. The drawback of this heating method is the problem of excitation of the magneto-sonic wave through the plasma boundary layer from the antenna located along the wall, without exceeding its voltage standoff. The amount of coupling depends on the antenna excitation and the surface admittance at the antenna output due to the plasma profile. The paper deals with the optimization of the antenna excitation by the use of sections of traveling-wave antennas (TWAs) distributed all along the reactor wall between the blanket modules. They are mounted and fed in resonant ring system(s). First, the physics of the coupling of a strap array is studied by simple models and the coupling code ANTITER II. Then, after the study of the basic properties of a TWA section, its feeding problem is solved by hybrids driving them in resonant ring circuit(s). The complete modeling is obtained from the matrices of the TWA sections connected to one of the feeding hybrid(s). The solution is iterated with the coupling code to determine the loading for a reference low-coupling ITER plasma profile. The resulting wave pattern up to the plasma bulk is derived. The proposed system is totally load resilient and allows us to obtain a very selective exciting wave spectrum. A discussion of some practical implementation problems is added.

The behavior of neutron emissions during ICRF minority heating of plasma at EAST

Ion cyclotron radio frequency (ICRF) wave heating is a primary method to heat ions in the Experimental Advanced Superconducting Tokamak (EAST). Through neutron diagnostics, effective ion heating was observed in hydrogenminority heating (MH) scenarios. At present, investigation of deuterium–deuterium (DD) fusion neutrons is mostly based on time-resolved flux monitor and spectrometer measurements. When the ICRF was applied, the neutron intensity became one order higher. The H/H + D ratio was in the range of 5–10%, corresponding to the hydrogen MH dominated scenario, and a strong high energy tail was not displayed on the neutron spectrum that was measured by a liquid scintillator. Moreover, ion temperature in the plasma center (Ti) was inversely calculated by the use of neutron source strength (Sn) and the plasma density based on classical fusion reaction equations. This result indicates that Ti increases by approximately 30% in L-mode plasma, and by more than 50% in H-mode plasma during ICRF heating, which shows good agreement with x-ray crystal spectrometer (XCS) diagnostics. Finally, the DD neutron source strength scaling law, with regard to plasma current (IP) and ICRF coupling power (PRF) on the typical minority heating condition, was obtained by statistical analysis.

Performance comparison of the two ICRF antennas in EAST

The experimental advanced superconducting Tokamak has two suits of ion cyclotron radio frequency heating systems, in which the two antennas are of different structures. Their performance is assessed and compared by CST microwave studio. The radiating capacity of antennas and the arcing around them are estimated. The impurity release is analyzed by the radio frequency (RF) potential in the plasma sheath. The simulation results show that the radiating capacity for the folded antenna (I-port) is better than those for the double loops antenna (B-port). However, the folded antenna is worse than the double loops antenna in terms of breakdown. Moreover, the impurity production is relevant to spectrum shaping. The RF potential at (0, π, π, 0) phasing with the peak of spectrum k // = 8.5 m−1 is lower than the one with other phases. The impurity is reduced obviously when the folded antenna is powered with (0, π, π, 0) phasing.

Tungsten transport and sources control in JET ITER-like wall H-mode plasmas

A set of discharges performed with the JET ITER-like wall is investigated with respect to control capabilities on tungsten sources and transport. In attached divertor regimes, increasing fueling by gas puff results in higher divertor recycling ion flux, lower divertor tungsten source, higher ELM frequency and lower core plasma radiation, dominated by tungsten ions. Both pedestal flushing by ELMs and divertor screening (including redeposition) are possibly responsible. For specific scenarios, kicks in plasma vertical position can be employed to increase the ELM frequency, which results in slightly lower core radiation. The application of ion cyclotron radio frequency heating at the very center of the plasma is efficient to increase the core electron temperature gradient and flatten electron density profile, resulting in a significantly lower central tungsten peaking. Beryllium evaporation in the main chamber did not reduce the local divertor tungsten source whereas core radiation was reduced by approximately 50%.

Measurements of fast ions and their interactions with MHD activity using neutron emission spectroscopy

Ion cyclotron radio frequency (ICRF) heating can produce fast ion populations with energies reaching up to several megaelectronvolts. Here, we present unique measurements of fast ion distributions from an experiment with 3rd harmonic ICRF heating on deuterium beams using neutron emission spectroscopy (NES). From the experiment, very high DD neutron rates were observed, using only modest external heating powers. This was attributed to acceleration of deuterium beam ions to energies up to about 2–3 MeV, where the DD reactivity is on a par with that of the DT reaction. The high neutron rates allowed for observations of changes in the fast deuterium energy distribution on a time scale of 50 ms. Clear correlations were seen between fast deuterium ions in different energy ranges and magnetohydrodynamic activities, such as monster sawteeth and toroidal Alfven eigen modes (TAE). Specifically, NES data showed that the number of deuterons in the region between 1 and 1.5 MeV were decaying significantly during strong TAE activity, while ions with lower energies around 500 keV were not affected. This was attributed to resonances with the TAE modes.

An analysis of the influence of impurities on fast particle attenuation and on fast ion spectral shape in LHD

Neutral particle fluxes measured by neutral particle analyzers can provide information about the ion temperature as well as the non-Maxwellian anisotropic ion distribution tails from neutral beam injection and ion cyclotron radio frequency heating. In the case of multidirectional diagnostics employing high resolution atomic energy spectrometers, the neutral atomic flux source is not localized in contrast to pellet charge exchange or diagnostic neutral beam methods. The correct interpretation of such measurements from plasma in a complex toroidally asymmetric geometry, like that of LHD, requires careful numerical modeling of the neutral flux formation. Previously a measured neutral flux calculation scheme was developed and was used for the LHD geometry and a suitable analytic expression for ionization cross sections sigma(s)(z)(E,n(e),T(e),Z(eff)) of impurities was formulated by Janev et al. [Nucl. Fusion 29, 2125 (1989)]. In this paper, the attenuation of fast particles by impurities is incorporated into the neutral flux calculation scheme and the influence of impurities on the calculated neutral flux spectra is shown. Finally, the behavior of the calculated and experimental suprathermal particle distributions is compared for pure hydrogen and for argon impurity seeded plasmas.

Chord integrated neutral particle diagnostic data analysis for neutral beam injection and ion cyclotron radio frequency heated plasma in a complex Large Helical Device geometry

Energy and angle-resolved measurements of charge exchange neutral particle fluxes from the plasma provide information about Ti, as well as non-Maxwellian substantially anisotropic ion distribution tails due to neutral beam injection (NBI) and ion cyclotron radio frequency (ICRF) heating. The measured chord integral neutral flux calculation scheme for the Large Helical Device magnetic surface geometry is given. Calculation results are shown for measurable atomic energy spectra corresponding to heating-induced fast ion distributions from simplified Fokker-Planck models. The behavior of calculated and experimental suprathermal particle distributions from NBI and ICRF heated plasma is discussed in the context of the experimental data interpretation.

‘Burning plasma’ diagnostics for the physics of JET and ITER

JET's recent experimental programme proved that ‘burning plasma diagnostics’, i.e. neutron, alpha particle, He ash, and fuel mixture measurements, can provide very useful information about crucial physical aspects of great reactor relevance. First of all, several of these diagnostics can improve the diagnostic capability of the ion fluid significantly. During TTE spatially resolved neutron measurements at JET were essential in obtaining the isotopic composition and the transport of the hydrogen isotopes, allowing a direct comparison between the measured transport coefficients and the neoclassical theory. The neutron emission profiles can also give crucial indications for assessing the merits of various heating schemes and their current drive capability. Neutron spectroscopy in its turn provides a clear and direct measurement of the temperature and the velocity distribution of the fuel ions. For example, the dependence of the toroidal velocity from the ion cyclotron radiofrequency heating phasing was clearly seen during TTE. The requirements of accurate neutron measurements are also promoting considerable research in detector technology, in particular in the fields of compact spectrometers and solid state detectors. ‘Burning plasma’ diagnostics can also strongly contribute to the physics of energetic particles and their interaction with the main plasma. γ-Ray spectroscopy is now an established method to determine the spatial localization, to visualize the trajectories of the alpha particles and the fast deuterons and to obtain estimates of their slowing down. A completely new method to detect the energetic particles, exploiting the line intensity ratio of extreme ultraviolet radiation emitted by suitable extrinsic impurities, is also being pursued. This technique allows investigating the energy range below 600 keV, extremely interesting for the study of wave–particle interactions.

The JET-Enhanced Performance programme: More heating power and diagnostic capabilities in preparation for ITER

Since early 2000, the JET-Enhanced Performance (JET-EP) programme has been aiming at optimising JET for ITER-relevant plasma operations, from 2005 onwards. The overall heating capability of JET will be increased to 40 MW by upgrading the neutral beam system and building a new ITER-like ion cyclotron radio-frequency heating (ICRH) antenna with particularly stringent specifications. The new divertor will be able to absorb more than 300 MJ per shot and allow running high-triangularity ITER-like scenarios with a greater flexibility with respect to different plasma configurations. The control of extreme plasma shapes will be reinforced and a new disruption mitigation system using a very fast gas valve will be provided. The diagnostic capability will be enhanced by several new systems designed to address a number of crucial physical phenomena for ITER. To study tritium retention further, new technologically challenging erosion-redeposition diagnostics will be installed, particularly in the divertor region, both real time and integrating. New neutron detectors using the latest advances in scintillators and data recording techniques, detectors for fast α particles with high pitch angle and energy resolution, high-resolution Thomson scattering, with a 20 Hz repetition rate, which will provide temperature and density profiles with a spatial accuracy close to 2 cm are the leading items of this programme. New halo sensors will be installed to better understand disruption phenomena. New high-signal to noise bolometric tomography and an ambitious IR viewing system using a state of the art camera will also be added. This paper presents an overview, emphasising the main objectives and pointing out the various technological challenges and innovations.

ICRF coupling and edge density profile on Tore Supra

In addition to the propagation and absorption of the waves into the plasma, a crucial point in ion cyclotron radiofrequency heating (ICRH) is to know how the energy is coupled to the plasma. The coupling efficiency is experimentally estimated through measurements of the antenna coupling resistance and it is of primary importance to precisely establish its dependence on the plasma parameters. The coupling resistance is highly connected to the density profile, which determines the cut-off positions of the magnetosonic waves for a given radiated wave number spectrum. For a large size tokamak like Tore Supra these cut-off densities range between 1018 and 1019 m−3, corresponding to the plasma edge. In this study, the RF coupling is investigated with respect to the plasma edge density profile measured by X-mode reflectometry and a characteristic cut-off position of the magnetosonic wave. We show that the edge density is not only a function of the average density but also depends in a complex way, on various edge plasma regimes, the power deposition, or the kind of limiter materials or divertor perturbation used to control the edge. Over a large variety of plasma edge regimes, it is shown how the distance between a characteristic ICRH cut-off density layer and the antenna is the relevant parameter to address the problem of the RF coupling efficiency. Simulation of the coupling process with the three-dimensional antenna code Ion Cyclotron ANTenna compares well with RF measurements, provided that the experimental density profiles are used in the computations.

Overview of JET results

Scientific and technical activities on JET focus on the issues likely to affect the ITER design and operation. Our understanding of the ITER reference mode of operation, the ELMy H-mode, has progressed significantly. The extrapolation of ELM size to ITER has been re-evaluated. Neoclassical tearing modes have been shown to be meta-stable in JET, and their beta limits can be raised by destabilization (modification) of sawteeth by ion cyclotron radio frequency heating (ICRH). Alpha simulation experiments with ICRH accelerated injected 4He beam ions provide a new tool for fast particle and magnetohydrodynamic studies, with up to 80–90% of plasma heating by fast 4He ions. With or without impurity seeding, a quasi-steady-state high confinement (H98 = 1), high density (ne/nGW = 0.9–1) and high β (βN = 2) ELMy H-mode has been achieved by operating near the ITER triangularity (δ∼ 0.40–0.5) and safety factor (q95∼ 3), at Zeff∼ 1.5–2. In advanced tokamak (AT) scenarios, internal transport barriers (ITBs) are now characterized in real time with a new criterion, . Tailoring of the current profile with lower hybrid current drive provides reliable access to a variety of q profiles, lowering access power for barrier formation. Rational q surfaces appear to be associated with ITB formation. Alfvén cascades were observed in reversed shear plasmas, providing identification of q profile evolution. Plasmas with ‘current holes’ were observed and modelled. Transient high confinement AT regimes with H89 = 3.3, βN = 2.4 and ITER-relevant q < 5 were achieved with reversed magnetic shear. Quasi-stationary ITBs are developed with full non-inductive current drive, including ∼50% bootstrap current. A record duration of ITBs was achieved, up to 11 s, approaching the resistive time. For the first time, pressure and current profiles of AT regimes are controlled by a real-time feedback system, in separate experiments. Erosion and co-deposition studies with a quartz micro-balance show reduced co-deposition. Measured divertor thermal loads during disruptions in JET could modify ITER assumptions.

Role of sawtooth in avoiding impurity accumulation and maintaining good confinement in JET radiative mantle discharges

Impurity injection in the JET ELMy H-mode regime has produced high-confinement, quasi-steady-state plasmas with densities close to the Greenwald density. However, at large Ar densities, a sudden loss of confinement is observed. A possible correlation between loss of confinement and the observed MHD phenomena, both in the core and in the edge of the plasma, was considered. The degradation in confinement coincided with impurity profile peaking following the disappearance of sawtooth activity. In addition, impurity density profile analysis confirmed that central MHD modes prevented impurity peaking. Experiments were designed to understand the role of sawtooth crashes in re-distributing impurities. Ion-cyclotron radio frequency heating was used to control the central q-profile and maintain sawtooth activity. This resulted in quasi-steady-state, high-performance plasmas with high Ar densities. At and high Ar injection rates, quasi-steady-states, which previously only lasted <1τE, were now maintained for the duration of the heating (Δ t ∼ 9τ E). The increased central heating may have an additional beneficial effect in opposing impurity accumulation by changing the core power balance and modifying the impurity transport as predicted by neo-classical theory.

Predictive simulations of radio frequency heated plasmas of Tore Supra using the Multi-Mode model

Multichannel integrated predictive simulations using the Multi-Mode transport model are carried out for radio frequency heated Tore Supra tokamak discharges in which helium is the primary ion component. Lower hybrid heated discharges in which the total current is driven noninductively [X. Litaudon et al., Plasma Phys. Controlled Fusion 43, 677 (2001)] and a discharge with ion cyclotron radio frequency heating of the hydrogen minority ions [G. T. Hoang et al., Nucl. Fusion 38, 117 (1998)] are simulated. The simulations of these discharges represent the first test of the Multi-Mode model in helium plasmas with dominant electron heating. Also for the first time, the particle transport in Tore Supra discharges is computed and the density profiles are predicted self-consistently with other transport channels. It is found in these simulations that the anomalous transport driven by trapped electron mode turbulence is dominant compared to the transport driven by the ion temperature gradient turbulence. The feature of the Multi-Mode model to calculate the impurity transport self-consistently with other transport channels is used in this study to predict the influence of carbon impurity influx on the discharge evolution.

Progress in ergodic divertor operation on Tore Supra

Upgrade of the Tore Supra ergodic divertor (ED) has led to significant progress in ED physics. Pulse durations of 30 s with LHCD have been achieved demonstrating the heat exhaust capability of both the actively cooled technology at hand and of this specific divertor concept. The disruptive limit governed by the stochastization of the outer magnetic surfaces is found to occur for a value of the Chirikov parameter reaching two on the magnetic surface q = 2+(3/12). This experimentally observed robustness allows one to operate at very low safety factor on the separatrix (q∼2). Numerical analysis of ballooning turbulence in a stochastic layer indicates that the decay of the density fluctuations is associated with an increase of the fluctuating electric drift velocity. This results in an enhanced cross-field transport in the vicinity of the target plates. This lowering of confinement appears to be compensated by an intrinsic transport barrier on the electron temperature. The three-dimensional response of the temperature field is computed with a fluid code. The code can recover the intrinsic transport barrier at the separatrix, reported experimentally, together with small amplitude temperature modulations in the divertor volume. Experimental evidence for the three density regimes (linear, high recycling and detachment) is reported. The low critical density values for transitions between these regimes indicate that similar parallel physics governs the axisymmetric and ED, despite the open configuration of the latter. Measurement and understanding of these density regimes provide a means for feedback control of plasma density and an improvement in ion cyclotron radiofrequency heating coupling scenarios. Experimental data also indicated that particle control with the vented target plates is effective. Increase of both impurity control and radiation efficiency are reviewed. Global power balance has been analysed in order to account for non-axisymmetric radiation. These results, taken together, confirm the large radiation capability of the ED.