Ion Cyclotron Range Of Frequencies Heating Experiments Research Articles

Finite elements method-based ICRF wave heating simulation integrating with SOL plasma for EAST tokamak

Ion cyclotron range of frequency (ICRF) wave heating simulation is often carried out in the core plasma region. However, the inclusion of scrape-off layer (SOL) plasma in the simulation model may lead to a new physical phenomenon and needs to be studied. In this paper, we apply a finite elements method based on the approach of Vallejos et al (2019 Nucl. Fusion 59 076022), to simulate ICRF wave heating to account realistically for SOL plasma of the EAST. In the presence of the density pedestal near the last closed flux surface, a kind of cavity mode is observed for the case of low parallel wave number. Near the ion–ion hybrid resonance (IIR) layer in the SOL region, mode conversion from fast waves to slow waves takes place. ICRF wave coupling characteristics are roughly consistent with the prediction of the dispersion relation except for some small deviations, which may be caused by fast wave reflection in the high-field side. Approximately on-axis heating of H ion is observed and the power deposition zone broadens with parallel wave number increasing. Wave energy dissipation in SOL plasma is less than 7% and localized near the IIR region. Furthermore, the comparison between D(H) and D(He-3) minority heating scenarios is also carried out. The results and conclusions in this paper can provide a theoretical reference for ICRF heating experiments and may supply a new insight into the form of ICRF waves in the plasma edge.

Results of ICRF Heating Experiments from the EAST 2010 Campaign**supported partly by National Natural Science Foundation of China (Nos. 11105179, 11375235 and 11375236) and partly by the JSPS-NRF-NSFC A3 Foresight Program in the Field of Plasma Physics (No. 11261140328)

Radio frequency (RF) plasma heating in ion cyclotron range of frequencies (ICRF) was successfully performed on the Experimental Advanced Superconducting Tokamak (EAST). This is mainly because lithium wall conditioning was routinely used to reduce both impurity and hydrogen (H) recycling and to improve the ICRF power absorption. Mainly ICRF heating of the H minority regime at 27 MHz has been applied in deuterium plasmas. The ion cyclotron resonance heating (ICRH) is found to depend strongly on plasma preheating. The ICRH efficiency can be much improved in conjunction with the lower hybrid wave (LHW). Effective ion and electron heating was observed with the H minority heating mode. The increase of the stored energy reached 30 kJ in L-mode plasma by using the ICRF power of 1.0 MW alone when the H cyclotron resonance layer was at plasma center.

Lower hybrid current drive and ion cyclotron range of frequencies heating experiments in H-mode plasmas in Experimental Advanced Superconducting Tokomak

An ion cyclotron range of frequencies (ICRF) system with power up to 6.0 MW and a lower hybrid current drive (LHCD) system up to 4 MW have been applied for heating and current drive experiments in Experimental Advanced Superconducting Tokomak (EAST). Significant progress has been made with ICRF heating and LHCD for realizing the H-mode plasma operation in EAST. During 2010 and 2012 experimental campaigns, ICRF heating experiments were carried out at the fixed frequency of 27MHz, achieving effective ions and electrons heating with the H minority heating (H-MH) mode. The H-MH mode produced good plasma performance, and realized H-mode using ICRF power alone in 2012. In 2010, H-modes were generated and sustained by LHCD alone, where lithium coating and gas puffing near the mouth of the LH launcher were applied to improve the LHCD power coupling and penetration into the core plasmas of H-modes. In 2012, the combination of LHCD and ICRH power extended the H-mode duration up to over 30 s. H-modes with various types of edge localized modes (ELMs) have been achieved with HIPB98(y, 2) ranging from 0.7 to over unity. A brief overview of LHCD and ICRF Heating experiment and their application in achieving H-mode operation during these two campaigns will be presented.

ICRF Heating Experiments in Both East and West Anchor Cells on GAMMA 10

In the GAMMA10 tandem mirror, waves with ion cyclotron range of frequency (ICRF) have been used for plasma production and heating. Magneto Hydro Dynamic (MHD) stability is kept with the anchor cells which have minimum-B configuration. In a typical discharge, plasmas in the anchor cell are heated with ICRF waves excited by Type-III antennas installed in the central cell. In order to produce high performance plasma, anchor heating should be enhanced. Double-Arc Type (DAT) antennas are installed in both east and west anchor cells. In this study, direct heating experiments in both east and west anchor cells with DAT antenna are performed. DAT antenna is driven with the different frequency from Type-III antenna and driven with the same frequency. In the same frequency case, the phase difference between both antennas can be changed. In both the different and the same frequency cases, the increase of the line density is clearly observed in the central and anchor cells. Stronger effects are observed in the east anchor than in the west anchor.

Recent ICRF Heating Experiments for a Divertor Simulation Study on GAMMA 10

In GAMMA 10, a divertor simulation study has been started with open magnetic field configuration in the end region. High heat and particle fluxes are required along the magnetic field line to the end region. Plasmas with high ion-temperature of several keV and strong temperature anisotropy of more than 10 have been produced by using ion-cyclotron range of frequency (ICRF) heating in the central cell. Direct anchor heating experiments with new anchor antennas have been performed and the enhancement of the MHD stabilization has been observed. High energy ions whose energy is more than 50 keV have been observed in the end-loss ions. The axial transport of high-energy ions due to loss processes other than the classical Coulomb scattering has been discussed. Alfvén-ion-cyclotron (AIC) waves are spontaneously excited owing to such the strong temperature anisotropy and considerable energy transport along the magnetic field line due to the AIC waves is expected. In this review, recent ICRF heating experiments for the divertor simulation study are described.

High Power RF Transmitters for ICRF Applications on EAST

An Ion Cyclotron Range of Frequency (ICRF) system with a radio frequency (RF) power of 4 × 1.5 MW was developed for the Experimental Advanced Superconducting Tokamak (EAST). High RF power transmitters were designed as a part of the research and development (R&D) for an ICRF system with long pulse operation at megawatt levels in a frequency range of 25 MHz to 70 MHz. Studies presented in this paper cover the following parts of the high power transmitter: the three staged high power amplifier, which is composed of a 5 kW wideband solid state amplifier, a 100 kW tetrode drive stage amplifier and a 1.5 MW tetrode final stage amplifier, and the DC high voltage power supply (HVPS). Based on engineering design and static examinations, the RF transmitters were tested using a matched dummy load where an RF output power of 1.5 MW was achieved. The transmitters provide 6 MW RF power in primary phase and will reach a level up to 12 MW after a later upgrade. The transmitters performed successfully in stable operations in EAST and HT-7 devices. Up to 1.8 MW of RF power was injected into plasmas in EAST ICRF heating experiments during the 2010 autumn campaign and plasma performance was greatly improved.

Design and Test of the Fast Ferrite Tuner for ICRF Heating in EAST

Ion cyclotron range of frequency (ICRF) heating has been used in tokamaks as one of the most successful auxiliary heating tools and has been adopted in EAST. To ensure the steady operation of the ICRF heating system in EAST, the research and development of the fast ferrite tuner (FFT), which aimed to achieve real-time impedance matching of transmitter to antenna, has been carried out. The design and analysis of the FFT is an iterative process where multiple parameters have to be taken into account. The dimensions of the FFT should be chosen as a compromise between relative equivalent electrical length and high-power performance by using the finite element method and numerous computer simulations. The first prototype aimed at achieving a response time of milliseconds and operation with a peak power of 300 kW, which will inform us about the radio-frequency and the high-power performance of such a ferrite tuner. The bench test results have demonstrated that the FFT with a tuning speed of [approximately]200 ms is faster than the traditional methods, and it can be one of the candidates for the real-time impedance matching of the ICRF heating system in EAST. The high-power performance of the FFT should be tested in the EAST 2012 spring campaign. To be fit for the real-time impedance matching for ICRF heating experiments, development of a new prototype, which aims at a response time of 0.5 ms, an insertion loss of <1%, and operation with a peak power of 1.5 MW, is in progress.

Development of the High Radio Frequency Power Amplifiers for ICRF Heating in EAST

High radio frequency (rf) power amplifiers were designed as a part of research and development of an ion cyclotron range of frequency (ICRF) system that aimed at long-pulse operation at the megawatt level in a frequency range of 25 to 70 MHz. A study on the high-power amplifiers for ICRF heating in Experimental Advanced Superconducting Tokamak (EAST) is presented. To realize the design with a compact structure, a double coaxial cavity was employed as the output circuit of the final power amplifier (FPA) for tuning and matching, and the strip line was adopted for the input impedance matching circuit of the drive power amplifier (DPA). A double-stub tuner matching network with a variable-length U-link was used to obtain the impedance matching between the DPA and the FPA. To ensure the stable operation of the amplifiers, a grounded-grid configuration was chosen, and precautions were taken to suppress all parasitic oscillations of the anode output circuit. The rf power amplifiers performed successfully in stable operation at the megawatt level at each integer frequency from 25 to 70 MHz during the tests, and a rf power of 1.5 MW was achieved in a matching dummy load. The test results show a good agreement with the calculated values. The amplifiers operated reliably in long-pulse mode in EAST, and the total rf power of [approximately]1.8 MW was injected into plasmas in EAST ICRF heating experiments in the 2010 autumn campaign.

Current Status of ICRF Heating Experiments on EAST

Radio frequency (RF) heating in the ion cyclotron range of frequencies (ICRF) is one of the primary auxiliary heating methods for EAST. The ICRF system provides 6 MW power in primary phase and will be capable of 10 MW later. Three 1.5 MW ICRF systems in a frequency range of 25 MHz to 70 MHz have already been in operation. The ICRF heating launchers are designed to have two current straps with each driven by a RF power source of 1.5 MW. In this paper a brief introduction of the ICRF heating system capability in EAST and the preliminary results in EAST are presented.

Advanced impedance matching system for ICRF heating using innovative twin stub tuner and frequency variation

Ion cyclotron range of frequency (ICRF) heating has been a reliable tool for steady-state plasma heating with high RF power of several tens of megawatts. However, a sudden increase in the reflected RF power during ICRF heating experiments with ELMy H-mode plasmas is an issue which must be solved for future fusion experimental devices or fusion reactors. This paper describes an innovative ICRF heating system using a frequency feedback control to reduce the reflected power in response to the rapid change in the plasma impedance in the ELMy H-mode plasma. A twin stub tuner has been newly invented for this purpose. The feasibility of keeping the reflected RF power fraction at a low level, e.g. 1%, is demonstrated even with a large change in plasma resistance, e.g. 2 ∼ 8Ω. Calculated and experimental results are presented for the conventional double stub tuner impedance matching system equipped with the twin stub tuner.

Wave-Particle Studies in the Ion Cyclotron and Lower Hybrid Ranges of Frequencies in Alcator C-Mod

This paper reviews the physics and technology of wave-particle-interaction experiments in the ion cyclotron range of frequencies (ICRF) and the lower hybrid (LH) range of frequencies (LHRF) on the Alcator C-Mod tokamak. Operation of fixed frequency (80 MHz) and tunable (40- to 80-MHz) ICRF transmitters and the associated transmission system is described. Key fabrication issues that were solved in order to operate a four-strap ICRF antenna in the compact environment of C-Mod are discussed in some detail. ICRF heating experiments utilizing the hydrogen (H) and helium-3 (3He) minority heating schemes are described, and data are presented demonstrating an overall heating efficiency of 70 to 90% for the (H) minority scheme and somewhat lower efficiency for (3He) minority heating. Mode conversion electron heating experiments in D(3He), D(H), and H(3He) discharges are also reported as well as simulations of these experiments using an advanced ICRF full-wave solver. Measurements of mode-converted ion cyclotron waves and ion Bernstein waves using a phase contrast imaging diagnostic are presented and compared with the predictions of a synthetic diagnostic code that utilizes wave electric fields from a full-wave solver. The physics basis of the LH current profile control program on Alcator C-Mod is also presented. Computer simulations using a two-dimensional (velocity space) Fokker Planck solver indicate that ~200 kA of LH current can be driven in low-density H-mode discharges on C-Mod with ~3 MW of LHRF power. It is shown that this off-axis LH current drive can be used to create discharges with nonmonotonic profiles of the current density and reversed shear. An advanced tokamak operating regime near the ideal no-wall β limit is described for C-Mod, where ~70% of the current is driven through the bootstrap effect. The LH power is coupled to C-Mod through a waveguide launcher consisting of four rows (vertically) with 24 guides per row (toroidally). A detailed description of the LH launcher fabrication is given in this paper along with initial operation results.

Strong electron heating in CHS ICRF heating experiments

An ion cyclotron range of frequencies (ICRF) heating experiment was performed on the Compact Helical System (CHS) device in order to identify the problems in establishing this as a reliable heating method. Radiofrequency heating was applied to plasmas with two ion species produced by ECH. Stored energy increased upto 2.2 kJ with the application of a 590 kW ICRF heating pulse, giving a heating efficiency comparable to that of NBI heating. More importantly, the `flat-top' of the stored energy was maintained during the pulse. Good heating was realized with 30% proton and 70% deuteron plasmas at high density (4*1019 m-3). It was found that the best discharges are dominated by electron ICRF heating, which seems to be the reason for the successful results. On the contrary, a flat-top in stored energy has never been obtained in discharges where ion heating dominates. The loss of high energy ions with large pitch angles produced by ion heating appears to degrade the performance and limit the duration of the heating. A similar degradation was encountered in previous ICRF heating experiments in helical systems. A newly designed detector was used to study the loss of the helically trapped particles

Measurements of H/(H+D) Ratios in ICRF Heating Experiments of CHS

The ratio of hydrogen (minority) to deuterium in the ion cyclotron range of frequency (ICRF) heating experiment has been measured using visible spectroscopy. The large increase in the stored energy during ICRF heating is observed with boronization, compared with titanium gettering, mainly because of the reduction of impurity radiation. The minority ratio, however, is not controllable due to the increase in hydrogen amount. After the electron cyclotron resonance discharge cleaning using deuterium gas the hydrogen amount has been successfully reduced. As a result it has been more effective than He-glow discharge cleaning. When the hydrogen minority decreases, the ion heating is expected. The maximum stored energy is obtained with the minority ration of 30% at a line-averaged density of 3.3 x 10{sup 13} cm{sup -3}. 4 refs., 7 figs.

Fast-wave ion cyclotron resonance heating experiments on the Alcator C tokamak

In this paper, minority regime fast-wave ICRF (Ion cyclotron range of frequency) heating experiments conducted on the Alcator C tokamak [Nucl. Fusion 26, 1665 (1986)] are described. Up to 450 kW of rf (radio frequency) power at frequency f=180 MHz was injected into plasmas composed of deuterium majority and hydrogen minority ion species at magnetic fields of B0≂12 T, densities 0.8≤n̄e≤5×1020 m−3, and minority concentrations 0.25≲ηH≤8%. Typical ion temperatures were TD(0)∼1 keV, while, depending on density, typical electron temperatures were in the range Te(0)∼1.5–2.5 keV. Central deuterium ion temperature increases of ΔTD(0)=400 eV were observed at n̄e=1×1020 m−3, while significantly smaller ion temperature increases were observed at higher densities. At the highest densities, ion heating became insignificant due in part to a limitation on power handling by the antenna that became more severe with increasing density. Significant electron heating was not observed at any density. Heating of the minority species at low densities indicated severe losses, but at higher densities it was consistent with efficient collisional coupling of the rf power from the hydrogen minority species to the deuterium majority species. Analysis of the deuterium power balance indicated no significant change in the deuterium thermal transport properties as a function of rf power. This may be consistent with the fact that the rf power never exceeded the Ohmic heating (OH) contribution. However, a very strong dependence of the deuterium transport properties on density was observed, and appears to be the dominant process limiting the effectiveness of ICRF heating at high density.