Electron Cyclotron Heating Experiments Research Articles

Kinetic theory of parametric decay instabilities near the upper hybrid resonance in plasmas

Parametric decay instabilities (PDIs) near the upper hybrid resonance layer are studied with a 1D framework. In a uniform plasma, the kinetic nonlinear dispersion relation of PDI is numerically calculated for parameters corresponding to electron cyclotron heating experiments at the ASDEX-U tokamak, in which O-mode radiation was converted to X-mode radiation by reflection from the high-field sidewall. The forward scattering processes driven by X-mode and linearly converted electron Bernstein waves (EBWs) are investigated and found to lead to a primary PDI where the pump waves decay into lower hybrid waves and sideband EBWs. A frequency shift of 930 MHz is obtained for the sideband EBWs in the primary PDIs. Subsequently, the sideband EBWs can decay into a low-frequency ion Bernstein quasi-mode (IBQM) and a secondary EBW, where the dominant forward scattering channel is the first-order IBQM with a frequency close to twice the ion cyclotron frequency. The decay channels obtained by numerical calculation can explain the characteristics of the signal observed in ASDEX-U experiments. The threshold of the pump electric field strength required to excite the primary PDI in the presence of plasma inhomogeneity is also estimated.

A Radiometer for Plasma Diagnostics in a Magnetic Mirror GDT

A radiometer developed at Budker Institute of Nuclear Physics (Siberian Branch, Russian Academy of Sciences) for diagnostics of the efficiency of the electron cyclotron plasma heating in a mirror magnetic trap (GDT) is discussed. The radiometer is based on the superheterodyne principle. The characteristics of the instrument such as the operating range, sensitivity, frequency and time resolution are adapted to the requirements of the experiments. The receiver is equipped with a system for protection against stray radiation from powerful gyrotrons that are used in the electron cyclotron heating experiments. The radiometric measurements of the plasma self-emission carried out at the GDT facility showed that the new information channel helps to interpret the processes of resonant microwave plasma heating more accurately.

Tripolar vortices in ion-temperature-gradient mode with non-Maxwellian electrons in an inhomogeneous magnetoplasma

We consider a low frequency plasma comprising of Kappa distributed electrons and Maxwellian ions embedded in an external magnetic field in toroidal ion-temperature-gradient driven modes. A set of nonlinear equations are derived in the presence of equilibrium density, temperature, and magnetic field gradients. In the nonlinear regime, solutions in the form of tripolar vortices are derived by using Braginskii’s transport equations. It has been observed that the scale lengths over which the nonlinear vortex structures form get modified in the presence of Kappa distributed electrons. In tokamak the present study is applicable where non-Maxwellian population has been observed in electron cyclotron heating experiments and resonant frequency heating.

Observation of the inward propagation of spontaneous toroidal flow from the plasma boundary in LHD

Spontaneous generation of toroidal flow from the separatrix and its inward radial propagation in association with the change in the electron temperature gradient have been observed near the plasma boundary in the modulated electron cyclotron heating (MECH) experiment in the Large Helical Device. The observations presented in this paper provide strongly the supports of the hypothesis for the conversion of the poloidal flow to the toroidal one at the plasma peripheral region, exhibiting a clear delay response on MECH in the time derivation of the toroidal flow (V̇ϕ≡∂Vϕ/∂t) to the poloidal one (V̇θ≡∂Vθ/∂t) at a normalized radius of ρ≈ 0.97. The ratio of order unity for |V̇ϕ/V̇θ| during a conversion phase from poloidal flow into toroidal one is consistent with that predicted by a quasi-stationary theoretical model based on the turbulent instability, regardless of its sign and spatial structure. The present work demonstrates a new dynamic response in the plasma momentum transport and represents a significant confirmation of its non-local nature.

Development and application of a ray-tracing code integrating with 3D equilibrium mapping in LHD ECH experiments

The central electron temperature has successfully reached up to 7.5 keV in large helical device (LHD) plasmas with a central high-ion temperature of 5 keV and a central electron density of m−3. This result was obtained by heating with a newly-installed 154 GHz gyrotron and also the optimisation of injection geometry in electron cyclotron heating (ECH). The optimisation was carried out by using the ray-tracing code ‘LHDGauss’, which was upgraded to include the rapid post-processing three-dimensional (3D) equilibrium mapping obtained from experiments. For ray-tracing calculations, LHDGauss can automatically read the relevant data registered in the LHD database after a discharge, such as ECH injection settings (e.g. Gaussian beam parameters, target positions, polarisation and ECH power) and Thomson scattering diagnostic data along with the 3D equilibrium mapping data. The equilibrium map of the electron density and temperature profiles are then extrapolated into the region outside the last closed flux surface. Mode purity, or the ratio between the ordinary mode and the extraordinary mode, is obtained by calculating the 1D full-wave equation along the direction of the rays from the antenna to the absorption target point. Using the virtual magnetic flux surfaces, the effects of the modelled density profiles and the magnetic shear at the peripheral region with a given polarisation are taken into account. Power deposition profiles calculated for each Thomson scattering measurement timing are registered in the LHD database. The adjustment of the injection settings for the desired deposition profile from the feedback provided on a shot-by-shot basis resulted in an effective experimental procedure.

Toroidal ion-temperature-gradient driven vortices in an inhomogeneous magnetoplasma with non-Maxwellian electrons

Nonlinear equations which govern the dynamics of low-frequency toroidal ion-temperature-gradient driven modes (i.e., ω≪ωci, where ωci is the ion gyro-frequency) are derived in the presence of equilibrium density, temperature, and magnetic field gradients. In the nonlinear case, solutions in the form of dipolar vortices and vortex street are presented for a plasma comprising of Maxwellian ions and nonthermal electrons that are embedded in an external magnetic field. By using Braginskii's transport equations for the Maxwellian ions and Kappa distributed electrons, the coupled mode equations for the system under consideration are derived. The results have been applied in Tokamak plasmas, and it has been observed that the scale lengths over which the nonlinear vortex structures form get modified in the presence of Kappa distributed electrons. The present study is also applicable to tokamaks and stellarators where non-Maxwellian population has been observed in resonant frequency heating, electron cyclotron heating experiments, and in runaway electrons.

Soft x-ray intensity profile measurements of electron cyclotron heated plasmas using semiconductor detector arrays in GAMMA 10 tandem mirror.

Temporally and spatially resolved soft x-ray analyses of electron cyclotron heated plasmas are carried out by using semiconductor detector arrays in the GAMMA 10 tandem mirror. The detector array has 16-channel for the measurements of plasma x-ray profiles so as to make x-ray tomographic reconstructions. The characteristics of the detector array make it possible to obtain spatially resolved plasma electron temperatures down to a few tens eV and investigate various magnetohydrodynamic activities. High power electron cyclotron heating experiment for the central-cell region in GAMMA 10 has been started in order to reduce the electron drag by increasing the electron temperature.

Higher Harmonics in the Perturbative Transport Study in TJ-II ECH Plasma

We analyzed the higher harmonics of temperature perturbation in the modulated electron cyclotron heating experiment on TJ-II plasma. The higher harmonics (e.g., 5th harmonic) exhibited significantly weaker decay in amplitude as they propagated in radius as compared with the prediction by diffusive model. The change in the time derivative of temperature at the onset (and turning-off) of the heating power propagates in radius with very little temporal smoothening.

High Power ECH Experiments for Direct Bulk Electron Heating in Central-Cell Region Plasmas on GAMMA 10 Tandem Mirror

High power electron cyclotron heating (ECH) experiment for the central-cell region in GAMMA 10 has been started in the hot ion mode operation (Ti = several keV). Transmission system for the central-cell ECH, including a taper, mirrors and polarizers, has been designed and installed for efficient direct bulk electron heating. ECH system with a twister and a circular polarizer for central-cell region has the advantages for controllable polarization of incident wave. Recently, a high power gyrotron of ~500 kW at 28 GHz has been developed in collaboration with Japan Atomic Energy Agency (JAEA) and this high power gyrotron was installed in central-cell region of GAMMA 10. This high power and efficient ECH system was applied to central-cell electron heating and significant increment of soft X-ray and diamagnetism signals during a pulse of ECH operation was obtained. In this paper, central ECH experimental results in the hot ion mode operation are reported.

Status and Plan of GAMMA 10 Tandem Mirror Program

Recent progress and near future plan of GAMMA 10 efforts are presented. With high power plug electron cyclotron heating (ECH) up to ~400 kW, the ion confining potential of more than 2 kV was confirmed. The drift type low frequency fluctuations were suppressed by the positive radial electric field produced by plug ECH. It is found that the efficient EC heating on mirror devices from a strong B field side requires the minimization of the stray microwave in addition to the 100% X-mode excitation to avoid the enhancement of the ion loss. The development of a gyrotron, the key tool of these ECH experiments, has been made in collaboration with NIFS (National Institute for Fusion Science), More than 1.5 MW for more than 1s has been demonstrated at 77GHz. The plan of the boundary plasma research program with modification of GAMMA 10 is in progress. The new program includes the physics and technology studies of the divertor and SOL plasmas and PWI relevant to torus plasmas like ITER. The high heat flux experiments using the open end mirror throat has been started and we have obtained successful preliminary data, which include the heat flux of 8 MW/m2.

Operating experience on six 110 GHz, 1 MW gyrotrons for ECH applications

Since 1999, CPI has shipped six, 110 GHz, 1 MW, 5 s pulsed gyrotrons to General Atomics (GA) for use in electron cyclotron heating experiments on the DIII-D tokamak. As a result of extensive testing of these gyrotrons at CPI and GA, a wealth of information has been obtained regarding the long-term performance of the original electrical and mechanical design of the tubes. All of the gyrotrons have achieved long-pulse operation and five of the six gyrotrons achieved 1 MW output power levels with the nominal beam voltage and current of 80 kV and 40 A, respectively. Experience with the electron guns and interaction cavities has been quite good, though the low efficiency experienced by one gyrotron may be attributed to a gun or cavity irregularity. The internal converters employed in the gyrotrons have produced high-quality Gaussian output beams with internal diffractions losses of 5–6.5%. After several years of operation, the first three 110 GHz gyrotrons experienced collector failures due to cyclic fatigue. Improved analyses and diagnostics have been employed to avoid these problems in the future. Two problems have been experienced with the diamond output windows. These problems have been eliminated by changing braze techniques and procedures. Based on the experience obtained on these six gyrotrons, recommendations can be made for future gyrotron development work.

Study of Efficient Electron Cyclotron Wave Coupling with Grooved Mirror Polarizer in the GAMMA 10 Tandem Mirror

An improvement of electron cyclotron heating (ECH) system in the central cell of the GAMMA 10 tandem mirror is carried out by the use of two grooved mirrors. Two grooved mirrors consist of a twister and a circular polarizer. In case of the ECH system in the central cell region, fundamental ECH is examined and launched microwave beam from the antenna propagates through the two parabolic mirrors. The incident microwave is radiated from the second mirror at a position with magnetic field intensity B=2.0 T and with an incident angle of 35.1 degrees. Two grooved mirrors can generate suitable elliptical polarizations for both of ordinary mode (O-mode) and extraordinary mode (X-mode) in oblique launch with a slanted magnetic field line. The first results of the polarization control of the incident wave for high power ECH experiments are reported.

Compact antenna for two-dimensional beam scan in the JT-60U electron cyclotron heating/current drive system

A compact antenna system was designed and fabricated to enable millimeter-wave beam scanning in the toroidal and poloidal directions of the JT-60U tokamak for electron cyclotron heating (ECH) and electron cyclotron current drive (ECCD) experiments. The antenna consists of a fast movable flat mirror mounted on the tokamak vacuum vessel and a rotary focusing mirror attached at the end of the waveguide that is supported from outside the vacuum vessel. This separate support concept enables a compact structure inside a shallow port (0.68×0.54×0.2m) that is shared with a subport for an independent diagnostic system. During a plasma shot, the flat mirror is driven by a servomotor with a 3-m-long drive shaft to reduce the influence of the high magnetic field on the motor. The focusing mirror is rotated by a simple mechanism utilizing a push rod and an air cylinder. The antenna has been operated reliably for 3 years after a small improvement to the rotary drive mechanism. It has made significant contributions to ECH and ECCD experiments, especially the current profile control in JT-60U.

Heat Pinches in Electron-Heated Tokamak Plasmas: Theoretical Turbulence Models versus Experiments

Two fluid turbulence models, the drift wave based quasilinear 1.5D Weiland model and the electromagnetic global 3D nonlinear model CUTIE, have been used to account for heat pinch evidence in off-axis modulated electron cyclotron heating experiments in the Rijnhuizen Tokamak Project. Both models reproduce the main features indicating inward heat convection in mildly off-axis cases. In far-off-axis cases with hollow electron temperature profiles, the existence of outward convection was reproduced only by CUTIE. Turbulence mechanisms driving heat convection in the two models are discussed.

Understanding sawtooth activity during intense electroncyclotron heating experiments on TCV

Different types of central relaxation oscillations areobserved in the presence of ECH dependingon the location of the deposited power. In the TCV tokamak, normal sawteeth, i.e. triangular sawteeth similar toohmic sawteeth, and saturated sawteeth are observed with central ECHpower deposition, while giant sawteeth and `humpback'oscillations occur when heating close to the sawtooth inversionsurface of the local soft X ray emissivity. New measurements with hightemporal resolution show that the crash phase of these sawtooth typesis accompanied by a reconnection process associated with an m/n = 1 resistiveinternal kink mode. After the sawtooth crash, full magneticreconnection is observed in normal and in saturated sawteeth, whilefor giant and humpback sawteeth the reconnection process is incompleteand poloidally asymmetric temperature profiles persist after the crash. The detailed dynamics of the magnetic island associatedwith the resistive internal kink mode are described by a displacementfunction which is inferred from the experimental data. In normalsawteeth, the kink mode is destabilized just before the crash, whilein all other sawtooth types a magnetic island exists for a significantfraction of the sawtooth period. The different types of sawteeth havebeen simulated using a numerical code based on a theoretical modelwhich describes the evolution of the electron temperature in thepresence of localized heat sources and of a magnetic m/n = 1/1 island.

The Temperature in the RTP Tokamak under Feedback Control by Regulating the Output of a 110-GHz Gyrotron

In addition to wave properties like polarization, harmonic number, high- or low-field side launching, and launch angle, absorption of electron cyclotron waves depends strongly on plasma parameters, such as the temperature. Therefore, we have introduced a new way of operating our 110-GHz gyrotron for electron cyclotron heating and electron cyclotron current-drive experiments. In the Rijnhuizen Tokamak Project (RTP), a system for feedback control of the plasma temperature by regulating the output of the gyrotron has come into operation recently. Other parameters, i.e., the Shafranov parameter λ = βpol + li/2 - 1, and the loop voltage can be under feedback control as well. The control system is described and some test results are given.

Perturbative tests of theoretical transport models using cold pulse and modulated electron cyclotron heating experiments

It is difficult to discriminate between various tokamak transport models using standardized statistical measures to assess the goodness of fit with steady-state density and temperature profiles in tokamaks. This motivates consideration of transient transport experiments as a technique for testing the temporal response predicted by models. Results are presented comparing the predictions from the Institute for Fusion Studies—Princeton Plasma Physics Laboratory (IFS/PPPL), gyro-Landau-fluid (GLF23), Multi-mode (MM), Current Diffusive Ballooning Mode (CDBM), and Mixed-shear (MS) transport models against data from ohmic cold pulse and modulated electron cyclotron heating (ECH) experiments. In ohmically heated discharges with rapid edge cooling due to trace impurity injection, it is found that critical gradient models containing a strong temperature ratio (Ti/Te) dependence can exhibit behavior that is qualitatively consistent both spatially and temporally with experimental observation while depending solely on local parameters. On the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)], off-axis modulated ECH experiments have been conducted in L-mode (low confinement mode) and the perturbed electron and ion temperature response to multiple heat pulses has been measured across the plasma core. Comparing the predicted Fourier phase of the temperature perturbations, it is found that no single model yielded agreement with both electron and ion phases for all cases. In general, it was found that the IFS/PPPL, GLF23, and MS models agreed well with the ion response, but not with the electron response. The CDBM and MM models agreed well with the electron response, but not with the ion response. For both types of transient experiments, temperature coupling between the electron and ion transport is found to be an essential feature needed in the models for reproducing the observed perturbative response.

High power density electron cyclotron experiments in the L2M stellarator

The results of electron cyclotron heating (ECH) experiments in the L-2M stellarator are presented. The main goal of the experiments is to investigate the physics of ECH and of plasma confinement at high values of the volume heating power density. A current free plasma is produced and heated by extraordinary waves at the second harmonic of the electron cyclotron frequency ( omega 0=2 omega ce). The experimental results are compared with the empirical LHD scaling and with numerical simulations of plasma confinement and heating processes based on neoclassical theory using the full matrix of transport coefficients, including some additional anomalous corrections

Transition phenomena observed during edge electron cyclotron heating in the Heliotron-E helical device

Abstract An edge electron cyclotron heating (ECH) experiment has been performed on the Heliotron-E helical device using 35 GHz and 53 GHz gyrotrons. Transition phenomena have been observed during the 53 GHz edge ECH. The Hα emission and the outflux to the wall dropped, and then the core electron density increased by 20%–60%. The electron stored energy seems to be improved up to 20% after the transition. This phenomenon is found to depend strongly on the edge heating power, the target plasma density and the limiter position. A possible mechanism is discussed in terms of high energy electrons produced by the edge ECH.

106 GHz electron cyclotron heating experiment on Heliotron-E

Abstract A new electron cyclotron heating (ECH) system has been installed on the Heliotron-E helical device. The gyrotron output 106 GHz TE 12,2 mode is converted into a gaussian beam by the quasi-optical Vlasov convertor. The gaussian beam is coupled to the HE 11 waveguide mode, then transmitted by the corrugated waveguides. The transmitted beam is well focused and launched to the Heliotron-E device. Plasma has been produced and heated by this 106 GHz ECH. The second harmonic breakdown was investigated and compared with the fundamental harmonic breakdown. The improvement of the particle confinement was observed during 106 GHz second harmonic ECH. Compared with the 53 GHz fundamental ECH phase, the averaged electron density was increased by 20% without gas puffing. The outflux to the divertor plate was reduced.