Cyclotron Resonance Heating Experiments Research Articles

Broadening of the power deposition profile in on-axis X2 ECRH experiments in the L-2M stellarator and the low-threshold parametric decay instability of the pump wave

The effect of considerable broadening of the power deposition profile in on-axis X2 electron cyclotron resonance heating experiments observed in the L-2M stellarator is interpreted in the paper as one of the consequences of the low-threshold parametric decay instability of the pump wave, resulting in excitation of ion and electron Bernstein daughter waves. The instability is considered under the specific L-2M experimental conditions and shown to be absolute. The threshold of this instability is exceeded in the experiment. This finally allows explaining, at least qualitatively, the effect of nonlocal microwave power deposition detected in L-2M.

Triple Langmuir probe calibration in TOMAS ECRH plasma

In the TOMAS device, a triple Langmuir probe is used to measure the electron temperature and density. The accuracy of this measurement depends on correct determination of the effective collecting area of the probe, which depends on complex plasma transport processes. The probe can be calibrated by electron cyclotron resonance heating experiments using the cut-off density of the ordinary wave (O-wave). This threshold only depends on the frequency of the injected wave, and the occurrence of this phenomenon is clearly visible in the temperature evolution. The value of density is consequently known at this point and can be used to calibrate the density measurements of the triple Langmuir probe.

Low-Threshold Induced-Side-Scattering Instability at the Tokamak Edge Transport Barrier Predicted in O-Mode Electron Cyclotron Resonance Heating Experiments

The trapping of a lower hybrid wave in the tokamak edge transport barrier is predicted, reducing by 3 orders of magnitude the excitation threshold for the absolute parametric decay instability that leads to side scattering of the ordinary microwave pump in electron cyclotron resonance heating (ECRH) experiments. This process is similar to the stimulated Raman scattering instability in laser physics and can result in substantial anomalous scattering of the pump wave, like in laser fusion experiments. The corresponding broadening of the ECRH power deposition profile can reduce the ability of this method to control the neoclassical tearing modes both in present day machines, as ASDEX-Upgrade, where the theory can be checked, and in fusion reactors such as ITER and DEMO.

Characterization of injection and confinement improvement through impurity induced profile modifications on the Wendelstein 7-X stellarator

Pulsed injections of boron carbide granules into Wendelstein 7-X stellarator (W7-X) plasmas transiently increase the plasma stored energy and core ion temperatures above the reference W7-X experimental programs by up to 30%. In a series of 4 MW electron cyclotron resonance heating experiments, the PPPL Probe Mounted Powder Injector provided 50 ms bursts of 100 μm granules every 350 ms at estimated quantities ranging from approximately 1 mg/pulse to over 30 mg/pulse. For each injection, the stored energy was observed to initially drop and the radiated power transiently increased, while the radial electron density profile rose at the edge as material was assimilated. Once the injected boron carbide was fully absorbed, the density rise transitioned to the core while the stored energy increased above the previous baseline level by an amount linearly correlated with the injection quantity. During the injection, the ion temperature gradient steepened with peak core ion temperatures observed to increase from a nominal 1.7 keV to over 2.6 keV for the largest injection amounts. Enhanced performance is accompanied by a reversal of the radial electric field at ρ < 0.3, indicating that the core transport has switched to the ion root. These observations are suggestive of a change in transport and provide further evidence that externally induced profile modifications provide a possible path to enhanced W7-X performance metrics.

First spatially resolved measurements of the D-3He α-particle source with the upgraded JET gamma-ray camera.

The Joint European Torus (JET) gamma-ray camera has been recently upgraded with the installation of new gamma-ray detectors, based on LaBr3(Ce) scintillation crystals, which add spectroscopic capability to the existing system allowing measurements with good energy resolution (5% at 0.622MeV), a dynamic range from hundreds of keV up to about 30MeV, and high counting rate capabilities of MCps. First gamma-ray measurements during the C38 campaign of the JET have been successfully carried out, in particular, in D-3He plasmas from three-ion ion cyclotron resonance heating experiments, where the detection of 16.4MeV γ-rays from D + 3He → γ + 5Li reactions with the gamma-ray camera upgrade allowed determining the spatial profile of alpha particles born in D + 3He fusion reactions.

Density Region of Low-Threshold Parametric Decay Instabilities in Electron Cyclotron Resonance Heating Experiments in Tokamak

The density conditions under which low-threshold parametric decay instabilities occur are analyzed by a physical model. Calculations show that low-threshold parametric decay instabilities occur in the density region. Magnetic field effects on the density region are analyzed. The maximum width of the density region is found. The plasma temperature has little effect on the density region. In order to avoid the low-threshold parametric decay instabilities, the daughter wave’s frequency must be far from the UH frequency inside magnetic island. It’s the key to control the magnetic field inside the magnetic island.

On possibility of strong anomalous damping of an ordinary pump wave in the electron cyclotron resonance heating experiments due to the low-power-threshold parametric decay instability

We analyze the low power-threshold parametric decay instability of an ordinary wave in the first electron cyclotron harmonic frequency range, which leads to the excitation of a localized upper hybrid (UH) wave and a lower hybrid wave. We investigate its transition into the saturation mode due to both the pump wave depletion and the cascade of successive secondary decays. A set of equations describing this nonlinear frequency down-conversion of the primary UH wave is derived and then solved both analytically and numerically under the conditions typical of the off-axis O1-mode electron cyclotron resonance heating experiments in the FTU tokamak. The possibility of a strong anomalous absorption of the pump power in the case of even-step cascade of secondary decays is discovered.

Reduction of anomalous absorption in the extraordinary mode second harmonic electron cyclotron resonance heating experiments

The possibility of suppressing low-power-threshold parametric decay instabilities excited at plasma auxiliary heating with an extraordinary wave at the second harmonic of the electron cyclotron resonance and reducing the anomalous absorption associated with this instability is considered. It is theoretically demonstrated that increasing the microwave pump beam width and its power allows reducing the related anomalous absorption rate.

Numerical investigations of parametric decay into trapped waves in magnetized plasmas with a non-monotonic density background

Parametric decay instabilities (PDIs) exciting daughter waves trapped inside a magnetized plasma with a non-monotonic density profile are investigated numerically. The investigation is motivated in particular by observations of low threshold PDI signatures during second harmonic electron cyclotron resonance heating experiments in magnetically confined fusion experiments. We use the particle-in-cell code EPOCH to study conversion of a fast X-mode pump wave into a combination of half frequency X-mode and electron Bernstein waves and identify two regimes where PDIs can excite trapped electrostatic waves. Above the second harmonic upper hybrid (UH) density, a PDI known also as a two plasmon decay (TPD) instability excites a pair of UH waves that we locate in frequency and wavenumber space. At lower densities, a PDI known as stimulated Raman scattering may produce one trapped and one returning X-mode daughter wave with a much slower growth rate than the TPD instability. In both cases, we show that the frequency separation of the daughter waves depends on the density in a predictable manner. With little loss from the decay region, the trapped daughter waves become unstable with respect to secondary parametric instabilities (PIs), leading to distinctly different phases of the UH spectrum. Unlike the primary instability, the secondary PIs are shown to depend on ion dynamics. Furthermore, we observe escaping waves near the 3/2 pump frequency resulting from tertiary PIs in agreement with recently proposed backscattering during magnetically confined fusion experiments.

Possible transition from modest to strong anomalous absorption in X2-mode electron cyclotron resonance heating experiments in toroidal devices

We analyze both analytically and numerically, two cases of low-power-threshold two-upper hybrid-plasmon parametric decay instability saturation when the odd- or even-step cascade of secondary decays is possible. We have found that giant anomalous absorption (more than 80%) of pump power leads to its depletion in the regime when the instability is saturated due to the even-step cascade of decays of primary upper hybrid waves. This can make the power deposition profile different from that predicted by linear theory and provides an alternative explanation for the evident broadening observed under electron cyclotron resonance heating in toroidal devices.

Possibility of strong anomalous absorption of microwaves in electron cyclotron resonance heating experiments in fusion devices

The effect of strong anomalous absorption (up to three-quarters of the pump power) of microwaves in electron cyclotron resonance heating (ECRH) experiments in toroidal devices is predicted. It originates from the low-power-threshold two-plasmon parametric decay instability, for which the only effective saturation mechanism is provided by the pump wave depletion. This makes the power deposition profile different from that predicted by the linear theory and provides an alternative interpretation for its evident broadening observed on ECRH in toroidal devices.

Tokamak research at the Ioffe Institute

Recent research at three small tokamaks with different parameters located at the Ioffe Institute—the spherical tokamak Globus-M, the large aspect ratio tokamak FT-2 and the compact tokamak TUMAN-3M—are reviewed. This overview covers energy confinement (Globus-M and FT-2), L–H transition (TUMAN-3M and FT-2), Alfvén waves (Globus-M and TUMAN-3M), ion cyclotron emission (TUMAN-3M), major plasma discharge disruption (Globus-M) and scrape-off layer (Globus-M) studies. A full-f global gyrokinetic modeling benchmark using synthetic diagnostics in FT-2 is described. Anomalous absorption and emission in electron cyclotron resonance heating experiments due to the parametric excitation of localized upper hybrid waves are analyzed theoretically. Progress in the development of the neutral particle analysis, gamma-ray spectrometry and divertor Thomson scattering combined with laser-induced fluorescence diagnostics for ITER is discussed. The status of the new Globus-M2 spherical tokamak is reported.

Low-threshold absolute parametric decay instability in the monotonous density profile region in O1-mode ECRH experimental conditions

The possibility of the low-threshold absolute parametric decay instability excitation by the ordinary wave in the presence of a monotonous plasma density profile is analyzed. It is shown that in the cases of daughter upper hybrid and ion Bernstein waves the energy circulation between two nearby decay points can lead to an absolute instability at the pump power of less than half a megawatt. The instability saturation due to the ion Bernstein wave amplitude-dependent stochastic damping effect is considered and the rate of anomalous pump absorption is estimated. This scenario can explain the observations of anomalous scattering in the O1-mode electron cyclotron resonance heating (ECRH) experiments at the FTU (Frascati Tokamak Upgrade) tokamak in the presence of a monotonous density profile.

Saturation of the low-threshold parametric decay of the O-mode pump in the fundamental harmonic ECRH experiment

The saturation regime of the low-power-threshold ordinary (O) wave parametric decay instability leading to the excitation of a localized upper hybrid (UH) wave and a low-hybrid wave is analysed. The cascade of secondary decays of a daughter UH wave to a localized UH wave and ion Bernstein wave is considered as presumably the most important nonlinear mechanism responsible for the transition to a saturation regime. A set of equations describing this nonlinear frequency down-conversion is derived and then solved both analytically and numerically under the conditions typical of the on-axis fundamental harmonic ordinary mode (O1-mode) electron cyclotron resonance heating experiments at the FTU tokamak.

Analysis of higher harmonics on bidirectional heat pulse propagation experiment in helical and tokamak plasmas

In this contribution we analyze modulation electron cyclotron resonance heating (MECH) experiment and discuss higher harmonic frequency dependence of transport coefficients. We use the bidirectional heat pulse propagation method, in which both inward propagating heat pulse and outward propagating heat pulse are analyzed at a radial range, in order to distinguish frequency dependence of transport coefficients due to hysteresis from that due to other reasons, such as radially dependent transport coefficients, a finite damping term, or boundary effects. The method is applied to MECH experiments performed in various helical and tokamak devices, i.e. Large Helical Device (LHD), TJ-II, Korea Superconducting Tokamak Advanced Research (KSTAR), and Doublet III-D (DIII-D) with different plasma conditions. The frequency dependence of transport coefficients are clearly observed, showing a possibility of existence of transport hysteresis in flux-gradient relation.

Theory of anomalous backscattering in second harmonic X-mode ECRH experiments

A quantitative model explaining generation of the anomalous backscattering signal in the second harmonic X-mode electron cyclotron resonance heating (ECRH) experiments at TEXTOR tokamak as a secondary nonlinear process which accompanies a primary low-threshold parametric decay instability (PDI) leading to excitation of two—upper hybrid (UH)—plasmons trapped in plasma is developed. The primary absolute PDI enhancing the UH wave fluctuations from the thermal noise level is supposed to be saturated due to a cascade of secondary low-threshold decays of the daughter UH wave leading to excitation of the secondary UH waves down-shifted in frequency and the ion Bernstein wave. A set of equations describing the cascade is derived and solved numerically. The results of numerical modelling are shown to be in agreement with the analytical estimations of the growth rate of the initial and secondary parametric decays and the saturation level. The generation of backscattering signal is explained by coupling of the daughter UH waves. The fine details of the frequency spectrum of the anomalously reflected extraordinary wave and the absolute value of the observed backscattering signal in the second harmonic X-mode ECRH experiments at TEXTOR are reproduced.

Fusion Research in Ioffe Institute

In this paper we present the fusion related activities of the Plasma Physics Division at the Ioffe Institute. The first experiments on lower hybrid current drive (LHCD) in a spherical tokamak performed at the Globus-M tokamak (R = 0.36 m, a = 0.24 m, Bt = 0.4 T, Ip = 200 kA) with a novel poloidally oriented grill resulted in an RF driven current of up to 30 kA at (100 kW, 2.5 GHz), exceeding the modelling predictions. At the FT-2 tokamak (R = 0.56 m, a = 0.08 m, Bt = 3 T, Ip = 30 kA) experiments with a traditional toroidally oriented grill revealed no strong dependence of the LHCD density limit on the H/D ratio in spite of LH resonance densities differing by a factor of 3. Microwave Doppler reflectometry (DR) at the Globus-M, and DR and heavy ion beam probe measurements at the tokamak TUMAN-3M (R = 0.53 m, a = 0.24 m, Bt = 1.0 T, Ip = 190 kA) demonstrated geodesic acoustic mode (GAM) suppression at the L to H transition. Observations at FT-2 using Doppler Enhanced Scattering showed that the GAM amplitude is anti-correlated both spatially and temporally to the drift turbulence level and electron thermal diffusivity. For the first time turbulence amplitude modulation at the GAM frequency was found both experimentally and in global gyrokinetic modelling. A model of the L–H transition is proposed based on this effect. The loss mechanisms of energetic ions' (EI) were investigated in the neutral beam injection (NBI) experiments on Globus-M and TUMAN-3M. Empirical scaling of the 2.45 MeV DD neutron rate for the two devices shows a strong dependence on toroidal field and plasma current justifying the Bt and Ip increase by a factor of 2.5 for the proposed upgrade of Globus-M. Bursts of ∼1 MHz Alfvenic type oscillations correlating with sawtooth crashes were observed in ohmic TUMAN-3M discharges. The possibility of low threshold parametric excitation of Bernstein and upper hybrid waves trapped in drift-wave eddies resulting in anomalous absorption in electron cyclotron resonance heating (ECRH) experiments in toroidal plasmas was identified theoretically. A novel method of radial correlation Doppler reflectometry is shown to be capable of measuring the turbulence wave-number spectrum in realistic 2D geometry. On the progress in design and fabrication of three diagnostics for ITER developed in the Ioffe institute is reported: neutral particle analysis, divertor Thomson scattering and gamma spectroscopy.

Low-threshold absolute two-plasmon decay instability in the second harmonic electron cyclotron resonance heating experiments in toroidal devices

The effect of the X-mode parametric decay into two short wavelength upper hybrid (UH) plasmons propagating in opposite directions is analyzed. Due to the huge convective power loss of both the UH plasmons along the inhomogeneity direction, the power threshold of the convective parametric decay instability (PDI), which can be excited in the presence of a monotonous density profile is derived to exceed the gyrotron power range currently available. In the presence of the magnetic island possessing the local density maximum at its O-point the daughter UH plasmons can be trapped in the radial direction that suppresses their energy loss from the decay layer in full and makes the power threshold of the convective two-plasmon PDI drastically (three orders of magnitude) lower than in the previous case. The possibility of the absolute PDI being due to the finite size of the pump beam spot is demonstrated as well. The power threshold of the absolute instability is shown to be more than two orders of magnitude lower than the threshold of the convective instability at the monotonous density profile.

On the possibility of reducing the instability threshold of a parametric decay of an extraordinary wave into two upper hybrid waves in an inhomogeneous plasma

A parametric decay instability (PDI) of an extraordinary wave leading to excitation of two upper hybrid (UH) plasmons at frequencies close to half the pump wave frequency is analyzed. It is shown that the two-plasmon PDI power threshold can be significantly reduced under conditions of electron cyclotron resonance heating (ECRH) experiments in toroidal magnetic devices, where the plasma density profile is often nonmonotonic, which leads to the localization of UH waves.

Observation of a new high-β and high-density state of a magnetospheric plasma in RT-1

A new high-β and high-density state is reported for a plasma confined in a laboratory magnetosphere. In order to expand the parameter regime of an electron cyclotron resonance heating experiment, the 8.2 GHz microwave power of the Ring Trap 1 device has been upgraded with the installation of a new waveguide system. The rated input power launched from a klystron was increased from 25 to 50 kW, which enabled the more stable formation of a hot-electron high-β plasma. The diamagnetic signal (the averaged value of four magnetic loops signals) of a plasma reached 5.2 mWb. According to a two-dimensional Grad-Shafranov analysis, the corresponding local β value is close to 100%.