Ion Cyclotron Range Of Frequencies Research Articles

Self-consistent calculation of the effects of RF injection in the HHFW heating regimes on the evolution of fast ions in toroidal plasmas

A critical question for the use of ion cyclotron range of frequency (ICRF) heating in the ITER device and beyond is interaction of fast waves with energetic ion populations from neutral beam injection (NBI), fusion reactions, and minority ions accelerated by the RF waves themselves. Several experiments have demonstrated that the interaction between fast waves and fast ions can indeed be strong enough to significantly modify the NB ion population. To model the RF/fast ion interaction and the resulting fast ion distribution, a recent extension of the full wave solver TORIC v.5 that includes non-Maxwellian effects has been combined with the Monte Carlo NUBEAM code through an RF “kick” operator. In this work, we present an initial verification of the NUBEAM RF “kick” operator for high harmonic fast wave (HHFW) heating regime in NSTX plasma.

Structural Analysis of High-Field-Side RF Antennas During a Disruption on the Advanced Divertor eXperiment (ADX)

The Advanced Divertor and RF tokamak eXperiment (ADX) is a compact, high-field device proposed by the MIT Plasma Science and Fusion Center and collaborators, which will address critical gaps in world fusion research on the pathway to fusion energy. In addition to developing and testing new divertor concepts at reactor level magnetic field strengths and power densities, ADX will test new antenna concepts for lower hybrid current drive (LHCD) and ion cyclotron range of frequency (ICRF) heating systems. In particular, ADX will be purpose-built to allow antennas to be positioned on the high magnetic field side of the torus, i.e., on the inner wall. With antennas placed at this location, plasma-wall interactions are greatly reduced and favorable RF wave physics projects to dramatic improvements in current drive efficiency and current profile control as well as very effective scenarios for RF heating and flow drive. Initial designs for the high-field-side LHCD and ICRF antennas have been completed and are analyzed to determine the loads induced during a full-current plasma disruption. While locating antennas at the inner wall is beneficial from an RF standpoint, it exposes them to a higher toroidal field which, when combined with the eddy currents caused by a disrupting plasma, will lead to higher loads. Using COMSOL Multiphysics, a model of the ADX vessel and coils is created to predict the magnetic fields, eddy currents, and loads acting on the antennas during a disruption. Structural models are then run to predict the stresses and to provide guidance for design improvement, such as determining where structural reinforcements may be necessary.

Effect on the Tritium Breeding Ratio due to a distributed ICRF antenna in a DEMO reactor

This thesis reports results of MCNP-5 calculations, with the nuclear data library FENDL-2.1, to assess the effect on the Tritium Breeding Ratio (TBR) due to a distributed Ion Cyclotron Range of Frequencies (ICRF) antenna integrated in the blanket of a DEMO fusion power reactor. A preliminary design of the antenna with a reference configuration of the DEMO reactor was used together with a parametric analysis for different parameters that strongly affect the TBR. These are the type of breeding blanket (Helium Cooled Pebble Bed, Helium Cooled Lithium Lead and Water Cooled Lithium Lead), the covering ratio of the straps of the antenna (the ratio between the surface of all the straps and the projected surface of the antenna slot: 0.49, 0.72 and 0.94), the antenna radial thickness (20cm and 40cm), the thickness of the straps (2cm, 4cm and a double layer of 0.2cm plus 2.5cm with the composition of the First Wall), and finally the poloidal position of the antenna (0°, which is the equatorial port, 40° and 90°, which is the upper port). For an antenna with a full toroidal circumference of 360°, located poloidaly at 40° with a poloidal extension of 1m and a total First Wall surface of 67m2, the reduction of the TBR is −0.35% for a HCPB blanket concept, −0.53% for a HCLL blanket concept and −0.51% for a WCLL blanket concept. In all cases covered by the parametric analysis, the loss of TBR remains below 0.61%. Such a distributed ICRF antenna has thus only a marginal effect on the TBR for a DEMO reactor.

Implementation of the new multichannel X-mode edge density profile reflectometer for the ICRF antenna on ASDEX Upgrade.

A new multichannel frequency modulated continuous-wave reflectometry diagnostic has been successfully installed and commissioned on ASDEX Upgrade to measure the plasma edge electron density profile evolution in front of the Ion Cyclotron Range of Frequencies (ICRF) antenna. The design of the new three-strap ICRF antenna integrates ten pairs (sending and receiving) of microwave reflectometry antennas. The multichannel reflectometer can use three of these to measure the edge electron density profiles up to 2 × 1019 m-3, at different poloidal locations, allowing the direct study of the local plasma layers in front of the ICRF antenna. ICRF power coupling, operational effects, and poloidal variations of the plasma density profile can be consistently studied for the first time. In this work the diagnostic hardware architecture is described and the obtained density profile measurements were used to track outer radial plasma position and plasma shape.

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.

Resonance in fast-wave amplitude in the periphery of cylindrical plasmas and application to edge losses of wave heating power in tokamaks

Heating magnetically confined plasmas using waves in the ion-cyclotron range of frequencies typically requires coupling these waves over a steep density gradient. This process has produced an unexpected and deleterious phenomenon on the National Spherical Torus eXperiment (NSTX): a prompt loss of wave power along magnetic field lines in front of the antenna to the divertor. Understanding this loss may be key to achieving effective heating and expanding the operational space of NSTX-Upgrade. Here, we propose that a new type of mode, which conducts a significant fraction of the total wave power in the low-density peripheral plasma, is driving these losses. We demonstrate the existence of such modes, which are distinct from surface modes and coaxial modes, in a cylindrical cold-plasma model when a half wavelength structure fits into the region outside the core plasma. The latter condition generalizes the previous hypothesis regarding the occurrence of the edge losses and may explain why full-wave simulations predict these losses in some cases but not others. If valid, this condition implies that outer gap control is a potential strategy for mitigating the losses in NSTX-Upgrade in addition to raising the magnetic field or influencing the edge density.

Theoretical analysis of the EAST 4-strap ion cyclotron range of frequency antenna with variational theory**Project supported by the National Magnetic Confinement Fusion Science Program, China (Grant No. 2015GB101001) and the National Natural Science Foundation of China (Grant Nos. 11375236 and 11375235).

A variational principle code which can calculate self-consistently currents on the conductors is used to assess the coupling characteristic of the EAST 4-strap ion cyclotron range of frequency (ICRF) antenna. Taking into account two layers of antenna conductors without lateral frame but with slab geometry, the antenna impedances as a function of frequency and the structure of RF field excited inside the plasma in various phasing cases are discussed in this paper.

ICRH induced particle losses in Wendelstein 7-X

Fast ions in W7-X will be produced either by neutral beam injection (NBI) or by ion-cyclotron resonant heating (ICRH). The latter presents the advantage of depositing power locally and does not suffer from core accessibility issues (Drevlak et al 2014 Nucl. Fusion 54 073002). This work assesses the possibility of using ICRH as a fast ion source in W7-X relevant conditions. The SCENIC package is used to resolve the full wave propagation and absorption in a three-dimensional plasma equilibrium. The source of the ion-cyclotron range of frequency (ICRF) wave is modelled in this work by an antenna formulation allowing its localisation in both the poloidal and toroidal directions. The actual antenna dimension and localization is therefore approximated with good agreement. The local wave deposition breaks the five-fold periodicity of W7-X. It appears that generation of fast ions is hindered by high collisionality and significant particle losses. The particle trapping mechanism induced by ICRH is found to enhance drift induced losses caused by the finite orbit width of trapped particles. The inclusion of a neoclassically resolved radial electric field is also investigated and shows a significant reduction of particle losses.

Development and test of decoupler for ICRF antenna in EAST

Ion Cyclotron Range of Frequency (ICRF) heating has been adopted in EAST tokamak as one of main auxiliary heating methods. The ICRF antenna usually consists of multiple launching elements because of limited port and space of tokamak device. Mutual coupling between straps has been observed in previous EAST ICRF current drive experiments. Due to adverse effects of such mutual coupling, many issues induced by cross power cannot be ignored, such as power imbalance in feed lines, high voltage standing wave ratio (VSWR), and etc.To restrain such mutual coupling, A device named decoupler was developed and tested in EAST ICRF system. According to the admittance matrix of load, three assembly positions (oscillation position, optimum position, and smooth position) along transmission line for the decoupler were taken into account and tested. The test results showed that ohmic dissipation in decoupler could not be neglected, which partly influenced the decoupling performance. The oscillation position and optimum position could restrain such adverse effects of ohmic dissipation and showed good decoupling performance. However, they cannot ensure the steady operation during H-mod due to the load variation. Finally, the smooth position has been adopted for EAST I port antenna because of steady decoupling performance comprised with engineering error and load resilience, which sincerely enhance the capability of system operation.

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.

Maximization of ICRF power by SOL density tailoring with local gas injection

Experiments have been performed under the coordination of the International Tokamak Physics Activity (ITPA) on several tokamaks, including ASDEX Upgrade (AUG), JET and DIII-D, to characterize the increased Ion cyclotron range of frequency (ICRF) antenna loading achieved by optimizing the position of gas injection relative to the RF antennas. On DIII-D, AUG and JET (with the ITER-Like Wall) a 50% increase in the antenna loading was observed when injecting deuterium in ELMy H-mode plasmas using mid-plane inlets close to the powered antennas instead of divertor injection and, with smaller improvement when using gas inlets located at the top of the machine. The gas injection rate required for such improvements (~0.7 × 1022 el s−1 in AUG, ~1.0 × 1022 el s−1 in JET) is compatible with the use of this technique to optimize ICRF heating during the development of plasma scenarios and no degradation of confinement was observed when using the mid-plane or top inlets compared with divertor valves. An increase in the scrape-off layer (SOL) density was measured when switching gas injection from divertor to outer mid-plane or top. On JET and DIII-D, the measured SOL density increase when using main chamber puffing is consistent with the antenna coupling resistance increase provided that the distance between the measurement lines of sight and the injection location is taken into account. Optimized gas injection was also found to be beneficial for reducing tungsten (W) sputtering at the AUG antenna limiters, and also to reduce slightly the W and nickel (Ni) content in JET plasmas. Modeling the specific effects of divertor/top/mid-plane injection on the outer mid-plane density was carried out using both the EDGE2D-EIRENE and EMC3-EIRENE plasma boundary code packages; simulations indeed indicate that outer mid-plane gas injection maximizes the density in the mid-plane close to the injection point with qualitative agreement with the AUG SOL density measurements for EMC3-EIRENE. Field line tracing for ITER in the 15 MA QDT = 10 reference scenario indicates that the planned gas injection system could be used to tailor the density in front the antennas. Benchmarking of EMC3-EIRENE against AUG and JET data is planned as a first step towards the ITER SOL modelling required to quantify the effect of gas injection on the SOL density in front of the antennas.

3D simulations of gas puff effects on edge density and ICRF coupling in ASDEX Upgrade

In recent experiments, a local gas puff was found to be an effective way to tailor the scrape-off layer (SOL) density and improve the ion cyclotron range of frequency (ICRF) power coupling in tokamaks. In order to quantitatively reproduce these experiments, to understand the corresponding physics and to optimize the gas valve positions and rates, simulations were carried out with the 3D edge plasma transport code EMC3-EIRENE in ASDEX Upgrade. An inter-ELM phase of an H-mode discharge with a moderate gas puff rate (1.2 × 1022 electrons s−1) is used in our simulations. We simulated cases with gas puff in the lower divertor, the outer mid-plane and the top of the machine while keeping other conditions the same. Compared with the lower divertor gas puff, the outer mid-plane gas puff can increase the local density in front of the antennas most effectively, while a toroidally uniform but significantly smaller enhancement is found for the top gas puff. Good agreement between our simulations and experiments is obtained. With further simulations, the mechanisms of SOL density tailoring via local gas puffing and the strategies of gas puff optimization are discussed in the paper.

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)].

A new B-dot probe-based diagnostic for amplitude, polarization, and wavenumber measurements of ion cyclotron range-of frequency fields on ASDEX Upgrade

A new B-dot probe-based diagnostic has been installed on an ASDEX Upgrade tokamak to characterize ion cyclotron range-of frequency (ICRF) wave generation and interaction with magnetized plasma. The diagnostic consists of a field-aligned array of B-dot probes, oriented to measure fast and slow ICRF wave fields and their field-aligned wavenumber (k(//)) spectrum on the low field side of ASDEX Upgrade. A thorough description of the diagnostic and the supporting electronics is provided. In order to compare the measured dominant wavenumber of the local ICRF fields with the expected spectrum of the launched ICRF waves, in-air near-field measurements were performed on the newly installed 3-strap ICRF antenna to reconstruct the dominant launched toroidal wavenumbers (k(tor)). Measurements during a strap current phasing scan in tokamak discharges reveal an upshift in k(//) as strap phasing is moved away from the dipole configuration. This result is the opposite of the k(tor) trend expected from in-air near-field measurements; however, the near-field based reconstruction routine does not account for the effect of induced radiofrequency (RF) currents in the passive antenna structures. The measured exponential increase in the local ICRF wave field amplitude is in agreement with the upshifted k(//), as strap phasing moves away from the dipole configuration. An examination of discharges heated with two ICRF antennas simultaneously reveals the existence of beat waves at 1 kHz, as expected from the difference of the two antennas' operating frequencies. Beats are observed on both the fast and the slow wave probes suggesting that the two waves are coupled outside the active antennas. Although the new diagnostic shows consistent trends between the amplitude and the phase measurements in response to changes applied by the ICRF antennas, the disagreement with the in-air near-field measurements remains. An electromagnetic model is currently under development to address this issue.

Feedback control of plasma density and heating power for steady state operation in LHD

For steady state operation, the feedback control of plasma density and heating power system was developed in the Large Helical Device (LHD). In order to achieve a record of the long pulse discharge, stable plasma density and heating power are needed. This system contains the radio frequency (RF) heating power control, interlocks, gas fueling, automatic RF phase control, ion cyclotron range of frequency (ICRF) antenna position control, and graphical user interface (GUI). Using the density control system, the electron density was controlled to the target density and using the RF heating power control system, the RF power injection could be stable. As a result of using this system, we achieved the long pulse up to 48min in the electron density of more than 1×1019m−3. Further, the ICRF hardware experienced no critical accidents during the 17th LHD experiment campaign in 2013.

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.

Validation of full-wave simulations for mode conversion of waves in the ion cyclotron range of frequencies with phase contrast imaging in Alcator C-Mod

Mode conversion of fast waves in the ion cyclotron range of frequencies (ICRF) is known to result in current drive and flow drive under optimised conditions, which may be utilized to control plasma profiles and improve fusion plasma performance. To describe these processes accurately in a realistic toroidal geometry, numerical simulations are essential. Quantitative comparison of these simulations and the actual experimental measurements is important to validate their predictions and to evaluate their limitations. The phase contrast imaging (PCI) diagnostic has been used to directly detect the ICRF waves in the Alcator C-Mod tokamak. The measurements have been compared with full-wave simulations through a synthetic diagnostic technique. Recently, the frequency response of the PCI detector array on Alcator C-Mod was recalibrated, which greatly improved the comparison between the measurements and the simulations. In this study, mode converted waves for D-3He and D-H plasmas with various ion species compositions were re-analyzed with the new calibration. For the minority heating cases, self-consistent electric fields and a minority ion distribution function were simulated by iterating a full-wave code and a Fokker-Planck code. The simulated mode converted wave intensity was in quite reasonable agreement with the measurements close to the antenna, but discrepancies remain for comparison at larger distances.