Electron 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.

Design and experimental results of a 28 GHz, 400 kW gyrotron for electron cyclotron resonance heating

A high-power 28 GHz gyrotron has been successfully developed at the Institute of Applied Electronics, China Academy of Engineering Physics. This gyrotron was designed for electron cyclotron resonance heating (ECRH) in the spherical tokamak XL-50. A diode magnetron injection gun was designed to produce the required gyrating electron beam. The gyrotron operates in the TE8,3 mode in a cylindrical open cavity. An internal quasi-optical mode converter was designed to convert the operating mode into a fundamental Gaussian wave beam and separate the spent electron beam from the outgoing microwave power. A tube has been built and successfully tested. The operational frequency of the tube is 28.1 GHz. For beam parameters at an accelerating voltage of 71 kV and beam current of 16 A, the gyrotron has delivered an output power of 400 kW, with a pulse length of 5 s. The output efficiency is about 50% with a single-stage depressed collector. The gyrotron has been installed on the XL-50 and has played an important role in the ECRH experiments.

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.

Methods for Reducing Anomalous Losses in ECRH Experiments at Second Resonance Harmonic

An approach is analyzed that makes it possible to reduce anomalous absorption in experiments on electron cyclotron resonance heating (ECRH) at the second harmonic of the electron cyclotron resonance. The anomalous absorption is associated with excitation of the low-threshold parametric decay instability of the extraordinary pump wave at the local maximum of the nonmonotonic density profile. The general case is considered, which corresponds to nonlinear excitation of only one localized daughter upper-hybrid wave occurring as a result of the primary decay process. It is shown that, due to rather low instability threshold, it could hardly be completely suppressed in ECRH experiments using megawatt microwave beams. However, an increase in the radius of the pump wave cross section can considerably reduce the corresponding anomalous absorption.

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.

Observation of second harmonic electron cyclotron resonance heating and current-drive transition during non-inductive plasma start-up experiment in QUEST

Noninductive plasma current start-up using 2nd harmonic electron cyclotron resonance heating (ECRH) with oblique radio frequency (RF) injection is demonstrated in a Q-shu University experiment with steady-state spherical tokamak. A strong transition was observed in the heating and plasma current ramp-up. The initial bulk electron heating regime exhibits T ebulk ∼ 140 eV and no hard x-ray (HXR) emission with a low I p of ∼15 kA; it abruptly transitions to a regime that exhibits a low T ebulk of ∼10 eV and a strong HXR emission with a high I p of ∼50 kA. This behavior is distinctly different from that observed in previous fundamental ECRH experiments. The mechanism of the heating and current drive transition are investigated considering wave power absorption and plasma power balance. The results indicate that the transition is caused by the favorable heating of tail electrons where the RF power absorption at the 2nd harmonic increases nearly linearly with T etail, while the power transfer from the tail electrons to the bulk electrons decreases with 1/T etail 0.5. This causes a rapid transition to a state with high T etail while reducing T ebulk towards colder ion temperature. The understanding of the transition mechanism helps to consider plasma current start-up using 2nd harmonic ECRH for tokamak reactors such as JT-60 SA and ITER.

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.

X-mode beam broadening in turbulent plasma

It is shown that plasma turbulence at the edge of a fusion machine can lead to significant broadening of an X-mode microwave beam, which also impacts the performance of the heating systems and diagnostics. Based on a similar method developed for O-mode polarization, the expression for the mean microwave power distribution after the beam passes through the plasma edge turbulence is obtained, and its spatial and probabilistic distributions are analyzed. As this polarization is used for collective Thomson scattering (CTS) on ITER and for electron cyclotron resonance heating (ECRH) on ASDEX Upgrade, numerical simulations of beam broadening due to the edge turbulence are performed for the two conditions of plasma parameters, related to the planned CTS experiments on ITER and to ECRH experiments on ASDEX Upgrade. Ideas to reduce the beam broadening induced by the edge turbulence in the CTS case have been tested and are summarized in the conclusion.

Thermographic reconstruction of heat load on the first wall of Wendelstein 7-X due to ECRH shine-through power

Electron cyclotron resonance heating (ECRH) is a powerful and flexible plasma heating technique that serves as the main heater at Wendelstein 7-X (W7-X) and will be used at ITER for start-up, heating, current drive and mitigation of plasma instabilities. In the case of poor or degraded microwave absorption, which is expected in the O2-mode heating scenario, a significant part of the beam directly hits the wall, leading to local overheating and potential damage. The ECRH shine-through power is mostly reflected onto the targets; only a small fraction is really absorbed through ohmic losses (typically 3% for graphite at 140 GHz). The ohmic losses do not only depend on the material properties and the frequency, but also on the polarization of the wave and the angle of incidence. This paper presents a thermographic analysis of ECRH experiments at W7-X, including heat load and temperature simulations of the first wall that include ECRH shine through. Two O-mode ECRH experiments with both a high temperature rise of the first wall and different angles of beam incidence on the wall’s surface are depicted. One experiment has 775 kW of power modulation (5 Hz) with mixed polarization (45% O-mode, 55% X-mode) and an EC beam angle almost normal to the first wall. The second has 550 kW of steady EC power with O-mode polarization, a shallow beam angle and increased power absorption by the material. It is shown that infrared thermography is a useful tool for measuring shine-through power and protecting wall components.

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.

Low-power-threshold parametric decay instabilities of powerful microwave beams in toroidal fusion devices

We discuss the experimental conditions responsible for a drastic decrease in the power threshold of parametric decay instabilities under auxiliary electron cyclotron resonance heating (ECRH) in toroidal magnetic fusion devices when the upper hybrid (UH) resonance for the pump wave is absent. We show that for a finite-width pump in the presence of a nonmonotonic (hollow) density profile occurring due to plasma equilibrium in the magnetic islands or anomalous particle fluxes from the ECR layer, 3D localization of one or both daughter waves is possible. This localization leads to the full suppression of daughter wave energy losses from the decay layer and a substantial increase in the nonlinear pumping efficiency. This decreases the power threshold of nonlinear excitation, which can be easily overcome in current ECRH experiments utilizing 1 MW microwave beams. Different scenarios of extraordinary and ordinary wave decays are investigated. The secondary decays of primary daughter waves and pump wave depletion are considered as the most effective mechanisms leading to the transition of primary instability to the saturation regime. The proposed theoretical model was shown to be able to describe the anomalous phenomena discovered in ECRH experiments in different toroidal fusion devices all over the world.

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.

Commissioning of Electron Cyclotron Resonance Heating (ECRH) system on tokamak Aditya-U

The Aditya tokamak has been upgraded to Aditya-U by changing it’s vacuum vessel from rectangular to circular to accommodate the diverter coil for shaped plasma. The 42 GHz ECRH (Electron Cyclotron Resonance Heating) system has been integrated with the tokamak. The system is capable to deliver 500 kW power for 500 ms duration. The ECRH system with modified layout has been integrated successfully on Aditya-U; the burn pattern close to tokamak port ensures good Gaussian beam. Earlier the Aditya Tokamak used to operate at lower magnetic field (Max. 1.0 T) and the ECRH assisted low-loop voltage (˜6 V) plasma start-up experiments were carried out at second harmonic. Now the upgraded Aditya is operated at higher magnetic field up to 1.5 T. Initial ECRH experiments have been carried out at fundamental harmonic at magnetic field from 1.2 T to 1.35 T. Approximately 150 kW to 250 kW power in fundamental O-mode is launched from low field side. The ECH pulse of ˜40 ms is launched at the flat-top of plasma current. The effect of ECR heating is seen as some increase in soft X-ray signal. In some small plasma shots, the EC power is launched after the shot and EC breakdown effect has been observed.

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.