ICRF Heating Research Articles

The Role of Combined ICRF and NBI Heating in JET Hybrid Plasmas in Quest for High D-T Fusion Yield

Combined ICRF and NBI heating played a key role in achieving the world-record fusion yield in the first deuterium-tritium campaign at the JET tokamak in 1997. The current plans for JET include new experiments with deuterium-tritium (D-T) plasmas with more ITER-like conditions given the recently installed ITER-like wall (ILW). In the 2015-2016 campaigns, significant efforts have been devoted to the development of high-performance plasma scenarios compatible with ILW in preparation of the forthcoming D-T campaign. Good progress was made in both the inductive (baseline) and the hybrid scenario: a new record JET ILW fusion yield with a significantly extended duration of the high-performance phase was achieved. This paper reports on the progress with the hybrid scenario which is a candidate for ITER longpulse operation (∼1000 s) thanks to its improved normalized confinement, reduced plasma current and higher plasma beta with respect to the ITER reference baseline scenario. The combined NBI+ICRF power in the hybrid scenario was increased to 33 MW and the record fusion yield, averaged over 100 ms, to 2.9x1016 neutrons/s from the 2014 ILW fusion record of 2.3x1016 neutrons/s. Impurity control with ICRF waves was one of the key means for extending the duration of the high-performance phase. The main results are reviewed covering both key core and edge plasma issues.

Modelling of combined ICRF and NBI heating in JET hybrid plasmas

During the 2015-2016 JET campaigns many efforts have been devoted to the exploration of high performance plasma scenarios envisaged for ITER operation. In this paper we model the combined ICRF+NBI heating in selected key hybrid discharges using PION. The antenna frequency was tuned to match the cyclotron frequency of minority hydrogen (H) at the center of the tokamak coinciding with the second harmonic cyclotron resonance of deuterium. The modelling takes into account the synergy between ICRF and NBI heating through the second harmonic cyclotron resonance of deuterium beam ions which allows us to assess its impact on the neutron rate RNT . We evaluate the influence of H concentration which was varied in different discharges in order to test their role in the heating performance. According to our modelling, the ICRF enhancement of RNT increases by decreasing the H concentration which increases the ICRF power absorbed by deuterons. We find that in the recent hybrid discharges this ICRF enhancement was in the range of 10-25%. Finally, we extrapolate the results to D-T and find that the best performing hybrid discharges correspond to an equivalent fusion power of ∼7.0 MW in D-T.

ICRF-Induced Changes in Floating Potential and Ion Saturation Current in the EAST Divertor

Injection of waves in the ion cyclotron range of frequencies (ICRF) into a tokamak can potentially raise the plasma potential via RF rectification. Probes are affected both by changes in plasma potential and also by RF-averaging of the probe characteristic, with the latter tending to drop the floating potential. We present the effect of ICRF heating on divertor Langmuir probes in the EAST experiment. Over a scan of the outer gap, probes connected to the antennas have increases in floating potential with ICRF, but probes in between the outer-vessel strike point and flux surface tangent to the antenna have decreased floating potential. This behaviour is investigated using field-line mapping. Preliminary results show that mdiplane gas puffing can suppress the strong influence of ICRF on the probes’ floating potential.

Electromagnetic simulations of JET ICRF ITER-like antenna with TOPICA and SSWICH asymptotic codes

Multi-megawatt Ion Cyclotron Range of Frequencies (ICRF) heating is routinely used in the JET tokamak. To increase the ICRF heating power available from the A2 antennas, the ICRF ITER-Like Antenna (ILA) was reinstalled for the 2015 JET ITER-like wall experimental campaign. The application of high levels of ICRF power typically results in increased plasma wall interaction which leads to the observation of enhanced influx of metallic impurities in the plasma edge. It is assumed that the impurity production is mainly driven by the parallel component of the Radio-Frequency (RF) antenna electric near-field, E// (parallel to the confinement magnetic field of the tokamak), that is rectified in a thin boundary layer (RF sheath). Torino Polytechnic Ion Cyclotron Antenna (TOPICA) code was used to compute E// field maps in front of the ILA and between its poloidal limiters in the presence of plasma using measured density profiles and various antenna feedings. E// field maps calculated between the poloidal limiters were used to estimate the poloidal distribution of RF-sheath Direct Current (DC) potential in this private region of the ILA and make relative comparison of various antenna electrical settings. For this purpose we used the asymptotic version of the Self-consistent Sheaths and Waves for Ion Cyclotron Heating Slow-Wave (SSWICH-SW) code. These estimations can help to study the formation of RF sheaths around the antenna and even at distant locations (∼3m away).

ICRF mode conversion in three-ion species heating experiment and in flow drive experiment on the Alcator C-Mod tokamak

In recent three-ion species (majority D and H plus a trace level of 3He) ICRF heating experiments on Alcator C-Mod, double mode conversion on both sides of the 3He cyclotron resonance has been observed using the phase contrast imaging (PCI) system. The MC locations are used to estimate the species concentrations in the plasma. Simulation using TORIC shows that with the 3He level <1%, most RF power is absorbed by the 3He ions and the process can generate energetic 3He ions. In mode conversion (MC) flow drive experiment in D(3He) plasma at 8 T, MC waves were also monitored by PCI. The MC ion cyclotron wave (ICW) amplitude and wavenumber kR have been found to correlate with the flow drive force. The MC efficiency, wave-number k of the MC ICW and their dependence on plasma parameters like Te0 have been studied. Based on the experimental observation and numerical study of the dispersion solutions, a hypothesis of the flow drive mechanism has been proposed.

IShTAR ICRF antenna field characterization in vacuum and plasma by using probe diagnostic

RF sheath physics is one of the key topics relevant for improvements of ICRF heating systems, which are present on nearly all modern magnetic fusion machines. This paper introduces developement and validation of a new approach to understanding general RF sheath physics. The presumed reason of enhanced plasma-antenna interactions, parallel electric field, is not measured directly, but proposed to be obtained from simulations in COMSOL Multiphysics® Modeling Software. Measurements of RF magnetic field components with B-dot probes are done on a linear device IShTAR (Ion cyclotron Sheath Test ARrangement) and then compared to simulations. Good resulting accordance is suggested to be the criterion for trustworthiness of parallel electric field estimation as a component of electromagnetic field in modeling. A comparison between simulation and experiment for one magnetic field component in vacuum has demonstrated a close match. An additional complication to this ICRF antenna field characterization study is imposed by the helicon antenna which is used as a plasma ignition tool in the test arrangement. The plasma case, in contrast to the vacuum case, must be approached carefully, since the overlapping of ICRF antenna and helicon antenna fields occurs. Distinguishing of the two fields is done by an analysis of correlation between measurements with both antennas together and with each one separately.

Synergetic heating of D-NBI ions in the vicinity of the mode conversion layer in H-D plasmas in JET with the ITER like wall.

This paper discusses the extension of the ‘three-ion’ species ICRF technique for heating mixture plasmas using fast injected NBI ions as resonant ‘third’ species. In this scenario the ICRF power is absorbed by the fast beam ions in the vicinity of the mode conversion layer where the left-hand polarized RF electric field E + is strongly enhanced. The ions in the beam velocity distribution that have a Doppler-shifted resonance close to the mode conversion layer efficiently absorb RF power and undergo acceleration. We show first experimental observations of ICRF heating of D-NBI ions in H-D plasmas in JET with the ITER-like wall. In agreement with theoretical predictions and numerical modelling, acceleration of the D-NBI ions in this D-(DNBI )-H scenario is confirmed by several fast-ion measurements. An extension of the heating scheme discussed here is acceleration of T-NBI and D-NBI ions in D-T plasmas, offering the potential to further boost the Q -value in future D-T campaigns in JET.

Modeling of the Traveling Wave Antenna in view of the ICRF heating of DEMO

An upgraded version of the fast semi-analytical code ANTITER is used to model the refueled TWA sections of any arbitrary number of straps facing a low coupling plasma profile. The code computes the Y, Z and S matrices of the antenna array from which the performances of the TWA sections and of their feeding system are deduced as a function of the geometry of the TWA. The model incorporates the feeding of the section by a resonant ring circuit recirculating the output power of each section. Approximate scaling laws guiding the design are derived for the antenna main parameters. A first optimization of the design for the DEMO antenna section is made and the resulting power capability of a DEMO system evaluated.

Power Compensation for ICRF Heating in EAST**supported by the National Magnetic Confinement Fusion Science Program of China (No. 2015GB101001) and National Natural Science Foundation of China (Nos. 11575237, 11375235, 11375236)

The source system covering a working frequency range of 24 MHz to 70 MHz with a total maximum output power of 12 MW has already been fabricated for Ion Cyclotron Range of Frequency (ICRF) heating in EAST from 2012. There are two continuous wave (CW) antennas consisting of four launching elements each fed by a separate 1.5 MW transmitter. Due to the strong mutual coupling among the launching elements, the injection power for launching elements should be imbalance to keep the k‖ (parallel wave number) spectrum of the launcher symmetric for ICRF heating. Cross power induced by the mutual coupling will also induce many significant issues, such as an uncontrollable phase of currents in launching elements, high voltage standing wave ratio (VSWR), and impedance mismatching. It is necessary to develop a power compensation system for antennas to keep the power balance between the feed points. The power balance system consists of two significant parts: a decoupler and phase control. The decoupler helps to achieve ports isolation to make the differential phase controllable and compensate partly cross power. After that, the differential phase of 0 or π will keep the power balance of two feed points completely. The first power compensation system consisting of four decouplers was assembled and tested for the port B antenna at the working frequency of 35 MHz. With the application of the power compensation system, the power balance, phase feedback control, and voltage standing wave ratio (VSWR) had obviously been improved in the 2015 EAST campaign.

Toroidal rotation induced by asymmetric cyclotron resonance absorption in minority ICRF-heated tokamak plasmas

A new mechanism of ion cyclotron range of frequency (ICRF)-induced rotation is proposed to explain the toroidal rotation with minority ICRF heating without net momentum injection. For ICRF waves launched with the symmetric spectrum, a nonlinear toroidal force can be generated through the asymmetric absorption of the toroidal wave momentum, which is due to the finite toroidal rotation of minority ions. This ICRF-induced toroidal force can drive a significant toroidal rotation of bulk ions.

Observation of Central Toroidal Rotation Induced by ICRF on EAST**supported by the National Magnetic Confinement Fusion Science Program of China (Nos. 2013GB112004 and 2015GB103002), National Natural Science Foundation of China (Nos. 11175208, 11305212, 11375235, 11405212 and 11261140328), the Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology (2014FXCX003) and Brain Korea 21 Program for Leading Universities

Core plasma rotation of both L-mode and H-mode discharges with ion cyclotron range of frequency (ICRF) minority heating (MH) scheme was measured with a tangential X-ray imaging crystal spectrometer on EAST (Experimental Advanced Superconducting Tokamak). Co-current central impurity toroidal rotation change was observed in ICRF-heated L- and H-mode plasmas. Rotation increment as high as 30 km/s was generated at ∼1.7 MW ICRF power. Scaling results showed similar trend as the Rice scaling but with significant scattering, especially in L-mode plasmas. We varied the plasma current, toroidal field and magnetic configuration individually to study their effect on L-mode plasma rotation, while keeping the other major plasma parameters and heating unchanged during the scanning. It was found that larger plasma current could induce plasma rotation more efficiently. A scan of the toroidal magnetic field indicated that the largest rotation was obtained for on-axis ICRF heating. A comparison between lower-single-null (LSN) and double-null (DN) configurations showed that LSN discharges rendered a larger rotation change for the same power input and plasma parameters.

Study of toroidal flow generation by ion cyclotron range of frequency minority heating in the Alcator C-Mod plasma

The averaged toroidal flow of energetic minority ions during ICRF (ion cyclotron range of frequencies) heating is investigated in the Alcator C-Mod plasma by applying the GNET code, which can solve the drift kinetic equation with complicated orbits of accelerated energetic particles. It is found that a co-directional toroidal flow of the minority ions is generated in the region outside of the resonance location, and that the toroidal velocity reaches more than 40% of the central ion thermal velocity (Vtor ∼ 300 km/s with PICRF ∼ 2 MW). When we shift the resonance location to the outside of |r/a|∼0.5, the toroidal flow immediately inside of the resonance location is reduced to 0 or changes to the opposite direction, and the toroidal velocity shear is enhanced at r/a ∼ 0.5. A radial diffusion equation for toroidal flow is solved by assuming a torque profile for the minority ion mean flow, and good agreements with experimental radial toroidal flow profiles are obtained. This suggests that the ICRF driven minority ion flow is related to the experimentally observed toroidal rotation during ICRF heating in the Alcator C-Mod plasma.

Data and Information Management Control System for ICRF Heating on EAST

For HT-7 and EAST, the ion cyclotron range of frequency (ICRF) heating has been adopted as one of the most efficient auxiliary heating methods. At the ICRF experiment, scientists and engineers need to analyze experimental data and record information. But these data and information have been stored in a variety of formats, and these files have been stored in the different workstation computers. Now, these data and information are organized, centrally stored and released by the data and information management control system (DIMCS). The DIMCS provides a remote access platform, which permits the scientists and engineers browse all the experimental data and information, and allows record information. This improves the work efficiency. Meanwhile, DIMCS provides data services for the ICRF heating system upgrades.

Simulation Study of Toroidal Flow Generation of Minority Ions by Local ICRF Heating

The toroidal flow generation of minority ions by the local ion cyclotron range of frequencies (ICRF) heating is investigated in a tokamak plasma by applying the GNET code, which can solve the drift kinetic equation in the 5-D phase space. An asymmetry of velocity distribution function in the parallel direction is found and two types of toroidal averaged flow of minority ions are observed. One is the sheared flow near the RF power absorption region depending on the sign of k||, and the other is the toroidal flow, which is larger than the previous one, independent of the sign of k||. It is found that the k||-sign-independent toroidal flow is generated by the net toroidal motion of energetic tail ions and that the k||-sign-dependent flow is related to the mechanism proposed by Ohkawa [Phys. Plasmas 12, 094506 (2005)].

The RF conditioning of vacuum feedthrough for high power ICRF heating in KSTAR

The high-power coaxial vacuum feedthrough for the ICRF antenna of KSTAR tokamak was designed, fabricated and tested. It utilizes two cylindrical vacuum barriers formed of an alumina ceramic. Two cylinders are coaxially disposed between tapered coaxial conductors to form a vacuum sealed connection between a pressurized coaxial transmission line and an ICRF antenna located within a vacuum chamber of KSTAR. A vacuum feedthrough is one of the most critical parts of an ICRF antenna. It must transmit RF power while keeping the antenna in high vacuum, and support antenna elements against electromagnetic forces and thermal stress. Frequently, high-power pulsed ICRF experiments have been limited due to breakdown at the vacuum feedthrough. In order to study a higher standoff capability, the vacuum feedthrough is tested and conditioned in the vacuum chamber with a high power RF test stand. A fast interlock circuit is also utilized to reduce breakdown energy. The amplitude of RF breakdown voltage of the conditioned vacuum feedthrough is measured to be ∼55kV in the normal vacuum pressure of 1–5×10−5mbar. The breakdown voltage is not changed severely up to 5×10−3mbar. This paper will show the results of high power RF test and conditioning aiming for higher reliability at high RF power.

Wave excitation by nonlinear coupling among shear Alfvén waves in a mirror-confined plasma

A shear Alfvén wave at slightly below the ion-cyclotron frequency overcomes the ion-cyclotron damping and grows because of the strong anisotropy of the ion temperature in the magnetic mirror configuration, and is called the Alfvén ion-cyclotron (AIC) wave. Density fluctuations caused by the AIC waves and the ion-cyclotron range of frequencies (ICRF) waves used for ion heating have been detected using a reflectometer in a wide radial region of the GAMMA 10 tandem mirror plasma. Various wave-wave couplings are clearly observed in the density fluctuations in the interior of the plasma, but these couplings are not so clear in the magnetic fluctuations at the plasma edge when measured using a pick-up coil. A radial dependence of the nonlinearity is found, particularly in waves with the difference frequencies of the AIC waves; bispectral analysis shows that such wave-wave coupling is significant near the core, but is not so evident at the periphery. In contrast, nonlinear coupling with the low-frequency background turbulence is quite distinct at the periphery. Nonlinear coupling associated with the AIC waves may play a significant role in the beta- and anisotropy-limits of a mirror-confined plasma through decay of the ICRF heating power and degradation of the plasma confinement by nonlinearly generated waves.

High Power Heating and Steady State Operation in the Large Helical Device

Recent advances in the high power and steady state heating system and experiment results of the Large Helical Device (LHD) are reviewed in this paper. Plasma performance is extended largely through high power NBI, ECH and steady state ICRF heating devices, and improved operation techniques. The NBI of a 28 MW has extended the plasma parameter regime such as ion ITB plasmas, has a central ion temperature of more than 8 keV, and the extremely high-density plasmas ten times higher than the tokamak limit. An ECH system with seven gyrotrons (total power of 4.6MW) has been operated for pre-ionization and plasma heating. The high electron temperature regime was extended toward a higher density regime and a central electron temperature of 13.5 keV was achieved with a line-averaged electron density of ne = 1 x 1019 m-3. Steady state operation plasma with ne = 1.2 x 1019 m-3, ion and electron temperature of 2 keV, and plasma sustainment time of 48 min was achieved with ICH and ECH heating power of 1.2 MW for majority helium with minority hydrogen.

High-Density Plasma Production in the GAMMA 10 Central Cell with ICRF Heating on Both Anchor Cells

In the GAMMA 10 tandem mirror, divertor simulation experiments that utilize particle flux toward the west end region (called end-loss flux) have been implemented. Since a positive correlation has been reported between the end-loss flux and the central-cell density, an increase of the central-cell density is important for obtaining a higher end-loss flux on the divertor simulation experiments. By arranging the ion cyclotron range of frequency (ICRF) systems so as to excite strong ICRF waves in both anchor cells simultaneously, we have succeeded in producing high-density plasmas (line density of 1.2 × 1014 cm−2) in both anchor cells. As a result, a higher central-cell density of 4.4 × 1012 cm−3 and a higher end-loss flux of more than 1023 m−2s−1 have been obtained. One of the possible mechanisms of the high density production is a formation of positive potentials on both anchor cells. Plasmas in the central cell are confined due to those potentials.

Extension of Operational Regimes with ICRF Heating on GAMMA 10/PDX

Recent ICRF heating experiments performed in GAMMA 10/PDX are reported. Owing to antenna-phasing technique and four ICRF antennas installed in the mirror cells other than the central cell, the operational regime of GAMMA 10/PDX is significantly extended. Several ICRF heating schemes to increase the parameters (plasma density in the central cell, particle flux flowing to the machine end and its ion temperature parallel to the field lines) are demonstrated. These results present positive progress in the ongoing divertor simulation experiments on GAMMA 10/PDX.

Electron Cyclotron Emission Imaging Observations of m/n = 1/1 and Higher Harmonic Modes during Sawtooth Oscillation in ICRF Heating Plasma on EAST**Supported by the National Magnetic Confinement Fusion Energy Program of China under Grant Nos 2013GB106002 and 2014GB109002, and the National Natural Science Foundation of China under Grant Nos 10990210 and 11275200.

The m/n = 1/1 and its higher harmonic modes are observed in sawtooth oscillations by using the novel high-resolution 2D ECE imaging system on the experimental advanced superconducting Tokamak (EAST). Higher harmonic modes are appearing for a short time during the crash phase of sawtooth oscillation in lower βp plasma, which is not the preferable position in the poloidal cross section. These modes generate sharp pressure points on the inversion radius during the crash phase of sawtooth oscillation. Furthermore, reconnection events proceed in two distinctive phases. In the first phase, a small amount of heat is expelled through the weak reconnection while in the second phase the remaining large quantity of heat and particles emerged rapidly from the hot core to the peripheral region of the inversion radius. In addition, these harmonic modes are only found before and after the ICRF pulse, while in the ICRF pulse only the (1,1) mode exists in the sawtooth oscillation.