Electron Cyclotron Wave Injection Research Articles

Numerical study of plasmas start-up by electron cyclotron waves in NCST spherical tokamak and CN-H1 stellarator

Abstract According to the physics of tokamak start-up, this paper constructs a zero-dimensional (0D) model applicable to electron cyclotron (EC) wave assisted start-up in NCST spherical torus (spherical tokamak) and CN-H1 stellarators. Using the constructed zero-dimensional model, the results obtained in this paper under the same conditions are compared and validated against references results for pure hydrogen plasma start-up in tokamak. The results are in good agreement, especially regarding electron temperature, ion temperature and plasma current. In the presence of finite Ohmic electric field in the spherical tokamak, a study on the EC wave assisted start-up of the NCST plasma at frequency of 28 GHz is conducted. The impact of the vertical magnetic field B v on EC wave assisted start-up, the relationship between EC wave injection power P inj, Ohmic electric field E, and initial hydrogen atom density nH0 is explored separately. It is found that under conditions of Ohmic electric field lower than ITER (~ 0.3 V m-1), EC wave can expand the operational space to achieve better plasma parameters. Simulating the process of 28 GHz EC wave start-up in the CN-H1 stellarator plasma, the plasma current in the zero-dimensional model is replaced with the current in the poloidal coil of the stellarator. Plasma start-up can be successfully achieved at injection powers in the hundreds of kilowatts range, resulting in electron densities on the order of 1017 to 1018 m-3.

Numerical study on the peeling–ballooning modes with electron cyclotron wave injection

The influence of electron cyclotron wave (ECW) injection with different deposition positions and injection powers on the evolution of peeling–ballooning (P–B) modes is studied with the BOUT++ code, in which the energy deposition and current drive of the ECW are calculated using a ray tracing code. It is shown that the changes in the profiles of plasma pressure, current density, and resistivity induced by ECW injection can significantly influence the linear property and the nonlinear evolution of P–B modes. For the linear simulation, the ECW deposited at the top of the pedestal makes the high toroidal mode number (n) P–B modes more unstable; however, it stabilizes the medium-n to high-n P–B modes when the ECW is deposited at the middle of the pedestal, and the ECW deposited at the bottom of the pedestal decreases the growth rate of P–B modes with medium-n. Further investigation shows that the injected ECW influences the characteristic of linear P–B modes by changing the diamagnetic effect, magnetic shear, pressure gradient, current density, resistivity, and so on. It is known from the nonlinear simulation that the energy loss caused by the edge localized mode (ELM) with ECW injection deposited at the top of the pedestal is nearly the same as that in the case without ECW injection, while the ECW deposited at the middle and bottom of the pedestal is helpful to decrease ELM energy loss. According to the analyses of the time evolution of the P–B mode toroidal spectrum, the physical mechanism of the decrease in ELM energy loss in the simulation is that ECW injection suppresses the most unstable toroidal harmonic of the P–B mode. On the other hand, the influence of ECW injection on P–B modes becomes more obvious when the power of the injected ECW increases. Moreover, the influence of current driven by the ECW on P–B modes is studied separately in this paper, which plays a different role from the bootstrap current.

Modeling very high electron heating by radio frequency waves on EAST

In the 2018 EAST experimental campaign, a very high central electron heating, fully-non-inductive discharge with the core electron temperature over 9 keV has been achieved. Such high central electron heating was realized by injecting radio frequency waves, including 1.8 MW lower hybrid wave (LHW) and 0.8 MW electron–cyclotron waves (ECW). Experimental diagnosis indicates two different time scales characterizing the electron heating process, a rapid and a slow rise of the central electron temperature after the injection of ECW. In this work, integrated modeling is performed to investigate the physical mechanisms of such high electron heating. Five characteristic phases during the increase of the electron temperature are chosen for modeling. In phase 1, the electron heating is by LHW alone. The modeling confirms that the LHW can only sustain the core electron temperature T e ≈ 5.5 keV, which is consistent with the experiment. In phase 2, the electron temperature increases rapidly after the first 0.4 MW ECW is injected. The result shows that the rapid increase of the electron temperature is due to the interaction between the ECW and the electrons. With the increase of the electron temperature, the electron flux induced by the trapped electron modes (TEMs) and the electron temperature gradient driven modes (ETGs) is enhanced in the core region. In phase 3, the electron temperature increases slowly after phase 2. It is found that the slow increase is mainly due to the flattening of the density profile. The flattening of the density profile can decrease the thermal diffusivity of the electrons mainly induced by the TEMs leading to a higher electron temperature for a given heating source. In phase 4, the electron temperature again increases rapidly after the second 0.4 MW ECW is injected. The physical mechanism is similar to that in phase 2. In phase 5, the LHW power deposition of the LHW remains almost unchanged compared to that in phase 4 since the electron temperature is sufficiently high. The slow rise of the electron temperature is caused by the improvement of the electron energy confinement as thermal diffusivity of the electrons is decreased due to the flattening of the electron density profile, which is similar to the main reason in phase 3.

Observation of an edge coherent mode and poloidal flow in the electron cyclotron wave induced high βp plasma in QUEST

Fluctuations are measured in the edge and scrape-off layer (SOL) of QUEST using fast visible imaging diagnostic. Electron cyclotron wave injection in the Ohmic plasma features excitation of low frequency coherent fluctuations near the separatrix and enhanced cross-field transport. Plasma shifts from initial high field side limiter bound (inboard limited, IL) towards inboard poloidal null (IPN) configuration with steepening of the density profile at the edge. This may have facilitated the increased edge and SOL fluctuation activities. Observation of the coherent mode, associated plasma flow, and particle out-flux, for the first time in the IPN plasma configuration in a spherical tokamak may provide further impetus to the edge and SOL turbulence studies in tokamaks.

Experiments on magneto-hydrodynamics instabilities with ECH/ECCD in FTU using a minimal real-time control system

Experiments on real time control of magneto-hydrodynamic (MHD) instabilities using injection of electron cyclotron waves (ECW) are being performed with a control system based on only three real time key items: an equilibrium estimator based on a statistical regression, a MHD instability marker (SVDH) using a three-dimensional array of pick-up coils and a fast ECW launcher able to poloidally steer the EC absorption volume with dρ/dt = 0.1/30 ms maximum radial speed. The MHD instability, usually a tearing mode with poloidal mode number m and toroidal mode number n such that m/n = 2/1 or 3/2 is deliberately induced either by neon gas injection or by a density ramp hitting the density limit. No diagnostics providing the radial localization of the instabilities have been used. The sensitivity of the used MHD marker allows to close the control loop solely on the effect of the actuator’s action with little elaboration. The nature of the instability triggering mechanism in these plasma prevents that the stabilization lasts longer than the ECW pulse. However when the ECW power is switched on, the instability amplitude shows a marked sensitivity to the position of the absorption volume with an increase or decrease of its growth rate. Moreover the suppression of the dominant mode by ECRH performed at high plasma density even at relatively low power level facilitates the development of a secondary mode. This minimized set of control tools aim to explore some of the difficulties which can be expected in a fusion reactor where reduced diagnostic capabilities and reduced actuator flexibility can be expected.

Numerical simulation of electron cyclotron current drive characteristics on EAST

Electron cyclotron current drive (ECCD) will be applied in the EAST tokamak during its the new campaign. In order to provide theoretical predictions for relevant physical experiments, some numerical simulations of ECCD with the parameters of EAST have been carried out by using TORAY-GA code based on the understanding of ECCD mechanisms. ECCD efficiencies achieved in different plasma and electron cyclotron (EC) wave parameters are given. The dependences of ECCD characteristics on EC wave injection angle, toroidal magnetic field, plasma density, and temperature are presented and discussed.

Electron Bernstein wave heating by electron cyclotron wave injection from the high-field side in LHD

In the Large Helical Device (LHD), evident electron Bernstein wave (EBW) heating was successfully performed. The experiment was carried out using the electron cyclotron heating (ECH) system that was upgraded by installation of high-power, long-pulse 77 GHz gyrotrons. The EBW heating was achieved by a mode conversion from injected EC wave to EBW, by the so-called slow-XB technique where an X-mode wave is injected to the plasma from the high magnetic field side. The specific magnetic configuration of LHD provides a good opportunity to realize the slow-XB technique, which is generally difficult for tokamaks. With the slow-XB technique, increases in kinetically evaluated electron energy Wpe and electron temperature Te were observed in overdense plasmas. An electron heating in the so-called super dense core plasma in LHD, which is characterized with an internal diffusion barrier and a steep density gradient at the plasma core, was successfully demonstrated in the plasma core region where the central electron density ne0 of 17 × 1019 m−3 was about 1.2 times higher, at the beginning of the EC-wave injection, than the left-hand cut-off density of applied 77 GHz EC waves.

Tungsten accumulation in H-mode plasmas of JT-60U

It has been observed that with increasing toroidal rotation velocity inside the q = 1 layer in the direction opposite to the plasma current, sawtooth activity becomes moderate and tungsten accumulation becomes significant. The tungsten accumulation level is significantly reduced from this trend in the case when electron cyclotron wave or high energy neutral beam is injected into the plasma core. In contrast, the tungsten accumulation is kept high by the electron cyclotron wave injection into the peripheral region.

Optimization of Electron Cyclotron Current Drive in the Magnetic Field Configuration of CHS

Second-harmonic electron cyclotron (EC) current drive experiments have been performed in the Compact Helical System (CHS). The driven current changes its direction according to the change of the EC-wave beam direction in agreement with an expectation from the Fisch and Boozer theory in the case of low-field-side injection of EC waves. The EC-driven current varies as a function of the magnetic axis position of CHS plasmas. The cause of the variation was experimentally investigated by a magnetic field scan. Setting the second-harmonic resonance layer near the magnetic axis was favorable to maximize the total EC-driven current. The main cause of the dependence of the driven current on the magnetic axis position is attributed to the change of distribution of the magnetic field along the beam path due to the change of the beam direction to aim at the magnetic axis in the three-dimensional helical magnetic field of the CHS.

Temporal variation of density fluctuation and transport in reversed shear plasmas on JT-60U

Further transport reduction is induced by a pellet or electron cyclotron (EC) wave injection after the formation of an internal transport barrier in JT-60U reversed shear plasmas. After pellet injection, the O-mode reflectometer signal drastically decreases, indicating reduction in the electron density fluctuation level. The ion thermal diffusivity decreases by one order of magnitude and reaches the neoclassical level. The effective particle diffusivity also decreases by one order of magnitude, but the electron thermal diffusivity remains constant. After EC wave injection, when no reduction of the O-mode reflectometer signal is observed, the electron thermal diffusivity decreases by a factor of ∼2–3, while decreases in the ion thermal diffusivity and the effective particle diffusivity are small. These results indicate that the ion heat transport and particle transport were coupled with the measured density fluctuation, but electron heat transport was decoupled. Ion heat and particle transport and electron heat transport were dominated by different types of fluctuation.

Plasma production by low-field side injection of electron cyclotron waves in a simple magnetized torus

The mechanisms leading to ionization of neutral gas by means of microwaves in the electron cyclotron range of frequencies in a simple toroidal plasma configuration are investigated in the TORPEX device (Fasoli A et al 2003 Bull. Am. Phys. Soc. 48 119). The role of the upper hybrid resonance, where most of the ionization events occur once the plasma is formed, is clearly demonstrated. The fast phenomena related to ionization are discriminated from the slower mechanisms leading to the stationary background profiles by measuring the density perturbation caused by a modulation of the injected microwave power. The same method permits reconstruction of the spatial profile of the particle source. The ionization processes are then related to experimental control parameters such as the injected microwave power.

Steady-state tokamak operation, ITB transition and sustainment and ECCD experiments in TRIAM-1M

Experiments aiming at ‘day long operation at high performance’ have been carried out in TRIAM-1M. The record value of the discharge duration was updated to 5 h and 16 min. Steady-state tokamak operation is studied under the localized plasma wall interaction conditions. The distributions of the heat load, the particle recycling flux and impurity source are investigated to understand the co-deposition and wall pumping. The formation and sustainment of an internal transport barrier (ITB) in enhanced current drive mode has been investigated by controlling the lower hybrid driven current profile by changing the phase spectrum. An ITER relevant remote steering antenna for electron cyclotron wave injection was installed and a relativistic Doppler resonance of the oblique propagating extraordinary wave with energetic electrons driven by lower hybrid waves was studied.

Runaway electron behaviour during electron cyclotron resonance heating in the Frascati Tokamak Upgrade

The behaviour of high energy runaway electrons during the flat-top phase of electron cyclotron (EC) resonance heated (ECRH) discharges has been analysed in the Frascati Tokamak Upgrade. The decay of the electric field due to plasma heating has been observed to result in a rapid fall in the electron energy that frequently leads to quenching of the runaway population. The efficiency of this runaway mitigation scheme and its dependence on the plasma conditions has been investigated with particular emphasis on the required electric field drop during EC wave injection.

Magnetohydrodynamic Stability of Improved Confinement Plasmas in JT-60U

Progress in the understanding of magnetohydrodynamic (MHD) stability is summarized on JT-60U tokamak discharges with improved confinement such as the (hot-ion) H-mode, high-βp mode, high-βp H-mode, and reversed shear discharges. Transport barriers, which are essential for the improved confinement, play key roles in the local and global MHD stability owing to the local large pressure gradient and the related bootstrap current. Disruptive β limits of these discharges are consistent with theoretical ideal kink-ballooning stability limits with low toroidal mode numbers n. Achievable β limit is improved by broadening of the pressure profile with high plasma internal inductance, plasma shaping, and wall stabilization. Edge localized modes (ELMs) and barrier localized modes (BLMs), which are associated with edge and internal transport barriers, respectively, are analyzed carefully. Resistive interchange modes with n ≤ 3 are excited in the negative shear region in reversed shear discharges with the internal transport barrier and lead to major collapse occasionally through nonlinear coupling with a tearing mode in the positive shear region. MHD characteristics of low m/n (m: poloidal mode number) tearing modes, which are attributed to the neoclassical tearing mode, are investigated. Stabilization of tearing modes and control of sawtooth activity are demonstrated using the fundamental O-mode electron cyclotron wave injection. Resistive wall modes associated with current-driven and pressure-driven low n external kink modes are identified.

Electron temperature perturbations measured by electron cyclotron emission diagnostic systems in JT-60U

Electron cyclotron emission (ECE) diagnostic systems in JT-60U for the measurement of electron temperature perturbations and the results of the measurement are described. In 1999, the heterodyne radiometer system was upgraded, and the number of the channels was doubled to 24. It can now measure ECE ranging from 176 to 200 GHz. The heterodyne system enables the measurement of the location and structure of the magnetic island, which is observed in the steady state high β p H-mode discharges. Behavior of an m/ n=1/1 mode and sawtooth oscillations during electron cyclotron heating and electron cyclotron current drive is investigated by using the grating polychromator. Here, m and n are poloidal and toroidal mode numbers, respectively. It is found that magnetic island width with m/ n=1/1 is decreased during electron cyclotron (EC) wave injection. It is also found that sawtooth period is significantly expanded by EC wave injection near the q=1 surface. The obtained maximum sawtooth-free period was about four times longer than the theoretically predicted one.

Current drive and magnetic turbulence experiments with electron cyclotron and lower hybrid waves on the Versator-II tokamak

Electron cyclotron (EC) waves were injected into partially lower hybrid current driven (LHCD) discharges on the Versator-II tokamak to test the theoretically predicted current drive synergism between EC and LH waves. For discharges with line averaged thermal electron density ⟨ne⟩ ≈ 8 × 1012 cm-3, the EC waves increased the plasma current and decreased the loop voltage. In contrast, for discharges with ⟨ne⟩ ⩽ 6 × 1012 cm-3, the EC waves decreased the plasma current and increased the loop voltage. This reduction in the plasma current by the EC waves at low density was generally accompanied by an increase in the limiter hard X ray emission and a clamping or reduction in the 70 GHz plasma emission, suggesting that confinement of the suprathermal current carrying electrons was degraded by the EC waves. This enhanced loss of the high-parallel-energy current-carrying LH-driven electron tail may have been caused by EC-wave-induced magnetic turbulence, which was observed to increase during EC wave injection by up to an order of magnitude in the frequency range of 50 to 400 kHz. The observed turbulence levels during EC wave injection and the average parallel energy of the current carrying electrons were higher for the lower density discharges, making the current carrying electrons more susceptible to loss caused by magnetic turbulence. Simple estimates show that the observed EC induced magnetic turbulence can explain the poor observed EC/LH current drive efficiency and its dependence on the thermal electron density.

Electron cyclotron current drive in a lower hybrid current drive plasma

Electron cyclotron (EC) current drive experiments were carried out in lower hybrid current drive (LHCD) plasmas on the WT-3 tokamak. Using oblique injection of EC waves, plasma current is ramped up in a frequency range ω/Ωe ≃ 1.2-1.4, where Ωe is the EC frequency at the plasma centre. This ramp-up is ascribed to selective EC heating of fast electrons in LHCD plasmas via the fundamental EC resonance at the upshifted frequency due to the Doppler effect. Fast electron momentum distributions are estimated from bremsstrahlung hard X ray spectra. These distributions indicate that the parallel momentum input to the fast electrons in the relativistic energy range from oblique EC waves is effective for efficient EC current drive. Current ramp-up is also observed in the second harmonic frequency range ω/ Ωe ≃ 1.9-2.2. In this case, bulk electrons are heated as well as fast electrons and no definite cause of current ramp-up is clarified.

Non-inductive current drive using second harmonic electron cyclotron waves on the WT-3 tokamak

A plasma current of up to 70 kA has been sustained in WT-3 discharges by second harmonic (2Ωe) electron cyclotron (EC) waves alone, with zero loop voltage, after shutting off the Ohmic heating power. Further, in the case of high power EC wave injection, ramp-up discharges have been obtained. Pulse height analysis of hard X-rays in the line of sight at various angles to the toroidal field shows that the velocity distribution function of the high energy tail electrons is asymmetric in the toroidal direction. The 2Ωe EC wave is mainly absorbed by the tail electrons, and a 2Ωe EC driven current is generated by enhancing the asymmetry of the distribution. The figure of merit of 2Ωe EC current drive (ECCD) is ηEC(2) = (3.2-6.4) × 10−2 (1019 A/W·m2), which is one order of magnitude smaller than that of lower hybrid current drive in WT-3. This low value of ηEC(2) can be attributed to low confinement of the current carrying, high energy tail electrons produced by 2Ωe ECCD.

Particle balance during the injection of electron cyclotron waves in Thor Tokamak

The empirical analysis of the electron density evolution during ECRH is given. It is shown that when recycling from the walls is low and the neutral gas injection rate is kept constant, during ECW injection the line density drops mainly as a consequence of a net decrease in the total particle number, whereas profile reshaping occurs only for some conditions. It is also shown that the phenomenon is of resonant nature but not a by-product of plasma heating. By considering the fast time scale on which the particle balance changes and by comparing the profile density evolution during ECRH to that following a step-wise reduction of the gas injection rate, it is inferred that particle confinement is degraded in the presence of EC waves. The consistency of this conclusion with all the experimental evidence is positively verified.

Microwave injection in heliac device TJ-II

An analysis of the cut-off and resonance positions for the helical axis 'heliac' device TJ-II has been performed stressing the peculiarities of the injection of electron cyclotron waves into this unusual magnetic configuration. The optimum ways of injection were determined as well as the effects of plasma density on wave propagation and the density limit for waves at the 2nd harmonic in the X-mode and at the fundamental frequency in the O-mode.