Electron Cyclotron Wave Power Research Articles

Suppression of the m/n = 2/1 tearing mode by electron cyclotron resonance heating on J-TEXT

Abstract Stabilization of tearing modes and neoclassical tearing modes is of great importance for the tokamak operation. Electron cyclotron wave (ECW) has been extensively used to stabilize the tearing modes with the virtue of the highly localized power deposition. Complete suppression of the m/n = 2/1 tearing mode (TM) by electron cyclotron resonance heating (ECRH) has been achieved successfully on the J-TEXT tokamak. The effect of ECW deposition location and power amplitude on the 2/1 TM suppression have been investigated. It is found that the suppression is more effective when the ECW power is deposited closer to the rational surface. Partial suppression of the 2/1 TM by ECRH has been observed and the island width is reduced by 65% of the initial island width. The island rotation frequency is increased when the island width is reduced. The experimental results show that the temperature perturbations increase inside the magnetic island produced by the local heating plays a dominant role on tearing modes suppression. As the ECW power increased, the 2/1 island is suppressed to smaller islands width and the flow shear also plays a stabilizing effect on small magnetic islands. With the stabilizing contribution of heating and the flow shear, the 2/1 TM can be completely suppressed.

Particle pump-out induced by trapped electron mode turbulence in electron cyclotron heated plasmas on XuanLong-50 spherical torus

Particle pump-out effects induced by low-frequency (<200 kHz) density fluctuations were observed in solely electron cyclotron wave (ECW)-heated plasmas on the spherical torus XuanLong-50 (EXL-50) without a central solenoid. The intensity of the relative density fluctuations increases with increasing ECW power and decays when the ECW is turned off while sustaining the plasma current. The electron densities are maintained relatively high and steady when the density fluctuations are completely absent, indicating that the outward transport of electrons is dominated by the particle pump-out effect of the ECW. The density fluctuations are modulated by a supersonic molecular beam injection pulse and the modulation amplitude decreases with increasing electron density at the same ECW injection power and decreasing ECW power at the same electron density, respectively. Analysis revealed that a critical value of electron temperature gradient (ETG) triggers the density fluctuations, and the intensity of the relative density fluctuations is positively correlated with the ETG and approximately inversely proportional to the effective collision frequency. With plasma parameters similar to those of EXL-50 experiments, the HD7 code simulations demonstrate that trapped electron mode (TEM) turbulence can be excited by ETG higher than the critical value observed in the experiment. In addition, the dependence of the mode growth rate (supposed to be proportional to the saturation level of fluctuations in quasi-linear theory) and the measured intensity of the density fluctuations is comparable. The simulated outward particle flux integrated over the poloidal wave number spectrum is significant and proportional to ETG. These observations demonstrate that the density fluctuation is TEM turbulence, which is driven by ETG and induces particle pump-out when the electron density/effective electron collision frequency is low. The potential relevance of this work with the controls of plasma profiles, impurities, helium ash, and heat transport in future reactors of similar low effective collision frequency is also discussed.

The development of 1 MW ECRH system on J-TEXT

A 1 MW electron cyclotron resonance heating system has been designed and developed for J-TEXT tokamak not only to enhance the total heating power but also to provide another possible way to control the plasma parameters. The system is composed of two gyrotrons, both with an output power of 500 kW electron cyclotron wave power. The development progress of the system is elaborated and discussed. The commissioning tests for the gyrotrons were implemented, and each gyrotron realized its standard output successfully. With the assistance of this system, a variety of experiments have been carried out on J-TEXT, mainly related to plasma heating, plasma disruption, plasma start-up, current drive and so on.

Recent progress of the ECRH system and initial experimental results on the J-TEXT tokamak

In order to broaden the range of the plasma parameters and provide experimental conditions for physical research into high-performance plasma, the development of the electron cyclotron resonance heating (ECRH) system for the J-TEXT tokamak was initiated in 2017. For the first stage, the ECRH system operated successfully with one 105 GHz/500 kW/1 s gyrotron in 2019. More than 400 kW electron cyclotron (EC) wave power has been injected into the plasma successfully, raising the core electron temperature to 1.5 keV. In 2022, another 105 GHz/500 kW/1 s gyrotron completed commissioning tests which signifies that the ECRH system could generate an EC wave power of 1 MW in total. Under the support of the ECRH system, various physical experiments have been carried out on J-TEXT. The electron thermal transport in ECRH plasmas has been investigated. When ECRH is turned on, the electron thermal diffusivity significantly increases. The runaway current is elevated when a disruption occurs during ECRH heating. When the injected EC wave power is 400 kW, the conversion efficiency of runaway current increases from 35% to 75%. Fast electron behavior is observed in electron cyclotron current drive (ECCD) plasma by the fast electron bremsstrahlung diagnostic (FEB). The increase in the FEB intensity implies that ECCD could generate fast electrons. A successful startup with a 200 kW ECW is achieved. With the upgrade of the ECRH system, the J-TEXT operational range could be expanded and further relevant research could be conducted.

Investigation of electron cyclotron current drive efficiency on the J-TEXT tokamak

Electron cyclotron current drive (ECCD) efficiency research is of great importance for the neoclassical tearing mode (NTM) stabilization. Improving ECCD efficiency is beneficial for the NTM stabilization and the ECCD power threshold reduction. ECCD efficiency has been investigated on the J-TEXT tokamak. The electron cyclotron wave (ECW) power scan was performed to obtain the current drive efficiency. The current drive efficiency is derived to be approximately η 0 = (0.06–0.16) × 1019 A m−2 W−1 on the J-TEXT tokamak. The effect of the residual toroidal electric field has been included in the determination of the current drive efficiency, which will enhance the ECCD efficiency. At the plasma current of I p = 100 kA and electron density of n e = 1.5 × 1019 m−3, the ratio of Spitzer conductivity between omhic (OH) and ECCD phases is considered and the experimental data have been corrected. The correction results show that the current drive efficiency η 1 caused by the fast electron hot conductivity decreases by approximately 79%. It can be estimated that the driven current is approximately 24 kA at 300 kW ECW power.

Electron cyclotron current drive under neutral beam injection on HL-2M

Based on OMFIT framework and HL-2M parameters, this paper comprehensively considers the changes in plasma density, temperature, and other transport quantities caused by the interaction of neutral beam injection (NBI) and electron cyclotron wave (ECW) with plasma. The changes in the Shafranov shift of the plasma magnetic surface center are also evaluated. Theoretically, the influence of NBI on the deposition location and current drive efficiency of the ECW is studied. According to the findings, NBI affected the position location and efficiency of the electron cyclotron current drive (ECCD) deposited on both high field side (HFS) and low field side (LFS). NBI can relocate the ECW power deposition location to the core and increase the current drive efficiency when the ECW power is deposited on the LFS. When the NBI power increases to 7 MW, the ECCD deposition location can shift to the core by roughly 0.15 normalized small radii, and the current drive efficiency can be improved by 1.3 times. Moreover, as NBI power increases, the radial region where the dimensionless current drive efficiency equals to zero gets closer to the plasma edge. When ECW power is deposited on the HFS paraxial, increasing NBI power causes the ECW deposition location to move toward the plasma edge, thus lowering current drive efficiency. This trend is caused by an increase in NBI power, which can increase the Shafranov shift of the plasma center, increase the electron density, and change the electron temperature. These studies hold great significance for achieving more effective current drive and controlling the plasma current profile and neoclassical tearing mode instability.

Power threshold and saturation of parametric decay instabilities near the upper hybrid resonance in plasmas

Parametric decay instabilities (PDIs) occur for large-amplitude waves in quadratically nonlinear media, where they provide a limit of validity of linear theories and allow efficient coupling between different, well-defined wave modes. We investigate PDIs near the upper hybrid resonance in plasmas by injection of high-power electron cyclotron (EC) waves at the ASDEX Upgrade tokamak. Our measurements of PDIs have an unprecedented frequency resolution, far below the ion cyclotron frequency, allowing the first observations of secondary and tertiary PDIs during the saturation phase in a controlled laboratory setting. Furthermore, we are for the first time able to systematically compare theoretical predictions of the EC wave power thresholds, which must be exceeded to excite such PDIs, with experimental observations, validating the theory. Our findings are relevant for EC wave heating and current drive in tokamaks and stellarators, including future fusion power plants, as well as in low-temperature laboratory and industrial plasmas, inertial confinement fusion, and ionospheric modification experiments.

Numerical analysis of the optimized performance of the electron cyclotron wave system in a HL-2M tokamak**Project supported by the National Natural Science Foundation of China (Grant Nos. 11375085, 11405082, 11505092, 11475083, and 11375053), the National Magnetic Confinement Fusion Science Program of China (Grant Nos. 2013GB104004, 2013GB111000, 2014GB107000, and 2014GB108002), and the Natural Science Foundation of Hunan Province, China (Grant No. 2015JJ4044).

The capabilities of current drive, neoclassical tearing mode (NTM) stabilization, and sawtooth control are analyzed for the electron–cyclotron wave (ECW) system in a HL-2M tokamak. Better performance of the upper launcher is demonstrated in comparison with that of a dropped upper launcher, in terms of JEC/Jbs for NTM stabilization and IECCD/(Δρtor)2 for sawtooth control. 1-MW ECW power is enough for the 3/2 NTM stabilization, and 1.8-MW ECW power is required to suppress 2/1 NTM in a single null divertor equilibrium with 1.2-MA toroidal current with the upper launcher. Optimization simulation of electron–cyclotron current drive (ECCD) is carried out for three mirrors in an equatorial port, indicating that the middle mirror has a good performance compared with the top and bottom mirrors. The results for balanced co- and counter-ECCD in an equatorial port are also presented.

Numerical studies of electron cyclotron wave current drive on HL-2A tokamak

The electron cyclotron wave (ECW) current drive (CD) for the HL-2A tokamak is investigated numerically with a new ray-tracing and Fokker-Planck code. The code is benchmarked with other well-tested linear and quasilinear codes and is then used to study the electron cyclotron current drive on the HL-2A tokamak. The wave propagation, power deposition, and driven-current profiles are presented. The effect of electron trapping is also assessed. It is found that quasilinear effects are negligible at the present ECW power levels and that when both waves are injected at an angle of 20° on the plasma equatorial plane, the CD efficiency for the HL-2A saturates at ∼0.029 × 1020 A/W/m2 and ∼0.020 × 1020 A/W/m2 for the 0.5 MW/68 GHz first harmonic ordinary (O1) and 1 MW/140 GHz second harmonic extraordinary (X2) modes, respectively. The effects of the plasma density, temperature, and wave-launching position on the driven current are also investigated analytically and numerically.

EC Radiative Transport in the Presence of Anisotropic Distributions of Suprathermal Electrons

The RAYTEC code for electron cyclotron (EC) wave radiative transport modelling of fusion plasmas which earlier had been generalised to include a population of suprathermal electrons to cover EC radiative transport in the presence of an anisotropic distribution of suprathermal electrons in the form of a one-sided loss- or anti-loss-cone in pitch angle, has been modified and, furthermore, now also covers inverted loss-cone and anti-loss-cone distributions. This offers the possibility of dealing with anisotropic distributions symmetric in the electron velocity parallel to the magnetic field, as typical for the case of EC wave heating. Practical issues related to this approach are displayed and the impact of various choices of suprathermal populations on net EC wave power emission from a fusion plasma is evaluated and discussed.

RAYTEC: a new code for electron cyclotron radiative transport modelling of fusion plasmas

As it was recognized that local electron cyclotron (EC) wave power losses can be a competitive contribution to the 1D electron power balance for reactor-grade tokamak plasmas in regimes as anticipated for steady-state operation, a systematic effort is ongoing to improve the modelling capability for the radial profile of EC wave emission. This effort aims at generating a hierarchy of codes that cover the non-local behaviour of EC wave transport for inhomogeneous plasmas and in the presence of reflecting walls with increasingly improved accuracy and also provide sufficient computational efficiency for being usable in 1D transport studies. The recently developed code RAYTEC, which explicitly addresses the geometrical effects present in toroidal plasmas with arbitrary cross-section, is described and used to investigate the impact of elongation of the plasma cross-section and of toroidicity on the angular dependence of the EC radiation field, on the profile of the net EC wave power density lost from the plasma and on the total EC power loss for ITER-like plasma conditions. Furthermore, a comparison is made with the results of simpler models in use to describe both local and total EC power losses as well as with ones obtained from analytical formulae that are introduced on the basis of Trubnikov's formula for EC power emission.

Benchmarking of Codes for Calculating Local Net EC Power Losses in Fusion Plasmas

The codes SNECTR, CYTRAN, CYNEQ, and EXACTEC are compared in view of the calculation of the profile of the net electron cyclotron (EC) wave power density emitted for different electron temperature profiles and average temperatures of relevance for reactor-grade magnetoplasmas. The effects of either specularly or diffusely reflecting walls are assessed for a cylindrical plasma with circular cross-section, specular reflection, as assumed in EXACTEC, providing a lower bound to the net EC wave power losses in the hot plasma core (and therefore, as a rule, also to the total EC power loss) as well as to reabsorption in the edge plasma. The assumption of isotropy of the radiation intensity in the plasma that is adopted in both CYTRAN and CYNEQ (which cannot be justified a priori) is discussed and found to be adequate for strong diffuse reflection. However, it overestimates the net EC power loss in the plasma core for weakly as well as for specularly reflecting walls by up to 20%. The full transport code SNECTR (no longer in active use), for specular reflection, and the exact cylindrical code EXACTEC are in excellent agreement with each other while for strong diffuse reflection EXACTEC is found to underestimate the net EC power loss typically by 20%. EXACTEC, CYTRAN, and CYNEQ are confirmed to be well suited for use in systematic transport simulations of fusion plasmas.

Stabilization of neoclassical tearing modes by electron cyclotron current drive in JT-60U

Results of active control of neoclassical tearing modes (NTMs) by electron cyclotron current drive (ECCD) in JT-60U are described. Growth of an NTM with poloidal mode number m = 3 and toroidal mode number n = 2 has been suppressed by ECCD inside the sawtooth inversion radius in the co-direction, showing the possibility of the coexistence of sawtooth oscillations and a small-amplitude m/n = 3/2 NTM without large confinement degradation. Stabilization of an m/n = 2/1 NTM by ECCD at the mode rational surface has been demonstrated with a small ratio of the current density driven by the electron cyclotron (EC) wave to the local bootstrap current density (∼ 0.5). In addition, dependence of the stabilization effect on ECCD location has been investigated in detail. It has been found that an m/n = 2/1 NTM can be completely stabilized with the misalignment of the ECCD location less than about half of the full island width, and that the m/n = 2/1 NTM is destabilized with the misalignment comparable to the full island width. Time-dependent, self-consistent simulation of magnetic island evolution using the TOPICS code has shown that the stabilization and destabilization of an m/n = 2/1 NTM are well reproduced with the same set of coefficients of the modified Rutherford equation. The TOPICS simulation has also clarified that EC wave power required for complete stabilization can be significantly reduced by narrowing the ECCD deposition width.

EC + LH current drive efficiency in the presence of an internal transport barrier

In this paper we study the effects of the presence of an internal transport barrier (ITB) on the current drive efficiency and power deposition profiles in the case of electron cyclotron waves interacting with an extended tail generated by lower hybrid (LH) waves. We study the subject by numerically solving the Fokker–Planck equation, with temperature and density profiles corrected along the time evolution at each collision time, based on the actual time-evolving electron distribution function. The results obtained show that the LH and electron cyclotron (EC) power absorption profiles and the current driven by the combined action of both types of waves are weakly dependent on the depth of the ITB, slightly more dependent on the level of magnetic turbulence and much more dependent on the level of EC wave power.

Modification of the current profile in DIII-D by off-axis electron cyclotron current drive

Localized non-inductive currents due to electron cyclotron wave absorption have been measured on the DIII-D tokamak. Clear evidence of non-inductive currents is seen on the internal magnetic field measurements by motional Stark effect spectroscopy. The magnitude and location of the non-inductive current is evaluated by comparing the total and ohmic current profiles of discharges with and without electron cyclotron wave power. The measured current agrees with Fokker-Planck calculations near the magnetic axis, but exceeds the predicted value as the location of the current drive is moved to the half-radius.

A proposal for ECRH on JET†

In this paper the layout and preliminary design is described of an installation for the injection of electron-cyclotron wave power into JET. The installation consists of 12 gyrotrons, each 70 GHz, 200 kW RF-output power and a pulse duration of 10 s. A layout for the installation is given using vertical ports at the top and the bottom of the vacuum vessel for launching the RF power into the JET plasma. The electric power for the gyrotrons will be supplied by two high voltage transformer rectifiers with controllable output voltage. Each of the two modulators will use a series tube to regulate the required voltage of 80 kV for six gyrotrons. Possibilities to modulate the RF power are discussed. The entire system will be operated by means of the computerized control and data acquisition system (CODAS). The protection system that ensures a safe and reliable operation of the gyrotrons is described. The microwave considerations for the predesign of the transmission lines and launching systems are covered.