Ion Cyclotron Resonance Heating Research Articles

Investigation of the Thomson scattering-ECE discrepancy in ICRF heated plasmas at Alcator C-Mod

This paper reports on new experiments at Alcator C-Mod that were performed in order to investigate the long-standing, unresolved discrepancy between Thomson scattering (TS) and electron cyclotron emission (ECE) measurements of electron temperature in high temperature tokamak plasmas. Ion cyclotron range of frequency (ICRF) heating is used to produce high temperature conditions where the type of TS-ECE discrepancy observed in the past at JET and TFTR should become observable. At Alcator C-Mod, plasmas with Te(0) up to 8 keV are obtained using ion cyclotron resonance heating (ICRH), ICRF mode conversion heating and a combination of the two heating methods in order to explore the hypothesis that the presence of ICRH-generated fast ions may be related to the discrepancy. In all high temperature cases, the TS and ECE measurements of electron temperature agree to within experimental uncertainties. We find no evidence for the type of discrepancy reported at JET and TFTR. These results show that the TS-ECE discrepancy does not depend on high temperatures alone and also that the presence of ICRH-generated fast ions is insufficient to cause the TS-ECE discrepancy.

Theoretical analysis of the ICRH antenna's impedance matching for ELMy plasmas on EAST

AbstractA well-optimized design of an ion cyclotron resonance heating (ICRH) antenna is very important for steady-state plasma heating with high radio frequency (RF) power of several tens of megawatts. However, a sharp decrease in the coupling RF power because of impedance mismatch of ICRH system is an issue that must be resolved for present-day fusion reactors and International Thermonuclear Experimental Reactor. This paper has theoretically analyzed the ICRH antenna's impedance matching for ELMy plasmas on experimental advanced superconducting tokamak (EAST) by the transmission line theory. The results indicate that judicious choice of the optimal feeder location is found useful for adjustable capacitors' tolerance to the variations of the antenna input impedance during edge-localized mode (ELM) discharge, which is expected to be good for the design of ICRH antenna system and for real-time feedback control during ELM discharge on EAST.

Poloidal variation of high-Z impurity density due to hydrogen minority ion cyclotron resonance heating on Alcator C-Mod

In the Alcator C-Mod tokamak, strong, steady-state variations of molybdenum density within a flux surface are routinely observed in plasmas using hydrogen minority ion cyclotron resonant heating. In/out asymmetries, up to a factor of 2, occur with either inboard or outboard accumulation depending on the major radius of the minority resonance layer. These poloidal variations can be attributed to the impurity's high charge and large mass in the neoclassical parallel force balance. The large mass enhances the centrifugal force, causing outboard accumulation while the high charge enhances ion-impurity friction and makes impurities sensitive to small poloidal variations in the plasma potential. Quantitative comparisons between existing parallel high-Z impurity transport theories and experimental results for r/a < 0.7 show good agreement when the resonance layer is on the high-field side of the tokamak but disagree substantially for low-field side heating. Ion-impurity friction is insufficient to explain the experimental results, and the accumulation of impurity density on the inboard side of flux surface is shown to be driven by a poloidal potential variation due to magnetic trapping of non-thermal, cyclotron heated minority ions. Parallel impurity transport theory is extended to account for cyclotron effects and shown to agree with experimentally measured impurity density asymmetries.

First neutron spectrometry measurements in the ASDEX Upgrade tokamak

A compact neutron spectrometer based on the liquid scintillator BC501A has been installed on the ASDEX Upgrade tokamak. The aim is to measure neutron energy distribution functions as footprints of fast ions distribution functions, generated mainly via Neutral Beam Injection (NBI) in present day tokamaks. A flexible and fast software has been developed to perform digital pulse shape separation and to evaluate pulse height spectra. First measurements of count rates and pulse height spectra show a good signal to noise ratio for integration times comparable to the NBI slowing down time and to the energy confinement time. Due to the perpendicular line of sight, D-d fusion with perpendicular NBI is detected more efficiently and the line broadening of the 2.45 MeV neutrons is higher. Ion Cyclotron Resonance Heating (ICRH) combined to NBI exhibits a synergy effect, with count rates higher than the sum of the counts due to NBI and ICRH separately. Although the collimator is designed to screen gammas as much as possible, some qualitative gamma analysis is also possible, providing information in case of runaway electrons during disruptions. The experimental campaign for the characterisation of the system (detector + acquisition system) is complete and the determination of the response function is in progress.

Deuterium beam acceleration with 3rd harmonic ion cyclotron resonance heating in Joint European Torus: Sawtooth stabilization and Alfvén eigenmodes

Experiments on accelerating NBI-produced deuterium (D) beam ions from their injection energy of ∼110 keV up to the MeV energy range with 3rd harmonic ion cyclotron resonance heating were performed on the Joint European Torus [P. H. Rebut and B. E. Keen, Fusion Technol. 11, 13 (1987)]. A renewed set of nuclear diagnostics was used for analysing fast D ions during sawtooth stabilization, monster sawtooth crashes, and during excitation of Alfvén eigenmodes (AEs) residing inside the q = 1 radius. The measurements and modeling of the fast ions with the nonlinear HAGIS code [S. D. Pinches et al., Comput. Phys. Commun. 111, 133 (1998)] show that monster sawtooth crashes are strongly facilitated by the AE-induced re-distribution of the fast D ions from inside the q = 1 radius to the plasma edge.

The Ion Cyclotron, Lower Hybrid and Alfven Wave Heating Methods

This lecture covers the practical features and experimental results of the three heating methods. The emphasis is on ion cyclotron heating. First, we briefly come back to the main non-collisional heating mechanisms and to the particular features of the quasilinear coefficient in the ion cyclotron range of frequencies (ICRF). The specific case of the ion-ion hybrid resonance is treated, as well as the polarisation issue and minority heating scheme. The various ICRF scenarios are reviewed. The experimental applications of ion cyclotron resonance heating (ICRH) systems are outlined. Then, the lower hybrid and Alfvén wave heating and current drive experimental results are covered more briefly. Where applicable, the prospects for ITER are commented.

Production and diagnosis of energetic particles in FAST

The Fusion Advanced Study Torus (FAST) has been proposed as a possible European satellite facility to study fast-ion physics in deuterium plasmas under conditions relevant to a burning plasma. Energetic minority ions (H or 3He) accelerated by ion cyclotron resonance heating (ICRH), with dimensionless parameters close to those of fusion-born alphas in ITER, can be produced in FAST via 30 MW power ICRH minority heating. This work provides a first assessment of the extent to which the 3He fast-ion population can be diagnosed in FAST with a set of dedicated diagnostics for confined fast particles. Neutron emission spectroscopy (NES), gamma-ray spectroscopy (GRS) and collective Thomson scattering (CTS) diagnostics have been reviewed with a description of the state-of-the-art hardware and a preliminary analysis of the required lines of sight. The results of the analysis, based on numerical simulations of the spatial and energetic particle distribution function of the ICRH-accelerated ions for the standard FAST H-mode scenario, suggest that NES and GRS measurements can provide information on the fast 3He population effective tail temperature, with time resolutions in the range 20–100 ms. The proposed CTS diagnostic can measure the fast-ion parallel and perpendicular temperature with a spatial resolution of 5–10 cm and a time resolution of 10 ms. The paper provides a scientific basis for the prediction of the production and diagnosis of energetic ions in FAST.

Improved Pellet Charge Exchange Measurements in Large Helical Device

The pellet charge exchange technique (PCX), which is a combination of the compact neutral particle analyzer and an impurity pellet, is a unique method to observe the radial energetic particle distribution. There are not only charge exchange reactions between the hydrogen in the pellet and a proton, but also between the partially ionized carbon in the pellet and the proton. The neutralization factor from energetic ion to neutral particle could be deduced from the electron temperature and the electron density of the pellet cloud. The radial profiles of energetic particle distribution were measured and compared in various ion cyclotron resonance heating (ICH) plasmas. The energetic particle flux significantly increased at the resonance layer created by the ICH. PCX provides more precise information about the resonance layer than conventional neutral particle diagnostics.

Survey of the TS-ECE Discrepancy and recent investigations in ICRF heated plasmas at Alcator C-Mod

This paper reports on a new investigation of the long-standing, unresolved discrepancy between Thomson Scattering (TS) and Electron Cyclotron Emission (ECE) measurements of electron temperature in high temperature tokamak plasmas. At the Alcator C-Mod tokamak, ion cyclotron range of frequency (ICRF) heating is used to produce high temperature conditions where the TS- ECE discrepancy, as observed in the past at JET and TFTR, should appear. Plasmas with Te(0) up to 8 keV are obtained using three different heating scenarios: Ion Cyclotron Resonance Heating (ICRH), ICRF mode conversion heating and a combination of the two heating methods. This is done in order to explore the hypothesis that ICRH-generated fast ions may be related to the discrepancy. In all high temperature cases at C-Mod, we find no evidence for the type of discrepancy reported at JET and TFTR. Here we present the C-Mod results along with a summary of past work on the TS-ECE discrepancy.

Integrated modelling of ICRH in a quasi-axisymmetric stellarator

We apply the code package SCENIC to a two field-period quasi-axisymmetric stellarator. Ion cyclotron resonance heating (ICRH) is applied both on the high- and low-field side to a 1% 3He minority in a deuterium plasma. It is shown that due to toroidal variations, the results are considerably different from similar tokamak studies. In particular, toroidal variations in power deposition and pressure are created and accentuated during radio frequency heating, such that modifications to the magnetic equilibrium depend on toroidal angle. We demonstrate that due to enhanced particle loss, low-field side heating is significantly less efficient than high-field side heating, and that toroidally trapped particles impose upper power limits for efficient radio frequency injection.

Sawtooth control in ITER using ion cyclotron resonance heating

Numerical modelling of the effects of ion cyclotron resonance heating (ICRH) on the stability of the internal kink mode suggests that ICRH should be considered as an essential sawtooth control tool in ITER. Sawtooth control using ICRH is achieved by directly affecting the energy of the internal kink mode rather than through modification of the magnetic shear by driving localized currents. Consequently, ICRH can be seen as complementary to the planned electron cyclotron current drive actuator, and indeed will improve the efficacy of current drive schemes. Simulations of the ICRH distribution using independent RF codes give confidence in numerical predictions that the stabilizing influence of the fusion-born alphas can be negated by appropriately tailored minority 3He ICRH heating in ITER. Finally, the effectiveness of all sawtooth actuators is shown to increase as the q = 1 surface moves towards the manetic axis, whilst the passive stabilization arising from the alpha and NBI particles decreases.

Optimizing ion-cyclotron resonance frequency heating for ITER: dedicated JET experiments

In the past years, one of the focal points of the JET experimental programme was on ion-cyclotron resonance heating (ICRH) studies in view of the design and exploitation of the ICRH system being developed for ITER. In this brief review, some of the main achievements obtained in JET in this field during the last 5 years will be summarized. The results reported here include important aspects of a more engineering nature, such as (i) the appropriate design of the RF feeding circuits for optimal load resilient operation and (ii) the test of a compact high-power density antenna array, as well as RF physics oriented studies aiming at refining the numerical models used for predicting the performance of the ICRH system in ITER. The latter include (i) experiments designed for improving the modelling of the antenna coupling resistance under various plasma conditions and (ii) the assessment of the heating performance of ICRH scenarios to be used in the non-active operation phase of ITER.

Ion heat transport studies in JET

Detailed experimental studies of ion heat transport have been carried out in JET exploiting the upgrade of active charge exchange spectroscopy and the availability of multi-frequency ion cyclotron resonance heating with 3He minority. The determination of ion temperature gradient (ITG) threshold and ion stiffness offers unique opportunities for validation of the well-established theory of ITG driven modes. Ion stiffness is observed to decrease strongly in the presence of toroidal rotation when the magnetic shear is sufficiently low. This effect is dominant with respect to the well-known ωE×B threshold up-shift and plays a major role in enhancing core confinement in hybrid regimes and ion internal transport barriers. The effects of Te/Ti and s/q on ion threshold are found rather weak in the domain explored. Quasi-linear fluid/gyro-fluid and linear/non-linear gyro-kinetic simulations have been carried out. Whilst threshold predictions show good match with experimental observations, some significant discrepancies are found on the stiffness behaviour.

Spectral Broadening of Characteristicγ-Ray Emission Peaks fromC12(He3,pγ)N14Reactions in Fusion Plasmas

The spectral broadening of characteristic $\ensuremath{\gamma}$-ray emission peaks from the reaction $^{12}\mathrm{C}(^{3}\mathrm{He},p\ensuremath{\gamma})^{14}\mathrm{N}$ was measured in $D(^{3}\mathrm{He})$ plasmas of the JET tokamak with ion cyclotron resonance heating tuned to the fundamental harmonic of $^{3}\mathrm{He}$. Intensities and detailed spectral shapes of $\ensuremath{\gamma}$-ray emission peaks were successfully reproduced using a physics model combining the kinetics of the reacting ions with a detailed description of the nuclear reaction differential cross sections for populating the $L1\mathrm{\text{\ensuremath{-}}}L8$ $^{14}\mathrm{N}$ excitation levels yielding the observed $\ensuremath{\gamma}$-ray emission. The results provide a paradigm, which leverages knowledge from areas of physics outside traditional plasma physics, for the development of nuclear radiation based methods for understanding and controlling fusion burning plasmas.

Simultaneous analysis of ion and electron heat transport by power modulation in JET

Heating power modulation experiments using ion cyclotron resonance heating (ICRH) in the 3He minority scheme have been performed in the JET tokamak to investigate heat transport properties. This RF scheme provides a dominant localized ion heating, but also some electron heating, and therefore both ion and electron heat channels were modulated. This allows us to carry out a simultaneous transport analysis of ion and electron heat transport channels, including transient transport phenomena. This also provides an experimental assessment of the ICRH heat sources of the 3He scheme. The modulation approach, so far widely used for electron transport studies, has been validated for ion heat transport in these experiments and yields results on stiffness and threshold of the ion temperature gradient (ITG)-driven ion heat transport. The results for the electron channel demonstrate the importance of the ITG-driven, off-diagonal, contribution to electron heat transport in plasmas with significant ion heating.

Neutron Flux Measurements in an ICRF Mode Conversion Regime Heating Plasmas on HT-7

Ion cyclotron resonance heating experiments using antenna in the high field side (HFS) have been carried out on HT-7 in different target plasmas. Unlike a standard-mode conversion heating scheme with dominant electron heating, anomalous ion heating and DD neutron fluxes higher than those estimated from thermal ions were observed in the present experiments with the ion-ion hybrid resonant layer near the center of plasma. The features of ion cyclotron range frequency (ICRF) antenna in HFS and experiments suggest that this is most probably due to the nonlinear 3/2 harmonic deuterium heating by the mode-converted ion Bernstein wave, which could produce a high energy tail on ion energy distribution.

3D electromagnetic optimization of the front face of the ITER ICRF antenna

In the framework of the ion cyclotron resonance heating (ICRH) antenna development for ITER, a design based on an external matching concept has been proposed [1]. We present in this work a series of electromagnetic simulations of this design performed with the commercial code CST Microwave Studio [2]. On the one hand, we explore how various geometrical modifications of some parts of the antenna (the straps and the four-port junction) can practically double the RF power coupled to the plasma. This optimization is supported by transmission line analysis. On the other hand, we treat the important question of the opportunity to tilt the straps in the toroidal direction to follow the plasma curvature as close as possible. We show that a configuration with two toroidal segments is sufficient and that further segmentation is not necessary. This work also underlines significant progress in the realism of ICRH antenna modelling and the importance of considering realistic load shaping in the models.

Direct electron heating experiment on the Aditya tokamak using fast waves in the ion cyclotron resonance frequency range

Second harmonic heating experiments using fast waves are carried out on the Aditya tokamak in the ion cyclotron resonance frequency (ICRF) range with the help of a 200 kW, 20–40 MHz RF heating system, which is developed indigenously. Significant direct electron heating is observed in a hydrogen plasma. The rise in electron temperature is prompt with the application of RF power and the increment in electron temperature increases linearly with RF power. A corresponding increase in plasma beta and hence an increase in stored diamagnetic energy are also observed in the presence of RF power. The low-Z impurity radiation and electron density do not increase significantly with RF power. The direct electron heating by fast wave in Aditya is also predicted by the ion cyclotron resonance heating code TORIC.

Diamagnetic loop measurement in Korea Superconducting Tokamak Advanced Research machine

Diamagnetic loop (DL), which consists of two poloidal loops inside the vacuum vessel, is used to measure the diamagnetic flux during a plasma discharge in the Korea Superconducting Tokamak Advanced Research (KSTAR) machine. The vacuum fluxes in the DL signal can be compensated up to 0.1 mWb by using the coefficients, which are obtained from experimental investigations, in the vacuum flux measurements during vacuum shots under same operational conditions of magnetic coils for plasma experiment in the KSTAR machine. The maximum error in the diamagnetic flux measurement due to the errors of the coefficients was estimated as ∼0.22 mWb. From the diamagnetic flux measurements for the ohmically heated circular plasmas in the KSTAR machine, the stored energy agrees well with the estimated kinetic energy within the discrepancy of 25%. When the electron cyclotron heating, the neutral beam injection, and the ion cyclotron resonance heating are added to the ohmically heated limiter plasmas, the additional heating effects can be clearly observed from the increase of the stored energy evaluated in the DL measurement.

New high power CW test facilities for ITER ICRH components testing

First CW test bed, devoted for ion cyclotron resonance heating (ICRH), is being built at CEA Cadarache. It has been designed for testing the ICRH antenna sub assemblies under ITER relevant conditions (vacuum, cooling and RF). This paper presents a technical overview of these facilities and discusses their future operations in the framework of the ITER ICRH European R&D program.