Lower Hybrid Wave Injection Research Articles

Long-pulse high-performance H-mode plasmas achieved on EAST

A record duration of a 310 s H-mode plasma (H98y2 ∼ 1.3, ne/nGW ∼ 0.7, fBS > 50%) has been recently achieved on experimental advanced superconducting tokamak (EAST) with metal walls, exploiting the device's improved long-pulse capabilities. The experiment demonstrates good control of tungsten concentration, core/edge MHD stability, and particle and heat exhaust with an ITER-like tungsten divertor and zero injected torque, establishing a milestone on the path to steady-state long-pulse high-performance scenarios in support of ITER and CFETR. Important synergistic effects are leveraged toward this result, which relies purely on radio frequency (RF) powers for heating and current drive (H&CD). On-axis electron cyclotron heating enhances the H&CD efficiency from lower hybrid wave injection, increasing confinement quality and enabling fully non-inductive operation at high density (ne/nGW ∼ 70%) and high poloidal beta (βP ∼ 2.5). A small-amplitude grassy edge localized mode regime facilitates the RF power coupling to the H-mode edge and reduces divertor sputtering/erosion. The high energy confinement quality (H98y2 ∼ 1.3) is achieved with the experimental and simulated results pointing to the strong effect of Shafranov shift on turbulence. Transport analysis suggests that trapped electron modes dominate in the core region during the record discharge. The detailed physics processes (RF synergy, core-edge integration, confinement properties, etc.) of the steady-state operation will be illustrated in the content. In the future, EAST will aim at accessing more relevant dimensionless parameters to develop long-pulse high-performance plasma toward ITER and CFETR steady-state advanced operation.

Kinetic theory of parametric instabilities of lower hybrid waves in tokamaks in the electromagnetic framework

Parametric instabilities during the injection of lower hybrid waves into tokamak edge plasmas have received considerable attention for affecting the current drive at high density, mainly through spectral broadening. Here, kinetic theory of parametric instabilities was revisited in the electromagnetic framework. From Vlasov–Maxwell equations in the guiding center formalism, the theory of parametric instabilities in the electromagnetic framework is presented with appropriate nonlinearity treatment. Numerical calculations are performed under realistic lower hybrid current drive experimental parameters. Comparison between the electrostatic and electromagnetic results shows that the electromagnetic corrections have a stabilizing effect on the parametric process in all decay channels. The quantitative differences in growth rates, as well as in the most unstable regions, might have a non-negligible impact on the amplification factor of the parametric instabilities. However, with the dependence of parametric instabilities on the local plasma parameters, calculations show the same tendency between the electromagnetic and electrostatic model in a wide parameter range.

Nonlinearity in parametric instabilities during the injection of lower hybrid waves into tokamak plasmas

Nonlinear kinetic theory of parametric instabilities is developed and applied to study the parametric process during the injection of lower hybrid waves into tokamak plasmas. Four approaches containing different nonlinear terms are numerically solved and compared. For the resonant mode decay, the quasi-linear treatment is enough for the daughter branches. However, for the quasi-mode decay, the contribution from the nonlinear response of quasi-mode should be included in the governing equations of the sidebands. The results with correct nonlinear terms can recover those from the usually used fluid-kinetic hybrid approach in the electrostatic limit, but the kinetic theory and its simplification method developed here could be more generally applied to study parametric instabilities within the full electromagnetic framework.

Observation of Alfvén eigenmodes driven by fast electrons during lower hybrid wave heating in EAST plasmas

Toroidal Alfvén eigenmodes (TAE) and ellipticity induced Alfvén eigenmodes (EAE) driven by fast electrons are observed for the first time in steady-state EAST plasmas heated by lower hybrid wave (LHW). For the analyzed discharge four modes appear after the injection of LHW. Comparing experimental observations with simulation, high frequency modes are located near and low frequency mode is near . The two positions are very close to where most of the heating power is deposited and population inversion occurs. Analysis of resonance condition between particles and waves indicates that barely-passing, deeply trapped and barely-trapped electrons can drive the mode.

Bifurcation in lower hybrid wave absorption induced by modification of electron temperature and formation of steady internal transport barrier

As the constraint imposed by the wave propagation condition limits the maximum allowed k//-upshift (k// is the component of the wave vector parallel to the magnetic field), the lower hybrid wave (LHW) absorption is bounded in the region defined by the strong Landau-damping limit and the boundary of wave propagation domain. This absorption mechanism causes interplay of the distribution of the absorbed LH power with the modification of plasma configuration, which constitutes nonlinearity in the LHW propagation and absorption. The HL-2A discharge with LHW and neutral beam injected is modeled using the gyro-Landau fluid transport model, in which radiation is deliberately enhanced to modify the electron temperature. Because of the nonlinearity of the LHW propagation and absorption, the LH power deposition jumps from one stationary state to another through an intermediate unstable state, generating bifurcation in the LHW absorption. Onset of the LHW bifurcation closely correlates with the modification of electron temperature, showing that the change of the electron temperature is an essential ingredient in producing the bifurcation. The change of the current profile during the transition period between the two stationary LHW absorption states results in enhancement of the E × B shearing flow arising from toroidal rotation, which causes the shearing rate of E × B flow to exceed the growth rate of drift ballooning instability, triggering transport barrier development and achieving a steady internal transport barrier in the 2nd stationary LHW phase. It is suggested that a modification of electron temperature can be a plausible way to control the plasma confinement in the tokamak discharge with LHW injection.

Isotopic effect of parametric instabilities during lower hybrid waves injection into hydrogen/deuterium plasmas

Based on the local dispersion relation, the parametric instability (PI) was numerically investigated for the injection of lower hybrid waves (LHWs) into hydrogen and deuterium plasmas separately. Numerical calculations under typical scrape-off layer parameters in tokamak plasmas show that both the unstable regions of the PI and the values of growth rates are close for two cases, in spite of the decaying channel of the ion sound quasimode or ion cyclotron quasimode (ICQM). These numerical results could be understood by the analyses based on the fluid model. Parameter dependences are also similar for hydrogen and deuterium plasmas. For example, the ICQM growth rate increases with an increasing density, a decreasing temperature, and a decreasing magnetic field in deuterium plasmas as it does in hydrogen plasmas. The isotopic effect of the PI during the LHW injection is weak. As a result, the lower hybrid current drive efficiency at a high density in deuterium plasmas cannot be much improved over hydrogen plasmas if the PI process dominates the behavior of LHWs at the plasma edge.

Simulation of Lower Hybrid Current Drive for DEMO

Lower hybrid (LH) wave is not only convenient to generate a flat or reversed magnetic shear profiles, but also helps one to explore scenarios for steady-state tokamak operation with improved confinement. Here with LSC code (lower hybrid simulation code), we calculate density and temperature profiles, relative power of injected wave and current wave lunch for two options of DEMO at the launched LH wave frequency 5 GHz. Two plasma scenarios pertaining to two different DEMO options, known as pulsed (option 1) and steady-state (option 2) models, have been analyzed. We perceive that power deposition by using lower hybrid wave injection mainly takes place near the edge of plasma and approximately in more peripheral region for both of options but has approximately higher efficiency for option 1 compared to option 2. About current wave lunch, a major part of that is close to the plasma edge for both of options. We have some considerable parts that reach to internal layers for option 1 and then current drive mainly takes place in a wider, more peripheral region for option 1.

Parameter study of parametric instabilities during lower hybrid wave injection into tokamaks

A parameter study of the parametric instability (PI) during lower hybrid wave injection into tokamak plasmas is performed based on its local dispersion relation. The drift kinetic model and the fluid model are employed, where both the E × B and E∥ coupling terms are retained. The E∥ coupling term is significant at a lower plasma density ne or a smaller oblique angle δ1 and then influences the density dependence of the PI. At a lower ne the PI decaying into the ion sound quasimode (ISQM) dominates, while at a higher ne the dominant PI is that decaying into the ion cyclotron quasimode (ICQM). A higher electron temperature Te can diminish the PI growth rate for both ISQM and ICQM types, since an increasing Te weakens the PI driving term, just like turning down the injecting power. A large background magnetic field B0 may reduce the PI growth rate, but this effect is rather weak on the ISQM-type PI at a lower ne.

Neutron energy spectrum measurements with a compact liquid scintillation detector on EAST

A neutron detector based on EJ301 liquid scintillator has been employed at EAST to measure the neutron energy spectrum for D-D fusion plasma. The detector was carefully characterized in different quasi-monoenergetic neutron fields generated by a 4.5 MV Van de Graaff accelerator. In recent experimental campaigns, due to the low neutron yield at EAST, a new shielding device was designed and located as close as possible to the tokamak to enhance the count rate of the spectrometer. The fluence of neutrons and γ-rays was measured with the liquid neutron spectrometer and was consistent with 3He proportional counter and NaI(Tl) γ-ray spectrometer measurements. Plasma ion temperature values were deduced from the neutron spectrum in discharges with lower hybrid wave injection and ion cyclotron resonance heating. Scattered neutron spectra were simulated by the Monte Carlo transport Code, and they were well verified by the pulse height measurements at low energies.

A compact stilbene crystal neutron spectrometer for EAST D-D plasma neutron diagnostics

A new compact stilbene crystal neutron spectrometer has been investigated and applied in the neutron emission spectroscopy on the EAST tokamak. A new components analysis method is presented to study the anisotropic light output in the stilbene crystal detector. A Geant4 code was developed to simulate the neutron responses in the spectrometer. Based on both the optimal light output function and the fitted pulse height resolution function, a reliable neutron response matrix was obtained by Geant4 simulations and validated by 2.5 MeV and 14 MeV neutron measurements at a 4.5 MV Van de Graaff accelerator. The spectrometer was used to diagnose the ion temperature in plasma discharges with lower hybrid wave injection and ion cyclotron resonance heating on the EAST tokamak.

Modification of ordinary-mode reflectometry system to detect lower-hybrid waves in Alcator C-Mod

Backscattering experiments to detect lower-hybrid (LH) waves have been performed in Alcator C-Mod, using the two modified channels (60 GHz and 75 GHz) of an ordinary-mode reflectometry system with newly developed spectral recorders that can continuously monitor spectral power at a target frequency. The change in the baseline of the spectral recorder during the LH wave injection is highly correlated to the strength of the X-mode non-thermal electron cyclotron emission. In high density plasmas where an anomalous drop in the lower hybrid current drive efficiency is observed, the observed backscattered signals are expected to be generated near the last closed flux surface, demonstrating the presence of LH waves within the plasma. This experimental technique can be useful in identifying spatially localized LH electric fields in the periphery of high-density plasmas.

Modelling of Profile Control with Lower Hybrid Wave Injection in the HL-2A Single-Null Divertor Plasma

Separatrix of the single-null diverted plasma produced in the HL-2A tokamak is determined by a fast algorithm for identifying plasma boundary. Higher power LH wave (1.5MW) is injected into the diverted plasma with a slightly asymmetric spectrum. Plasma heating by electron Landau interaction results in operation scenarios of preferentially dominant electron heating. Current profile control under the dominant electron heating condition is investigated with numerical simulation. Due to the off-axis driven current, an RS q-profile is formed, and an enhanced confinement regime with steep electron temperature gradient is produced. The clear decrease of the electron thermal conductivity in the vicinity of the shear reversal shows that an electron-ITB is developed. When higher LH power injects into the target plasma that is heated by neutral beam injection, the ion temperature has a large increment in addition to the high increase of electron temperature. The temperature profiles indicate that an enhanced core confinement is established with both ion- ITB and electron-ITB developed.

Particle transport analysis in lower hybrid current drive discharges of JT-60U

Particle transport is modified in lower hybrid current drive discharges of JT-60U. The density profile becomes broad during the lower hybrid wave injection and the profile change depends on the injected wave spectrum. Particle transport coefficients (diffusion coefficient and profile peaking factor) were evaluated using gas-puff modulation experiments. The diffusion coefficient in the current drive discharges is about three times larger than that in the ohmic discharges. The profile peaking factor decreases in the current drive discharges and the evaluated values are consistent with the measured density profiles.

Fast reciprocating probe system for local scrape-off layer measurements in front of the lower hybrid launcher on JT-60U

A fast reciprocating probe system with a long drive shaft was incorporated into a multi-junction lower hybrid (LH) wave launcher on JT-60U in order to investigate an improved coupling mechanism of the radio frequency wave to the core plasma. The system has been operated reliably over a horizontal scan of 25 cm in 1.5 s using a compact pneumatic cylinder drive and springs. A double probe measurement provided the scrape-off layer plasma profile between the last closed flux surface and the first wall with the spatial resolution of 1−2 mm measured with a laser displacement gauge. The profiles of the electron density ne and temperature Te were in good agreement with those obtained with a triple probe method. During the LH wave injection with good coupling to the core plasma, an increase in the local Te was observed in front of the LH launcher mouth. The local ne was (7−10)×1016 m−3, consistent values needed for the good coupling.

Launcher heat load on high power LHCD experiments in JT-60U

Heat loads on a lower hybrid wave launcher as well as on the first wall during high power auxiliary heating and their origin are investigated. It is found that the heat load on the launcher is moderate during injection of lower hybrid wave into a ohmic plasma. However, it is significantly enhanced by the existence of high energy banana-trapped particles and localized near the equatorial plane. In JT-60U, lower hybrid wave can be coupled to the plasma at a launcher-plasma distance of ∼ 20 cm without any significant heat load on the launcher grill. Nevertheless, the heat load due to banana-orbit particles may impose a constraint on the position of a launcher.

Bootstrap current generation by symmetrically injected lower-hybrid waves in a tokamak plasma.

High poloidal beta (\ensuremath{\epsilon}${\mathrm{\ensuremath{\beta}}}_{\mathit{p}}$\ensuremath{\sim}1) plasma equilibria produced by injection of lower-hybrid waves with symmetric spectra (\ensuremath{\Delta}\ensuremath{\varphi}=\ensuremath{\pi} phasing of waveguides) show evidence of substantial non-Ohmic current generation (zero loop voltage). X-ray data indicate that, while up to 30% of the current may be carried by a highly anisotropic electron tail (\ensuremath{\varepsilon}g30 keV), the bootstrap current generated by an enhanced population of intermediate energy electrons (\ensuremath{\varepsilon}\ensuremath{\simeq}10 keV) could account for the remaining 70% of the current.

Combined operation of lower hybrid current drive and heating and neutral beam injection on ASDEX

Current drive and heating with lower hybrid (LH) waves were combined with neutral beam injection (NBI) heating on ASDEX up to total powers of 4.2 MW. The same LH current drive efficiency as in Ohmic target plasmas was obtained. For the energy confinement time, similar parametric dependences on heating power, plasma current and qa value as in plateau-like scaling were found for combined operation of NBI and LH current drive in the L-mode. H-modes were triggered routinely by injection of LH waves during NBI. Peaked current density profiles give better global confinement than broad profiles in combined NBI and LH current drive

Analysis of electron cyclotron emission from nonthermal gas discharges

Abstract Nonthermal ASDEX plasmas created by injection of lower hybrid waves have been analyzed by simulating the observed electron cyclotron emission (ECE) spectra with the help of a two‐component (low‐ and high‐energy) plasma in the perpendicular slab approximation. Numerical simulations confirm the two‐component description of the nonthermal discharges and provide estimates of the population and energy of the suprathermal electrons.

Study of the parametric instabilities in the lower hybrid frequency range in the FT tokamak

An overall view of the phenomenology observed with radiofrequency (RF) probes during lower hybrid wave injection is presented for several operational regimes of the FT tokamak. The main emphasis is on the high plasma density regime where the effects of the launched waves on the main plasma become negligible. A rather rich spectrum from about 0 MHz to beyond the injected frequency is observed in response to the RF pulse. Many secondary peaks, or sidebands, shifted from the pump by multiples of the ion cyclotron frequency evaluated at the outer torus edge appear, while the pump spectrum is strongly broadened. Evidence of the primary importance of the border plasma conditions is recognized. Using a numerical code, the authors satisfactorily reproduce part of the experimental data concerning the decay process giving rise to the sideband. The decay of the pump into a quasi-mode strongly damped on ions is considered, and the convection losses due to the finite extension of the pump are taken into account. The key to the good agreement between the numerical simulations and the experiment is to attribute a prominent role to the plasma edge and to its variation with the main plasma parameters, i.e. density and current.