Ion Cyclotron Radio Frequency Research Articles

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.

Simulation of high-power electromagnetic wave heating in the ITER burning plasma

The next step toward fusion as a practical energy source is the design and construction of ITER [R. Aymar et al., Nucl. Fusion 41, 1301 (2001)], a device capable of producing and controlling the high-performance plasma required for self-sustaining fusion reactions, i.e., “burning plasma.” ITER relies in part on ion-cyclotron radio frequency power to heat the deuterium and tritium fuel to fusion temperatures. In order to heat effectively, the radio frequency wave fields must couple efficiently to the dense core plasma. Calculations in this paper support the argument that this will be the case. Three-dimensional full-wave simulations show that fast magnetosonic waves in ITER propagate radially inward with strong central focusing and little toroidal spreading. Energy deposition, current drive, and plasma flow are all highly localized near the plasma center. Very high resolution, two-dimensional calculations reveal the presence of mode conversion layers, where fast waves can be converted to slow ion cyclotron waves. When minority ions such as deuterium or helium-3 are used to damp the launched waves, these ions can be accelerated to high energies, forming suprathermal tails that significantly affect the wave propagation and absorption. By neglecting the toroidal localization of the waves and the finite radial excursion of the energetic particle orbits, the quasilinear evolution of these suprathermal ion tails can be simulated self-consistently in one spatial dimension and two velocity dimensions.

Boronization during the first plasma operation on EAST

Both ion cyclotron rf and glow discharge boronization have been successfully used for wall conditioning on EAST tokamak device. The whole process is monitored continuously by residual gas analyzer and film thickness monitor. These diagnostics provide detailed information about the boronization. High hydrogen inventory level observed after boronization maybe due to the boronization material used (C2B10H12). Ion cyclontron rf conditioning is proved to be an efficient wall conditioning method for superconducting device because it could be carried out under toroidal magnetic field. In this paper, the procedure of boronization is described, and subsequently sample analysis and the effect on plasma operation are introduced. Conclusion is given at the end.

Energetic Particle Physics Studies on Alcator C-Mod

Energetic particle physics is studied in Alcator C-Mod in reactor relevant regimes with high density and equilibrated electron and ion temperatures. Stable Alfvén eigenmodes are excited with low-power active magnetohydrodynamic antennas in the absence of a significant energetic particle tail to directly measure the damping rate of the modes. Stable toroidal Alfvén eigenmode (TAE) damping rates between 0.5% < γ/Ω < 4.5% have been observed in diverted and limited plasmas. Alfvén eigenmodes are destabilized with high-power hydrogen minority ion cyclotron radio frequency (ICRF) heating (PICRF < 6 MW) in lower-density plasmas in the current rise and in relatively high-density ([bar]ne < 2.5 × 1020 m-3) H-mode plasmas, which creates an energetic hydrogen ion tail with calculated energies up to 400 keV. Low toroidal mode number (n < 4) unstable modes are observed in the current rise with magnetic pickup coils at the wall and phase contrast imaging density fluctuation measurements in the core. Observations of energetic particle modes or TAEs that decrease in frequency and mode number with time up to a large sawtooth collapse indicate that fast particles play a role in stabilizing sawteeth. Alfvén eigenmodes can also be used as diagnostics to precisely constrain the q profile and provide a qualitative measure of the fast particle distribution time evolution.

Electromechanical analysis of the ITER ion cyclotron antenna structure and components

3D finite-element models of the ITER ion cyclotron RF antenna assembly have been implemented in order to perform electromagnetic and stress analyses and to calculate the applied electromagnetic loads, torques and forces on the antenna structure during plasma disruption events. The electromagnetic analysis has been carried out as follows: first the antenna outer structure is analyzed (support structure, neutron shield and Faraday shield), the magnetic field variation penetrating the outer structure is then computed and applied as forcing field to the antenna inner structure (four port junction, coaxial lines and straps). The stress analysis is carried out, for both the inner and outer structure, by computing in any element the cross product of the eddy current distribution with the local static magnetic field and by deriving stresses and deformations. Results of the analyses have shown that mechanical stresses are below the allowable limits in all the components of the antenna except the Faraday shield which needs some design improvements because of the very large displacements and stresses.

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.

Mitigation and control of the particle pinch in the Electric Tokamak

The Electric Tokamak [R. J. Taylor, T. A. Carter, J.-L. Gauvreau et al., Nucl. Fusion 45, 1634 (2005)] operates at high plasma density (one and a half times the Greenwald limit) due to a strong particle pinch. However, particle accumulation causes several problems. The operation of the machine can suffer several violent disruptions that hinder the study of many plasma phenomena. Plasma motion and large density swings are undesirable because they alter continuous processes, leaving only transient regimes to study. Particle source and local temperature control can defeat the fundamental mechanisms of this ‘‘electric’’ pinch. If edge fueling feedback is not sufficient to induce quiescent behavior, the fast ion loss caused by second harmonic ion-cyclotron rf injection functions as a particle sink deep within the outer plasma cross section. By linking these strong effects to the fueling feedback, stable medium density (2×1018particles∕m3) plasmas can be sustained for several seconds. This new regime yields surprisingly long and calm discharges.

TOPICA: an accurate and efficient numerical tool for analysis and design of ICRF antennas

The demand for a predictive tool to help in designing ion-cyclotron radio frequency (ICRF) antenna systems for today's fusion experiments has driven the development of codes such as ICANT, RANT3D, and the early development of TOPICA (TOrino Polytechnic Ion Cyclotron Antenna) code. This paper describes the substantive evolution of TOPICA formulation and implementation that presently allow it to handle the actual geometry of ICRF antennas (with curved, solid straps, a general-shape housing, Faraday screen, etc) as well as an accurate plasma description, accounting for density and temperature profiles and finite Larmor radius effects. The antenna is assumed to be housed in a recess-like enclosure. Both goals have been attained by formally separating the problem into two parts: the vacuum region around the antenna and the plasma region inside the toroidal chamber. Field continuity and boundary conditions allow formulating of a set of two coupled integral equations for the unknown equivalent (current) sources; then the equations are reduced to a linear system by a method of moments solution scheme employing 2D finite elements defined over a 3D non-planar surface triangular-cell mesh. In the vacuum region calculations are done in the spatial (configuration) domain, whereas in the plasma region a spectral (wavenumber) representation of fields and currents is adopted, thus permitting a description of the plasma by a surface impedance matrix. Owing to this approach, any plasma model can be used in principle, and at present the FELICE code has been employed. The natural outcomes of TOPICA are the induced currents on the conductors (antenna, housing, etc) and the electric field in front of the plasma, whence the antenna circuit parameters (impedance/scattering matrices), the radiated power and the fields (at locations other than the chamber aperture) are then obtained. An accurate model of the feeding coaxial lines is also included. The theoretical model and its TOPICA implementation have been fully validated against measured data both in vacuo and in plasma-facing conditions for real-life structures.

Impact of real-time magnetic axis sweeping on steady state divertor operation in LHD

Steady state divertor operation with high performance plasmas (ne ∼ 0.7 × 1019 cm−3, Ti ∼ 2 keV) was demonstrated for half an hour in the Large Helical Device (LHD), the superconducting helical device (R = 3.6–3.9 m, a = 0.6 m, B = 3 T, l/m = 2/10). The high performance plasmas have been sustained with an averaged heating power of 680 kW and achieved an injected energy of 1.3 GJ. This required both advanced technological integration of heating systems and divertor heat flux control. In particular, optimization of divertor heat flux distribution along the divertor leg trace on divertor plates and real-time magnetic axis sweeping (R = 3.67–3.7 m) have allowed LHD to access a steady state regime with a margin of safety for the actively cooled divertor plates. The distribution of divertor heat load along the traces was investigated with calorimetric measurements and it was found that there was a localized heat load connected with the loss of high-energy ions produced by ion cyclotron radio frequency near-fields. Orbit analysis shows that the behaviour of high-energy ions is qualitatively in good agreement with the experimental result. Long-pulse discharges were terminated by radiation collapse due to penetration of metallic flakes into the plasma.

‘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.

Active and fast particle driven Alfvén eigenmodes in Alcator C-Mod

Alfvén eigenmodes (AEs) are studied to assess their stability in high density reactor relevant regimes where Ti≈Te and as a diagnostic tool. Stable AEs are excited with active magnetohydrodynamics antennas in the range of the expected AE frequency. Toroidal Alfvén eigenmode (TAE) damping rates between 0.5%&amp;lt;γ∕ω&amp;lt;4.5% have been observed in diverted and limited Ohmic plasmas. Unstable AEs are excited with a fast ion tail driven by H minority ion cyclotron radio frequency (ICRF) heating with electron densities in the range of n¯e=0.5–2×1020m−3. Energetic particle modes or TAEs have been observed to decrease in frequency and mode number with time up to a large sawtooth collapse, indicating the role fast particles play in stabilizing sawteeth. In the current rise phase, unstable modes with frequencies that increase rapidly with time are observed with magnetic pick-up coils at the wall and phase contrast imaging density fluctuation measurements in the core. Modeling of these modes constrains the calculated safety factor profile to be very flat or with slightly reversed shear. AEs are found to be more stable for an inboard than for central or outboard ICRF resonances in qualitative agreement with modeling.

Properties of field emission from high voltage RF antenna

A high voltage (up to 80 kV for 20 ms and 110 kV for 10 ms) resonant RF probe operating at 52 MHz is used to study the influence of surface effects and plasma on the operation of high voltage ion cyclotron RF antennas for nuclear fusion devices with magnetic confinement.From the measured DC charging characteristic of the blocking capacitor, the field emission currents are derived. The measurements are performed both with and without plasma at He pressure p⩽0.1 Pa, lower than the threshold pressure for ignition of a high voltage RF glow discharge in the system. Effects of conditioning and presence of plasma on the field emission properties (effective emission area ARF and amplification factor βRF) are studied.

Calculation of coupling to the electron Bernstein wave with a phased waveguide array

Conventional electron cyclotron heating using the O- and X-modes to carry energy from the plasma edge to the cyclotron resonance layer is not possible for high density, low magnetic field devices (RFPs and STs, for example), since these modes are evanescent in all but the very edge of the plasma. As an alternative, we consider coupling to the electron Bernstein wave (EBW) with a phased waveguide array, the mouth of which is inserted to the vicinity of the upper hybrid resonance at the edge of the plasma. The calculation of the waveguide reflection coefficient is similar to the lower hybrid coupling problem solved by Brambilla, but the character of the plasma surface admittance is quite different for the EBW from that for the lower hybrid wave. Two models for the surface admittance are compared. In the first, the lowest-order EBW is included in the calculation, while in the second, the cold plasma model (which does not have any mode corresponding to the EBW) with weak collisions is used. The surface admittances obtained in those two models for parameters relevant to coupling experiments performed in the Madison symmetric torus (MST) are compared, and found to agree closely, despite the very different physics in the models. A significant asymmetry with respect to the direction mutually perpendicular to the static magnetic field and the radial direction (the poloidal direction in tokamak geometry, the toroidal direction in the edge of the RFP) is found. This phenomenon is related to the well-known up/down asymmetry in fast wave launch in the ion cyclotron radio frequency and to the up/down asymmetry previously reported for the fast wave in the lower hybrid radio frequency. The effect is very strong in this situation due to the density gradient scale length being much shorter than the local wavelength (violation of WKB condition) in the coupling region.

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.

Improved Edge Confinement due to Ion Cyclotron Radio Frequencyboronization in the HT-7 Tokamak

The phenomena of improved edge confinement due to ion cyclotron radiofrequency boronization were observed with a fast reciprocatingLangmuir probe in the HT-7 tokamak. A strong shear layer of radialelectric field was produced in the plasma edge region, which resultedin the formation of an edge transport barrier. Consequently, theedge profiles of electron density were steepened, and both the absoluteand relative fluctuation levels were suppressed by the shearingE×B flows. Concomitant reduction of the coherencebetween electron density and poloidal electric field fluctuations andthe change of their cross-phase resulted in turbulent particle fluxdropping by more than a half at the plasma edge. This demonstrates thede-correlation effect of turbulence and its contribution to edgetransport. The results presented here suggest a link between wallconditions and boundary plasma physics, especially an interactionbetween atomic processes and turbulence through the formation of radialelectric field shear at the plasma edge.

Development of coatings to improve RF voltage stand-off

For a reliable operation of Ion Cyclotron RF (ICRF) systems on fusion devices with magnetic confinement at high power, an improvement of the voltage stand-off of RF transmission lines and antennas is required. Coatings for RF conductors have been developed to increase the voltage stand-off. Stainless steel is often used as a basis material for the antenna components. The technique to reduce magnetic permeability of steel was elaborated. A technological process for deposition of TiN, TiC, W, Ta coatings with and without Cu sublayer was developed and tested. The coatings on the components of a RF probe were prepared for experiments with high RF voltage. The work function of the deposited coatings was measured. The application of the coating technology to the ICRF systems is discussed.

Neutralization loss of high energy particles in the plasma boundary of LHD

The neutralization loss of high-energy particles is studied by investigating the density dependence of the energetic neutral flux from large helical device plasmas of hydrogen and helium discharges. Clear difference between the density dependence of energetic particles originated from neutral beams and those produced by ion cyclotron radio frequency (ICRF) is observed. One of the causes of this difference is the spatial profiles of energetic particles characteristic to each heating scheme. The spatial distributions of H 0, He 0, He + are calculated as a function of the neutral source temperature entering across the Last Closed Flux Surface (LCFS) including six charge exchange processes, assuming a cylindrical geometry (ACHEN-Code). While the observed density dependence of energetic neutral flux escaping from ICRF-heated helium plasmas is well reproduced by the calculation with the incoming neutral temperature of about 10 eV, that from NBI-heated helium plasmas shows more modest dependence than the calculation.